What cosmic rays offer in return for all those headaches is the extraordinary experience of sampling pieces of a star. “We believe that galactic cosmic rays come from groups of massive young stars called OB associations,” Binns says.
These stars blaze brilliantly and evolve a thousand times as rapidly as our sun. As they grow unstable, in as little as 6 million years, they expel much of their outer layers. Then, in a final frenzy, the stars go supernova, “and we believe that the primary accelerator of the cosmic rays is the shock wave from these supernovae.” One star pops off, then another and another, until all the massive stars in the cluster have exploded and stirred up a hornet’s nest of cosmic rays.
Those cosmic rays then spend 10 million years, give or take, speeding through interstellar space until some of them reach our planet. A handful of those, in turn, happened to reach Super-TIGER. The incoming cosmic rays are the very same atoms that blew out of those distant stars — not just light from the stars, not a byproduct of the stars, but bits of the stars themselves.
“We’re detecting particles that started out hundreds of light-years away that have traveled for millions of years, and after all that time they’re coming to an end here in our detector,” Murphy marvels. That direct connection makes cosmic rays incredibly powerful tools for testing our ideas about how stars form, evolve and die.
Most of the cosmic rays that reach Earth are hydrogen nuclei, but those are so common that, by themselves, they do not carry much distinctive information. Binns discards them and focuses instead on rare but much more communicative heavy atoms, ranging in mass from zinc to molybdenum — the “trans-iron” elements that make up the TI in TIGER — which are produced in the nuclear furnaces of extremely massive stars.
We all have a deep investment in that stellar process. Earlier generations of such stars produced the heavy elements that make up Earth; that’s also where the iron in your blood and the calcium in your bones came from. But Binns has a unique interest in those heavy elements because they act like an identity stamp, telling him which cosmic rays came from the OB stars and which came from surrounding material that just got caught up in the flow.
The whole theory of how OB stars evolve, explode and give rise to cosmic rays is based on ideas that still have question marks in them. Binns peppers his language with “we believe.” The goal of Super-TIGER is to put periods on those sentences.
Tracking a Black Hole
On Dec. 9, 2012 (Dec. 8 in Antarctica), Super-TIGER began a daylong ascent on a helium balloon 200 times the volume of the Goodyear blimp. The launch was carefully timed. For just a couple of months around the middle of Antarctic summer, an air current tidily circles the South Pole, ensuring that any experiment bobbing along in that vortex will tend to stay above the continent.
Super-TIGER managed nearly three laps around the pole before the winds gave out, setting a long-duration record for a heavy-lift balloon and collecting more cosmic rays than the scientists had dared hope. There was only one serious mishap: A freak failure of all four solid-state memory drives aboard the experiment (“very surprising to me,” Binns says drily) scuttled some of their results.
Even so, the team recovered at least four-fifths of their high-priority data. Some of the rest may turn up when the drives are physically recovered and returned to the lab.
Based in part on findings from TIGER, Super-TIGER’s smaller predecessor, astrophysicists have begun filling in the story of the clusters of the young, massive OB stars. Early on, when the cluster begins to condense from a collapsing cloud of gas, the brightest and shortest-lived stars blow out fierce winds that energize the first set of cosmic rays and expel material from the star itself.
The supernova explosions that follow further whip up those shock waves, and then violently accelerate the material from previous supernovas as well. Measurements from TIGER seem to show that about 20 percent of cosmic rays consist of heavy elements expelled by the stars themselves. The rest appears to be material that had been sitting quietly between the stars before getting caught up in the crossfire.
The results from Super-TIGER will add many details. They will also make it possible to investigate an especially exotic possibility. Some of the cosmic rays it detected might have come not from manic stars but from a black hole, lurking right here within our galaxy, that sucks in nearby material and whips it nearly to the speed of light.
If such a monster, known as a microquasar, does exist, Super-TIGER’s detectors should see an anomalous signal, too powerful even for a supernova. “There would be a funny bump in the energy spectrum,” Murphy says. “If we could find a microquasar nearby, wow, that would be pretty cool.”
Traces of that black hole might already be stored away on Binns’ computer, or locked in a crippled data drive sitting atop an Antarctic glacier. Or the evidence may yet be streaking Earthward at thousands of miles per second, unknown until the next time Super-TIGER rises high in the sky, gazing out into the depths of space.