An explosion in a galaxy, far, far away
1 min read 25 May 2023, 10:27 PM ISTA Type II supernova, formed by the violent explosion of a star 10-50 times larger than our Sun, has been detected close to Ursa Major. Known as SN 2023ixf, it is relatively easily seen through small amateur telescopes and is located in the Pinwheel Galaxy, which is only about 21 million light years away, making it the closest supernova detected in five years. Supernovas are important as they expel carbon and other heavy elements that later condense into planets, making them fundamental to life.
PremiumThe time has come," the Walrus said, “to talk of many things. Of shoes, and ships, and ... supernovas, why not?"
No, Lewis Carroll’s Walrus did not say that, but there’s never a bad time to talk about supernovas. And right now is a particularly good time, because one has just ... well, how do I put this? Exploded? But supernovas don’t explode. Stars explode to form supernovas. So supernovas just ... happen.
And yet a word like “happen" can’t capture the cataclysmic, earth-shattering nature of this event. In fact, when our Sun turns supernova, the Earth will indeed be shattered. That adjective is accurate. So I wouldn’t hang around waiting for it to happen, a few billion years from now. I’d be winging it to some other habitable planet well in advance.
But the time has also come to be serious. You’ve heard the word “supernova". You may have heard of the supernova that a Japanese amateur astronomer, Koichi Itagaki, detected only a week ago. It was subsequently also seen on images taken a couple of days earlier. Late on 19 May, a team of astronomers used a telescope in Spain to confirm Itagaki’s sighting. They classified it as a “Type II" supernova—one formed by the violent explosion of a star that’s between 10 and 50 times larger than our Sun. It has a name: SN 2023ixf.
Any supernova is exciting news. But this one has caught the fancy of the world’s astronomers for a few reasons.
First, it is relatively easily seen through even small amateur telescopes. It is also pretty easy to locate in the night sky—lying very close to Ursa Major or the Great Bear, the familiar constellation that’s like a giant question mark. In fact, if I was not in Bombay as I write this, and instead somewhere with far darker skies, I think I would be able to see SN2023ixf through my telescope, at least as a small dot in the northern sky.
Second, it is in one of the arms of the Pinwheel Galaxy, known as M101. This is a spiral galaxy that’s facing us, so we can see the entire spiral through a telescope, so the Pinwheel is an always photogenic, always gorgeous sight.
Third, the Pinwheel is only about 21 million light years away, which means SN2023ixf is the closest supernova we’ve detected for five years.
So what are supernovas, and why should we care about them? In a very real sense, I’m writing this and you’re reading it because of some long-ago supernova. Stars contain various elements, among them carbon, which is fundamental to life. But as the great Stephen Hawking once wrote:
“... that carbon is still a long way from forming ordered aggregates of chemical compounds of the type that can enjoy a glass of Bordeaux ... or ask questions about the universe. For beings such as humans to exist, the carbon must be moved from inside the star to friendlier neighbourhoods. That is accomplished when the star, at the end of its life cycle, explodes as a supernova, expelling carbon and other heavy elements that later condense into a planet."
Thus the short version of what just happened in the Pinwheel Galaxy: a massive star ran out of fuel, and bang! We have supernova SN2023ixf.
Though let’s be clear about that phrase “just happened". More accurate would be to say that we’ve just observed the explosion, for it actually happened a while ago. 21 million years ago, in fact. Remember that the Pinwheel is that many light years away, meaning it takes light from there that many years to reach us on Earth. So when this star exploded and expelled carbon into space, it also sent light shooting through space. 21 million years later, some of that light reaches us on Earth, and we “see" the supernova.
Let’s also be clear: the sight of this explosion is not like one we’d see on Earth—no huge mushroom cloud, no rocks flying everywhere. I mean, it is unimaginably more powerful than anything we’d see on our planet. But it is also so unimaginably distant that all we can detect, really, is a bright dot of light where there wasn’t one previously. This is typical of supernova sightings. Of course the star that became SN2023ixf was in that spot, but the Pinwheel Galaxy is too far away for us to be able to discern an individual star there. Until it turned supernova. This is how Itagaki found SN2023ixf.
Think for a moment of what’s implied here. We never saw the star itself because it is so far away. But Itagaki caught sight of it in its death throes, when it turned into a supernova. He found it with a telescope, of course, but still—it is now visible. That means astronomers are able to calculate its “apparent magnitude", a measure of how bright it appears to us.
A few lines to explain the idea of magnitude: On this scale, the lower the number, the brighter the object is in our night sky. The brightest star we can see, Sirius or the Dog Star, has a magnitude of -1.33; Polaris, the North Star, is at 2.0; the planet Venus, at its brightest, reaches -4.6; and the faintest stars we can see with our naked eye have a magnitude of 7.2. To put all this in perspective, one unit of magnitude means a factor of brightness of about 2.5. So Polaris, 3.33 magnitudes less than Sirius, is 3.33 x 2.5 = 8.3 times fainter than Sirius. Similarly, Venus is over 8 times brighter than Sirius.
So on this scale, SN2023ixf has a magnitude of 10.8. Maybe that doesn’t impress you much. But maybe this will: that’s about the magnitude of Messier 102, which is about twice as far from us as SN2023ixf. But get this: Messier 102 is a galaxy, containing about 100 billion stars. So this one obscure faraway star turned into a supernova that’s as bright as an entire galaxy.
Indeed, think about that for a moment.
In fact, the last known supernova in our own Milky Way Galaxy, SN1604 in 1604, was, for several months, brighter than all the stars in the sky. At an estimated magnitude of -2.25, it was over twice as bright as Sirius.
If SN2023ixf was in the Milky Way rather than in the Pinwheel, it would likely be just as spectacular a sight as SN1604 must have been, 400 years ago. If it was in our solar system, though ... let’s just say, you wouldn’t be reading this.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
