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World VLBI Array Image of the youngest supernovae remnant made using observations from 20 telescopes in November 1998
World VLBI Array Image of the youngest supernovae remnant made using observations from 20 telescopes in November 1998.




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Starburst Galaxies and Supernovae

The nearby galaxy M82 is currently undergoing a rapid phase of star formation and is, as a result, called a Starburst Galaxy. Most stars, like our own Sun, live for billions of years, but a small number are tens of times more massive and evolve very rapidly, living for only a few million years or so. When they die, they do so in a spectacular fashion, blowing themselves apart in a titanic explosion called a supernova. So, paradoxically, the signature of such a region of star formation is the explosive death of massive stars.

The advantage of radio observations

Because starburst galaxies like M82 contain a lot of obscuring dust it is very difficult for optical telescopes to investigate the regions of star formation. However, radio waves can pass unhindered through the dust enabling radio astronomers to look right into the violent heart of the galaxy.

Using MERLIN, the Very Large Array and VLBI observations to study M82

The false colour image of the central region of M82 was made by combining the data from the VLA with those from MERLIN to give a map showing broad scale structure allied to great detail. The peaks of radio emission, shown as white spots on the map, indicate the sites of supernova remnants. Due to the great distance of the galaxy (10 million light years) this map is not able to resolve detail in the remnants but, knowing their precise location, it was then possible to use global arrays of telescopes to image them in unprecedented detail. They are seen to be expanding shell-like clouds of gas shining brightly at radio wavelengths. Comparing images of the same young supernovae made in 1986 and 1997, Jodrell Bank astronomers found that the shells of gas are expanding at speeds of up to 20,000 km/s. The youngest supernova exploded only about 35 years ago but, hidden behind a curtain of dust, was never seen by optical telescopes.

The evolution of supernovae

In M82, radio astronomers have seen the evolution of the expanding fireball from a supernovae explosion. As our own galaxy is now in a relatively quiescent phase of star formation, there are no young supernovae to observe close by, and thus these observations of M82 give us a unique chance to study them in the early phase of their evolution. This is important as it is thought that most galaxies, including our own, have gone through a similar phase of rapid star formation when the Universe was young. So, by studying M82, we can learn about the early history of our own galaxy, the Milky Way.

Subaru (top) and Merlin/VLA (bottom) images of the galaxy M82. The radio image covbers the heart of the galaxy as shown by the blue box overlaid onto the optical image. The white spots are the supernova remnants.


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