Ken Tapping, September 21, 2011

On August 24, astronomers detected the explosion of a giant star, the brightest seen from Earth in at least 20 years. The explosion, called a supernova, took place in the galaxy Messier 101, which lies about 21 million light years away. That means the explosion actually took place 21 million years ago, but the galaxy is so far away that the light from the explosion is only now reaching us. The energy released is so powerful that, for a period of a few days to a month or so, the dying star will shine more brightly than all the billions of other stars in its galaxy combined. This makes the star relatively easy to spot. Lots of amateur astronomers keep an eye on distant galaxies to spot any sudden increase in brightness. However, there are two unique things about this supernova — it is bright enough to be visible through a good pair of binoculars or a small telescope, and it is well placed for observation, in the Big Dipper.

These days, the Big Dipper lies in the North-western sky. Find the two stars at the end of the handle. The one at the end of the handle is called Alkaid, and the next one in, Mizar, is double to the naked eye. Its partner is called Alcor. If you have a telescope, look closely at Mizar and you will see that it has another partner, much closer in, making it a triple star. Messier 101 forms an equilateral triangle (one with three sides all equal in length) with those stars, above the handle. If the sky is dark enough, and your binoculars or telescope is large enough, Messier 101 will appear as a faint, fuzzy blob, with a bluish-white star visible in it. If you monitor this star over the coming days and weeks, you will see it fade away, leaving only the fuzzy blob, which is the combined light of all the billions of surviving stars in that galaxy.

Stars obtain energy by nuclear fusion, the conversion of light elements, such as hydrogen, into heavier ones, like carbon, phosphorus and iron. Eventually they have to run out of fuel. This happens sooner for more massive stars because they burn their fuel significantly faster. For stars like the Sun, their ends are relatively less dramatic. They sneeze away the outer layers, leaving their hot cores exposed. These remains, called white dwarf stars, have no fuel and, over millions or billions of years, slowly cool off. Stars with more than about 1.4 half times the mass of the Sun have much more dramatic endings. Their cores get more compressed and grow increasingly hotter until instabilities cool the core and reduce the pressure, so that the outer layers become unsupported. The unfortunate star then collapses and blows up in one of the largest explosions occurring in the modern universe.

The new supernova, classified as Type 1a, is a little different. The star was originally much like the Sun, maybe a bit more massive, but it had a partner orbiting close by. It reached old age and sneezed off its outer layers, becoming a white dwarf. Then its partner started to age and sneeze off its outer layers too. Some of this material was grabbed by our white dwarf which accumulated a large collection of fuel on its surface. Its mass eventually exceeded 1.4 times the mass of the Sun, causing it to become unstable and to blow itself apart.