Gamma rays from monster stars

By Phil Plait | February 7, 2007 10:56 pm

Update (February 15, 2007): When I originally wrote this, I was a bit confused over the cosmic rays generated by the stellar winds as opposed to the gamma rays which were generated. I have edited this post to fix that correction. In general I dislike just changing things without making it clear where the mistakes were, but the edits were in a bunch of places, and I didn’t want to totally blow the flow of the article. But this stuff is cool, so maybe I’ll write another post about the difference between cosmic and gamma rays. A lot of folks confuse them, so it’s worth talking about.

As I write this, I’m attending a symposium for GLAST, the Gamma Ray Large Area Space Telescope, a mission I’ve blogged about before. It’s scheduled to launch for November 2007, so scientists are beginning to gather and talk about what this machine will do. That’s what this symposium is about.

A quick primer: GLAST will observe gamma rays, which are the highest energy form of light. Only incredibly energetic events can generate gamma rays, like exploding stars, matter falling into a black hole, ultradense neutron stars with fantastically strong magnetic fields (like, a trillion or a quadrillion times the strength of the Earth’s magnetic field), or subatomic particles smashing into each other at very nearly the speed of light. Get used to big adjectives because I’ll be using quite a few here.

We had a press conference earlier with three really interesting news items. I was already in on the action since my boss here is the press officer for such events. I wound up organizing the press releases and rewriting them as well to make them publicly accessible. So for the first two items I’ll simply send you to the press page.

But the third one is just too cool, and I want to expound on it a bit.

Cosmic rays are extremely high energy particles that shoot around in space (not to be confused with gamma rays, which are a form of light — be careful as you read this story, since it involves both cosmic and gamma rays!). They irritate most astronomers because they hit our detectors and screw up the image by making it look like TV static. But they also tell an important tale. They have so much energy that it’s actually rather difficult to understand where they come from. In general, it’s thought that the only environment with enough energy to generate these little guys is near a supernova, an exploding star.

But now cosmic rays have been detected, indirectly, coming from an unusual source: normal stars. Well, kind of normal. Astronomers using HESS (the High Energy Stereoscopic System; a series of cosmic ray telescopes in Namibia) detected high-energy gamma rays coming from a cluster of stars called Westerlund 2. This cluster is young – it still has lots of gas in it — and vigorously making new stars. One of these new stars is called WR20a… except it’s not really one star, it’s actually a binary system with two stars orbiting each other. Both of stars are what we call main sequence stars, stars like the Sun which are fusing hydrogen into helium in their cores. But what stars these are! Both of them are absolute monsters: each has more than 80 times the mass of the Sun. This is close to the upper limit to the mass a star can have without tearing itself apart, so having two of these behemoths so close together is really incredible. Also, they are so close they are practically touching each other. They make a complete orbit in less than four days! For comparison, Mercury takes 88 days to circle the Sun.

Here is an image of Westerlund 2, taken by the Spitzer Space Telescope. WR20a is marked.

When I heard about them, the hairs on the back of my neck literally stood up. This is one incredible pair of stars.

Stars this massive generate a mighty wind, like a solar wind but vastly more powerful. Both these titans together blow a wind so powerful that it has carved an enormous bubble in the cluster gas. This bubble was blown so forcefully it has actually blown outside of the cluster and into the gas that lies between stars in the Galaxy, what astronomers call the interstellar medium. The stellar wind from WR20a is very fast and powerful, but it’s less dense than the ISM, and when a low density wind slams into a denser cooler medium, chaos ensues. There is incredible turbulence and shock waves set up, and this is prime ground for making cosmic rays. Subatomic particles are accelerated to incredibly high speeds from all this turbulence (no doubt magnetic fields are involved as well), and they become, by definition, cosmic rays.

Cosmic rays are really hard to detect directly. They get deflected by the Galaxy’s magnetic fields, for examples, and generally lose a lot of energy before they can get from where they are created to the Earth. However, cosmic rays can make gamma rays! If a cosmic ray, which is really just a really fast subatomic particle, slams into another particle that is not moving as quickly, you can get all sorts of subatomic "fragments" from the collision. One such particle is a pion. Pions are really unstable, which means that once created they quickly decay (in about 10-16 seconds!), and when they decay they create a gamma ray. The upshot of this is that if you can detect gamma rays from cosmic sources, it means that you might actually be seeing cosmic rays at work.

So astronomers using HESS heard about this binary star WR20a in the cluster, realized it might make cosmic rays, and pointed the HESS telescopes at the cluster – mind you, no one had ever thought that normal stars could make cosmic rays, so this was a gamble. What they found was amazing: a glow of gamma rays around the cluster, clear evidence that cosmic rays are being generated (no other possible source of gamma rays was found, indicating they must be from cosmic rays making decaying pions). Moreover, the emission was extended, meaning it was spread out. If it came from the stars themselves, they’d see the emission as a point source, a dot. But since it was spread out, and in fact coming from a region bigger than the cluster, they knew the cosmic rays were coming from where the stellar wind was slamming into the ISM. This is the first time we have ever seen cosmic rays coming from anywhere other than supernovae.

Here is a diagram of what they saw (click it for a bigger version). The glow from the gamma rays (and thus the cosmic rays) was coming from a patch of sky about the same size as the full Moon.

Something about systems like this really gets me going. Two monster stars, totaling 160+ solar masses, tossing each other around their orbits twice a week (twice a week! Holy Haleakale!), blasting out thousands of times the Sun’s energy, and essentially bellowing their stellar winds with such vicious strength that they are reshaping the space around them for thousands of cubic light years.

It boggles me that such a thing can even exist, let alone be observed, understood, and even have its behavior predicted.

Man, humans are smart. I’m so proud to be one of ’em.


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