Adam Block is an astronomer who dedicates a lot of his life to using decently big telescopes to create gorgeous images of all kinds of objects, though mostly deep-sky things like nebulae and galaxies. 

He sent me an image recently that, I have to admit, really confused me. I had to do a lot of reading to understand it, and to be honest my grasp on it is still a little shaky. But it’s so cool I have to show it to you.

Behold, NGC 7531:

[Click here to get a much larger 2,000 x 2,900 pixel version, which you want to do.]

Whoa. It’s obviously a lovely spiral galaxy; the arms look blue because gas clouds (which you can see as pinkish dots) are actively forming stars in them. Most of the newborn stars are low mass, like the Sun or smaller, but a few are massive and blue. They’re incredibly luminous, outshining all the other stars combined, so the arms tend to look blue. These massive stars don’t live long, so in the galactic center, where star formation ceased long ago, they have all exploded, leaving behind redder stars, coloring the center more ruddy. 

All the individual stars you see are in our own Milky Way galaxy, and we have to look past them to see the much more distant galaxy; NGC 7531 is about 75 million light-years away, so it’s far, far in the background of those stars.

But… what’s that weird umbrella-shaped plume above the galaxy?

Yeah, good question. Structures like this were first seen in the 1950s, though at the time no one really knew what they were. More modern research has shown them to be the leftover remnants of a smaller galaxy, which made the very bad decision to collide with the larger spiral.

I’ve written about galaxy collisions many times in the past, usually when two decent-sized galaxies slam into each other. But far more common is when a small dwarf galaxy (sometimes called the bullet) runs into a larger major galaxy (the target). Both are affected, but very differently. [Adam also wrote a bit about this in his image description he posted on the terrific AstroBin website.]

Sometimes, when the small galaxy rams right through the middle of a disk galaxy (a galaxy with a flat disk of stars, gas, and dust around the middle, like the Milky Way or really any spiral galaxy) the gravity of the bullet galaxy can pull stars toward the center of the target galaxy. When the bullet galaxy passes, the stars are then pulled away, creating an expanding ring that can grow quite big. The classic example of this is the wonderful and weird Cartwheel Galaxy.

But the smaller galaxy is affected too. If it’s small and compact it may pass through relatively unscathed, but if it’s bigger and more diffuse, then the target galaxy’s gravity will change it profoundly. As the small galaxy approaches, the side closer to the target galaxy feels a stronger gravitational pull toward it then the center and trailing side. This is called the tidal effect, and it stretches the galaxy out like taffy. It elongates. 

This continues as it passes through the bigger galaxy, and it can get really elongated. If the orbit of the bullet galaxy is extremely elliptical, almost a straight line (called a radial orbit), it can drop into the big galaxy, pass through, continue on for a ways slowing the whole time, and then fall back into the target galaxy. If this happens then the tidal effect can work multiple times, completely pulling apart the bullet galaxy.

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