On April 11, 2024, astronomers got a gift: a supernova in a nearby galaxy. A massive star exploded in the galaxy NGC 3621, a spiral a little more than 21 million light-years from Earth.

What’s great about this supernova — called Supernova 2024ggi — is that because it was close it was relatively easy to study. It was bright enough to get great spectra, which can tell us vast amounts about the explosion.

When the core of a massive star collapses, a lot of complicated physics goes on that winds up releasing vast amounts of energy that tear their way up through the star and blast the outer layers away. The supernova brightens rapidly, taking just a few days to reach peak brightness, then starts to fade. A lot of important stuff happens in those few days as it brightens, so getting early detection is important. 2024ggi was spotted by the ATLAS telescope system a mere six hours after the star blew up, which is incredible (here’s a journal paper with tons of info about the supernova). 

But nowadays we can do even better. For nearby supernovae like 2024ggi it’s possible, even likely, that the host galaxy was observed by Hubble, sometimes years before the explosion. When that happens, we may also be able to see the progenitor, the star that exploded, before it exploded. We can also see what kind of environment the star was in, too.

That happened with 2024ggi! Hubble not only observed the host galaxy NGC 3621, but actually did so several times (as did the Spitzer Space Telescope, which sees in the infrared). That’s important because it means astronomers can see how the star behaved before it exploded as well. A team of Chinese astronomers took a look at the Hubble and Spitzer data, and were able to find interesting things [link to journal paper].

For one thing, the star is clearly a red supergiant in the images, a bloated, cool, and extremely luminous massive star. Massive stars don’t live long, running through the nuclear fuel in their cores rapidly. As they start to run out, they swell up and turn red. In the case of the star that would eventually become supernova 2024ggi, the astronomers determined it was probably about 900 times wider than the Sun — well over a billion kilometers across! That means it would have stretched as far as the orbit of Saturn. That’s a big star. It likely initially had a mass of about 13 times the Sun’s, which is typical for such a star.

And it was blasting out energy to the tune of over 80,000 times the Sun’s luminosity. And that was before it exploded. Anything nearby would’ve already been cooked. Once the supernova blast wave hit it, well. The definition of overkill.

Interestingly, the star was seen to vary in brightness, something we now understand is common before red supergiants explode. Betelgeuse, for example, changes brightness on several different cycles, and that’s not including the time it blew out a huge cloud of dust that dimmed the star dramatically in 2019/2020. In the case of 2024ggi, though, not much dust was seen. That’s something of a surprise.

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