“Keep up the good work, if only for a while, if only for the twinkling of a tiny galaxy.” -Wislawa Szymborska
You all know about shooting stars.
Seen from Earth, mostly during meteor showers, these aren’t stars at all, but are tiny fragments of rock that hit the Earth’s atmosphere, and streak across it, leaving a bright fireball as it burns up. If you’re a great (and lucky) astrophotographer, you can nab a picture of one close up as it burns up.
But this principle, that a fast-moving object traveling through space will run into whatever matter is in its way, leads us to some amazing observations. It hardly matters whether you’re shooting a meteor through the Earth’s atmosphere or a bullet through water; the results are spectacular.
The fast-moving object runs into the matter, collides with it, and feels the resistive effects of whatever it’s passing through. For pretty much anything, it heats up and slows down.
What do I mean by “pretty much anything?” Take the star, Mira, for example. (Full story here.) Zooming through our galaxy at a whopping 64 km/s relative to the rest of the stars and gas in there, Mira’s “matter” that it passed through is all of the interstellar gas and dust.
But while it looks like a normal star in the visible (bottom), the Ultra-Violet light (taken by Galex, top) shows the dramatic effects of friction. A small fraction of the outer layers — where the gas is most diffuse — actually get stripped off of the star, leaving a wake of hot gas behind it! Mira, in other words, is a real star that’s actually shooting through our galaxy.
But what about a shooting galaxy?
Our large, spiral galaxy is pretty lonesome. Sure, a couple of million light years away lies Andromeda, another large spiral. But for tens of millions of light years, that’s it. No other large galaxies. Yet, if you look at the image above, you’ll see that in some regions of space, there are many, many large galaxies clumped close together. This is just a small part of the Virgo Cluster, the closest large cluster of galaxies to us, with about 1,300 galaxies within about 7 million light years of one another.
On the outskirts of the Virgo cluster is an innocuous, nondescript galaxy named IC 3418. Have a look.
Unlike the other galaxies in the cluster, however, IC 3418 is being pulled towards the center of the cluster for what appears to be the very first time! It’s moving at over 1,000 km/s towards the center of Virgo, or about 0.3% the speed of light.
And it’s an entire galaxy!
So where’s its tail, you ask? Where’s all the evidence of this “friction” we’re supposed to see? Well, just like with Mira, you need to look in Ultraviolet light once again.
(Thanks again, Galex!) Not only is there a tail there, but if we look close at this galaxy, we can see that something spectacular is happening to the gas getting left behind in the wake of this galaxy.
Under its own gravity, the stripped gas is combining with the intergalactic medium and collapsing to form stars! These are hot young stars, and those are the incredibly bright spots you see in the Ultraviolet image above. In other words, a shooting galaxy leaves not a tail, not some dust, but a trail of new stars in its wake.
And that’s what a shooting galaxy looks like!