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SYFY WIRE Bad Astronomy

Spectacular Chaos From Two Galaxies (Literally) in Flagrante Delicto

By Phil Plait

The universe is a fairly amazing place, if you know where to look. The good news is, you can look pretty much anywhere and be amazed by what you see. The even better news is that some of the things you'll come upon will raise the hair on the back of your neck and grind your sense of scale to dust.

For proof, I present to you a Hubble Space Telescope image of the cosmic train wreck that is NGC 2623, a massive collision of galaxies taking place over a stretch of a million trillion kilometers of space:

I know, right? [Click the image to embiggen.]

NGC 2623 is about 250 million light years away, a soul-crushing distance, but Hubble's eye is keen indeed and can tease out fantastic details. This image is actually composed of several different observations by the orbiting observatory, lovingly assembled by amateur astronomer Martin Pugh as part of the Hubble Legacy Archive.

There's so much to see here! What you're looking at is not really one galaxy, but not really two, either. It used to be two fully mature and fairly large galaxies, but we've caught them in flagrante delicto (literally, since this means "in blazing offense"), right in the middle of their ridiculously photogenic collision.

A galaxy is typically composed of billions of stars, many also festooned with huge clouds of gas and dust. Our own Milky Way is a familiar spiral galaxy, and the two original galaxies making up NGC 2623 probably started out as spirals as well. By happenstance, they were traveling on paths that brought them very close together ... too close. The gravity of one galaxy started tugging on the other, the force growing inexorably as the two neared. Orbits distorted, shapes changed, and the end result is the creation of those two, long streamers of material called "tidal tails," drawn out of each galaxy by the gravity of the otherâand each more than 50,000 light years long. They loop around due to the collision being a glancing blow; the galaxies didn't slam into each other head-on, but instead passed close and then looped around, slowly spiraling into each other like lovers embracing.

But this is hardly a tender interaction. Huge clouds of gas collided, smacking into one another at high velocity, compressing them. Collapsing clouds form stars, and that is evident as the blue color toward the tails. Young, massive stars are hot and shine blue. They also don't live long, just a few million years, so we know this collision happened in the not-so-distant past. In fact, using various techniques to date the stars in the collision, it looks like this whole catastrophe started less than 100 million years ago. Fairly recently, if your calendar keeps galactic time.

The core of the collision is just a complete mess. Huge arcs of gravitationally-stripped gas wrap around it, and it's studded with the bright blue sites of furious star formation. The reddish and darker regions are due to dustâcomplex molecules that are very good at absorbing light are blocking the view behind them. Dust is created by stars, both when they form and when they die. This huge splurging swath of it cutting across to the upper left of the core is another sure sign that baby stars are popping out at a tremendous rate.

Amazingly, even with all this violence occurring, it's highly unlikely that even two stars will physically collide! The scale here is immense, with stars separated by distances of trillions of kilometers. On that scale, stars are specks, so tiny that the odds of two smacking into each other are essentially zero. Gas clouds are millions of times bigger, which is why they can collide to form more stars.

Itâs chaos on an epic scale.  But it'll settle down. In another few hundred million years, the two galaxies will actually merge, becoming one, somewhat bigger galaxy, probably a puffy, cotton-ball shaped elliptical-class galaxy. In a way that'll be too bad; all the drama and beauty of this vast event will fade. But such is life in our Universe.

And one more thing. Sure, this galaxy is a long way off, and you may be thinking, "There but for the grace of gravity go I." But don't get too smug: In 4 billion years, our fate is the same. The Milky Way will collide with the equally ginormous Andromeda Galaxyâweâre barreling toward each other at a rate of hundreds of kilometers per second. And when we do eventually collide all those eons from now, we'll wind up looking a whole lot like NGC 2623. If it makes you feel any better, perhaps some alien civilization millions of light years away will see us and gasp in awe (or whatever they do in awe) as they soak in the beauty of our two galaxies becoming one.

OK, fine, that doesn't make me feel any better, either. Still, it's fascinating that these collisions happen at all. When we observe them we learn a lot about how galaxies behave, and the eventual fate of our own ⦠as well as get an eyeful of some of the most gorgeous astronomical artwork in the Universe.

Tip o' the eyepiece to Astronomy Picture of the Day. Image credit: NASA/ESA/Hubble/Martin Pugh, used by permission.

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