Observing the Astronomical A-List This chapter will provide you with guidance for observing 50 of the best objects to view in the heavens. These 50 objects were first introduced in the 'Astronomical A-List'. Many are visible to the unaided eye, most with binoculars and all with a small telescope. There are many catalogues of celestial objects. The one that is most relevant to amateur astronomers is the Messier Catalogue produced by Charles Messier in the 1700s to provide a catalogue of diffuse objects which might be confused with comets. It contains many of the best objects to observe in the northern sky including star clusters, supernova remnants and galaxies. But, as Messier was observing from Paris, it cannot include the wonderful objects in the southern skies. A recent catalogue, the Caldwell Catalogue compiled by Patrick Moore, does include southern sky objects but neither includes individual star systems that can be very interesting to observe such as multiple and variable stars. The object of the A-List is to be totally inclusive so that if an amateur astronomer has observed the majority then they will have seen an example of almost every type of interesting celestial object. Why 50 object? The list tries to include the all of the different types of objects – but only the best. The Messier and Caldwell catalogues include many globular clusters but perhaps only five stand out: M13, M92 and M15 in the northern sky and, even better, Omega Centuri and 47 Tucanae in the southern sky. All these have been included. The same reasoning was applied to the other types of objects too, and the list ended up at 50. Some entries are pairs of objects that can be seen together in a single telescope field of view such as the galaxies M81 and M82. So the actual number of individual objects is actually higher than 50. Here are some statistics. Twenty-five of the objects are in Messier's Catalogue and twelve are in the Caldwell Catalogue. This leaves 13 that occur in neither. These are mostly star systems that are well worth observing, such as Algol, the 'demon' star that 'winks' due to its occultation by its companion star every 2.867 days. [BOX] Each entry gives the Messier or Caldwell number, the common name (if any) and the type of object. Following this is a sequence of bold letters. These indicate how the object may be viewed: E The unaided eye B Binoculars L Telescope at low power M Telescope at medium powers H Telescope at high power. The positions of the object are for equinox 2000. Best time to find the objects Some of these objects can be seen whenever they are reasonably high in the sky when it is clear. Other objects, particularly galaxies, will require dark and transparent skies to be seen well. What does this mean? A dark sky is one when the Moon is not too bright (a thin crescent will not matter, a full moon will) and there is not too much light pollution – that is away from towns or cities. A transparent sky is one where there is little water vapour or dust in the atmosphere to scatter light. Not only do both of these reduce the apparent brightness of objects but they also reflects any light pollution – making their effect even worse. Nights following heavy rain are often ideal as the dust has been washed out of the atmosphere. Other objects, such as close binary stars, need a night of good seeing when the atmosphere is steady and the stars do not twinkle too much. Not surprisingly, objects look at their best on nights when the atmosphere is both transparent and steady but, sadly, these do not occur to often. How to find these objects in the night sky The introduction to each constellation indicates when the objects within them can be seen at their best. This is simply when they are highest in the sky so that you are observing through the minimum amount of atmosphere. The charts will show you where the objects are in relation to the brighter stars of the constellation. In these charts north is to the top, south to the bottom, east to the left and west to the right. In some cases, such as the Pleiades Cluster, no further instructions are needed, in others the object may be very close to a bright star and so can be located easily. Where the object is away from any bright stars there are three ways that can be used to find them: 1. Star hopping. One starts with an obvious bright star and follows a series of instructions to move from star to star to find the object in question. 2. The geometrical method. The object may make, for example, a right angle or equilateral triangle with two other stars so, having found these stars, perhaps by star hopping, one can visualise where the object should lie in the sky and point the telescope towards that region. 3. Scanning in Right Ascension (RA) or Declination (Dec). This requires an equatorially mounted telescope. Suppose an object is either precisely north or south of an obvious star. If this star is first centred in the finder scope or telescope field of view, and the RA axis locked, then moving the telescope in Dec, up or down as appropriate, will bring you to the desired object. By using the Dec setting circle it is often possible to offset the telescope pointing by the correct number of degrees so it will immediately appear in the field of view using a low power eyepiece. Just the same technique will bring you to objects that have the same Dec as an obvious bright star. In this case you can set the RA setting circle to zero so reading off the offset directly – note that the RA circle is graduated in minutes of time not angle; 1 hour is equivalent to 15Ί, 4 minutes of time = 1Ί. (When an object is on or close to the meridian (the north-south line) RA and Dec equate to Azimuth and Altitude respectively so this technique can also be used with an Alt/Az telescope mount.) The information about each object will indicate how you can best find them using the techniques described above. A word of warning: The details provided about each of the objects contain many facts such as brightness, size, distance and age. These are not always well known and different sources give different values. The text indicates when a fact is not known that accurately and gives the values that are believed to be most accurate. Please do not be too surprised if you come across differing values in other books or on the web! One aim of the A-list is for it to be used to encourage amateur astronomers, especially youngsters, to make their own observations to gain Bronze, Silver and Gold observing awards by submitting logbooks of their observations by e-mail. The certificates will be awarded jointly by the University of Manchester’s Jodrell Bank Observatory and the UK’s Society for Popular Astronomy. [Auth: Is this information available on the website? If so, I think we could omit this section because the info will be available on the web.] We would like the observers, should they wish, to submit a photograph of themselves and their telescopes from which we will build up a gallery of images on the A-List web pages. Observers under the age of 18 will be able to get Junior Awards, including gold, by observing solely from their home location. Astronomy societies are encouraged to help their younger members to gain the awards. For adults to gain a Gold Award, they will have to have made observations in both hemispheres so it will not be gained lightly! Observers who submit exceptional observing logs will gain starred awards – a Gold* award will be something to be really proud of! Full details on how to submit observing logs for the awards can be found at the following web address: www.jb.man.ac.uk/public/A-List/ [Again, we can put the details in the back of the book under Further Reading and Information Sites] (Note: In the following text the words 'Milky Way' are used in two ways: with capitals when they refer to our galaxy, which is called the Milky Way, and with small letters when it refers to the band of light across the sky which is how our galaxy appears to us.) The Constellations Andromeda and Triangulum These adjacent constellations contain two A-List objects: the galaxies M31 and M33. These, along with own galaxy, the Milky Way, are the three major galaxies in our Local Group of galaxies. The distribution of galaxies in the Local Group is shaped a little like a dumbbell with the Milky Way galaxy at the centre of one 'weight' with M31 and M33 at the heart of the other. This is why they appear relatively close in the sky. These galaxies are best seen during the months before Christmas, high overhead for northern observers, but low in the north for southern skywatchers. M31 – The Andromeda Galaxy Spiral Galaxy E B L M31 is the nearest large galaxy to us lying at a distance of 2.9 million light years. It is the largest galaxy in our Local Group and is a spiral galaxy of type Sb somewhat larger than our own Milky Way. (Type Sa spirals have a large nucleus and very tight spiral arms, type Sc have a small compact nucleus with very open arms. Type Sb lie in between.) With a magnitude of 3.4, M31 is visible to the unaided eye – the most distant object in the Universe that most people can see using just their eyes! The easiest way to locate it is to star hop from the star Alpheratz, ? Andromedae, which forms the north-western star of the square of Pegasus. First move two stars eastwards to the star Mirach then turn 90 degrees clockwise and move to the first bright star. Moving in the same direction by the same distance, you should then easily pick up a fuzzy white glow which is the nucleus of M31. If Pegasus is low in the sky, then a second way of finding it is to follow the 'arrow' made by the west-most three stars of Cassiopeia. Andromeda is 15 degrees (about three binocular fields of view) from the tip of the arrow. You can see the heart of M31 with your unaided eye as a fuzzy white patch of light, but it is best seen with binoculars under dark and transparent skies. Then, with well dark-adapted eyes, it is possible to get some feeling of the extent of the galaxy which stretches some 3 by 1 degrees in angular size – spanning half way across a typical binocular field. Using a telescope with a low power eyepiece is also rewarding as, if one slowly sweeps the field of view across the galaxy under dark skies, it is possible to pick out some of the dark dust lanes that cut across the faint starlight. M31 has two daughter galaxies M32 and M110. Both are elliptical and seen as tight fuzzy balls. M32, magnitude 8.4, appears closer to the centre of M31 and is a type E2 galaxy – almost spherical. M110, of type E6 and magnitude 8.5, is more obviously elongated. Position: 0h 42.7m +41o 16' M33 – The Triangulum Galaxy Spiral Galaxy B L M33 is a face-on type Sc spiral galaxy having a small compact nucleus and open spiral arms. It lies at a distance of 3 million light years and is the third largest galaxy in our Local Group. With a magnitude of 5.7, those with very acute eyesight under perfect conditions might just be able to observe it with the unaided eye but, as it is face-on to us, the light is well spread out and most will only ever observe it with binoculars or a telescope – and even with such optical aids, very dark and transparent skies are needed. From M31, retrace your step to the star Mirach, and continue on in the same direction for about the same distance – seven degrees, or between one and two binocular fields. In binoculars it appears like a little piece of tissue paper stuck on the sky – just a little brighter than the surroundings. Larger telescopes – say 8 inches (200mm) or more – will just show a hint of the open spiral arms surrounding the compact nucleus. With an overall diameter of about 60,000 light years, M33 is much smaller than M31 or our own Milky Way but it is more typical of spiral galaxies across the Universe. Position: 01h 33.9m +30o 39' The Constellation Aquila Altair, Aquila’s brightest star is the most southerly of the three stars that make up the summer triangle (Deneb in Cygnus, Vega in Lyra and Altair – see the Vulpecula star chart on pXX) so, not surprisingly, Aquila is best observed in August, September and October. It contains one A-list object, Eta ??) Aquila – the first Cepheid variable star to have been discovered. Eta (?) Aquilae Cepheid Variable Star E B M Eta Aquilae lies eight degrees (one and a half binocular fields) directly south of Altair. It is a bright Cepheid variable that varies in magnitude from 3.7 to 4.5 with a period of 7.2 days. Its variability was discovered by the English astronomer Edward Piggot in 1784. Try and observe it over a period of time and compare its brightness with Iota (?? Aquila four degrees away. (For northern observers put ? Aquila at the upper left of a binocular field and ? Aquila will be at the lower right – the opposite for southern observers). Iota Aquila has a magnitude of 4.36, so at minimum ? Aquila will be slightly less bright than ? (but probably not noticeably so), but at maximum it will be obviously brighter. Soon afterwards, Piggot's deaf mute neighbour, John Goodricke, discovered a second star, called ? Cephei, which varied in a similar way. These stars, which are some of the intrinsically brightest in the heavens, are unstable and oscillate in size and brightness with a very regular period. They became known as Cepheid Variables – after ? Cepheus, the second such star to be discovered. They have played an important role is measuring the size of the Universe. Early in the last century an American astronomer, Henrietta Leavitt, discovered that the brightness of these stars varied with a period which was in proportion to their absolute brightness. As they are very bright, they could be seen in quite distant galaxies. So by simply measuring their periods their absolute brightness can be found and hence the distance to the galaxy in which they are located. Observations of Cepheid Variables in remote galaxies by the Hubble Space Telescope have provided one of the best measurements of the scale size of the Universe to date. Position: 19h 52.6m +01o 00' The Constellation Auriga A constellation that climbs high in the northern sky after Christmas, but is only low above the northern horizon for southern observers. It lies along the plane of the milky way and, as a result, is where one might expect to find rich star fields and open star clusters – their stars recently formed from the dust and gas lying within the plane of our galaxy. Auriga contains three open star clusters M36, M37 and M38. The most impressive of these is M37 – an A-List object. M37 Open Star Cluster B M M37, the largest, richest and brightest of the three Auriga open star clusters, was observed by Charles Messier in 1764. The cluster covers a field of view 25 arc minutes across and has a visual brightness of 6.2, so it might just be glimpsed by the unaided eye under perfect conditions. It is easily seen through binoculars, lying just to the west of the line between ? Aurigae and El Nath, (?) Tauri. It provides a lovely view with a telescope at medium power and contains over 500 stars, of which some 150 are brighter than 12.5 magnitude, and thus individually visible in a small telescope under transparent skies. One can estimate the age of an open cluster by plotting its Hertzsprung-Russell diagram (a plot of luminosity against temperature) and seeing at what point stars are no longer seen on the main sequence – the region of the plot where stars spend the majority of their lives. More massive, and hence brighter, stars leave the main sequence sooner. M37 has at least a dozen Red Giant stars that have evolved away from the main sequence while the brightest stars still on the main sequence are just hotter than type F. This gives an estimated age of about 300 million years. The star cluster’s distance is of order 4500 light years. Given its angular diameter this gives an overall size of the cluster of about 23 light years. Position: 05h 52.4m +32o 33' Other Messier Objects in Auriga Sweeping north west of M37, binoculars will pick up the two other open clusters. M36 is reached first. It is smaller than M37, about 14 light years across, and contains perhaps 60 stars. The brightest are 9th magnitude and are more massive than those seen in M37. It is thus younger, around 25 million years old, and lies at about 4000 light years away. If you continue to move in the same direction M38 can be seen. It lies at a similar distance to M37 but, at magnitude 7.4, is less bright. At about 220 million years old, the brightest star amongst the 100 or so visible members of the cluster is a yellow giant of magnitude 7.9. Positions M36: 05h 36.3m +34o 08' M38: 05h 28.7m +35o 51' The constellation Cancer Cancer, which is a small and not very prominent constellation, lies between Gemini and Leo. It contains one A-List object – M44, the Beehive Cluster sometimes called Praesepe, the Latin for manger. Cancer is best seen during February, March and April. M44 – Beehive Cluster Open Cluster E B L M44 can be seen with the unaided eye as a misty patch just over 1 degree in diameter, extended slightly in a north-south direction. It lies in the triangle formed by ???? and ? Cancri. Overall it is of 3rd magnitude. However, the 15 brightest stars have magnitudes between 6.3 and 7.5, so those with keen eyesight should be able to resolve individual stars under perfect conditions! Binoculars or a small telescope resolve the misty patch into about 40 stars, whilst larger telescopes will show up to 200. Eighty are brighter than magnitude 10, and they range down to magnitude 14. It is at a distance of 577 light years and was formed about 730 million years ago. It is a little surprising that the Beehive Cluster was included in Messier's catalogue as this was designed to be a catalogue of objects that might be confused with a comet. It was added just before the first Messier catalogue was published in 1771. Another star cluster, the Pleiades – M45, also not typical of the Messier objects, was added too. It is suspected that these might have been included in his catalogue along with two nebulae in Orion so that it would contain more objects than the catalogue published by Lacaille in 1755! Position: 08h 40.1m +19o 59' Other object in Cancer Cancer contains a second open cluster M67, just two degrees to the west of Alpha Cancri. It is about five times further away than M44 and 6th magnitude overall. Why not look for it when you have viewed M44! Position: 08h 51.4m +11o 49' The constellation Canis Major This constellation lies to the south-east of Orion and contains the brightest star in the sky, Sirius. It includes just one A-List object, the open cluster M41. This will be best seen low in the south during the winter months for northern observers but comes almost overhead for southern observers during their summer. M41 Open Cluster E B M M41 is easily found four degrees almost exactly south of Sirius so that, for northern observers, if Sirius is at the top of the field of a pair of binoculars or finder scope, M41 will be seen towards the bottom. (Southern observers: put Sirius at the bottom of the field and look towards the top.) With an overall magnitude of 4.5, it should be visible to the unaided eye under dark skies. It contains around 100 stars of which 50 are in the range seventh to 13th magnitude and so should be visible in an amateur telescope. It has a beautiful orange-red star at its heart that makes a lovely colour contrast against the backdrop of fainter stars. This is a type K3 star of magnitude 6.9 and is about 700 times more luminous than our Sun. It is thought that M41 was observed by Aristotle in 325BC, and as such would have been the faintest object recorded in antiquity. It was added to Messier's catalogue in 1765. M41 lies at a distance of about 2300 light years and its age is estimated at 190–200 million years. Position: 6h 46m -20o 44' The Constellation Carina This southern constellation represents the keel of the ancient constellation Argo Navis – ship of the Argonauts. It contains Canopus, the second brightest star in the sky, and also the Eta Carina Nebula in which lies the massive star Eta (?) Carina. It is best observed in March to May when Carina comes almost overhead during the evening. C92 – Eta Carina Nebula Unstable star and nebula B M This nebula is one of the wonders of the southern sky and the largest diffuse nebula in our galaxy. About 260 light years across, it is 7 times the size of the Orion Nebula. The Eta Carina and Orion Nebulae are called HII regions as they contain clouds of atomic hydrogen that have been ionized by intense ultraviolet light from the many hot young O-type stars that are found within them – like those that make up the trapezium in the Orion nebula. Eta lies in one of the brightest regions of the milky way and is best located by starting at the constellation Crux and moving along the milky way westwards 13 degrees – about two and a half binocular fields. On the northern edge of the nebulosity lies the star Eta (?) Carina. It is about 100 times the mass of the Sun and many times its luminosity. A highly unstable star, it is ejecting material into the surrounding space making it highly variable in brightness. In 1843 it became the second brightest star in the sky, but then slowly faded from view, hidden by the ejected shroud of gas, reaching a minimum magnitude of 7.6 in 1968. Its brightness has since been rising and is now about 4.2 magnitude – so visible to the unaided eye again. The outburst in 1847 was the biggest explosion that any star has been known to survive and it resulted in the ejection of two lobes of expanding gas, now about 0.8 light years apart, which have been beautifully imaged by the Hubble Space Telescope. Position: 10hrs 43.8m -59 o 52'. Designer: Include HST Image if at all possible The Constellation Centaurus Though not as well known as Crux, the Southern Cross, there is no doubt that Centaurus is the most impressive southern constellation. The ninth largest constellation in the sky, over 100 of its stars are visible to the unaided eye. Centaurus lies too far south to contain any Messier objects. It does, however, contain three A-List objects: one of these, Alpha (?) Centauri, is the nearest star system to our Sun, another is the best globular cluster in the heavens and the third is a dynamic galaxy. Centaurus is best observed in the autumn when it comes almost overhead. Alpha (?) Centauri or Rigil Kentaurus Multiple star system E H To the eye, Rigil Kent looks like a single star of magnitude -0.3, the third brightest star in the sky. It lies at a distance of 4.35 light years. With a telescope it splits easily into two stars: the primary A is a G-type star very similar to our Sun which has an apparent magnitude of -0.04, while the secondary B is an orange K-type star of magnitude 1.2. They circle each other in a highly elliptical orbit with a period of 80 years having a mean separation of 24 AU (1 AU is the mean Earth-Sun distance – about the distance of Uranus from the Sun). They currently appear separated by 19 arc seconds and are thus easily split with a small telescope. There is a third member of the star system Alpha (?) Centauri C that is 13,000 AU from A and B. As it is measurably closer to us, at a distance of 4.22 light years, it is also called Proxima Centauri and is the nearest star to our solar system. Position: 14h 39.6m -60o 50' C80 – Omega (?) Centauri Globular Cluster E B M This is the most spectacular globular cluster in the heavens, appearing as a 4th magnitude 'fuzzy' star to the unaided eye. Moving north of Hadar, Beta (?) Centauri, by 5 and 10 degrees respectively, about two finder fields in total, are two 2nd magnitude stars Epsilon (?), the more southerly, and Zeta (?) Centauri. Omega (?) Centauri makes an almost equilateral triangle with these two stars on its eastern side. With an equatorially mounted telescope one can centre Zeta Centauri in a medium power telescope field, lock the declination axis and then sweep 28 minutes (of time – 7 degrees in angle) west in RA. This should bring Omega Centauri into the field of view. It contains perhaps 10 million stars in a region about 160 light years across. It lies at a distance of some 16,000 light years and has an apparent diameter of 30 arc minutes or more – the size of the full moon! It does not have a very bright central core as is found in 47 Tucanae. Omega Centauri is a wonderful sight in a telescope of 8 inches (200mm) or more, but still very rewarding even with a 4-inch (100mm) aperture if the sky is dark and transparent. With a mass of about 5 million solar masses it is 10 times more massive than the Milky Way's other large globular clusters such as M13 in Hercules and has a similar mass to some small galaxies! It is the most luminous globular cluster in our Milky Way and in our Local Group of galaxies is only outshone by the cluster named G1 in the Andromeda Galaxy. Position:13h 26.8m -47o 29' C77 – Centaurus A - NGC 5128 Active Galaxy E B M Centaurus A, so called because it is a very bright source of radio emission, is a large bright (7th magnitude) elliptical galaxy crossed by a very prominent dust lane. It is often rightly called a 'peculiar' galaxy and is one of the most interesting galaxies that we can observe. One possible scenario is that it may have 'eaten' a large spiral galaxy in the last few billion years. The distance to C77 is not well known but probably lies in the range 10-16 million light years. It forms a right-angled triangle with Zeta (?) Centauri and C80 Omega, (?) Centauri, (see instructions for finding both in the C80 description above). Using an equatorial telescope find three 3rd magnitude stars, ?? ? and???Centauri, forming part of a tight triangle five degrees north of Zeta (?) Centauri. With ? in the centre of a wide field of view, lock the declination axis and move westwards 24 minutes in right ascension (6 degrees) to bring C77 into the field of view. The galaxy subtends an area 17 by 13 arc minutes in size. With a small telescope and dark skies, both halves of the galaxy are visible with the dark dust lane about 40 arc seconds in width running across the middle. Larger telescopes and higher power will show two foreground stars superimposed on the southern half, the brighter being 12 and the fainter 13.5 magnitude. Centaurus A is one of the largest, most massive and luminous galaxies known. It harbours an 'active galactic nucleus' in which material is falling in towards a supermassive black hole – perhaps weighing 100 million times that of our Sun! Opposing jets of particles have left the active nucleus and radiation from them produces two vast lobes of radio emission above and below the galaxy. These make Centaurus A one of the strongest radio sources in the sky. Position: 13h 25.5m -43o 01' The Constellation Crux – The Southern Cross Though Crux is perhaps the best known of the southern constellations, it is not quite as prominent as one might expect. Lying right along the plane of the milky way it tends to recede into the great mass of stars under dark skies. It is, however, easily found by following the line from the very bright stars Alpha (?) through Beta (?) Centauri which leads directly to Crux. It includes three A-List objects. However one, the Coal Sack, is actually a lack of anything to see so should it really be called an object? Alpha (?) Crucis – Acrux Double star E H Alpha Crucis was too far south to have been given an ancient name, so Acrux is simply a combination of the A in alpha and Crux. Being of 1st magnitude (0.83), it is the 12th brightest star in the sky. Under high power, a telescope reveals it to be a binary system with two very similar B-type stars having magnitudes of 1.33 and 1.73, which are separated by 4 arc seconds. With surface temperatures of around 27,000K, they are highly luminous. In fact, the brighter star is itself a double, the two component stars orbit each other every 76 days but are too close to split with a telescope. Therefore Acrux is a triple star system. Position: 12h 26.6m -63o 06' C94 – The Jewel Box Open Cluster B L This is an open cluster, also called Kappa Crucis, which contains about 100 visible stars and is about 10 million years old. It lies some 7500 light years away and spans a 10 arc minute field of view so fills a volume of space about 20 light years across. Lying close to Beta (?) Crucis, it is easy to find and is best seen with binoculars or a telescope at low power. It contains many highly luminous blue-white stars along with a central red supergiant that makes a beautiful colour contrast. It was named the Jewel Box by Sir John Herschel who called it 'a gorgeous piece of fancy jewellery'. Position: 12h 53.6m -60o 21' C99 – The Coal Sack Dark nebula E B Just to the south of the Jewel Box is a pear-shaped region of obscuring, or dark, nebula, 7 degrees long by 5 degrees wide. Called the Coal Sack, it is a dense region of dust and gas about 2000 light years from us that is hiding the light from more distant stars. It is the most prominent and conspicuous dark nebula along the plane of the Milky Way and is easily picked out by eye as a big dark region against the bright band of light from the stars making up own galaxy. It will fill the field of view of all but the widest field binoculars. Position: 12h 52m -63o 18' The Constellation Cygnus The northern constellation Cygnus lies along the plane of the Milky Way and its brightest stars can get lost against the magnificent backdrop of stars if the sky is too dark and clear! These stars form what is called the Northern Cross. Cygnus contains over 11 open star clusters but none have made the A-List. Just two objects have: a beautiful double star system – perhaps the very best in the sky – and the faint nebulosity of a fading supernova explosion. Cygnus is best observed during the months from August to November. Albireo – Beta (?) Cygni Double star E B H This is perhaps the most beautiful double star in the sky with a wonderful colour contrast between the brighter component, at magnitude 3 and yellow-gold or amber in colour and its fainter, 5.1 magnitude, companion which is a vivid blue-green. They are separated by 34 arc seconds, so any telescope will split them even under the worst of seeing conditions. If 10x50 binoculars are held very steadily or mounted on a tripod they should also be able to show that Albireo is a double. (This is where image stabilized binoculars would be very useful!) They are 380 light years away and the primary component is a K-type star that has evolved off the main sequence while its fainter companion is a hotter but smaller, B-type, main sequence star. This pair provides with observable proof that stars evolve. As the two stars lie at the same distance we know that the yellow-gold star is six times more luminous than the blue star. Stars lying on the main sequence, burn hydrogen into helium in their cores. The more massive stars are brighter and hotter, emitting blue or white light, while the less massive ones are cooler and less bright, emitting yellow, orange or red light. A main sequence yellow star would therefore be far less bright than a blue star. So the yellow star in Albireo cannot be a main sequence star. In fact, it has evolved away from the main sequence and has become a yellow giant as it converts hydrogen and helium into heavier elements in its core. It has become far bigger in size and, although each square meter of the surface emits less light than that of the blue star, its surface area is so much larger, that overall it emits 6 times more light. Position: 19h 30.7m +27o 57' [Auht has a colour diagram showing the relative brightness and colour ] C33/34 – The Veil Nebula Supernova Remnant B L This is one of the most challenging of all the A-List objects to observe as it requires very dark and transparent skies to make out the faint nebulosity against the sky background. If you can find an observing site with little or no light pollution and the skies are transparent – perhaps when heavy rain has washed the dust out of the atmosphere – then parts of the Veil Nebula can be seen even with 8x40 binoculars. The Veil Nebula is the wispy nebulous remnant of a star that exploded some 5000 years ago in what is called a Type II supernova. For a brief period its brightness could have rivalled that of the Moon and it would have been visible in broad daylight. There are three main areas of nebulosity in a roughly circular outline: NGC 6992 and 6995, making up Caldwell 33, to the east with NGC 6960, C34, to the west. C34, though somewhat less bright than C33, is easier to find as it runs north-south 'through' the 4.2 magnitude star 52 Cygni. This star is 3 degrees south of Epsilon (?) Cygni, the eastern star of the Northern Cross. So centre this in your binoculars or finder and drop south by less than one field width to find 52 Cygni and, hopefully, the faint hints of nebulosity. The slightly brighter parts of the nebulosity that make up C33 are 2.5 degrees to the east and very slightly north so the whole of the veil can be easily encompassed using 8x40 or 10x50 binoculars. Using a telescope, only a 2-inch, wide-field, eyepiece and a short focal length could encompass the whole field. In any event, use your lowest power eyepiece. Using an equatorially mounted telescope, find 52 Cygni first and try to observe the nebulosity there, then, with the declination axis locked, sweep eastwards by 2.5 degrees. Lock the right ascension axis and inch up perhaps half a degree in declination to observe the nebulosity of C33. At the same declination as C33 moving two thirds of the way back towards C34, you may glimpse a third area of nebulosity running north-south through a slightly inclined chain of stars. Good hunting! Positions: C33 20h 56.4m +31o 43' C34 20h 45.7m +30o 43' The Constellation Dorado This constellation, close to the south celestial pole, is highest in the sky during the winter months. It lies south and a little east of the very bright southern star Canopus. Within its boundaries lies the Large Magellanic Cloud (LMC), an A-List object, whilst another, 30 Dorado, lies within the LMC itself. LMC – Large Magellanic Cloud Irregular Galaxy E B L The LMC is an irregular (or possibly barred spiral) galaxy that, at about 170–180,000 light years' distance, is the second closest galaxy to our own Milky Way (a dwarf elliptical in Sagittarius is closer). It is at least 50,000 light years in diameter and contains several billion stars. The LMC is the fourth largest galaxy in our local group of galaxies (dominated by the Milky Way, M31 and M33). To the unaided eye it appears just like a cloud seen against a dark sky, or a part of the milky way that has somehow become detached. It must have been known since humans first looked up at the southern sky but was 'discovered' by the Portuguese explorer Ferdinand Magellan in 1519. The LMC has an angular diameter of six degrees and will thus nicely fill the field of view of most binoculars. A telescope using at low power will be able to sweep across the galaxy picking up the many bright nebulae and open star clusters. The most spectacular of these merits an A-List entry in its own right. Position: 5h 23.6m -69 45' C103 – 30 Doradus Bright Nebula and Open Cluster B M The name 30 Doradus is used collectively for a bright nebula, commonly called the Tarantula nebula due to its similarity in appearance to the spider of that name, and the cluster of stars embedded within it. The name for this object comes from the fact that together they were first catalogued as a star. The Tarantula Nebula is an immense star forming region, vastly bigger than the Orion Nebula, where clouds of gas are being excited by the ultraviolet radiation emitted by the very hot young stars that have formed within it. In fact, were it as close as the Orion Nebula, it would cover an area of sky 30 degrees across! It is the most massive HII region, as these regions consisting mainly of ionized hydrogen are called, in the entire local group of galaxies. Lying at a distance of about 165–170,000 light years it is over 3000 light years across. (For comparison, the Orion Nebula is 40 light years across.) It contains a vast number of blue supergiants, type-O stars that are among the most massive and luminous known – around 100 times the mass of the Sun and perhaps 100,000 times as luminous. Blue supergiant stars have short lives and evolve quickly ending their lives in spectacular supernova explosions. Astronomers were treated to just such an event in 1987; the resulting remnant, 1987A, has been studied extensively ever since. To the unaided eye the 30 Doradus region, about the angular size of the full moon, appears rather like the Lagoon Nebula, M8, in Sagittarius (see pXX). Visually about 4th magnitude, it is best seen with binoculars and telescopes. The brightest stars within the cluster are between 14th and 12th magnitude and so can be seen individually under dark skies and good seeing. Even the smallest telescopes will reveal the complex structure, showing loops of excited gas and embedded stars. With telescopes of 8 or more inches aperture (>200mm) the visual appearance can even approach that seen in the wonderful photographs of the region. Position: 5h 38.6m –69o 05' The Constellation Gemini Gemini, best seen in the months around Christmas, lies north and east of Orion, its south-western part extending into the Milky Way. The heads of these two heavenly twins are delineated by the bright stars Castor and Pollux. Castor, although the fainter of the two, was given the Alpha designation by Bayer. It is a very interesting multiple star and A-List object. Gemini also contains an excellent open cluster close to the plane of our galaxy and a planetary nebula well worth seeking out. Castor – Alpha (?) Geminorum Multiple star system E H This is a visual double star made up of two blue-white stars, A and B, of 1.9 and 2.9 magnitudes respectively. The pair orbit each other every 400 years and are now as close as they ever get, making them somewhat of a challenge to split and requiring very good seeing. In fact their spectra show that each is itself a double star! Castor A is made up of two identical 2 solar mass stars orbiting each other every 9.2 days while the stars that make up Castor B orbit even faster, every 2.9 days. One minute of arc to the south will be seen a faint 9th magnitude star. This is also part of the Castor system and is itself a double star consisting of two M-type dwarf stars about 0.6 solar masses. Amazingly, they are only twice the diameter of our sun apart and orbit each other every 2 hours. So Castor is actually a sextuple star system that would look absolutely amazing should one be able to pass close enough on a space journey! Position 7h 34.6m +31o 53' M35 Open Cluster E B L Just north of the left foot of the northern-most twin, the unaided eye can see a hazy patch of light whose size is somewhat more than that of the full moon. Binoculars will resolve this into individual stars spread uniformly within the cluster. The brightest are eighth to 9th magnitude. It is probably best seen in a telescope at low power using a wide-field eyepiece. M35, also known as NGC 2168, is thought to contain about 500 stars within a volume about 24 light years across and lies at a distance of 2700 light years. Given a very dark site and good sky transparency you may also observe a small compact cluster, just 5 arc minutes across, that lies half a degree southwest of M35. This is NGC 2158, a cluster which, overall, has a visual magnitude of about 8 to 9. It is actually very similar is size to M35 but some six times more distant so appearing both smaller and fainter. Position: 6h 09m +24o 20' C39 – The Eskimo or Clown Nebula Planetary Nebula H This is a planetary nebula that has an unusually bright 10th magnitude central star which is a white dwarf. The nebula got its name from the fact that the central bright region forms a face (with the white dwarf as its nose) while a larger, more diffuse, outer ring makes up the fur hood of the Eskimo's parka or the ruff of the clown's outfit. Sadly, with a small telescope, this outer ring will not be visible except under very dark skies, but you will easily see the central region and white dwarf. An 8-inch (200 mm) telescope will give you a reasonable chance of detecting the outer envelope even in less good conditions. It lies quite close (2o 21') to the 3.5 magnitude star Delta (?) Geminorum. To the south and east of ? Geminorum lie three 5th magnitude stars: 56, 61 and the double star, 63 Geminorum forming a right angle triangle. The Eskimo nebula lies just 37 minutes of arc to the south and a little east of the left hand star of the double. With a low-power eyepiece it will appear as a 'star' just to the south of an 8th magnitude star just 1.6 arc minutes away – the pair forming an apparent double. Increasing the magnification will reveal the true nature of the nebula as a 10th magnitude central star surrounded by a bright region of nebulosity. Its visual magnitude is about 9th magnitude. Incidentally, ? Geminorum lies just 10 arc minutes from the plane of the ecliptic (the path of the Sun across the sky) and it was very close to both this star and the Eskimo Nebula that Clyde Tombaugh discovered the planet Pluto in 1930. As these notes were being written the point was reinforced as another planet, Saturn, passed just 4 arc minutes away from the star! Position: 7h 29.2m +20o 55' The Constellation Hercules In May, June and July the constellation Hercules comes almost overhead for those in the northern hemisphere. It lies roughly half way between the two bright stars Vega, in Lyra, and Arcturus, in Bootes. During those same months it can be seen just above the northern horizon from the southern hemisphere, but its low elevation may make the two A-List objects, M13 and particularly M92, difficult to observe from the southern hemisphere. (At Sydney, M92 rises 12 degrees above the horizon, M13 rises 20 degrees above.) M13 – Hercules Cluster Globular Cluster E B M This is arguably the finest globular cluster in the northern sky and was discovered by Edmund Halley in 1714. Charles Messier logged it as he tracked the path of a comet in 1779. However, with a visual magnitude of 5.8 (the noted observer, James O'Meara puts it at 5.3) it is visible to the unaided eye so may well have been seen in antiquity. M13 lies along the western side of the 'keystone' about 2.5 degrees south of the star Eta (?) Herculis – it is thus very easy to find. M13 contains several hundred thousand stars in a volume of space 145 light years across. It has an angular diameter of 20 arc minutes and lies around 25,000 light years from us. These statistics hide the sheer beauty of the cluster which needs a really dark and transparent sky and good seeing to observe at its best. (These do not often occur together!) The dark sky enables fainter stars to be seen and, if the atmosphere is steady, the stellar images will be 'tighter' allowing even fainter stars to stand out against the background glow of light from the cluster. It will look good in any telescope, however small, but the view through a 10- inch (250mm) or larger telescope under perfect conditions can take your breath away, with M13 looking almost three-dimensional in appearance. Curving arcs of stars appear to extend southeast and northwest branching out into the surrounding space. Several stars reach 11th and 12th magnitudes with 20 or more at 13th magnitude. Position: 16h 41.7m +36o 28' M92 Globular Cluster B M M92 is also a superb globular cluster, somewhat overshadowed by its near neighbour M13. With a visual magnitude of 6.5, it is just on the limit of unaided eye visibility but really needs binoculars or a telescope to observe. M92 is north and a little east of the north-eastern star of the keystone's, 3rd magnitude Pi (?) Herculis. With an equatorially mounted telescope, centre the finder or low-power field of view on this star and then move very slightly northeast to the 5th magnitude star 69 Herculis. Lock the RA axis then sweep north in declination just under 6 degrees. M92 will then be in the centre of the field. It lies at a distance of about 27,000 light years and has an angular extent of 14 arc minutes corresponding to a diameter of 109 light years. The total mass of the stars is of order 300,000 solar masses. At a very similar distance from us as M13, some of the brightest stars within it will be individually resolved with good seeing and transparent skies. Remember that fainter stars can be seen when 'averted vision' is used – just look slightly away from the centre of the cluster. Position 17h 17.1m +43o 08' The Constellation Leo Leo follows Orion across the sky so as Orion is setting, Leo is highest in the sky. It is seen best in March, April and May. Leo lies away from the obscuring dust of the milky way. This means that it is devoid of any open clusters or star-forming regions but, on the other hand, the lack of obscuring dust allows distant galaxies to be seen. Two pairs of Messier galaxies are included in the A-List. M65 and M66 Spiral galaxies B L M A pair of 9th magnitude spiral galaxies visible either together in a low-power eyepiece or individually at medium power. M65 is a type Sa spiral that lies at a distance of 35,000,000 light years and has a magnitude of 9.3. M66, considerably larger than M65, is a type Sb spiral lying slightly further away at 41,000,000 light years and fractionally brighter at magnitude 8.9. These two galaxies are located halfway between Theta (?) and Iota (?) Leonis, and just to the east of the 5th magnitude star 73 Leonis. They can be seen, along with a third galaxy, NGC 3628, just to the north, with a pair of 8x40 or 10x50 binoculars – providing that the sky is very dark and transparent. Sadly, in our light-polluted skies such conditions do not occur often. The 9th magnitude brightness given for these galaxies seems quite bright, but this is the integrated magnitude over the whole galaxy and each part only shines with the equivalent brightness of a 12th magnitude star. These 'faint fuzzies', as they are often called, are quite a challenge! Positions: M65 11h 18.9' +13o 05' M66 11h 20.2' +12o 59' M95 and M96 Spiral Galaxies L M This pair of galaxies, separated by just 42 arc minutes, are located close to another Messier galaxy, M105. They lie between the 5th magnitude stars 52 and 53 Leonis and are almost due east of Regulus. A good way to find them with an equatorially mounted telescope is to put Regulus towards the southern side of the finder scope or low-power telescope field of view, lock the declination axis and move just under 9 degrees east. This should bring you to M95, with M96 a further 42 arc minutes east of M95. M95 is a barred spiral of Hubble type SBb lying at a distance of 38,000,000 light years with an integrated visual magnitude of 9.7. Under ideal conditions it looks a little like Saturn; a central concentration of light – the nucleus – looking like the planet and the bar looking like the rings. This is visible with a 4-inch (100mm) telescope but a larger aperture will certainly help! M96 is slightly further away, at 41,000,000 light years, and is a type Sa galaxy with a magnitude of 9.2. It has a very condensed central core and can take on the appearance of an eye. These objects are not the easiest to see but, when you do succeed, reflect on the fact that you are looking back in time many millions of years! Positions: M95 10h 44.0' +11o 42' M96 10h 46.8' +11o 49' The Constellation Lyra This contains two A-List objects: a wonderful multiple star system and a planetary nebula that is easy to find and observe. These are best observed in July, August and September, but are sadly rather low above the northern horizon for observers in the southern hemisphere. Epsilon (?) Lyrae – The Double Double Multiple Star System E B H This is perhaps the best multiple star system that can be easily observed! Look first with binoculars and find Vega, at magnitude -0.04 the fifth brightest star in the heavens and one of the stars that make up the Summer Triangle. Centre the field of view on Vega and, to the east and a little north, the two 'stars', epsilon1 and epsilon?, that make up Epsilon Lyrae will be seen as a double, separated by 208 arc seconds. Once found you may, given keen eyesight, be able to 'split' the double using just your eyes. Each of the two 'stars' is just brighter than 5th magnitude. If you observe them with a telescope using reasonably high power when Lyra is high in the sky and the atmosphere is steady, you should see that each of the two 'stars' is itself a double, one orientated along the line joining the two major components, the other at right-angles to it. The former pair (epsilon1) are of magnitude 4.6 and 5.0 separated by 2.6 arc seconds with the latter (epsilon2) having magnitudes of 5.1 and 5.5 and separated by 2.3 arc seconds – a small separation, which is why the seeing has to be good to split them. They all appear white to the eye. Each pair is orbiting each other with periods of about 1200 (epsilon1) and about 600 (epsilon2) years, while the two pairs are also gravitationally bound and orbit each other about once every million years. Position: 18h 44.3m +39o 40' M57 – The Ring Nebula Planetary nebula H Perhaps the easiest planetary nebula to observe, the Ring Nebula looks like a smoke ring or doughnut. It lies just below the line joining Gamma ?????and Beta (?? Lyrae, being slightly closer to ? Lyrae. Just sweep a telescope between the two stars and, given reasonably dark skies, it should be immediately obvious. It has a magnitude of 8.8 and is 1.4 by 1.0 arc minutes in angular size. It is now believed that it is a shell or possibly a cylinder of bright glowing gas that was blown off when the progenitor star exploded leaving the very hot (~100,000K) white dwarf star that is seen at its centre in photographs. This bluish compact object, about the size of the Earth, is only 15th magnitude and so, while showing up well in photographs, will only be seen in the biggest backyard telescopes. Ultraviolet light emitted by the white dwarf excites the surrounding gas to glow. The most recent estimates give M57 an age of about 7000 years. It is thought to lie at a distance of just over 2000 light years and is about half a light year across – or 500 times the size of the solar system. Position: 18h 53.6m +33o 02' The Constellation Orion Probably the best known of all the constellations, Orion dominates the sky in January, February and March. Perhaps surprisingly, given its closeness to the milky way, it is rather devoid of deep-sky objects that merit inclusion the A-List. However, the one that does is one of the true highlights of the heavens. M42 – Orion Nebula Bright nebula E B L M H The Orion nebula, seen as a diffuse glow in the sword of Orion, is one of the most beautiful objects in the heavens. It is a region of star formation 1600 light years from us, the glowing gas excited by ultraviolet light from the young, very hot stars at its heart. It extends over a region of angular size 1 by 1.5 degrees but is part of a much larger cloud that covers much of the constellation, over 10 degrees across. Photographs show swathes of nebulosity such as 'Barnards Loop' arching around the M42 region. Seeing that M42 is one of the first deep-sky objects that most astronomers observe – and rightly so – it is surprising that there is no mention of it in ancient records. Apparently, not even Galileo observed it, even though, at about 4th magnitude, it is visible to the unaided eye under reasonably dark skies. The gases that make up the nebula are predominantly hydrogen and helium dating from the origin of the Universe along with nitrogen and oxygen produced by nuclear fusion as stars evolve. The electrons are stripped from their nuclei by the ultraviolet light and, as they re-combine, give off well-defined spectral colours: a pinky red for hydrogen and green and blue for oxygen. The eye is not sensitive to red so with a reasonably large telescope, the brighter parts of the nebulosity appear greenish in colour. Observing the nebula under dark and transparent skies with a telescope at low power shows the wonderful looping whirls of dust and gas seen across a wide region. Using a medium-power eyepiece, the central region of the nebula will be seen to harbour a number of bright stars; three almost in a line and 4 making up what is called the 'trapezium'. A dark dust cloud can be seen intruding in towards the bright inner core of the nebula surrounding the trapezium. This is called the 'fish's mouth'. Moving to high power will show the four stars, A to D, of the trapezium. The brightest is 5th magnitude and provides the vast majority of the ultraviolet light that excites the gas in the nebula. Two others are 6th magnitude with the faintest 8th magnitude. Using very high magnification under nights that are both dark and have good seeing (which are rather rare), even a 4-inch telescope will pick out a fifth, 10th magnitude, component called E, making up a flattened triangle with the closest pair of stars. It lies just outside the trapezium. There is a sixth, 10th to 11th magnitude, star called F on the opposite side of the trapezium but this is much harder to see. The Constellation Pegasus Pegasus is at its highest in the sky during September, October and November and its square – the body of the winged horse – is very well known. A very good test of sky transparency is to count the number of stars that you can see with your unaided eye within the square. If you count four, then the transparency is reasonably good, but if you can see more than four, then it will be an excellent night for hunting down faint galaxies and nebula! Pegasus is away from the plane of the Milky Way so is not rich in deep-sky objects but there are two A-List objects within its boundaries. M15 Globular Cluster B M M15 is one of the six globular clusters brighter than 7th magnitude in the northern sky. It lay some 30,000 light years away beyond the thick dust lanes and star clouds in Sagittarius. M15 is easily found by following the line of the two stars Theta (?) and Epsilon (?) Pegasi a further 4 degrees in a north-westerly direction. So, by placing Epsilon at the appropriate side of a binocular or finder scope field, M15 should be visible towards the other. With medium power, the cluster is seen to lie within a triangle of three stars, one seventh and two 8th magnitude. M15 lies at a distance of 37,000 light years. Visually it appears to have an angular extent of about 7 arc minutes and has a very bright and compact core. Its overall brightness of magnitude 6.2 means that, under ideal viewing conditions, it could be seen with the unaided eye. Its brightest individual stars are 12 to 13th magnitude, so may be picked out individually against the glow of the fainter stars in the cluster when observing with a large aperture telescope. A recent Hubble Space Telescope image shows that stars in the dense core of M15 are crowded together closer than anywhere else in our galaxy, except at its very heart. At least 30,000 stars pack into a volume just 22 light years across. This could be the result of a process called 'core collapse' or perhaps a massive black hole is hidden at its centre! Position 21h 30.0m +12o 10' 51 Pegasi Star with planet B Perhaps this is an odd choice for the A-List. There is nothing to see except a single 5.49 magnitude star that can be found just to the west (about 2 degrees) of the line joining the two stars at the western side of the square, Alpha (?) and Beta (?) Pegasi. It is a little nearer to Alpha. So why is it included? Simply because it has a place in history – the first Sun-like star around which a planet was detected. The planet, called 51 Pegasi b, has a mass of at least 0.47 times that of Jupiter. It orbits 51 Peg every 4.23 days; compare that with the 88 days that it takes Mercury to orbit the Sun! Its circular orbit has a radius of only 0.05 AU, just 7.5 million kilometres (4.6 million miles). So close to its star, its atmosphere would be extremely hot, perhaps 1000 degrees Celsius, and it would suffer extreme tidal forces. No one expected that a giant planet could exist so close to its sun – giving theoreticians much food for thought. So how was it discovered? The light reflected by it would be lost in the glare of its sun so it cannot be seen. However, both the planet and star orbit their common centre of gravity so 51 Peg is moving round in a small circle. This means that sometimes it is coming towards us and sometimes away from us giving a very small shift in its spectral lines due to the Doppler shift. It was this that was detected in the spectrum of 51 Peg so allowing the period and minimum mass of its planetary companion to be calculated. Since then, many other planets have been detected by this method, but 51 Pegasi b will always have the distinction of being the first! Position: 22h 57.6m +20o 46' The Constellation Perseus Perseus lies along the plane of the Milky Way, but as we view it, we are looking away from the galactic centre, so that there are fewer clusters within its boundaries than when looking towards Sagattarius or Scorpius. However, the two clusters that form the Perseus Double Cluster provide one of the best binocular sights in the heavens and Perseus also harbours a very interesting star system, Algol. Whilst, for northern observers, Perseus comes high overhead in the latter months of the year, for southern observers it only just rises above the northern horizon so that, whilst they should be able to observe Algol, sadly, the Double Cluster will lie below their horizon. C14 - The Double Cluster Twin Open Clusters E B L M Visible to the unaided eye as a hazy patch in the Milky Way, binoculars or a small telescope at low power can show both these two beautiful clusters in the same field of view. They are most easily found by sweeping with your eyes, binocular or finder scope to the east and a little south of Cassiopeia, following the line set by its bright stars Gamma (?) and Delta (?). The bright cores of the two clusters are separated by just less than one Moon diameter, 25 arc minutes, and together they cover over a degree of sky. Given their separation and individual visual brightness of between 4th and 5th magnitude, one should be able to see them as separate entities. But this is not usually the case. Surprisingly perhaps, the best chance to do so is by observing them just as twilight ends; when they first appear to the eye but the background stars of the Milky Way are still invisible. (In a similar fashion, the brighter stars of constellations – those that form the patterns that we learn – show up far more clearly under twilight or light-polluted conditions than when seen in really dark conditions. This is why you are advised to learn the shapes and locations of the constellations when the sky conditions are not too good!) The two clusters, also known as h and Chi (?) Persei, are a beautiful sight in 10x50 binoculars; each cluster having a bright centre and many individually resolved stars. With a low-power eyepiece both can be seen in the same field and then, moving up to medium power, each can be observed in detail. They lie in the Perseus spiral arm of the Milky Way some 7300 light years away, and were both formed about 3 million years ago. Position: 2h 20.5m +57o 08' Algol – Beta (?? Persie Eclipsing Binary E B Algol is one of the most remarkable and most famous individual stars in the sky. Its Arabic name is Al Ghul, which means the 'demon' star (Ghul is related to 'ghoul', a ghost). Why a demon? Because it winks! Every 2.87 days its brightness quickly drops from magnitude 2.1 to 3.4 and then rises again to 2.1 over a period of 10 hours. John Goodricke of York was one of the first astronomers who discovered its regular brightness variations in 1782–3. Much later, in 1881, astronomers realised that the effect could be caused by a binary system in which the orbital plane of the two stars was almost in line with the Earth, so that every 2.87 days there is a partial eclipse! This is when the fainter star of the two comes in front of the brighter. In between each major drop in brightness, there is a much smaller drop as the brighter star comes in front of the fainter. The primary star is a blue B-type star with a surface temperature of 12,000K. The secondary is a much larger but dimmer K-type orange giant star. Interestingly, the two stars do not seem to be following the normal rules of stellar evolution. More massive stars evolve faster than less massive ones, so the orange giant – which has evolved away from the main sequence – should be more massive than the blue primary star. But it has less mass! It appears that material may be flowing from the giant star (so reducing its mass) onto the normal star whose mass is thus increasing. Position: 3h 8.2m +40o 57' The Constellation Sagittarius Lying in the direction of the centre of the Milky Way, Sagittarius is one of richest constellations in the sky. Sadly, for observers in northern Europe, it tends to be lost low in the southern horizon during the short summer months. An excellent reason to travel to the southern hemisphere to observe it where it is high in the sky during the winter! Its main stars form the shape of a teapot. It contains 15 Messier objects, four of which have been included in the A-List. M8 – The Lagoon Nebula Diffuse Nebula and Open Cluster B L M This is perhaps one of the most beautiful regions of the Milky Way. It lies just over 5 degrees to the west of and slightly north of Lambda (?) Sagittarii (the top of the teapot's lid). So placing Lambda on the appropriate side of a binocular or finder field of view should bring M8 into the other side of the field. It is very hard to miss and, at about 5th magnitude, can be picked out using your eyes alone. The region is one and a half degrees wide with the main region of bright nebulosity at one end and a young open cluster, NGC 6530, towards the other. A dark east-west dust lane, most obvious at low and medium powers, may be why it was called the Lagoon Nebula, though it looks more like a river than a lagoon. The gas that glows in the bright emission regions is excited by the ultraviolet light from a very hot 6th magnitude star, 9 Sagittarii, and others, most of which are hidden by dust. The very brightest part of this region is named the 'hourglass nebula' after its shape. Though not seen in small telescopes, the nebula contains many small dark compact 'globules'. These are collapsing clouds of dust and gas, typically 10,000 AU across, which are new stars in their first stages of formation. Position: 18h 03.6m -24o 23' M17 – The Swan or Omega Nebula Diffuse Nebula B M M17 has many names as well as the two most common ones, given above. It is also called the Horseshoe or Lobster nebula – the latter particularly in the southern hemisphere. As in all bright nebulae, it is shining as a result of gas being excited by the ultraviolet light emitted from the hot young stars embedded within it. It is found right at the northern edge of Sagittarius on the boundary with Scutum, 9 degrees north and a little west of the star Lambda (?) Sagittarii, that delineates the top of the teapot's lid. So, starting with this star at the left (southern observers: right) of the field of a pair of binoculars or finder scope and move northwards from this star by about two field widths. This 6th magnitude patch of light, somewhat larger than the size of the full moon, is easily visible in binoculars and can even be picked out by the unaided eye under dark skies. A telescope shows the swan-shaped central region of the nebula whilst among fainter wisps of nebulosity below the swan, can be seen an open cluster of 9th magnitude blue stars. The brightest part of the nebula is about 15 light years across and contains sufficient gas to form hundreds of stars. Position: 18h 20.8m -16o 11' M20/M21 - Trifid Nebula Diffuse Nebula and Cluster B L M M20 lies just north of the Lagoon nebula (see above as to how to find it) and will be encompassed with both M8 and the open cluster M21 in a single 2 degree field – as can be observed by most small telescopes at low power. It was discovered by Charles Messier in 1764. He described it as a cluster of eighth to 9th magnitude stars within an envelope of nebulosity. It is called the Trifid Nebula, as it is seemingly split into three segments by dust lanes that radiate from the bright central region. It also looks like a clover leaf. Colour photographs show that as well as the emission nebulosity that appears red in photographs (emitted by excited hydrogen), there is also an extensive blue reflection nebula on its northern side. Moving north-east by one degree from M20 is M21, a 6.5 magnitude open cluster containing about 60 stars. There is a strong concentration towards its centre. The brightest stars are of type B0 – giant stars that have very short lives – so we know that this cluster must be very young, perhaps only 4.6 million years old. Positions M20: 18h 02.6m -23o 02' M21: 18h 04.2m -22o 30' The Constellation Scorpius Scorpius, with its tail arching below Sagittarius, includes Antares, the 13th brightest star in the sky. An irregular variable, its brightness varies from 1.02 to 0.86 magnitudes. Note that the constellation is called Scorpius not Scorpio – as used by astrologers! It lies in the plane of the Milky Way galaxy and is thus rich in both globular and open star clusters. Four of the latter are included in the A-List. It is low in the south for northern observers during summer, but high in the winter skies for those in the southern hemisphere. M6 - The Butterfly Cluster Open Cluster E B L M6 is a cluster of about 120 stars that suggests the outline of a butterfly with open wings. The brightest stars in the cluster are of magnitudes 6 to 7, with 60 greater than 11th magnitude. The brighter stars are blue-white with the exception of the very brightest of all; a yellow-orange semi-regular variable star that alters in brightness from 7 up to 5.5 magnitudes every 850 days or so. It makes a very nice colour contrast with the blue-white stars around it. Overall, the visual magnitude of the cluster is 4.2 so it should be easily visible to the unaided eye under dark skies. It lies 4.5 degrees north-west of M7 (see below) and is probably found most easily in binoculars or finder scope by scanning north-west from M7 when both will be seen in the same field of view. M6 extends over an area some 20 arc minutes across; two- thirds the angular diameter of the Moon. Its distance is difficult to estimate due to the obscuration of light by the dust in the Milky Way but is approximately 1600 light years away, which would make the cluster extend across a volume of space 12 light years in diameter. It is estimated to be 100 million years old. Position: 17h 40.1m -32o 13' M7 – Ptolemy's Cluster Open Cluster E B L This is by far the most obvious cluster in Scorpius, standing out in a very bright region of the milky way. Just imagine that the 'teapot' of Sagittarius, to the north-east, was pouring tea. M7 is just where you would put the cup! It is obvious in binoculars and, with an overall visual magnitude of 3.3, easily visible to the unaided eye. A splendid cluster, it was mentioned by Ptolemy in 130AD and it has been suggested recently that it be named after him – an excellent idea. The cluster contains about 80 stars spread over an angular field 80 arc minutes across and all shine brighter than 10th magnitude. Its brightest member is a yellow giant of 5th magnitude and a dozen or so are brighter than 7th magnitude. It is thought to be over twice as old as its neighbour M6, which makes it about 220 million years old. Position 17h 53.9m -34o 49' C76 – The Scorpius or Northern Jewel Box Twin Open Cluster E B L This is a name given to yet another wonderful region of the milky way at the western-end of the tail of the scorpion, almost due south of Epsilon (?) Scorpii. Sweep south from Epsilon Scorpii by approximately two binocular fields of view, passing on the way a pair of 3rd magnitude stars Mu 1 (?1) and Mu 2 (?2) and stop as Zeta (?? Scorpii, another double star system, enters the field of view. Interestingly, Zeta is one of the most luminous stars in our galaxy – over 100,000 times as bright as our Sun! Just north of Zeta (?? Scorpii is a bright and compact open cluster NGC 6231, north-east of which are two loose open clusters Cr 316 and Tr 24 – numbers 316 and 24 respectively in the catalogues of clusters compiled by P. Collinder and R. J. Trumpler in the 1930s. Continuing north is the second compact cluster NGC 6242. A bright region of nebulosity called IC4268 lies just to the north of the loose clusters. This region has all the appearances of a comet – the 'tail' arching north-west away from the 'coma' formed by NGC 6231 – so the region is also called the 'False Comet' Nebula. It marks one of the Milky Way's nearby spiral arms, some 6000 light years distant. NGC 6231 is very young cluster, perhaps 3.2 million years old. This is indicated by the fact that its brightest star is an O-type star of apparent magnitude 4.7. Such stars burn up their fuel very quickly and so have very short (but dramatic) lives! Position: 16h 54.2m -41o 50' The Constellation Taurus Taurus is highest in the sky during December and January, but easily visible in neighbouring months for those in the northern hemisphere. It forms part of a spectacular skyscape along with Orion, Canis Major and Gemini. Taurus contains three A-List objects; the two closest open clusters to us in the heavens and also a supernova remnant. C41 – The Hyades Cluster Open Cluster E B L This famous V-shaped cluster outlines the head of Taurus, the Bull, with the bright star Aldebaran as his eye. In fact, Aldebaran is not part of the cluster and lies at only half of its 150 light-year distance. Stars can be shown to be part of a cluster by measuring their 'proper motion' – the path they take across the sky. All the stars of the Hyades are moving along parallel paths eastwards towards a point close to Betelgeuse in Orion. In contrast, Aldebaran is moving southwards. The cluster has a central core about 10 light years in diameter with outer members spread over a volume some 80 light years across. It is thought to be about 730 million years old. Its angular extent is over five degrees across so is best seen with binoculars, but a telescope at low power can give a nice view of its core including a prominent double star. Position: 04h 26.9m +15o 52' M45 - The Pleiades Cluster Open Cluster E B L Perhaps the most beautiful open cluster in the heavens, it is often called the Seven Sisters. This name comes from a mythological story in which seven sisters were placed in the heavens to overcome their grief over the death of their father. Interestingly, almost no-one would ever see precisely seven stars. Most people actually see six stars with their unaided eye under dark skies. There is a significant drop in brightness between the sixth brightest star and the next four which still lie above the nominal unaided eye limit of 6th magnitude. So those that see more than six are likely to see nine or 10. Amazingly, under superb conditions with well dark-adapted eyes some have managed to discern up to 30! The cluster is about two degrees across and binoculars or a short focal length telescope at low power can encompass the whole of the cluster – one of the best sights in the sky. Particularly pretty is a little triangle of stars just besides Alcyone, the brightest star in the cluster, and a double star appearing almost in the centre of the cluster. The Pleaides cluster is very young, only 70–100,000,000 years old, and contains about 100 stars. It is just 380 light years away from us, so is a relatively close neighbour in space. Colour photographs show that the brighter stars of the cluster are surrounded by blue reflection nebulae. These are caused by starlight reflecting off dust grains in the space between the stars and so, not surprisingly, is most apparent close to the three brightest stars. If the sky is very transparent and dark, this nebulosity can be observed, but almost in reverse: the background sky at the very centre of the 'square' formed by the four brightest stars of the cluster appears distinctly darker than that close to the stars themselves. Position: 04h 26.9m +15o 52' M1 – The Crab Nebula Supernova remnant M This is one of two supernova remnants in the A-List (the other being the Veil Nebula in Cygnus) and is one of around 100 such objects known of in our galaxy. The progenitor star was seen to explode in AD1054 and is recorded in Chinese texts. There are no European records perhaps because, in the eyes of the church, the heavens were meant to be perfect and unchanging so the scribes, who were generally monks, could well have been reluctant to chronicle such events. Since the explosion, the gas that was ejected as the star exploded has been expanding out into space and is gradually getting fainter. The fact that it still glows in the visual part of the spectrum at all is due to a powerhouse at its centre – the Crab Pulsar. The core of the massive progenitor star collapsed down until it was no bigger than the size of a big city – but still weighing more that our Sun – and began to spin rapidly. Energy is radiated away from the region of space above its magnetic poles and it acts like an interstellar light house; two beams of light and radio waves sweeping around the sky 30 times a second. Some of the energy radiated away is keeping the nebula glowing. The nebula is a 6 by 4 minute of arc parch of nebulosity just one degree northwest of the 3rd magnitude star Zeta (?) Tauri. Binoculars or a telescope at low power can thus show both in the same field so, given dark skies, M1 easy to find with 8x40 or 10x50 binoculars. Higher magnification using a telescope may make it somewhat easier to see. The pulsar is one of two stars at the centre of the nebula, both about 16th magnitude. Given a 10-inch (250mm) telescope and a superb, dark sky, observing site with excellent seeing conditions it can even be observed – the most exotic object anyone could ever expect to see. Its name, the Crab Nebula, was given to it by the sixth Earl of Rosse who observed it with a 72 inch reflector, then the biggest telescope in the world, from Birr Castle in Ireland. In his drawing it appears somewhat like a horseshoe crab (some would say pineapple), hence the name. [Designer: It would be great if we could have Lord Rosse’s picture here – I have produced a version IM] Position: 04h 26.9m +15o 52' The Constellation Tucana This small constellation, close to the south celestial pole, is best seen in springtime. Though small, with few bright stars, it contains two of the jewels of the southern sky. SMC – The Small Magellanic Cloud Irregular Galaxy E B L This is a small companion galaxy to our own Milky Way. It lies at a distance of 210 to 250 thousand light years, making it the third nearest galaxy to us (after the Large Magellanic Cloud and a dwarf elliptical galaxy in Sagittarius). It covers an angular extent of 280 by 160 arc minutes appearing as a faint cloud against a dark sky. The SMC must have been known since antiquity but it was 'discovered' by the explorer Ferdinand Magellan in 1519. To the unaided eye it appears like a little piece of the milky way. Binoculars and telescopes will show that it contains both open star clusters and bright nebulae. The SMC contains a large number of hot blue stars. As these have a relatively short life it shows that the SMC has recently undergone a period of star formation. The SMC follows a nearly circular orbit around our galaxy. Knowing the period and diameter of the orbit enables a calculation to be made of the total mass of the Milky Way galaxy. This turns out to be substantially more than the calculated mass of normal matter (in the form of stars, planets, gas and dust) and thus indicates the presence of 'dark matter' – a form of matter about which little is yet known. Position: 00h 52.7m -72o 50' C106 - 47 Tucanae Globular Cluster E B M Close to the SMC lies the second largest and brightest globular cluster in the sky. With a visual brightness of very close to 4, it is easily seen with the unaided eye under dark skies – its stars covering an area about the size of the full moon. It ranks second to Omega Centauri but it is only 0.05 magnitudes fainter – a virtually insignificant difference. Both 47 Tucanae and Omega Centauri exceed the brightness of any other globular clusters circling our galaxy by a factor of three. It was not 'discovered' until 1751 when it was observed by Lacaille, but had been charted as a 'star' in Bayer's 1603 Uranometria. (It was Bayer who gave the brighter stars their Greek letter designations.) 47 Tuc (its common abbreviation) lies just over 13,000 light years from the Sun – one of the closest globular cluster to us. Its brightest stars are about 14th magnitude so they can be resolved with an 8inch (200mm) telescope as individual points of light against the diffuse background glow from fainter stars in the cluster. It has a well defined compact core. Globular clusters are composed of some of the oldest stars in our galaxy – some estimates give them an age of over 12 billion years! This helps put constraints on our models of the evolution of the Universe. It MUST be older that the oldest stars! Position: 00h 24.1 -72o 05' The Constellations Ursa Major and Canes Venatici Sadly, these two constellations are only visible to northern latitude observers. The 'Plough' or 'Big Dipper' – shown by the thicker lines in the chart – is one of the best known groups of stars in the sky and almost everyone knows that by following the line of the two stars, Merak and Dhube (known as the pointers), that make up the right hand side of the plough, brings you to the Pole Star, hence showing where true north is. The stars that make up the outline of the bear are not quite so obvious! Ursa Major contains two A-list objects along with some other interesting objects that are also shown on the chart. Canes Venatici is a very small and insignificant constellation that lies below the tail of the bear; but it does contain one of the most beautiful objects in the sky, the Whirlpool Nebula. Alcor and Mizar Visual and telescopic doubles E B M Mizar, Zeta (?) Ursa Majoris, is the middle star of the three stars that make up the tail of the bear. With good sight, you should be able to see that it has a close neighbouring star – 12 arc minutes away – which is called Alcor. The pair, easily seen in binoculars, together are called the 'horse and rider'. The two stars orbit each other about once every million years and, in 1650, were the first double star ever to be observed. If however one now observes the pair with a telescope it will be seen that Mizar is itself a double star comprising two white stars 14 arc seconds apart – easily split even when seeing conditions are not good. In fact (though not visible in any telescope) each of the two stars is itself a double star so Alcor and Mizar make up a quintuple star system! In a telescope field using a medium power eyepiece with both Alcor and Mizar in view, a third faint reddish star will be seen to form a flattened triangle with them. Called Sidus Ludovicianum, it was named after Ernst-Ludwig V by a (not very good!) observer who mistook it for a planet! Position: 13h 23.9m +54o 55' M81 and M82 Spiral and Irregular Galaxies B L M These form a pair of galaxies that are close together both visually and in their actual separation. With no bright stars close by they are not the easiest to find. It is probably best to use the line formed by Gamma (?? and Alpha (?? Ursa Majoris, the bottom left and top right stars of the plough, as a pointer. If one moves binoculars or a finder scope up and to the right along the line of these two stars, the two galaxies will be found 10 degrees – or two field diameters – from ? Ursa Majoris. Both galaxies lie at a distance of about 12 million light years and their centres are only some 150,000 light years apart. A telescope with a low power eyepiece will show both galaxies in the same field – they are 37 arc minutes apart – then higher powers can be used to observe each individually. M81, at magnitude 6.8, is one of the brightest galaxies visible in the sky and thus easy to observe with a small telescope. It is a type Sb spiral galaxy, the same type as the Andromeda Nebula, M31, with a bright nucleus and reasonably open spiral arms. A small telescope will show the bright core of the nucleus and, if the sky is dark and transparent, a hint of the spiral arm structure particularly at the upper right and lower left. M82 is an elongated irregular galaxy that is less bright overall, at magnitude 8.4. However, as it is smaller in area, it is still easy to make out as a thin cigar-like band of light. It seems that a 'recent' close encounter with M81 has initiated a major burst of star formation and it is thus known as a 'starburst' galaxy. Radio images of its core have shown many very young supernova remnants resulting from the explosive end to the massive stars that have been formed there. Positions: M81: 09h 55.6m +69o 04' M82: 09h 55.8m +69 o 41' M51 Spiral Galaxy in Canes Venatici M This is perhaps the finest example of a face on spiral galaxy that we can observe, certainly with an amateur telescope. It lies 3.5 degrees south-west from the left hand star of the Bear's tail, Eta (?? Ursa Majoris, and forms a right angle triangle with it and Mizar. The distance of M51, a type Sc galaxy, is still in some dispute; some put it as near as 15 million light years, but a recent determination gave 31 million light years. It was originally discovered by Messier himself in 1773. Later Lord Rosse observed it with his giant 72 inch telescope at Birr Castle in Ireland and made a wonderful drawing showing the extended spiral arm structure – the first time spiral structure had ever been observed in a galaxy. In fact, M51 is a pair of interacting galaxies, NGC5194, the large spiral, and NGC5195, its smaller and irregular companion. It is thought that the gravitational interaction of the smaller galaxy may have triggered the formation of the spiral arms in the larger. Under reasonably dark skies, a small telescope will show the nuclei of the two galaxies separated by 4.5 minutes of arc. Under really dark and transparent skies the spiral structure may just be discerned, particularly with telescopes of 8 inch (200 mm) or more in aperture. Position: 13h 29.9' +47o 12' You wrote: I have a version of Lord Rosse’s drawing] [Ian please supply the drawing if you haven't already given it to Alfred] Other Messier Objects The chart also shows four other Messier objects; three galaxies and one planetary nebula. M108 is a 10th magnitude type Sc galaxy very close to Merak, Beta (?? Ursa Majoris, whilst M109 is a type SBc spiral, magnitude 9.8, close to Gamma (?? Ursa Majoris. It has a prominent central bar of stars from the end of which flow the spiral arms – hence the 'B', for barred, in its Hubble classification SBc. M101 makes a nice triangle with Eta (?? and Zeta (?? Ursa Majoris at the end of the tail of the bear. It is a magnitude 7.9, type Sc, galaxy at a distance of 27 million light years. It is called the 'pin-wheel galaxy' due to its open arms, but a small telescope will probably only show the compact nucleus. Finally we have M97, the Owl Nebula. This is a planetary nebula close to M108. At magnitude 9.9 it will require a dark sky to be able to pick it out. The disk of glowing material has two prominent circular 'holes' which appear like the eyes of an owl so giving it its name. The Constellation Virgo Virgo is large constellation whose brightest star is 1st magnitude Spica but which contains few other bright stars. At the north of the constellation and crossing into Coma Berenices is a region of the sky called the 'realm of the galaxies'. It is in this direction that we are looking towards the centre of our local super cluster of galaxies, the Virgo supercluster of which our small 'local group' of galaxies is an outlying member. Here Messier logged 16 galaxies, and sweeping this region under dark skies is very rewarding. One galaxy from this region is in the A-List along with a second galaxy on the boundary with Corvus. A nice, but currently very challenging, double star system is also in the list. Virgo is highest in the sky from April to June. M87 Giant Elliptical Galaxy M M87 is a magnitude 8.6 giant elliptical galaxy – one of the largest galaxies in the universe – lying at a distance of 60 million light years at the centre of the Virgo cluster. There are no bright stars nearby and thus it is not too easy to find. This is compounded by the fact that it is surrounded by other galaxies, though these are a little less bright. It lies on the line between Beta (?? Leonis to the west and Epsilon (?) Virginis to the east and is almost halfway between them just over 10 degrees, about two finder fields of view, from ? Leonis. If your telescope is equatorially mounted and equipped with an eyepiece whose actual field of view is 1.5 degrees or more, then you can place ? Virginis at the top of the field (with inverted image) and lock the declination axis. (Southern observers: bottom of the field.) Then sweep westwards in right ascension by 31 minutes (7.75 degrees) and M87 should lie towards the bottom (southern: top) of the field. M87 will appear as a roughly spherical fuzzy disc. M87 is an exceptional galaxy. Though of similar diameter to our own Milky Way galaxy, being spherical rather than just a thin disc in shape, it contains vastly more stars, several trillion in number. Deep exposure photographs show that it extends far beyond the 7 arc minutes shown in most photographs, perhaps to half a degree across. It is surrounded by a swarm of globular clusters. M87 also has a jet visible on short exposure photographs. This jet is an outflow of material ejected from the region of a supermassive black hole at the centre of the galaxy. The jet is the source of intense radio emission and thus M87 is termed a radio galaxy and has been given the name Virgo A. position: 12h 30.8m +12o 24' M104 - Sombrero Galaxy Spiral Galaxy M H M104 is an 8th magnitude type Sa spiral galaxy seen almost edge-on. Type Sa spirals have a prominent nucleus surrounded by tightly wound spiral arms. It lies on almost the same declination as Spica, so centre Spica in the telescope field using a low power eyepiece, lock the declination axis and move westwards in right ascension by 45 minutes (RA is measured in time: 45 minutes corresponds to 11.25 degrees). One can also reach it by moving north and a little east by about one finder’s field of view from Delta (?) Corvi. In addition, a line of 7th magnitude stars leads up towards M104 from Gamma (?) Corvi reaching an arrow shaped asterism pointing a little to the west of M104. (See inset on star chart.) As with all galaxies, you really need a dark and transparent sky to observe it well. At low power it appears as a small oval patch of light. With higher powers the brilliant core becomes more obvious and a dark band, due to a prominent dust lane, appears to cross it – this dust lane forming the rim of the Sombrero Hat. Its dimensions are 9 by 4 arc minutes and M104 is thought to lie at a distance of 50 to 60 million light years. position: 12h 40.0' -11o 37' Gamma Virginis Porrima Double star E H Porrima is a magnitude 2.7 star lying 10 degrees from Spica in the direct line from Spica towards Beta (?) Leonis – the star at the hindquarters of the lion. The view with a telescope under high power can be stunning: a pair of identical twins, each being type F stars, white in colour, whose surface temperature is about 7000K, somewhat hotter than our Sun. Lying at a distance of 38 light years, they are 50% more massive than our Sun and about 4 times greater in luminosity. The two stars are orbiting each other with a period of 170 years having a separation of 40 AU, about the distance of Pluto from the Sun. Their angular separation in the sky reaches a minimum in 2005 when they will be less than 1 arc second apart making them very difficult to split. You will need a night of excellent seeing and first-class telescope optics to succeed in the next few years! However it will be very worthwhile observing them each coming year as you will gradually observe them moving apart – out to a separation of about 2 arc seconds by 2012. There are very few double stars where one can see significant differences in just a few years! Their maximum separation, of just under six arc seconds, will occur around 2080. Position: 12h 41.7m -1o 27' Other Messier galaxies. The region in the circle on the sky with Epsilon (?) Virginis and Beta (?) Leonis at either side (shown on the chart) will reveal a host of galaxies when observing with a well dark-adapted eye under dark skies. Scanning a telescope at medium power should enable you to see many other galaxies in Messier's catalogue. From north to south there are: M85, M100, M98, M91, M88, M99, M90, M86, M84, M89, M87, M58, M59, M60, M49 and M61. The Constellation Vulpecula Vulpecula is a small constellation that lies between Cygnus and Lyra to the north and Delphinus, Sagitta and Aquila to the south. It has no bright stars but does, however, contain two A-List objects! It comes highest in the sky during July, August and September. Brocchi's Cluster Open Cluster or Asterism B L Brocchi's Cluster is commonly called the 'Coathanger' because that is exactly what it looks like! Seen upside-down with binoculars from the northern hemisphere – the right way up for southern observers – it is best found by sweeping across the milky way from the bright star Altair, in Aquila, one third of the way towards Vega, in Lyra. It should be easily spotted against a darker region of the Milky Way called the Cygnus Rift. It is a grouping of around 40 stars, but only 6 or so of the brighter ones have a common motion through space – the definition of the stars forming a cluster – so it might better be termed an 'asterism' – a chance pattern of stars. It subtends one degree across, so binoculars or a telescope at low power will show it best. There is a very nice colour contrast between two red giant stars forming part of the handle and the whiter stars of the 'bar'. position: 19h 25.4m +20o 11' M27 Dumbell Nebula Planetary nebula B M In 1762 Charles Messier discovered the 27th object in his catalogue, describing it as an oval nebula without stars. Later John Herschel gave it its common name, the Dumbell Nebula. It was the first planetary nebula to be discovered and is the remnant of a giant star that exploded at the end of its life leaving a cloud of dust and gas surrounding the dying ember of its core – a so called white dwarf star about the size of the Earth. This white dwarf is still extremely hot, about 85,000K, and so is emitting ultraviolet light which is exciting the surrounding gas shell and making it glow. As with all nebulous objects, it is best searched for in dark and transparent skies. The easiest way to locate it is to first find Gamma (?? Sagitta, the 3.5 magnitude star at the head of the tiny flattened triangle making up the 'arrow' of Sagitta. This lies 10 degrees directly north of Altair in Aquila. If ? Sagitta is now placed at the bottom of a binocular or finder field of view, M27, which is 3 degrees north, should be visible towards the top. (Southern observers put ? Sagitta at the top and look towards the bottom.) The nebula is 8 by 6 arc minutes in angular size and 7.4 in magnitude so it should be picked up as a bright spot in 8x40 or 10x50 binoculars. A telescope at medium power will show it as an elongated nebula, rather like an apple core in shape, and an 8-inch (200mm) telescope or larger may well show the 13.5 magnitude central white dwarf star. Its distance is not well known, perhaps 1,250 light years, and measurements of its rate of expansion, some 6.8 arc seconds per century, gives an estimated age of three to four thousand years. position: 19h 25.4m +20o 11'