What are those strange particles raining down on our planet from the depths of outer space?
Victor Hess (center) rode up in a balloon in 1912 to prove that Earth is bombarded by radiation from outer space. Astronomers are only now making sense of those enigmatic "cosmic rays." (Credit: American Physical Society) Physicists have been wrestling with that question for a century now, but the past couple months have seen remarkable progress toward a meaningful answer. It's taken so long because researchers have had to overcome a lot of obstacles along the way. Even the name of the thing they are studying is confusing. The particles are formally known as cosmic rays even though they are not rays at all, but fragments of atoms that are moving at extremely high velocities. And those fragments are extremely difficult to study, because cosmic rays do not move in straight lines. They are electrically charged, so they bend to the will of the magnetic fields that snake almost everywhere through deep space. By the time a particular cosmic ray reaches Earth, its path may have nothing to do with the place where it started out. Looking at cosmic rays is like pointing a telescope into a set of funhouse mirrors. But cosmic rays exciting things, because they provide a direct record of what is happening in some of the wildest, most energetic, most violent parts of the universe. They get kicked around by massive young stars, supernova explosions, pulsars, and black holes. The most potent cosmic rays are so energetic that they challenge the limits of physics to explain how they came to be. All of which explains why a small, hardy group of researchers continues to obsess cosmic rays so long after Victor Hess's 1912 balloon flight that first proved the existence of mysterious radiation coming down from the sky in all directions. Some of the progress has come from the aggressively named Super-TIGER experiment, which flew up to the stratosphere over Antarctica and circled the south pole for over 55 days in December and January. That mission watched the skies from 25 miles up, lofted by a 39-million-cubic-foot helium balloon. Its detectors scanned specifically for heavy cosmic rays: atoms heavier than zinc, which come from highly evolved "Wolf-Rayet" stars heading toward their demise. The results of the latest experiment are still under study. (I will discuss them in greater detail in next month's Out There column in DISCOVER magazine.) But data from TIGER, a smaller precursor mission, have made it pretty clear that the high-drama Wolf-Rayet stars are intimately linked with the origin of cosmic rays.
The Super-TIGER experiment prepares for lunch in Antarctica in December, 2012, setting out to make sense of cosmic rays once and for all. Credit: Ryan Murphy/Washington University Robert Binns of Washington University in St. Louis filled me in on some of the details. "The Wolf-Rayet phase is an evolutionary phase of a massive star’s life when very high-velocity winds occur, and they drive off huge amounts of material in a very short time. The lifetime of this phase is only about 100,000 years; then at the end of that phase, the massive star core collapses and either undergoes a supernova or collapses to a black hole," he says. "We believe the primary accelerator of cosmic rays is the magnetic fields in the shockwave from these supernovas. And the source of the material, we think, is partly the wind material that’s blown off before the explosion that is then picked up by the expanding shock, and also just ordinary interstellar medium, gas and dust that’s laying around in that region." If Binns is correct, the atomic particles he is studying are the very ones swept up by distant supernovas--explosions that briefly flare up a billion times as bright as the sun--which then traveled across hundreds of light years of space before hitting Super-TIGER's detectors. Two other recent studies strongly bolster this view. One comes from NASA's Fermi space telescope. Unlike Super-TIGER, Fermi cannot detect cosmic rays directly. Instead it picks up gamma rays (similar to visble light, but millions to billions of times as energetic) that are produced by a chain reaction that occurs when the cosmic rays randomly slam into surrounding hydrogen atoms that are floating quietly in space. The resulting gamma-ray glow is like a tracer that points back to the locations where atoms are getting energized. Put another way, Fermi makes it possible to take away the funhouse mirror and see where cosmic rays are born. A large team headed by Stefan Funk of Stanford University and the Kavli Institute used the Fermi telescope to track the origin of cosmic rays back to two unusual nebulas, named the Jellyfish Nebula (IC 443) and W44. Both of these have previously been identified as the remains of supernova explosions; both remnants are associated with massive stars like the ones that pass through a Wolf-Rayet phase. Finding that the trail of cosmic rays leads right back to these locations clinches the conclusion by Binns and others that supernovas are, in fact, stirring up atoms and flinging them across the galaxys. Funk, unable to resist the inevitable cry of the scientist trying to explain a successful piece of detective work, calls this discovery the "smoking gun." If the Fermi data are the smoking gun, then another, parallel piece of research has just turned up the fingerprints as well. Serbian astronomer Sladjana Nikolic, a Ph.D. student at the Max-Planck Institute for Astronomy in Heidelberg, Germany, and her colleagues sought out another visual clue about where and how the cosmic rays are getting energized. Using the Very Large Telescope in Chile (one of the largest telescopes in the world and an amazing instrument, despite its prosaic name), she zeroed in on another supernova relic, known as SN 1006. At the outer edge of the fast-expanding cloud of gas she--right where the shock front should be--she detected hot, energized hydrogen nuclei. They are almost surely the precursors of the kinds of cosmic rays that Super-TIGER and other experiments detect when they reach the Earth millions of years later.
SN 1006, the remains of a stellar explosion seen a thousand years ago, has been fingered as a source of cosmic rays. This view is a composite of radio, light, and x-ray images. (Credit: NASA, NOAO, NRAO.) It's been a slow, painstaking process getting to this level of understanding of cosmic rays, but the implications are profound. Before, when I looked up at the night sky I perceived it as something chilly and remote. The stars were things fundamentally cut off from my life, both in space and in time. Now I know that I was wrong. Fragments of distant stars are striking Earth's atmosphere all the time. They mix with the air. I am breathing them in at this very moment, and so are you. Other atoms, born in earlier generations of stars, also made the journey across space. They mingled with a gas cloud that, 5 billion years ago, collapsed to form the sun, the Earth, and the other planets. As Carl Sagan memorably put it in his landmark series Cosmos, "The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff." Cosmic rays may seem esoteric, but each one is a reminder of who we are and where we came from. Follow me on Twitter: @coreyspowell
