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SYFY WIRE Bad Astronomy

Spotting Betelgeuse

By Phil Plait
An image of the star Betelgeuse taken in the infrared with the Infrared Optical Telescope Array reveals two huge hot spots on the surface. Credit: Haubois et al.

If you go outside around midnight tonight and look to the south (north for you standing-on-your-head southern hemispherites), it'll be hard to miss Orion standing tall over the horizon. If you look at the star at the upper left, marking his right arm, you might note that it glows a ruddy orange-red. That star is the famous Betelgeuse, one of the brightest in the night sky.

But your view of it probably isn't as good as that of some French astronomers who got this awesome shot of Betelgeuse:

An image of the star Betelgeuse taken in the infrared with the Infrared Optical Telescope Array reveals two huge hot spots on the surface. Credit: Haubois et al.

Cooool. Literally. Betelegeuse is a red supergiant, a massive star nearing the end of its life; in a few millennia (or a few hundred) it'll explode as a supernova. But for now it's a swollen monster, cooler than the Sun, but intrinsically a lot more luminous because, simply, there's so much of it. Even with our most powerful telescopes, most normal stars would be an unresolved dot at a distance of 640 light years. But because Betelgeuse is so frakkin' big, we can resolve using a technique called interferometry. This uses several different telescopes to collect light and adds them together in a way such that extremely small objects -- well, apparently small, that is -- can be resolved.

At its mind-numbing distance of over 6 quadrillion kilometers (4 quadrillion miles), mighty Betelegeuse is diminished to a mere 0.045 arcseconds across. To give you an idea of how small this is, the full Moon is about 1800 arcseconds across in the sky. An arcsecond is 1/3600th of a degree, and Betelgeuse is a tiny fraction of even that. Hubble's resolution is about 0.1 arcseconds, so Betelgeuse is unresolved even using that famous 'scope (though using some fancy tricks some features on the star can be seen using Hubble).

Obviously, interferometry is a powerful method for looking at big stars! Using it, the astronomers were able to see two large, bright features on the surface of Betelgeuse, most likely convection spots, where hot gas is bubbling up from the star's interior. The bigger of the two spots is about 500 K hotter than the rest of the 3600 K surface, and accounts for about 8.5% of all the light the star emits! The other is smaller and unresolved, and contributes about 5% of the light.

Mind you, the bigger of the two hot spots really is ginormous: It's bigger than the distance of the Earth from the Sun!

Did I mention Betelegeuse is frakkin' huge?

Techniques like this reveal a huge amount of information on objects that are otherwise far too small in apparent size to measure. We already knew Betelgeuse is a dynamic star -- it changes its brightness over time, for example -- but this particular image shows us the scale of the changes on the star's surface, which can lead to models of how its interior behaves, which in turn will help us understand how supergiant stars live out their lives and eventually explode. At 640 light years away, Betelgeuse can't hurt us when it goes supernova, but it'll be an amazing light show ... and the more we know about it, the better.