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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 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