The Hulking sky

By Phil Plait | March 24, 2009 7:35 am

[Note: this story was submitted to Digg twice, and the button above points to the newer submission. However, the other submission has more diggs, so if you’re inclined to, please Digg using this link! Thanks.]

Gamma rays are the 800 pound gorillas of photons*.

They are the highest energy form of light. Even the wimpiest gamma ray has about a 500,000 times the energy of a visible light photon, and so you might imagine it takes a pretty seriously scary event to make one. You’d be right: we’re talking exploding stars, black holes being born, flares from stars, super-duper magnetic fields, and the like. Nuclear bombs make them too, but compared those other events, a nuke detonating is small potatoes.

That’s why this map this new picture of the sky is so seriously cool: it’s the highest resolution map ever made of the entire sky in gamma rays:

The gamma-ray sky as seen by NASA’s Fermi

The map was made with Fermi (what used to be called GLAST when I worked on it), a gamma-ray observatory launched last year. It detects those bullet-like photons that have an energy from 15 million to 150 billion times the energy of the kind of photon you can see with your eye; the gamma rays used to make this map all had energies higher than 150 million times that energy.

That’s seriously high-energy indeed.

This image was made over the course of three months. Fermi sweeps across the sky, recording the energy and position of every gamma ray it detects. Over time, it builds up a map of the sky in gamma rays. Where it sees more, the map is bright, and where it sees fewer the map is dark.

And there’s so much to see in the image! The flat line across the center is actually our own Milky Way galaxy. The map has been set up in such a way that the center of the galaxy is the center of the map, and the flat disk we see edge-on because we’re inside it — what we actually do call the Milky Way when we see it at night — runs along the middle. All those gamma rays are coming from incredibly energized gas, old dead supernovae, black holes, ultra-dense neutron stars, and other exotic beasties.

But there are lots of other sources too. Go to the high-res version and look to the upper right. See that thin arc? That’s the Sun! Over time, as the Earth moves around the Sun (and Fermi sticks close to the Earth) the position of the Sun on the sky moves, and the way the map is laid out (in what we call galactic coordinates) the Sun’s motion translates to that arc over the three months it took to create this image. It’s funny — we’re at a minimum of solar activity right now, so in any given gamma ray snapshot of the Sun it would be invisible, lost among all the other solitary dots of gamma rays from the sky. But its motion betrays it.

A lot of the other bright blobby sources are supernova remnants — Geminga, the Crab, and Vela. The one labeled PSR J1836+5925 (left of center and just above the galactic disk) is a pulsar, a rapidly spinning neutron star that’s left over from a titanic supernova explosion. Most of the rest are distant galaxies, their central black holes voraciously consuming matter and spitting some of it out in the form of twin beams of energy; if they happen to be aimed right at us we see gamma rays from them, and Fermi detects them.

But perhaps the coolest things in the image, the things that makes scientists squee, are the blobs that are unidentified. Quite a few bright sources in the map are simply unknown, and not identified with any visible source. The reason we can identify so many objects in the map is because when we look with our "regular" telescopes, we see something there and can say, "Here be a supernova!" But those unidentified ones? Nope. We look with our optical telescopes and there’s nothing there.

And that’s what scientists live for. These may be a whole new class of object! Or, more likely, something we already know about but behaving in a weird way. A galaxy that spews out gamma rays, but no optical light. Or a pulsar that for some reason sends only super-high-energy light our way, and nothing else.

Or maybe, just maybe, it’s something no one has thought of before. How cool would that be? Gamma-ray bursts — unbelievably violent explosions that occur when a black hole is born, and which could vaporize our planet without even blinking — were totally unknown until the 1960s. So who knows what’s still out there?

With observatories like Fermi, we get a glimpse into the Universe’s more terrifying objects… and what we see is not only incredible, it’s also new. And that, my friends, is why we do this. We want to understand more and more, but, as a happy circumstance, the only way to do that is to find more things we don’t understand.

That’s where the fun is.


*Come to think of it, if you could actually turn an 800 pound gorilla into photon, it really would be a gamma ray. When you convert an object into energy (which is what a photon really is), the amount of energy you get out is huge for even a tiny amount of mass. An 800 pound gorilla, converted into energy, would explode with the same yield as about 8000 one-megaton nuclear bombs. By definition, any photon above about 500,000 times the energy of a visible light photon is a gamma ray, so if you could somehow squish all that energy from the suddenly energized simian, it would be a gamma ray. And it would be really ticked off.

Image Credit: NASA/DOE/Fermi LAT Collaboration

Comments (42)

  1. Jason Dick

    I find it curious that they didn’t mask the Sun from that image.

  2. jtradke

    So there’s all these super-duper high-energy killer-death photons out there… how come we’re not all dead? Is something in the atmosphere blocking them before they hit us? The last thing I need is a gamma-ray hole in my head.

  3. kkozoriz

    All you need is another 800 pound gorilla made of anti-matter. POOF! Gamma rays

  4. That’s the most awesome footnote I’ve seen in a long time :D

  5. Brian

    If they masked the sun from the image, what would they put in its place? It would be like the way our brain fills in our blind spot by blithely assuming what we would have seen in that location — it works most of the time, but it’s quite incongruous when it’s wrong. Our sun is probably covering up just more dim blue, but we don’t know that.

  6. The high-res image is amazing! The reds and oranges practically leap off the screen. Almost a 3D effect.

    (Also cool science.)

  7. If you look at the unlabeled high-res image, the sun’s arc doesn’t even really stand out. Not sure there would even be much point in masking it since (at least here around solar minimum) it’s not much of a gamma-ray source. You’re telling me it could have covered up any of those bright orange dots?
    http://www.nasa.gov/images/content/317876main_Fermi_3_month_unlabeled_new.jpg

  8. Pieter Kok

    The sun is not bright at all in the gamma ray spectrum, that’s why it does not need to be blocked. The arc of the sun (which is extremely cool, BTW) is incredibly faint. Also, we are not fried by the gamma rays because the overall intensity is too low. What it does, however (together with the Earth’s natural radioactivity), is induce mutations in our DNA, thus driving evolution.

    As for the energy of a gamma ray photon: if you have a dust particle that is just about visible with the naked eye, a single gamma ray photon can make it move over a macroscopic distance. Be very glad that hbar is so small…

  9. Yoweigh

    You forgot to close a tag after your endnote. All the body text after the first post is tiny.

  10. Nice title. “SKY SMASH!”

  11. I remember in my freshman optics class the professor telling us that the highest energy gamma rays packed the energy of a well hit tennis ball into a single photon. Wow! If you’ve ever been on the receiving end of a well hit tennis ball and consider that your typical light bulb emits somewhere around 10^18 photons per watt per second (that was back of the envelope, so I might be off by a few orders of magnitude–but it’s still a huge fracking number), you begin to realize just how unbelievably hot high energy gamma rays are.

  12. IVAN3MAN

    Attempting to fix the font in 3… 2… 1…
    p {font-size: 16}

  13. Cheyenne

    Epic footnote win at the end of the article!

  14. phunk

    Is it possible for a photon to have that an 800 lb gorrila worth of energy? Can a photon have a wavelength less than the plank length?

  15. TheBlackCat

    Just by chance you would expect it to catch a few gamma ray bursts, right? How did they rule those out, or did they?

  16. Torbjörn Larsson, OM

    A fontastic post, even by BA standards! :-o

    Though I dunno about the gorilla analogy. Considering the havoc gamma rays play with possible mutation and extinction events, it looks more like a playful chimp to me. Slings poo most of the time, playfight often, but can rip your fingers and testicles away when really mad.

    Or maybe, just maybe, it’s something no one has thought of before. How cool would that be? Gamma-ray bursts — unbelievably violent explosions that occur when a black hole is born, and which could vaporize our planet without even blinking — were totally unknown until the 1960s. So who knows what’s still out there?

    You get my hopes up that Fermi may give the answer to the Fermi paradox.

    IIRC there is a paper that shows how even a rare and fairly local sterilizer of young and sensitive biospheres “reset and synchronize the clock” on them. Thus explaining why we own Earth instead of renting it – “where are they” as Fermi asked. GRB’s doesn’t seem to have the right properties to me, but if there are other rare but sufficiently hurtful things out there Fermi may find them?!

    How cool wouldn’t that be!

  17. IVAN3MAN

    p {font-size: 14}

  18. Aha! Yes, I forgot to close a font tag after the image credit. Fixed.

  19. Mario

    Astonishng, this is exactly why I love this blog.

  20. Gary Ansorge

    phunk:

    The Plank length is the shortest length we can theoretically measure in this universe. Basically, what that is saying is that we can’t even theoretically have a ruler small enough to measure anything shorter than the Plank Length. Thus we can have no idea if an energetic particle has a shorter wavelength so I would say no to your question,,,

    GAry 7

  21. Jason Dick

    If they masked the sun from the image, what would they put in its place? It would be like the way our brain fills in our blind spot by blithely assuming what we would have seen in that location — it works most of the time, but it’s quite incongruous when it’s wrong. Our sun is probably covering up just more dim blue, but we don’t know that.

    To mask the Sun, all you need to is remove those gamma ray hits that, at that particular time, could potentially have been from the Sun. So you’d obviously lose some signal across the Sun’s path, but not very much. At least this way you get to see the stuff along the path of the Sun.

    Of course, you also need to modify the magnitude of the sky signal by the amount of time that any particular pixel was masked. But the Fermi team should already have the capacity to do this as it isn’t going to be viewing the entire sky an equal amount at all times.

    I have to suspect that the failure to remove the Sun was just a matter of timing and resources, and I’d bet that they will do it in future releases. The fact that the arc is noticeable at all means that they should most definitely mask it so as not to lose information about what’s going on in the Sun’s path.

  22. That? Is freaking gorgeous. And amazing. My little science groupie heart is seriously squeeing.

  23. The Sky turns big and green when it’s angry?

    Does anyone walk the Plank Length? (Note: proper spelling Planck)

    Is my coffee ready?

    J/P=?

  24. Crudely Wrott

    Given that the sun is such a poor gamma source it may not have seemed necessary to block it. And the detectors were probably calibrated to observe more robust sources.

    I wonder, though, if some thought might not have been given to purposely not blocking poor old Sol in order to demonstrate what a poor gamma source it is.

    At any rate, the resulting image certainly demonstrates the fact. Now we can all relax. There’s nothing to worry about.

    Ooops. I think I just broke the Unspeakable Law. The law states that no matter what it is that you mention, if it’s bad it happens and if it’s good, it goes away.

    Sorry. *dammit*

  25. QUASAR

    You’ve got to love images like that one! To think that w’re actually viewing objects as they were millions and billions of years ago never stops fascinating me!

  26. I’d like to see one that puts the galactic center on the sides of the image and thus has the center showing the view out of the galaxy – I mean, we know there’s going to be gamma rays from the galactic center, duh. The thing is that there’s all sorts of interesting clumps at the sides where it’s distorted – I’d like to get a look at that.

  27. Eric

    Jason Dick:
    The purpose of an image like this is to provide a visualization of the gamma-ray sky, i.e. everything we see. There’s no reason to mask out any source, including the sun. For the purposes of analysis, if you wanted to analyze sources that cross paths with the sun, you’ll obviously need to include it in your model. But removing any photons that could possibly have come from the sun doesn’t really work. The Fermi point spread function is quite broad at low energies; the class of “photons that could possibly have come from the sun” includes most of the photons within several degrees of it. And anyway, the sun is itself an interesting source of gamma rays. I’m pretty sure future pictures of this sort will continue to show it, along with everything else we see.

  28. Thanny

    jtradke:

    Gamma rays have a very short wavelength. Those headed towards Earth don’t get very far before smacking into an air molecule. Maybe a couple of kilometers at most, on average.

    That image is also quite amplified, too. Keep in mind that it’s the sky being photographed. Compare the image with what you see on a clear, dark night outdoors, and remember that gamma rays are much less common than visible photons.

  29. OK, now I’m a Digg member.
    You been Duggified!

  30. IVAN3MAN

    @ Richard Drumm,

    Like, er… what took you so long, man?

  31. BeinSilly

    , so if you could somehow squish all that energy from the suddenly energized simian, it would be a gamma ray. And it would be really ticked off.

    No, it ‘d just be dead – vapourised (or whatever you call transformed from solid straight into into energy & thus nothing sentient anyway .. wouldn’t it? ;-)

    Unless gamma rays have emotions?! ;-)

  32. BeinSerious briefly

    It’s funny — we’re at a minimum of solar activity right now, so in any given gamma ray snapshot of the Sun it would be invisible, lost among all the other solitary dots of gamma rays from the sky. But its motion betrays it.

    What? The Sun doesn’t emit many gamma rays when its quiet?

    I thought the Sun radiated at all wavelengths constantly -maybe more actively when its well being more active sure but I didn’t know it ever stopped emitting gamma waves – does it?

    What’s happening with the Sun now anyway? I heard somewhere it ain’t been going as predicted – been quieter than expected – or that we’ve just changed solar cycle but the latest is taking longer to get going properly or something like that ..

    .. Could we be heading towards a new “Maunder minimum” & if do what are the likely consequences?

  33. Grump

    What would we gain by masking the Sun in this image? Instead of a light-blue streak, we’d have a black end empty “hole” in the map. It’s not like we’d suddenly see the gamma sources behind the Sun! And it’s not like STEREO where the glare from the Sun would overwhelm the sensors, necessitating that little masking thingie (proper technical term, there.)

  34. Cheyenne

    “These may be a whole new class of object!”

    Quark stars baby. Quark stars. That would be utterly delightful if we could see one of those. You got your neutrons and your black holes (and all the variations thereof)- all amazing – but imagine a quark star added to the list of crazy things out there. Magnificent.

    Something going through the phase of crushing down from a neutron to a quark would theoretically release a ton of gamma rays, and probably not as much visible light. Possibility, all I’m saying….

  35. Those gamma rays also have the ability, as was mentioned previously, of altering your DNA. Evolution? Maybe. You also have the chance that the evolutionary shot to your DNA will produce a cancerous cell in yourt body.
    But, them’s the downsides of being out in space, and being without an atmosphere.

  36. Sili

    Some days I really hate the galaxy. It’d be so much more nify if we didn’t have all that gunk obscuring the view.

    Question: Doesn’t shorter wavelengths scatter more? Does that mean that gammaray sources have to be even more luminous to reach us compared to say an infrared thingummajig at the same distance?

  37. BeinSerious briefly

    No answers yet to my questions here. Anybody? …

  38. What would we gain by masking the Sun in this image? Instead of a light-blue streak, we’d have a black end empty “hole” in the map. It’s not like we’d suddenly see the gamma sources behind the Sun! And it’s not like STEREO where the glare from the Sun would overwhelm the sensors, necessitating that little masking thingie (proper technical term, there.).

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