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Yesterday (Dec. 20, 2025) at 15:03 UTC (10:03 Eastern US time) the Sun reached the southernmost point in the sky for the year, marking the moment of the December solstice (usually called the winter solstice, but that’s hemispherist since for those folks south of the equator it’s the summer solstice).

That means we just had the longest night of the year, and the shortest amount of daylight. Which brings up a point, as an aside: we call a 24-hour period a “day”, but we also say that the time the Sun is up in the sky “day” (as opposed to “night”). This is usually not confusing, except when you’re differentiating between that 24-hour planetary rotational time versus daytime as I am here. I mean, all days are the same length — 24 hours, depending on what kind of day you mean, and surprise, there are a lot^* — but at the December solstice we have the shortest length of time the Sun is up, so the terminology is ambiguous. I’ll note that the official term for the entire 24-hour day/night cycle is the wonderful Greek word nycthemeron, which actually is fun to say and seriously just a cool word. I wish we used it more. 

Anyway, I know this solstice is a sad event for many astronomers, since we love the night — OK, I’m kidding, though maybe not 100 percent kidding — but for normal people it’s a happy day with hope for the coming months of more sunshine. Many ancient peoples celebrated this day as midwinter, excited to know that every day for months will have a little more light, a promise of the spring yet to come.

So happy solstice, and may your days to come be brighter.

^* After putting this issue in the queue to post, I decided this statement was a bit too enigmatic to stand as is, but this issue is already running a bit long. So I’ll elaborate a bit on this topic tomorrow. Depending on what you mean by “tomorrow”, mwuhahaha.

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Sometimes, you just need a fantastic image of a magnificent face-on spiral galaxy to fill your eyeballs with gorgeousity and your brain with awesome science.

Happy to oblige:

WOW. A much larger 4,600 x 4,600 pixel version is available, too.

NGC 1309 is about 100 million light-years away in the constellation Eridanus, the river. It’s a fairly typical spiral galaxy, with somewhat messy arms unwinding away from the center. The arms look blue because that’s where ongoing starbirth occurs, and massive stars are bright and blue, so they dominate the color you see. The central region, called the nucleus or hub, is yellower since star formation stopped long ago there, and the only stars left are redder (massive stars die young, but less hefty redder ones last a long time). There’s also some nice dark dust lines tracing along the arms as well, collections of zillions of clouds of grains of material that are opaque to visible light.

Hubble Space Telescope, which was used to take this image, has observed this galaxy a bunch of times, because a couple of stars have blown up in it. One, called SN2002fk, was what we call a Type Ia supernova: a tiny white dwarf that blew up after collecting material from another star. White dwarfs are incredibly dense and have fantastically strong gravity; when enough material piled up it triggered a thermonuclear explosion that essentially used the whole star as a bomb. The amount of energy released is so huge it can outshine the rest of its host galaxy!

Type Ia supernovae are important because, once you correct for a handful of variables, they appear to all blow up with nearly exactly the same brightness. If you know how luminous they are then you can measure its apparent brightness and get its distance. Since these can be seen out to about a billion light-years, that gives us a way to measure the distances to galaxies!

The beauty here is that NGC 1309 is also host to a number of Cepheid variable stars, which are massive stars that pulsate, literally expand and contract over the course of a few days or weeks. This changes their brightness on a regular cycle, and it turns out the period of the change is related to how luminous the star is. So, once again, we can use that to determine the distance to the host galaxy. This method is pretty accurate, but can only be used out to a certain distance before the Cepheids get too faint to see well. One of the main reasons Hubble was built was to look at Cepheids in other galaxies to help us build up a database of galactic distances.

And once we have that, when a Type Ia goes off in one of these closer galaxies we can calibrate them better, and use them to get distances to galaxies too far to see their Cepheids. This bootstrapping method is called the cosmic distance ladder, with each rung based on the one beneath it.

I was not surprised to see my old colleague Adam Riess’s name on this image; he led one of the two teams that discovered dark energy in 1998 by observing distant Type Ia supernovae. He still works on getting more data understand this weird phenomenon, which is causing the expansion of the Universe to accelerate. We still don’t really understand it at all, so the more observations we get the better we can try to figure it out.

In the Hubble image of NGC 1309 you can see hundreds of smaller, redder galaxies in the background. Those galaxies are likely billions of light-years distant, and we only have one reliable method of getting their distances — their redshift, which may be the last rung of the distance ladder for galaxies. But that needs to be calibrated as best we can as well, and observations like this one of NGC 1309 will help.

And also? Dang, it’s just really pretty. That’s nice too.

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