For many thousands of years, new stars have appeared in the sky for a short time before disappearing again. These stars were called Nova, which is Greek for 'new'. Now we still use Nova to describe these stars, only it no longer means 'new'.
We now know what causes these new stars to appear. These stars are not new though, quite the opposite. They are old. So old in fact that they are dying. These stars are however much larger than our own Sun and will die in a much different way. Their death is much more spectacular than the death of our Sun but to understand how they die, one misconception about stars must be cleared up. Stars don't burn.
Stars are huge balls of hydrogen gas sitting in space. However burning is not the correct word for what they do. They fuse. They are fusing hydrogen into heavier elements. Our Sun is fusing hydrogen into helium. In about 5,000 million years from now, our Sun will have exhausted its supply of hydrogen. It will swell up becoming an enormous Giant Red Star, many times its former size. In doing this, it will swallow up the Solar System's first two planets, Mercury and Venus, and possibly swallow up Earth as well.
The Sun will then start to fuse helium into Carbon and Oxygen. When the supply of helium is used up, it will puff off its outer layers into space. The outer layers of gas form a cloud called a planetary nebula. All that will be left behind is an incredibly dense core about the size of the Earth and as bright as a thousand full Moons called a white dwarf star.
These larger stars, mentioned earlier, work slightly differently. After fusing helium into carbon and oxygen, they will fuse heavier elements. Their size determines how heavy their last element is. Their cores are much denser and so they will go out in an explosion. This is what we call a Nova. The star flares up for a short while as it explodes. Before it appeared to be invisible in the sky, it now is brighter and since it can now be seen, it looks as if a new star has appeared in the sky.
There are however even bigger stars and their end is even more dramatic. These really big stars are called supergiants and are the largest stars in the known Universe. These stars fuse elements all the way up to iron. These stars are unable to fuse anything heavier and so can fuse no longer.
Their explosion is much bigger and brighter and we call it a type II supernova. The star lights up brighter than all the other stars in the galaxy put together! Supernovas are so bright they can be seen in other galaxies. Many have. The first supernova to be seen in another galaxy was in 1885 by German astronomer Ernst Hartwig (1851-1923) who discovered a bright new star in the Andromeda galaxy. Some have been in our Galaxy but the last one was hundreds of years ago in 1680. This was called Cassiopeia A in the constellation of Cassiopeia. The last supernova observed in our Galaxy was Kepler's Supernova in the constellation of Ophiuchus in 1604. This was a different type of supernova however called a type 1a supernova.
These are brighter than type II supernovas and are formed differently but are always the same brightness. They are caused when a white dwarf star is pulling gas from a larger companion star until its mass has increased such that it cannot support itself. It collapses and destroys itself in a type 1a supernova.
Since 1680 the nearest supernova was a star that went supernova in 1987 in the nearest galaxy, the Large Magellanic Cloud. It was a blue supergiant called Sanduleak -69°202 and became known as Supernova 1987A. It was barely visible before the explosion and became almost as bright as Polaris the Pole Stars and at its maximum brightness shone as brilliantly as 250 million Suns. The brightest ever, recorded supernova was in 1006 called Supernova SN1006 and is now 3,500 light years away.
520 light years away in a nebula in the constellation of Gemini is the rapidly spinning neutron star of Geminga. Geminga spins at a rate of once every 0.237 seconds meaning that it spins four times a second. It gives off X-rays and gamma rays and is the third largest source of gamma rays although it is almost undetectable at other waves lengths. It is the nearest neutron star to us.
It was in 342,000 BC that Geminga went supernova. There wasn't really anything special about the Geminga supernova apart from the fact that at the time Geminga was worryingly close to us. It could have been less than 50 light years away.
A supernova at a distance of only 30 light years could destroy Earth's ozone layer and if it was just one light year away, the Earth would be as lifeless as a crater on the Moon.
However since Geminga was so near, early man must have seen an amazing spectacle. Geminga would have shone in the sky for months. It would have been visible by day and cast shadows at night, and rivalled the full moon! The points of Geminga would have stretched from horizon to horizon and Geminga would have turned night into day!
One of the most easily visible constellations in the night sky is Orion the hunter. He is easily found by the three bright stars that make up his belt. However the top left star in his shoulder is the supergiant star Betelgeuse, one of the biggest stars known to exist. It is so big that if you placed it where the Sun is, it would reach out as far as Jupiter. If you made a telephone call from one side to the other, it would be 55 minutes before your voice reached the other end of the line travelling at the speed of light. It is 730 times greater than the Sun and 631 million miles (1,000 million kilometers) in diameter.
Betelgeuse is only 400 light years away and was probably a B type star when it was in its main sequence. However when Betelgeuse goes supernova we will have a similar view as early man had when Geminga graced the sky.
For weeks, Betelgeuse will light the sky and be the astronomical sight of a third of a million years! Expelling the power of 1044 joules, Betelgeuse will be a sight not to be missed. It could go supernova tomorrow, it could go in 100 or 10,000 years time, but we can be sure it will be worth the wait.
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