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Busy week

While I've been busy visiting schools, teaching undergraduates and fighting with the wonderful public transport system in my part of the world, lots of far more exciting things have been happening in the rest of the Universe.

For a start, there have been some quite exciting supernovae, not least of which was SN 2006X in M100. This explosion was of the sort known as "type Ia" supernova. These occur in binary systems. When one star in the pair reaches the end of it's life and runs out of fuel in the core it throws off it's outer layers and leaves behind a small, cooler object known as a white dwarf. When the second star in the pair reaches the end of it's life, it swells up to become a red giant. If the stars are close enough, the outer layers of the second star fill what is known as the Roche lobe and the material starts to fall onto the white dwarf. As material falls onto the surface, it gets compressed and heats up. When enough material has accreted (been collected) on the surface an explosion will occur. This is what we see as a type Ia supernova. Why are these interesting? Well, cosmologists love them. Each type Ia explosion has the same absolute brightness, that is, if they were all placed at the same distance from the Earth, they would all have the same brightness. Thanks to this fact, if we can measure how bright they appear to us, then we can calculate how far away they are and so get a handle on how big the Universe is.

Around the same time that 2006X was discovered, another supernova was seen in UGC9265. This explosion, SN 2006W, was of a different sort known as a type II. These are the result of catastrophic collapses of stars greater than about eight times the mass of our Sun. When they run out of hydrogen (the main constituent of stars, the fusion of hydrogen nuclei to make helium is what produces all the heat and light which keeps us alive) in the core, they start to collapse. As the core shrinks, the pressure and temperature increases until it gets hot enough for the fusion of helium. When all the helium is used up the core shrinks again until carbon begins to fuse. This continues all the way up the periodic table as far as iron. Iron is pretty stable so, even though the temperature and pressure goes up, it can not fuse in the core. The collapse continues until the core is so dense that the outer layers still falling in can't go any further, effectively they hit a solid surface and rebound. This is what causes the explosion we see as a type II supernova. These are the kind of explosions that I study as they tend to be seen in active starforming regions such the cores of interacting galaxies. By watching the explosions and the resulting expanding shells of gas we can tell a lot about the interstellar medium - the "stuff" between the stars.

There was another exciting explosion spotted of February 12th. A nova known as RS Ophiuchi has begun another outburst, the first since 1985. Again, these explosions occur on the surfaces of white dwarf stars, due to the accretion of material, but they are less energetic that supernovae. RS Oph has exploded a number of times before and this time appears to have gone from a magnitude of 10.5 to 4.5 over a couple of days. That's naked eye visibility! If you have the skies for it, go take a look.

Posted by Megan on Friday 17th Feb 2006 (21:48 UTC) | Add a comment | Permalink


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