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.


Comments (29)

  1. Laguna2

    That is so cool, and besides that thank you for answering my Q&BA question (at least partly)

  2. Buzz Parsec

    “Gamma rays, which are the highest energy form of light…”

    Is there actually an upper limit to how much energy a photon can have? With sufficient energy, would it form a black hole? E = h*nu and E = mc^2, so the “rest” energy of a photon would be h (Planck’s constant) times its frequency divided by the speed of light squared. With a high enough frequency, this could be an enourmous equivalent mass, but is there really an upper limit? Wave length (lambda) would be c/nu, so m=h/(lambda*c). I forget the formula for the Scharzchild limit, but if the mass were large enough and the wavelength small enough… Or is this calculation just too naive?

    Maybe I should send this to Q&BA.

  3. naavis

    The mind boggles.

  4. Chris CII

    Could you please give the power of ten that corresponds to your description (trillion, quadrillion) because between the short scale and the long scale those words don’t mean anything sure anymore (except perhaps in the US)… For instance is the trillion 10^12 or 10^18 ?

    If anyone is puzzled by the question, see

  5. Tukla in Iowa

    Americans use the short scale, so I’m sure Phil means 10^12 and 10^15.

  6. Dunc

    Even we British use the short scale these days… Long scale has pretty much died out in English-speaking countries.

  7. Foggy

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

    This is precisely what makes astrophysics (or cosmology) so interesting. The universe is so big, that anything which can possibly happen, is happening somewhere. All we have to do is keep looking. The universe is our laboratory!

  8. Cindy


    How soon can GLAST look at that system? I imagine all the science time for the first round has been scheduled so will it have to wait until the second call for proposals? Is there “director’s discretionary time” like on HST or a “pretty pictures” run?

  9. Ausrick

    Wow! Two stars that are each about 80 solar masses an the orbit each other in just 4 days!!!! Twice a week!. Thats Awesome. (as in awe-inspiring, not an over used slang term for plain-old-cool ūüėČ )

    I wonder about how many rsuns they are, how far apart they are, how fast they travel relative to each other, If they’re blue or not (I’m assuming they are), what the distortions in space-time near them looks like because of their gravity… and of course the destructive kid in me wants to know what it will be like when they finish their main sequence and start fusing heavier elements… and what will happen eventually when one or both of them die! And I can try and imagine it from what I already know about stars but having two of these monsters right next to each other is just such an amazing situation that I want to hear all of you that know more about stars than me paint some pictures! :)

    People are interested by things that are bigger and better!, (or in some fields, smaller and better!). Give us extremes! There are black holes, and then there are Super Massive Black Holes. There are stars, and then there are Two Giant Monsters in a Tight Binary System! HoooAhhhh!!!! :)

  10. Alex Whiteside

    Finding strange things and trying to figure out what they’re up to. That’s what we all got into science for, right? My final-year dissertation isn’t about anything this cool (computational chemistry), but it’s got exactly the same appeal.

  11. PAS

    Amazing! Makes me think how unimaginative science fiction has been by comparison, when hard-working folks find things like this actually out there.

    One point of confusion, though: the press release speaks of gamma-rays, but you (Phil) expound on cosmic rays. I thought they were different (EM vs. particle radiation, respectively). Is the binary system emitting both, or is this an accidental substitution of terms?

  12. jackd

    Can’t the distance between the two stars be computed once you have the masses and orbital period?

    I wonder how much this configuration has distorted the shapes of the stars? Would they be egg-shaped? And how did they get in there? Could they have coalesced in such close proximity, or did they form farther apart?

  13. Kaptain K

    Minor (very minor) nitpick. You refer to H.E.S.S as a series of cosmic ray telescopes. The word “series” connotates a line or progression. Cluster or group would give a much better mental picture.

  14. (second attempt at posting)

    If we were directly receiving the stellar wind from these stars, we’d be getting cosmic rays (high-energy charged particles). The press release indicates that we’re not getting these charged particles; instead, they’re plowing into the interstellar gas medium, and the resulting shock-wave smashup is emitting gamma rays which we then see with GLAST.

    Shocks and turbulent motion inside a bubble can efficiently transfer energy to cosmic rays, providing a plausible mechanism for particle acceleration. In size and location, the gamma-ray source resembles the so-called “blister” as reported by Whiteoak & Uchida 1997, where the bubble opens up and the wind expands into the low-density ambient medium. Shock acceleration at the boundaries of the blister may enable particles to diffusively re-enter into the dense medium, thereby interacting in hadronic collisions and producing γ-rays. Similar scenarios were outlined over twenty years ago for supernova-driven expansion of particles into a low density medium. If one accepts such a scenario here, it might give the first observational support of energetic gamma-ray emission due to diffusive shock acceleration from supersonic winds in a wind-blown bubble created by WR 20a, or by the ensemble of hot and massive stars in Westerlund 2.

    Rule of thumb #420: nothing with the phrase “hadronic collisions” will be comprehensible to the lay reader. Nevertheless, the gist seems to be that the collisions of particles which would otherwise be counted as cosmic rays produces electromagnetic radiation in the gamma-ray part of the spectrum.

    If I’m reading the story right, then “cosmic rays” in Prof. BA’s third paragraph after the Spitzer picture should actually be “gamma rays”.

  15. Steve

    “Man, humans are smart. I’m so proud to be one of √Ę‚ā¨ňúem.”

    Hear hear! I get so sick of claims that aliens built Stonehenge or the Pyramids or Machu Picchu or the Nazca lines or Random Impressive Object X.

    People are smart. People have always been smart. Now imagine a smart person in a world without TV, the Internet, or brain-dead spoon-fed entertainment of any kind.

    What would this person do? Create. Invent. Organize. Build. What? Anything. Everything. All kinds of amazing things.

    This is my standard response to those who claim that only aliens could have built Random Impressive Object X: bollocks. People are smart.

  16. Richard Smith

    “Stars this massive generate a mighty wind, like a solar wind but vastly more powerful.”

    Which begs the question: Will this wind… be so mighty… as to lay low… the mountains… of the Earth?

  17. wright

    What a mind-blowing image: these two blazing giants, whirling around each other, their combined solar wind stressing and rippling the very interstellar medium…

    A magnificent discovery. This is a great time to live in.

  18. Lunatik

    With such massive stars in orbit so close about each other. What would happen if?…

    Age of both the stars are about same. (roughly) One of the pair, were to go over the tipping point of heavier elements to self annillation before the other did? (Say 1-10 year timespan overall.)

    Could/would the affects of the change of magnetics, gravity, ect. cause a possible sympathtic/cascade type alteration of the second star, producing a greater yield/damage than the one star could by itself?


  19. Douglas Watts

    Excellent post, Phil. Very concise & well-written.

  20. Jeff Rubinoff

    How far away is this star cluster? I didn’t see distance mentioned in the paper you linked to.

  21. Gary Ansorge

    Yeah, Buzz, this does sound like a good question for the QBA. As I recall, the minimum size of a quantum black hole is dependent on the Plank length so the maximum frequency of light should be dependent upon the minimum Plank time? That should set the energy min/maximums for quanta and black holes. But then, I’m still just getting over being sick so maybe I’m just having fever dreams,,,

    Gary 7

  22. Alex Whiteside

    IIRC when a photon (or anything else you’re using to make a measurement) reaches a high enough energy, it collapses the space around it into a black hole and it shuts itself out of the universe, meaning you lose whatever information it carried. Therefore there’s a lower limit (the Planck length) to the wavelengths we can use to make measurements, and therefore a “pixel size” to the universe. Which is utterly cool in itself.

    I’m not sure how that works though, as I always thought photons were massless and here they seem to have a gravity associated with them.

  23. Samba

    What are the size of the stars?

  24. Here is a visual of what a this sun would look like against our own. I think I did it right, I’m kind of tired so don’t hate me if I screw up.

  25. John Castner

    For those asking about the maximum energy of a photon and speculating about a pixel size to the universe, note that some current theories claim that information cannot be destroyed, even by a black hole (just scrambled quite effectively). This in turn means that the black hole carries that information somehow, apparently encoded in its “interface” with the rest of the universe, the event horizon (EH). This leads us to conclude that as the mass of a black hole increases, its EH increases in area at least enough to contain _all_ the information that the increase in mass contained (down to the last quantum number of the last electron), and that all information is somehow encoded in surfaces, not volumes. I may be horribly mangling the science on this one, but look to the cover article in an issue of Scientific American from about 2 years ago, saying “ARE YOU A HOLOGRAM? Quantum physics says the entire universe may be”. That article puts a “pixel size” on the universe. A black hole would have to have an EH with at least that much area, and perhaps a photon cannot get more energetic than its E=mc^2 equivalent of that corresponding mass. I’d love to see responses at @$tr0_jp

  26. Tom

    With a separation of about 0.25 AU, I would expect that the stars would cook off in quick succession. The first supernova would blow off a large portion of the outer layers of the second, thus dramatically changing the balance between the pressure at the center of the star from fusion) and the weight of the layers above it. Removing the outer layers would allow the core to expand QUICKLY.


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