To B[e] or not to B[e]

Contributed by
Aug 5, 2009
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Compared to some stars, our Sun is something of a wimp. It just sits there all by itself, shining all the time, hardly changing in temperature, brightness, or mass.

But not all stars are so boring. I present to you this amazing image of the star HD 87643:


[Click to embiggen.]

HD 87643 is the star near the middle of the picture, surrounded by the weird glowing material. And OK, so the name of the star is a little boring, but the star itself is very cool indeed. Or hot. And it's not even one star.

HD 87643 is what's called a B[e] giant star. The B means it's a hot star, and since it's a giant (or possibly even a supergiant) it will one day end its life in a titanic supernova explosion. The [e] means its spectrum has emission lines. Usually, when you take the spectrum of a star you see absorption lines, dips in the spectrum indicating the presence of various atoms or molecules (see my recent post about this for some details). You only see emission lines -- bright features in the spectrum -- if the star is surrounded by gas that is being lit up by the star making it glow just like a neon sign. It's very rare to have stars do this, but HD 87643 is one of 'em. B[e] stars are thought to have very strong stellar winds, which means they are blowing a lot of gas into space, and it's this gas that's glowing.

Another funny thing about HD 87643 is that it's blasting out infrared light, far more than a B star should. That's a giveaway that it's also surrounded by dust. The dust absorbs light from the star, warms up, and then re-emits that light as heat. So the dust glows brightly in the infrared, and it's that excess radiation that betrays its presence.

Zoomed VLT image of HD87643
But the really funny thing about the star is that it changes in brightness on a somewhat regular schedule, dimming and brightening every 50 years or so. Moreover, a close-up of the star image (shown here) indicates it's surrounded by spherical shells of material, which means it's blasting off vast amounts of mass every few decades! What the heck is going on here?

To figure that out, astronomers used an interferometer, a telescope designed to take incredibly high-resolution observations of the star (you can get all the details in their journal article). What they found was that HD 87643 is actually two stars!

HD 87643 separated into its two components
You can see that in the extreme close-up image here. One is the massive B star, and the other is a less massive, somewhat cooler star. They are separated by a mere 7 or 8 billion kilometers (4-5 billion miles) -- to give you a sense of scale, this is only about twice the distance of Neptune from the Sun. Given that the stars lie about 5000 light years away, this image is pretty phenomenal!

It's not well determined yet, but it seems that they orbit each other with a period of roughly 50 years, and the orbit is stretched out into a very elongated ellipse. Every half-century or so, the stars swing past each other and have a very close encounter. When they do, their mutual gravity rips material off each other, and this matter goes screaming off into space, expanding outward to make the smoke rings seen in the picture above. Some of this material stays in orbit around the more massive star, creating a thick ring of dust which absorbs the starlight and generates the IR excess.

What a phenomenal system is HD 87643! Two hot stars, one inevitably marching toward doom, the other on an orbit that drops it down into the maelstrom. When it does, chaos ensues as matter is flung outward into space, gas that glows like a beacon in the galaxy, a circular target with a bull's-eye in the center announcing the star system's odd presence.

It always amazes me that we can figure this stuff out just from a few observations of a star, especially one that's 50 quadrillion kilometers away! But in astronomy, with advanced technology comes advanced knowledge. We learn more and more about the sky around us, increasing our understanding of the Universe.

And it's a pretty cool place.

Image credit: ESO/F. Millour et al.