July 2013: Planet-builders

In the show this time, we talk to Dr. Zoë Leinhardt about planet formation, Stuart rounds up the latest news and we find out what we can see in the July night sky from Ian Morison and John Field.

The News

In the news this month: Astroseismology and Astroarcheology.

• A white dwarf is the hot stellar core left over from a star which has exhausted all its fuel for fusion and has blown away all its outer layers of gas via strong stellar winds. Commonly white dwarfs are composed of carbon and oxygen, the by-products of the helium fusion which takes place during the end of life stages of a star which is similar in mass to our Sun. However there is a different sort of white dwarf that exists, one formed in the core of a star which was never given a chance to undergo helium fusion because of a catastrophic event that stripped the outer layers of the star prematurely. These helium rich white dwarfs have much thicker atmospheres of hydrogen and masses much smaller than typical carbon-oxygen white dwarfs. What makes these sort of white dwarfs interesting to study is that they form in extreme environments such as in the centre of globular clusters, in orbit around super massive black holes millions of times the mass of the Sun and as the survivor of its companions supernova explosion. Currently the understanding for how a low mass, helium white dwarf forms is poor. Helium white dwarfs which show particular characteristic differences have been seen to both cool rapidly and retain their temperature consistently. To be able to understand how it is possible for helium white dwarfs to possess both of these properties at the same time requires that we find a star which we can study and know will one day form a helium white dwarf. Fortunately the Wide Angle Search for Planets project known as WASP came across just a such a star, an eclipsing binary system where one star was having it outer hydrogen layers stripped away so that one day all that will remain is its helium rich stellar core, a helium white dwarf. A team of astronomers since that time have been using instruments provided by the European Southern Observatory to study and model the star and make predictions as to what properties the helium white dwarf will have when it is formed. This is done by taking observations of both the light curve of the star and its spectrum over long periods. By doing this the astronomers are able to measure the waves that roll along the surface of the star and which pulsate from its core. These waves then give detailed insight into the internal structure of the star and the forming helium white dwarf hidden within the outer layers of hydrogen gas using similar techniques to seismologists investigating the internal properties of the Earth via waves through and along the ground caused by earthquakes. If the astronomers are able to successfully predict the properties of a helium white dwarf from its progenitor star then that will allow helium white dwarfs to be classified. This will allow astronomers to probe more easily the properties of extreme astronomical systems, such as the centre of globular clusters and the environments around super massive black holes. Also, the study of the age of pulsars relies upon a thorough understanding of helium white dwarfs since both types of stars are regularly found orbiting each other.

• Elliptical galaxies are a type of galaxy that are very different to the more familiar spiral-type galaxies like the Milky Way. First, they come in many different shapes and sizes, from tiny dwarf galaxies that may contain only a few tens of millions of stars to giant galaxies with thousands of billions of stars, which is vastly larger than the Milky Way. The structure of an elliptical galaxy is not dominated by the rotation, like spiral galaxies are, but instead all the stars are on independent trajectories that are bound together by all their mutual gravitational attraction. Another feature of elliptical galaxies that makes them vastly different to spiral galaxies is they do not form any new stars because they consume or blow away all the gas and dust required early on in their lives. This means that the red, bloated stars found within elliptical galaxies formed much earlier on in the universe, in a burst of very rapid and very efficient star formation that may last only a billion years and occurred about 11 billion years ago. However, the largest elliptical galaxies provide a small problem for galaxy formation theories, they have formed hundreds of billions of stars much earlier in the history of universe than should be possible. We know this because we can estimate how fast early galaxies are forming stars by measuring the bright infrared emission from dust clouds being heated by many short lived massive stars and also counting how many of these go supernova per year. One way in which very large elliptical galaxies can form is by mergers between two elliptical galaxies long after star formation has ceased. Since we see large elliptical galaxies have formed 7 billion years ago and we know they have ceased star formation 10 billions ago, this leaves only a few billion years in which elliptical galaxies can be merging together to form the giant elliptical galaxies. These sort of mergers have been observed, with many examples seen of galaxies tearing each other apart as they collapse together. However this method does not explain all the large elliptical galaxies, at best it can explain how two thirds of the large ellipticals came to be. Last month new research published a different method for which large elliptical galaxies could form, backed up by observations of galaxies swelling with the light from millions of newly forming stars. These early galaxies were much more turbulent than galaxies at the present time, for comparison the Milky Way may form about 1 star per year, whereas earlier galaxies could form hundreds per year. However to be able to form elliptical galaxies with hundreds of billions of stars requires that a galaxy has thousands of stars being produced per year and that there is a high efficiency of conversion of interstellar gas to form stars. In order for a galaxy to achieve the required star formation rate of over a thousand per year requires the collision and merger of two already violently active star forming galaxies, known as starburst galaxies. This is exactly what the astronomers have found, two starburst galaxies merging around 11 billion years ago. Although it is impossible to ever observe the product of these two galaxies, through simulations it is possible to predict that the star formation rate of both galaxies combined will be more than enough to produce the large elliptical galaxies observed in the more recent universe.

Interview with Dr. Zoë Leinhardt

Dr. Zoë Leinhardt from the University of Bristol, UK, talks to us about her research on the formation of and evolution of planets, asteroids, and comets through the use of numerical simulations. She explains how these planetary bodies form and produce the diversity of extrasolar planets observed with Kepler.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during July 2013.

Leo is setting in the West, along with Bootes containing the bright star Arcturus. To it's left we see Corona Borealis and further left, we can see Hercules. Still further to the East, we have the constellations of Cygnus, Aquila and Lyra containing the stars Deneb, Altair and Vega which, together, make up the Summer Triangle. Down to its lower left is a small constellation called Delphina the dolphin.

The Planets

• Jupiter is visible in the pre-dawn sky at a magnitude of around -1.9.
• Saturn will be seen in the south west after sunset and will dim through the month from +0.5 to +0.6 magnitudes.
• Mars will lie about 7 degrees above the northeastern horizon, about half an hour before sunrise, with a magnitude of +1.5 to +1.6. On July the 6th it will lie close to a thin crescent moon between the horns of Taurus and will pass close to Jupiter later in the month.
• Mercury will lie between the Earth and the Sun, at inferior conjunction on July the 9th but will only really be visible towards the end of the month. It will reach nearly zeroth magnitude on the 29th of July.
• Venus begins the month 11 degrees above the horizon at sunset, however as twilight ends it will be only 5 degrees above the horizon, but it's magnitude of -3.9 means it should be easily visible.

Highlights

• Within Hercules, 4 stars make up the keystone. Within this, a fuzzy blob known as M13 (a globular cluster) is visible through binoculars or a telescope.
• Towards Lyra, a little to the left of Vega, Epsilon Lyrae - a double star- is visible. With a telescope, you can see that each of those stars is also a double, making it the "double double".
• Below vega are two bright stars, Beta and Gamma Lyrae, and between them is the planetary nebula M57, the ring nebula. You will need a telescope to see this.
• The Dumbell Nebula, M27 in the constellation of Vulpecula is visible with binoculars.
• 3rd July: Venus lies in front of the Beehive Cluster.
• 12th July: The thin crescent Moon joins Venus and Regulus to make a straight line.
• July 22nd: before dawn, Mars and Jupiter come within a degree of one another. In the evening, Venus comes within 1.25 degrees of Regulus, in Leo.
• July 24th a lineup of saturn's moons is visible with a telescope.
• In the last 2 mornings of July, 45 mins before sunrise, Jupiter, Mars and Mercury are all visible together. Binoculars are needed to see Mercury, but do not use them after the Sun has risen.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during July 2013.

The brightest part of the Milky Way is visible in the south-east after sunset, in the constellations of Scorpius and Sagittarius. It hosts many bright star clusters and nebulae that can be observed during the long winter nights. At the apex of the Milky Way is Crux, or the Southern Cross, a diamond-shaped quartet of stars with a fifth, fainter star within. To MĔori in Aotearoa (New Zealand), it is Te Punga, the Anchor. To one side of this is a dark patch called the Coalsack Nebula, which is a cold and dark cloud of interstellar dust and gas that may one day form new stars. Running along the Milky Way to the east are the two bright stars Alpha and Beta Centauri, which point the way to Crux and also mark the hooves of Centaurus, a creature with a human torso and the body of a horse. The faint glow of Omega Centauri, a globular cluster containing millions of stars, is to the north of Beta Centauri, and its structure can be seen with binoculars or a telescope. Aquila the Eagle and Cygnus the Swan are found along the Milky Way to the north. Deneb, the brightest star in Cygnus, is on the northern horizon marking the Swan's tail. A line of stars lead up to Albireo, the Swan's head. A small telescope reveals this to be a double star with yellow and blue components. Cygnus is sometimes referred to as the Northern Cross. Aquila, to the east, is marked by a line of three stars, the brightest of which is the central star, Altair. NGC 6709 and NGC 6755 are two open star clusters among several within Aquila, and a number of planetary nebulae may also be seen with a telescope. Nearby is the bright star Vega, in the constellation of Lyra. Vega, Deneb and Altair form the Winter Triangle, known as the Summer Triangle in the northern hemisphere.

The Planets

• Saturn is well placed in the evening sky, appearing as a yellowish star near the blue-white star Spica in Virgo. A small telescope reveals the planet's rings and its largest moon, Titan.
• Venus is low in the west after sunset, shining brilliantly.
• Mars and Jupiter are in the pre-dawn sky and appear close together towards the end of the month, when the stars Aldebaran and Betelgeuse are just above them
• Mercury also joins Mars and Jupiter at the end of the month.

Odds and Ends

Voyager has almost left the solar system. Its next objective is locating the heliosphere and heliosheath, where the solar system comes to an end. There are three criteria for having passed these, solar particles decrease, the number of cosmic rays increases, and the solar magnetic field reverses direction. Recent Science papers have confirmed the first two (cosmic rays and solar particles), but the magnetic field has not been observed to reverse. The current region has been re-named the heliosheath depletion zone.

NASA have produced an image of the surface of Mars with more than 1 billion pixels. It is composed of more than 900 exposures from 3 cameras aboard the Curiosity rover. The image is taken from the Rocknest region with Martian mountains visible in the distance.

The European Space Agency (ESA) and artist Katie Paterson are attempting to bring art and science closer together with a fascinating project. Paterson's latest installation, Campo del Cielo, Field of the Sky involved recasting a melted-down meteorite into its original shape. After getting in touch with ESA, a small fragment of the piece is a candidate for going up to the ISS on the automatic transfer vehicle (ATV) Georges Lemaitre, thus "completing the cycle".

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