NGC3621 and SN2024ggi

A supernova almost “right next door”

Supernova SN2024ggi was detected on April 11th, 2024, in the nearby galaxy NGC3621. This galaxy is part of the group of galaxies nearby to the Milky Way. 22 million light years is (by human standards) a very, very, very big distance. By astronomical standards it’s the equivalent of the house next door.

Contrary to what people think, supernovae are very common - it’s just that the vast majority of them are so far away they are effectively invisible to us.

We have yet to observe a supernova within the Milky Way - on average there should be one such event every century due to the size and stellar density of our galaxy. The closest recorded supernova was SN1987a, in the Large Magellanic Cloud, about 160,000 light years from Earth. Stepping out a bit further, we have SN2023ixf in M101 (the ‘Pinwheel Galaxy’), about 21 million light years away.

M101 isn’t easily visible from where I am, so I wasn’t able to see, let alone image, this event. Although plenty of Northern Hemisphere astronomers and astrophotographers were able to. M101 is also a bit of a show-off, with 5 recorded supernovae since 1900.

This year, it’s our turn down here.

I heard about the SN from a post to the ‘Cloudy Nights’ astronomy forum and for once luck was on my side - clear skies were forecast for that night. So I had the rig out and ready to roll as the skies darkened and the stars emerged in the night sky.

I had just managed to take some test images that clearly showed the supernova and I was configuring the imaging run when I looked up to discover the cloud gods laughing at me. Yep - thick high-level clouds rolling in from the west. I waited an hour or so, and the clouds showed no sign of abating so I packed my gear away, sadly disappointed but really happy that I’d managed to see such a rare event.

Image 1 below is the stacked and processed data from my test images - a staggering 3 minutes and 5 seconds in total.

Image 1

A week of cloudy skies went by before I had another chance to get the telescope out. This time, the moon was approaching it’s maximum and I was doing battle with both the bright glare from our celestial neighbour as well as an out-of-focus guide scope (so my guiding was terrible in comparison to how it usually runs).

This time I managed to get a reasonable amount of image data - not super high quality because of the aforementioned moon glow, but also because NGC3621 is a small target and pretty much at the limit of my telescope’s reach. There was also some thin, high-level cloud which made a few of the images useless. On top of that, I discovered a fine layer of dew on the telescope lens as I was packing up.

All complaints aside, I have an image of a rare event that I’m really happy to have captured.

And, yes, this single dying star is brighter than all the other stars in that galaxy. Once the expanding cloud of stellar debris fades from sight, there will be a black hole or neutron star left behind in the centre of an expanding ‘bubble’ of gas and dust.

Image 2 data:

  • Gain: 100
  • Offset: 10
  • Temperature: 0 degrees C
  • Exposure: 30 seconds
  • Frames: 215 Lights, 50 Darks, 50 each of Flats and DarkFlats (107.5 minutes integration in total)
  • Filter: UV/IR cut
  • Processed with SiriL (stacking, pre-processing and star removal with Starnet++) then finished in Affinity Photo v2

Equipment: SW72ED @ 420mm / HEQ5-Pro / ASI183MC-Pro / SV165+SV305 / Kstars/Ekos

Image 2

For those of you who may be curious, SN2024ggi has been identified as a ‘Type II Supernova’, meaning a star much larger than our sun (between 8 and 50 times bigger) suffered from a rapid collapse and then massive explosion.

Wikipedia has a good write-up if you’re interested: https://en.wikipedia.org/wiki/Type_II_supernova