How to Make a Real Shooting Star!

“I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet. The function of man is to live, not to exist. I shall not waste my days trying to prolong them. I shall use my time.” –Jack London

Most of you have seen a shooting star before, also known as a meteor.

They happen most frequently during (surprise) meteor showers, but occasionally (and sparsely) during other times of the year. Whenever a small bit of astronomical dust from outer space runs into the atmosphere — even though it may be no bigger than a grain of sand — it burns up in the atmosphere from the tremendous heat of friction, creating a spectacular streak of light across the sky.

Of course, they’re not actual stars. And really, they’re not moving all that quickly. But when the Earth moves through that old, dusty part of space, those grains of cosmic dust are moving fast with respect to the Earth’s atmosphere, and the friction from that causes the shooting star you see. The dust comes mostly from old comet trails, and that’s why the same meteor showers (e.g., the Perseids, the Leonids, the Quadrantids, etc.) happen around the same time each year, as the Earth passes through the comet’s orbit.

But what about real stars? Normally, in our galaxy, although stars whip around the center of our galaxy at a mind-boggling 200+ kilometers per second out where we are, they all go around at more-or-less the same speed.

So does all the gas and dust, and so — with everything moving similarly — practically nothing “interesting” happens. But then we come to an interesting star: Mira. Mira is a binary star system, but so are lots of stars. Let’s take a look:

Yes, yes, very nice, Hubble. It’s a star, congratulations. I’m not impressed, and probably neither are you. In fact, if you look in visible light, there isn’t anything impressive going on.

It’s bright, I guess, being a red giant and all, but bright stars are a dime-a-dozen. Did you notice in the Hubble image (two up) that there was something weird-looking in the Ultra-Violet image? It turns out that Mira is moving fast: at about 64 km/s relative to the rest of its neighborhood! So NASA’s high resolution GALEX satellite took a series of images around Mira, and what did it find? Well, it made this mosaic, below, which you can click on for the ridiculously large full resolution version.

This is a real shooting star! Only a couple of hundred light years distant, it’s quite close to us, and moving fast relative to the interstellar medium. The “tail” you see from it comes from heat and friction as the star plows through this interstellar gas and dust at breakneck speeds.

So how the hell did it get to be moving this fast in the first place? Like nearly everything else in the Universe, I blame this on gravity, too. Let me explain.

At lots of places in the Universe, you get clusters of stars together: groups of three or more. You can have globular clusters (above, like M92) with up to millions of stars, open star clusters (below, like M7), with dozens to thousands of stars, or just a group of three or more stars in the same solar system.

What’s remarkable about all of these? They’re all unstable! This means that, given enough time, all of these systems will eventually start to kick out some of their stars due to gravitational interactions!

We give this a name: violent relaxation. When the old system spits a member out, all of the other members become more tightly bound (i.e., more relaxed), but that’s a lower-energy configuration, and we need to conserve energy. So where does it go? It becomes kinetic energy for the mass that gets spit out!

And that’s probably what happened to Mira: she was a part of a larger star system that she got spit out from, and that system is now more tightly bound than when she was in it!

So keep a close eye on all your tightly bound star clusters, because it’s only a matter of time before another one gets kicked out at a spectacular speed!