Luminous Red Nova (LRN) in the Andromeda Galaxy (M 31)
But one of the perks of shooting so many galaxies is acquiring lots of "reference images." Of course, it's not just a perk. Accruing reference images is done more or less by design, because in order to do supernova hunting, I need to have comparison images for every galaxy I shoot. Else, how would I know if I've found a supernova in a given galaxy? I need to compare tonight's image of a galaxy with an older one to see if there are any new stars in the galaxy.
As a matter of fact, it's even trickier than that! Depending on sky conditions, one night's image may not be as good as another night's image. Some of my so-called reference images turn out to be of poorer quality than images that I took of the same object on subsequent nights. So, there are times when I take an image of a galaxy and it looks like there's a new star in it, compared to my reference image. In that case, before getting too excited about finding a supernova, I'll search through other images I've taken of that galaxy and, inevitably, there will be a faint star in several of my prior images, even though it wasn't seen in my reference image. In that case, I'll replace my reference image with a better one so that I don't have to waste time on it in future searches. Because of this problem, I never delete any of my supernova-hunting images. I keep all of them as reference images. In addition to providing me with more comparison options of a given galaxy, this also gives me the ability to stack dozens of images of any galaxy I've shot over the years, to make a "pretty picture," if I want. I have more than 160,000 images of galaxies, nebulae, and star clusters, in my vast collection.
Supernovae aren't the only transient objects, however. If you're hunting for supernovae, they can be found in the distant galaxies. But transient objects such as comets, asteroids, long-period variable stars, and novae can pop up anywhere and at any time, and when they do, it's very convenient to have an older image to compare with a new "star" (or, transient object). For moving objects, like comets and asteroids, you can simply wait an hour, or so, and take another picture. The movement (displacement) of the object in your two images will reveal your target. But for stationary objects like supernovae, novae, and variable stars, whose change is in their brightness, you'll need a reference image of when they are dim, so that you can compare it with an image when they are bright. Unfortunately, that is a more difficult situation because you may have to wait several months for them to change appreciably.
Take, for instance, this Luminous Red Nova (officially dubbed "AT2025abao") that recently popped up in the Andromeda galaxy. I found out about it from a YouTuber (Tsula's Big Adventures). The nova was discovered in October, but I hadn't seen or heard about it. Kind of embarrassing since I've shot M 31 four times since the beginning of October. You'd think I'd have seen the nova during those comparisons when I was looking for supernovae.
As it turns out, the Andromeda Galaxy (M 31) is a special case. As one of the nearest galaxies to our Milky Way, Andromeda is quite large compared to the more distant galaxies. So large, in fact, that I can't fit the entire galaxy into my camera's field of view. So, my many reference images for the Andromeda galaxy didn't help me out here, because the nova happened to be in a region of Andromeda that my camera routinely misses! It's outside my field of view.
Is that a problem for me?
No! And I'll tell you why.
The Andromeda galaxy is the most famous galaxy of them all. The first-ever supernova was found in the Andromeda galaxy in 1885. And Edwin Hubble's discovery of a Cepheid variable in the Andromeda galaxy in 1925 was the proof needed to establish that the "spiral nebulae" like the Andromeda galaxy were, in fact, distant galaxies like the Milky Way. You could say that the Andromeda galaxy was the first galaxy to be recognized as a galaxy. Not only that, but the Andromeda galaxy is visible to the naked eye. As such, it was a fuzzy object known to astronomers long before the invention of the telescope.
Being such a large and bright galaxy, Andromeda is also a favorite target for astrophotographers. And that means that people are shooting Andromeda all the time. Thus, if a supernova ever pops up in the Andromeda galaxy again, I don't stand a chance at being the first one to spot it! So, I don't take the Andromeda galaxy very seriously as one of my supernova targets. I shoot it now and then, but there are so many foreground stars that I usually just take my picture, blink it with an older one, and then move on. I don't lose sleep over the fact that there might be supernovae outside my camera's field of view. I search the areas that I can record in a single image, and I don't waste time moving the telescope up and down trying to capture Andromeda in its entirety. Besides, if there's ever a supernova in Andromeda, I'll surely hear about it from YouTuber astrophotographers before I can come across it myself (just like this LRN)!
But there's another perk of supernova hunting that comes into play here. And that is something that I call the "acceptable image concept." Today's astrophotographers are obsessed with deep images. They'll spend 50 to 100 hours on a target to capture the faintest details possible. They shoot 3-minute subframe exposures through three or four filters and stack them up to create their fantastic images of the deep sky.
And I say, "Good for them! Their images are mind-boggling, and I applaud their efforts."
But I'm a supernova hunter. I can't afford to spend 50 hours on one target. That would take me several weeks to shoot one galaxy. I need to shoot hundreds of galaxies per week if I want to find a supernova. I don't need to take pretty pictures of galaxies. I just need to see a supernova. I don't care if the galaxy is underexposed. Back in 2015, I decided that I can get an "acceptable image" in 15 to 20 seconds. I don't need any more depth than that to spot a supernova in a galaxy.
After a couple of years of supernova hunting, I realized that I could apply my "acceptable image concept" to other deep-sky objects, as well. With my fast f/2 optical system, star clusters and nebulae look pretty good in 20-second exposures. I can capture more detail in a 20-second exposure than I can see by looking through the eyepiece. So, I began to shoot the Herschel objects, to see if I could capture all 2500 of them. It took me 12 years to do it, squeezing in a few Herschel objects each night while searching for supernovae. I now have reference images of all the Herschel objects, too!
During that time, I had a friend who made sketches of deep-sky objects with his 12-inch Dobsonian. We made side-by-sides of all the Messiers and the Herschel 400, as well as other deep-sky objects, comets and asteroids. He would email me a scan of his sketch, and I'd either use one of my 20-second exposure reference images or, if I hadn't shot the object before, I'd go out and shoot it on my next clear night. I'd scale and rotate my image to match his sketch, copy and paste it alongside his sketch and send it back to him. Our side-by-sides are fantastic observing aids that can be used with sky atlases to track down thousands of deep-sky targets. They give a very realistic expectation of what can be seen visually through a backyard telescope.
That was a lot of fun for me. Of course, living in Arizona, with clear nights aplenty, and using my home network to remotely control my telescope and camera in the back yard, I do all my astronomy in the comforts of my living room. But my friend, who preferred looking through the eyepiece, and living in the northeast of the US, had to be outside with his telescope all night in the wind, freezing temps, and dew/frost, drawing his targets with a dim, red flashlight. Sometimes the dew was so heavy that his paper couldn't handle the pressure of an eraser. I don't know if it was frustration with sketching under harrowing conditions, or if it was the depressing number of cloudy nights he faced, or perhaps his advancing age, but our collaboration came to an unfortunate end a few years ago.
In any case, one of the objects we did a side-by-side of happened to be NGC 206, a star cluster in the Andromeda galaxy. It was discovered by William Herschel, which makes it a Herschel object, but it did not make the condensed "Herschel 400" list. One day, my friend sent me his sketch of NGC 206 and after searching through my images, I discovered that the cluster is always at the edge of the field in my images. So, on my next night out, instead of targeting M 31, I had my Go-To telescope point at NGC 206 for the side-by-side.
Fast-forward to the other night, when I found out about the Luminous Red Nova in M 31. I took an image and faced the age-old problem I've always had with M 31! It's too large for my camera and telescope, even at f/2! NGC 206 was in the corner of the field of view, and the LRN was not in there at all. So, I centered on NGC 206. That helped to bring in the LRN, but how was I going to find an image to blink it with using the time filter?
NGC 206 jogged my memory. I recalled making the side-by-side of NGC 206 with my friend a few years ago, so I searched through my side-by-sides to find the date when I took the original picture. It was taken on August 1, 2019.
Why am I telling you this story?
Because I thought that you may be wondering why I needed to go all the way back to 2019, to find a suitable image to blink with my image of the LRN. After all, if I'm a supernova hunter, shouldn't I have more recent images of the Andromeda galaxy? And the answer is, "Yes!"
But since the LRN is on the outskirts of the Andromeda galaxy. There's only one other night when I shot the Andromeda galaxy off-center, and that was on August 1, 2019.
UPDATE - January 6, 2026:
I checked in on the nova a couple of nights ago and it was very nearly the same brightness as it was on December 18, 2025. I would bet owners of reasonably large (15-inch and larger) Dobsonians can still get an eyepiece view of the nova, if they're observing under dark skies. Here's my picture from January 4th, again blinked with my image from 2019. The nova is located to the right of the annotated quadrilateral.
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