Shocking star is shocking. Shocking, I say!

Bad Astronomy
The entire universe in blog form
Jan. 24 2011 4:48 PM

Shocking star is shocking. Shocking, I say!

In a recent post, I said that science is at its best when it reveals both the inner and outer beauty of nature. I can't think of a better example than this stunning picture of a runaway star from NASA's Wide-field Infrared Survey Explorer, aka WISE:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


OOOooo, pretty! Click to embiggen.

The star in question is the bright blue one in the center of the picture. Its name is Zeta Ophiuchi, and to the eye it's a non-descript reddish star in the constellation of Ophiuchus (yes, the same one that recently found itself entangled in the zodiac embroglio). You wouldn't look twice at it, but it hides some secrets. For one, it's not red, it's blue! It's actually a supergiant, hot, blue star that's probably 20 times the mass of the Sun. It looks reddish to the eye because there's quite a bit of interstellar dust between us and it, and that tends to make light look redder (much like junk in the air makes a sunset look red). In fact, it's only about 450 light years away; if there were no dust between us and it Zeta Oph would be one of the three or four brightest stars in the sky!

But it has another secret: it's a runaway star. It has a rather high speed compared to other stars, and we think we know why: it was once part of a binary system. It probably started off life with less mass, and it was orbiting a high mass star. The other star swelled up into a red supergiant, dumping vast amounts of material onto Zeta Oph. Then the star exploded as a supernova! When it did, it flung off Zeta Oph like a water droplet off a shaking dog. It wasn't the explosion itself that pushed on the star; it was centripetal acceleration. The two stars were probably orbiting each other at high speed, and when the second star blew up, Zeta Oph kept that speed and flew off into space!

We see several such high-mass runaway stars, and we think that's where they come from: they survived their partners going supernova.

But what the heck is all that stuff around it in the picture? WISE "sees" in infrared light, which is emitted by warm material like all that interstellar dust around Zeta Oph (which itself looks blue in this picture, but remember, it's false color, even if Zeta Oph coincidentally really is blue). Most of that dust appears green in the image. But nearer the star that dust is considerably brighter for two reasons. One is that the material is being heated by the star itself as it passes through. But also there's a fierce wind of material streaming off of Zeta Oph's surface, and that's ramming the dust, compressing it. Denser material, in this case, can glow more brightly than the thinner material around it.

This compression can be seen as the giant bow wave, the curve in the material. The wind from the star is moving faster than the speed of sound in the material, so the material is experiencing a shock wave.

I know, sound doesn't carry in space, but space isn't empty here. There is material, wispy and ethereal as it is, so sound waves can travel through it. And in this case, so can shock waves! Shock waves in space are common if you know where to look.

Right now, Zeta Oph is getting old. It probably has a lifespan measured in millions of years, so it may not be too much longer before it too explodes as a supernova. When it does, it'll outshine Venus in the sky, even through all that dust. And as it fades, we'd see the light from the blast traveling through the dust cloud, which is light years across, like an echo from the explosion itself. What a sight that would be!

I didn't know anything about Zeta Oph before this picture came out, but now I see that it has an amazing history, and an amazing future ahead of it. So I stand by my statement: science really is at its best when it reveals both the inner and outer beauty of nature.

Image credit: NASA/JPL-Caltech/UCLA

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