So a Galaxy Walks Into a Bar ...

Adam Block is one of my favorite astrophotographers. Now, he has a bit of an unfair advantage: the 0.81-meter Schulman Telescope at the top of a mountain in Arizona at his disposal. He’s also really good at finding interesting but lesser-known objects and has a serious knack for creating incredible images of them.

I’ve featured his images many, many times on this blog, but I think this may be the very best I’ve ever seen: the spiral galaxy NGC 1398.

Holy resonance-amplified stellar awesomeness!

NGC 1398 is a galaxy roughly the same size as our Milky Way, located about 65 million light-years away in the Fornax cluster, a cosmic city of hundreds of galaxies. It’s what’s called a barred spiral, due to the long rectangular feature right in the middle. It also has that peculiar ring around the bar, a second double-armed ring farther out, and then a couple of dozen fluffy-looking spurs.

Bars are pretty common in big disk galaxies; the Milky Way has one. They form due to the way gravity works in the disk. In our solar system, essentially all the mass is in the Sun, and the planets orbit it in nice, regular paths. But in a disk galaxy the mass is spread throughout the disk, and that changes things. If you disturb the disk (say a nearby galaxy passes, and its gravity distorts the disk a bit) that perturbation can grow, propagating through the stars and gas.

The math is a tad complex, but the end result is the bar pattern, like a traffic jam in the central galaxy. That bar itself has a peculiar gravitational field, and can affect stars and gas outside it.

The bar rotates around the center of the galaxy with some period. At a certain distance from it, stars and gas orbits at some small multiple of that period, like twice as long, or four times as long. This simple relationship, called a resonance, pumps up the stars and gas, a bit like the way pumping your legs on a playground swing at the right frequency can make you go higher.

That’s what creates the inner ring. It’s actually a pair of tightly wound spiral arms that overlap (you can see that a lot more clearly in a WISE image of the galaxy in the far infrared). The outer ring is also really just tightly constrained arms, too. Outside of that, the spurs are patchy—what’s called flocculent, which is just a cool word (it means patchy, like clumps of wool or cotton).

In Adam’s picture (a total of an astonishing 20 hours of exposure) you can also see a lot of smaller galaxies, almost certainly in the far distant background. I do mean “distant”: They’re a billion light-years away or more. The bright red star in the lower right, on the other hand, is in our galaxy, probably only a few hundred or thousand light-years away.

Quite the range in this one photo! And a spectacular example of what happens when you take a handful of simple ingredients—stars, gas, gravity, and a few laws of physics—and let them interact for a few billion years.