Betelgeuse shocker

By Phil Plait | November 19, 2008 7:12 pm

Distance crushes perspective. Objects hurtle through space at mind-numbing speeds, some moving so quickly they could cross the United States in just seconds; yet, due to their distance, we could wait thousands of years to be able to perceive their motion at all.

Unless, that is, they leave behind some tell-tale sign of their rapid movement. Space is not empty, and a star plowing through this ethereally thin gas at dozens of kilometers per second reveals itself. The gas gets compressed ahead of the star, and flows around it in graceful arcs. Like water flowing around the bow of a ship, such a formation is called a bow shock.

This shock wave can be invisible to the unaided eye, but when we train infrared telescopes on them they leap out of the picture. Behold the bow shock of Betelgeuse:

Infrared image showing Betelgeuse’s bow shock

This picture was taken by the Japanese satellite observatory Akari. It’s a composite of images taken at 65 (blue in the image), 90 (green) and 140 (red) microns. These are well beyond the human eye’s capacity to see; our sensitivity is about 0.3 to 0.7 microns.

Betegeuse is a red supergiant about 400 600 light years away (in the linked article, it’s mistakenly listed as 200 light years). As the superstar plows through the gas between stars (the interstellar medium, as we pros in the know like to call it), the gas forms a bow shock about three light years across. It’s not the physical star itself doing the compressing, it’s actually Betelgeuse’s stellar wind. Like our own Sun’s solar wind, Betelgeuse blows off a stream of particles, but it’s far denser. The expanding gas is what’s slamming into the interstellar medium and forming this beautiful structure.

I have never seen anything like this from Betelgeuse, and it’s incredible. The shape of the bow shock makes Betelgeuse’s direction of motion obvious enough. The star is moving about 30 km/sec across the gas, and the wind is expanding at about another 17 km/sec, so the collision is actually pretty fast on human scales, but relatively weak for such events (sometimes objects are ramming through gas at hundreds of km/sec).

Besides being a pretty picture, there’s science to be had here. Examining data like this can tell us how thick the matter is between stars near Betelgeuse, and how it behaves when smacked by an expanding star’s wind. One day, Betelgeuse will explode*, going supernova, and the matter from the explosion will expand and slam into the already-ejected material at a large fraction of the speed of light. Understanding that material before the star explodes helps us understand what happens after it explodes, and that in turn teaches us a lot about the way stars are born, live out their lives, and die. Considering we owe our existence to such supernovae — they created the iron in our blood and the calcium in our bones — I think that’s a field worthy of study.

* At that distance, when it blows up it only be a pretty light in the sky, and won’t be able to harm the Earth at all. See Chapter 3, "The Stellar Fury of Supernovae" in my book Death from the Skies!

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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