Sky Matters: Not all stars explode, but when they do it's a spectacular sight - requiring sophisticated equipment

Pic: iStock/NASA

During June we were fortunate enough at Blackrock Castle Observatory to have the opportunity to photograph a star exploding in a relatively nearby galaxy commonly known as the Pinwheel Galaxy. 

Such explosions, known as supernovae, happen about twice in every century in a galaxy as big as the Pinwheel, or indeed our home galaxy the Milky Way which is of comparable size. That’s pretty rare when you consider there are one hundred thousand million stars in either galaxy that could explode at any given time. 

But in fact the majority of stars are incredibly stable, spending most of their lives simply burning brightly and doing nothing of any great note. For example, our own Sun will continue as we see it today for at least another 4.5 billion years, having already clocked up about 5 billion years!

The supernova in the Pinwheel was caused by a massive star reaching the end of its lifetime. 

This would have happened when it ran out of fuel – usually a gas like hydrogen – causing it to cool quickly. When gas cools it contracts and this results in a cataclysmic implosion following by a massive explosion – and remember, stars are essentially big balls of gas. 

The brightness of a supernova can rival that of an entire galaxy for a number of weeks, although in the case of the Pinwheel the supernova doesn’t seem to have been quite that bright.

There’s no way to accurately predict when a star might go supernova, but one thing we do know is that it has to be about 2 to 3 times more massive than the Sun for this to be even possible. That also implies our Sun is incapable of becoming a supernova, which is reassuring.

The unpredictable nature of supernovae means astronomers have to scan thousands or millions of galaxies each night, just to catch one or two going off. Several telescopes across the globe are devoted exclusively to such scanning tasks in the hope of finding a supernova as close to the moment it explodes as possible. 

By doing so we can follow how it changes from the moment of “birth” and for many years afterwards.

Part of the reason for our fascination with these objects is the access they give us to how stars are composed inside. It’s a little bit like breaking a car apart and identifying that it has an engine and seats and a radio, etc., components that would be almost impossible to know about just by looking at it from the outside. 

Another reason is that the sheer violence of a supernova explosion allows us to test theories of the very fundamental behaviour of matter. This has potentially practical implications for ways to generate clean energy, for example.

The last visible supernova in our galaxy occurred in 1604. Records show that it seems to have been visible in the daytime sky, possibly rivalling the planet Venus in brightness. There was a more recent supernova around 1667, but there’s no record of it having been visible to the unaided eye. 

The most famous supernova was that of 1054. Records show it was visible for a few weeks in daytime. It was also named the “Crab Nebula” by the Third Earl of Ross using a 0.9m telescope that he had built, from his estate in Birr, County Offaly.

Observing the supernova in the Pinwheel Galaxy, which is fading now, requires relatively sophisticated equipment. By contrast, observing the so-called ‘supermoon’ on 3rd July will only require you to use your eyes. 

The moon may appear somewhat brighter and the tiniest bit larger, because it is at one of its regular closest approaches to Earth, but it would take an eagle-eyed observer to see the difference with the ‘usual moon’ which I consider to be ‘super’. 

Unlike ‘supernovae’, the occurrence of ‘supermoons’ can be predicted for thousands of years in advance, so if you miss the supermoon in July you’ll have three more chances before the year is out.