If you’ve looked at any astronomy-related news over the last few days, you’ll have spotted that a supernova has exploded in a nearby galaxy, and astronomers are getting excited about it. Supernovae are not uncommon (we discovered more than 2000 of them just last year), but some get astronomers more excited than others. Why is this one interesting? Read on.
We think of stars like the Sun as lasting forever. On human timescales, that’s ok since most of them have lifetimes of millions to billions of years – our own Sun has been shining for some five billion years, and probably has enough fuel to last another five billion or so. But they are transient things, like everything else in the Universe, just on timescales it is difficult for us mere mortals to grasp. Even astronomers struggle!
Stars are giant fusion reactors, fusing hydrogen in the core to make helium, and releasing energy in the process. It’s this energy release that powers the heat and light we get from the Sun, and what makes all the other stars shine. Stars continue this fusion process for as long as the conditions are right in the core. You need a lot of hydrogen under very high pressure and at very high temperatures. Eventually, the hydrogen runs out and a star starts to die.
The way a star ends its life depends very much on one parameter: its mass. The amount of stuff that makes up a star. It’s a simple parameter, but it determines an awful lot about a star’s properties, lifespan, colour, luminosity, end point, and much else. Most stars are mostly composed of hydrogen (~75%) and helium (~25%) with small amounts of other elements. Stars that have formed recently (in cosmic terms) have a greater proportion of these “other” elements, for reasons I’ll come to, but they are still mostly hydrogen and helium.
When a star runs out of hydrogen in the core, the fusion process slows down, reducing the amount of energy released per second. That energy, in the form of photons, keeps the star from collapsing under gravity for most of the star’s life. When the number of photons decreases, the star can no longer counteract the relentless pull of gravity, and it starts to contract. In science terminology, the star is no longer in hydrostatic equilibrium.
As the star begins to contract under gravity, the pressure in the core begins to rise, the temperature also goes up, and we get to the point where conditions in the core are extreme enough that the fusion of helium can begin. This process also releases energy (although not at the same rate) and stops the collapse for a time. There isn’t so much helium though, so the helium-fusing period of a star’s lifespan is relatively short. The shell of mainly hydrogen around this hotter core is now also hotter, and can start undergoing hydrogen fusion too. Again, it doesn’t last anywhere near as long as the original hydrogen burning phase – a phase we call the main sequence lifetime, and which makes up the longest part of any star’s lifetime.
For stars like the Sun, that’s almost it. The helium also runs out, the core starts to collapse again. As the outer layers expand and cool, the star becomes a red giant. The core continues to shrink, eventually becoming a hot, dense object known as a white dwarf. The outer layers eventually drift off to form a (poorly-named) planetary nebula, the white dwarf core slowly cools and fades away over billions of years.
But for stars greater than a certain mass (somewhere around eight times the mass of our own Sun) the end is much more dramatic. These stars continue the fusion process, with the core developing like the layers of an onion, with each progressive layer fusing heavier and heavier elements as they are produced by the layers above them, all the way across the periodic table, up to iron. Iron is interesting, as its the first element in the periodic table where you cannot release energy through fusing it to create a heavier element. To fuse iron, you have to add energy. There’s nowhere inside a star for that injection of energy to come from, so the fusion process stops.
If there’s no fusion in the core, there is nothing stopping the layers above collapsing under gravity. And that’s what happens. Catastrophically.
The outer layers start to fall as the source of photon pressure to keep them up diminishes, until the iron core collapses under its own weight*, resulting in energetic shockwaves that literally rip the star apart. This is what we call a core collapse supernova. The physics of this are fascinating, and the theoretical modelling has come a long way in recent years as computers have become more powerful. (Other types of supernova happen too, astronomers recently detected radio observations from a thermonuclear supernova for the first time, but that’s for another day.)
Now, supernovae are not rare events. As I mentioned at the top, we discovered over 2000 supernovae just in 2022, with many more than that identified as candidate supernovae. Many of these are discovered by automated surveys that scan the sky each night looking for new objects, or astronomical objects changing in brightness. But some are discovered by dedicated amateurs with good equipment and an excellent knowledge of the sky.
This supernova, SN2023ixf, was discovered by one well-known supernova hunter, Koichi Itagaki from Japan. He’s discovered quite a few over the years! It is located in one of the spiral arms of the galaxy M101, also known as the Pinwheel galaxy due to its appearance, and it is pretty gorgeous:
The galaxy is some 21 million light years away, so the light reaching us now (including the light from this explosion) left the galaxy 21 million years ago and has only just reached us. Looking out into space is looking back in time, we only ever see the Universe as it was in the past.
So, if we discover so many of them each year, why are astronomers excited about this one in particular? Well, for one simple reason: its proximity. While we discover a lot of supernovae, most galaxies are a lot further away than M101, so most supernovae are also much further away than this. The closer a supernova is, the more easily we can study them, and the more detail we can achieve with our observations. A nearby event means that our telescope have better resolution (our images have more detail in them) and we collect more photons (an event of the same brightness twice as far away will appear four times fainter).
This galaxy has also been host to several supernovae over the years. Since we’ve been watching, we’ve spotted at least five in M101, the most recent being SN2011fe discovered in (no surprise!) 2011.
What we know so far
SN2023ixf was first discovered on May 19th and is being observed by astronomers around the northern hemisphere, so what do we know so far?
It’s a core collapse supernova, having gone through the evolutionary stages described above. Estimates from early modelling suggest that the progenitor star might have been about 15 times more massive than the Sun before it exploded. It has been getting brighter, heading for a peak magnitude of around mag 10 – far too faint to be seen with the naked eye, you’ll need at least a 4-inch/20-cm aperture telescope (and some patience!) to spot it.
There are lots of images over on David Bishop’s excellent Bright Supernova catalog page for this object, and plenty of updates if you do a quick search. It will likely stay bright form some weeks, so (as Catherine Heymans said on 5 Live at 8.50am this morning), if you have a telescope gathering dust in the attic, you might want to drag it out and have a look.
Why are they important?
So what’s all the fuss? It may be close, but it’s just one of many hundreds of thousands – so what?
Supernovae are cosmologically important. That nuclear fusion process I described makes elements up to iron in the periodic table. To get anything heavier than that, you need even more extreme conditions than in the core of a massive star, and that’s where supernovae come in.
In a supernova explosion, the extreme conditions created in the shockwave and subsequent blast result in a rapid burst of nucleosynthesis, creating elements much heavier than iron, including many that are important to the creation of things like the computer you are using to read this!
All of those heavy elements, as well as the stuff up to iron created before the explosion itself, get thrown out into the surrounding gas that make up what astronomers call the interstellar medium. This process is called enrichment, adding heavy elements to a gas that is mostly made of hydrogen and helium.
Stars form from the interstellar medium. Over dense regions in the gas start to clump together, gravity increases and more material gets pulled in. Slowly, over hundreds of thousands to millions of years, the gas accumulates, the temperature goes up, the pressure in the centre increases, until you have a hot ball of gas where nuclear fusion can begin in the core – et voila, a star is born!
The gas around the core then goes on to form planets in many cases. If that gas cloud only contains hydrogen and helium, you only get gas giants forming. If the gas also contains heavy elements in sufficient quantities, produced in previous supernova explosions, then you get rocky planets forming too.
If you’ve ever heard Carl Sagan talk about us being made of star stuff, this is what he means. Much of the chemicals that make life possible, that make up the crust the Earth, that make up you and me, and our computers, were made inside massive stars and released into space in supernova explosions.
Our own galaxy, the Milky Way, is also forming stars, some of which are massive enough to go supernovae in this way. We actually haven’t had one in the Milky Way since 1604 so we’re rather overdue for one.
Some stars we know of are big enough to explode this way. Betelgeuse is a red giant that has got astronomers very excited in the last couple of years by behaving in a way that could signal its impending doom. It hasn’t exploded yet, but keep an eye on it.
If a supernova did explode in the Milky Way it would likely appear extremely bright due to its closeness. Astronomers would go bananas.
Supernovae are energetic explosions, generating blasts of neutrinos, as well as strong radiation across the electromagnetic spectrum – radio, infrared, visible, ultraviolet, X-rays, gamma rays. Any planets in the neighbourhood would get a good dose of radiation.
Don’t worry though, there are no stars in the vicinity of the Sun that are big enough to be a threat to life on Earth by going supernova. The supernova candidates are all far enough away to not be a threat. They would be pretty cool to watch though!
Edit: I just spoke to BBC Radio 5 Live about this supernova! Listen back (5.50pm): https://www.bbc.co.uk/sounds/schedules/bbc_radio_five_live
Edit: thanks to Jost Migenda for spotting my mistake!