In 2015, the folks with Hubble Space Telescope released what was at the time the largest and highest-resolution image of the Andromeda Galaxy ever taken. Because the galaxy is so close to us, a mere 2.5 million light-years — astronomers have a quirky idea of what “close” is — it looms large in our sky, about 3° across (the Moon is half a degree across, for comparison). That’s far, far larger than Hubble’s field of view, so hundreds of images were taken and put together into a mosaic. Even so, it only covered half of Andromeda’s span.

Last week, after many more years of observations, they have just released another mosaic, and this one covers the other half as well.

Check. This. Out.

Pretty, right? And the thing is, it looks a lot like every other photo of Andromeda that you see… but that’s because I had to shrink it way WAY down to fit this newsletter. A hint at how detailed it really is can be seen around the edges, which look jagged, like a saw blade. Each one of those teeth represents a single Hubble field of view, and each one is 16 megapixels.

The raw image mosaic used for science is a staggering 2.5 billion pixels in size, created from over 600 individual images. It took over 1,000 orbits of Hubble to make this shot! [Note: Astronomers use orbits as a unit of time measurement for observations because it’s convenient; each orbit is about 90 minutes but not all of it is used for actual observing. Using a space telescope is pretty complex.]

The final mosaic for public consumption is huge: 42,208 x 9,870 pixels! You could wallpaper a room with a printed image of it. Click here to get access to it; the TIFF is nearly a gigabyte in size, and even the JPG is 330 MB. 

So what does it show? Well, for starters, over 200 million stars can be seen in this image!

This is a small section of the image that I cropped from the lower right part of the mosaic. I also shrank it to fit the newsletter, but you can still see thousands and thousands of stars, so crowded it almost looks like a compression artifact or grain, like in a low-light image. But nope. Those are stars. This looks like a young cluster to me, due to the surplus of bluer stars. Massive stars tend to be blue, and they don’t live long compared to stars like the Sun, so seeing so many so close together indicates they formed together in one place: a star cluster.

I was also impressed by how well they captured M32, a small elliptical satellite galaxy of Andromeda’s, which you can see as a fuzzy ball to the upper left. But here it is in a lot more detail, and again I’ll note I shrank it a lot to fit here:

M32 is what’s called a compact elliptical galaxy, for obvious enough reasons. It’s only about 6,000 light-years across—tiny compared to Andromeda’s roughly 100,000 light-year size—but has millions of stars in it. Like most ellipticals, star formation ceased in it billions of years ago, so all the blue stars are long gone, leaving only yellow and redder ones behind. It happens to sit along the line of sight to a bluer part of the Andromeda galaxy, and the contrast is striking.

Again, as a reminder, those grains in the image are individual stars. Yegads.

The original mosaic of the northern part of the galaxy was taken as part of a program called PHAT, for the Panchromatic Hubble Andromeda Treasury, meaning it was a huge image of the galaxy taken using filters to isolate several colors (this helps us characterize the kinds of stars and objects in it), and meant as a trove for future astronomers to mine. The second mosaic is from PHAST, which is the Panchromatic Hubble Andromeda Southern Treasury, which I only mention because it makes me smile. The data for PHAT and PHAST are public, which means anyone can download them. 

You do need some image processing experience to work with them (they’re in FITS format which is universally used in professional astronomical observations, but not common otherwise), but the point is this was done for the benefit of the astronomical community, which I love. A lot of projects are like that; in general when you use Hubble or JWST you get to keep the data to yourself for a year to work on it, but after that it becomes public so that anyone can look at it. This gives the people who make the observations a chance to analyze them without being scooped, but also supports astronomy by releasing the data in a reasonable amount of time. It’s a great compromise, and a wonderful glimpse into how cooperative science be.

Andromeda is the closest large spiral to our own Milky Way, close enough that it’s visible by eye to northern hemisphere observers in the winter; it’s up right now after sunset high to the southwest. If you live in a dark site it’s relatively easy to spot, especially with binoculars. I took a bunch of photos of it recently using just my phonecam!

Being nearby means it’s exceptionally well studied, and an important part of our understanding of how galaxies behave, including our own (and it might—might—even collide with us someday). That also means powerful telescopes can see fine details, which these mosaics prove. Astronomers will be poring over these images for decades to come.

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