Supernova Saturday

Supernovae cause the biggest explosions in the universe, often out shining their host galaxies. There’s plenty of flavours of supernova and they aren’t all just big stars blowing up.

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As we all eagerly await the launch of NASA’s Mars InSight spacecraft we should fill the time considering some of most powerful explosions in the universe. Supernovae are amazing and have dazzled astronomers for centuries. They appear as bright stars that seem to arrive out of nowhere and then disappear sometime later. In the case of the Crab Nebula, it was recorded as a temporary bright star and now over one thousand years later we have a fantastic nebula to train our telescopes on. Our own solar system is made up of heavy elements that were probably formed in the cores of nearby stars billions of years before our own Sun was born. When these stars blew up in huge supernovae and spread their remnants all over the galaxy some of this mixed with the hydrogen dust clouds and ultimately condensed into areas of increased density, of which one become a protostar, and then our very own Sun and Solar System.

The study of supernovae is problematic because we don’t see that many of them nearby due to the dust in the Milky Way. SN1987A was reasonably close and has given scientists a fantastic opportunity to observe the material being ejected from the explosion over the last couple of decades. Fortunately there have been a lot of supernovae in our galaxy and there’s plenty of evidence visible of supernova remnants that scientists are using to try and piece together an understanding of the processes involved when a star goes supernova. When they happen in our own galaxy, we can often observe the source of the explosion and the remnant object such as the Crab Nebula with it’s powerful pulsar sending out waves in the surrounding nebula. Milisavljevic and Fesen considered what can be gleaned from studying the remnants of nearby supernovae through examination of the rings of material, they note that it gets trickier the older the supernovae are as the material has more time to interact wth the interstellar medium.

N103B, Supernova remnant (Credit: NASA, HST)

Stars don’t last forever and when they’re bigger than a few Solar masses then they die in a spectacular fashion. In recent years we have been able to observe some of the monster red giant stars that are in the last stages of there lives and these stars will put on a big show for us when they go supernova. This isn’t the only way they go bang though, as there’s another family of supernova which was very important in measuring the size of the universe. These are supernovae paired in binary systems where one star has gone through it’s life cycle and left a remnant white dwarf, usually rich in carbon and oxygen. The other partner in the binary pair advances through its own lifecycle and get bigger, becoming a red giant, then the outer material of that star gets drawn into the companion white dwarf. After a while the once quiet white dwarf starts gaining significant mass and when it hits the magic number of 1.38 solar masses (the Chandrasekhar Limit) it blows up in a giant thermonuclear explosion. The cool thing is that when this happens, and it happens quite a few times that are oberserved throughout the observable universe, then the explosion is kind of the same each time – known as a type Ia Supernova. The relative uniformity with these sorts of supernova allow scientists to have a standard measure that helps them measure all sorts of things about the universe, such as size, expansion rate and soon on. Jacco Vink from the University of Amsterdam studied the 1604 Kepler Supenova and confirmed it was Type Ia category by looking at the elements that were ejected by the explosion, basically the outer shells of the supernova were material from the companion star with oxygen and nitrogen and the central bits of the remnant nebula contain loads of iron and silicon from the white dwarf that went bad.

Impression of white dwarf stealing its neighbour’s mass (Credit: NASA)

Another type of supernova that is really interesting is when two white dwarfs merge and cause an explosion. This is called a double degenerate progenitor, basically two white dwarf stars somehow get stuck together and explode, but above the Chandrasekhar Limit. It was the observation of these supernova busting the 1.38 limit that lead scientists to proposing the double degenerate progenitor model. As it turns at there’s quite a few of these in the Milky Way and the observation matches the model that there should be one about every 100 years. The interesting aspect of these supernova is that they don’t leave any partner as both stars are consumed in the explosion. This is unlike the normal Ia supernovae where the white dwarf blows up and leaves a depleted and somewhat shell shocked binary partner.

Supernova have been quite handy for the distribution of heavy elements, including those that are contained is all life on this planet. Carl Sagan summed it up perfectly when he said we were made of star dust – we really are. Despite the positive effects on our existence of supernovae, things are not always so rosy. There have been some suggestion that one or some of the mass extinctions that have plagued the last 500 million years or so might have been caused by a supernova through the delivery of a huge dose of cosmic radiation. Gunther Korschinek from the Technical University of Munich looked at the effects of this extra radiation on organisms and found that it’s possible that an increased dose a couple of magnitude higher than the normal (which is quite a lot) could be fatal for fauna. For this to occur the supernova would have needed to be relatively close, within 20 parsecs (pc). A parsec is equal to about 3.26 light years. Another cause of mass extinction would be the depletion of ozone, but, again the supernova would have to be quite close at less than 8pc. So the jury is out on whether any of the mass extinctions were caused by a supernova and it would be tricky to get the proof. The good news is Betelgeuse, the one nearby star that we might see go supernova, is 150pc away so it’s unlikely to cause us any difficulties other than traffic problems for people stopping to watch the show.