by Christopher Wanjek
Hyperactive young stars in the Orion Nebula may help explain exotic isotopes in the solar system.
What do X-rays, meteoroids, infant stars in the Orion Nebula, and our solar system have in common? Perhaps much more than anyone thought.
Eric Feigelson of Penn State University stumbled onto a connection one day while his thoughts were far from the solar system, turned toward the vibrant neighborhood of young stars, hot gas, and caliginous dust of the Orion Nebula. This nebula, 1500 light-years away, is visible to the naked eye in the constellation Orion, a gem to behold with a good pair of binoculars or a telescope under dark skies.
In Orion, Feigelson inadvertently found a possible solution to a long-standing mystery about our own solar system: the presence of exotic isotopes locked away in meteoroids. Scientists have assumed that these short-lived isotopes -- special forms of atomic nuclei, such as aluminum-26 and calcium-41 -- were transported here by a nearby supernova. Only tenuous evidence for such an explosion exists, but what else could have made the isotopes? The isotopes are about as old as the solar system, and the Sun couldn't possibly have been powerful enough to create them.
Well, maybe we need to give the Sun a little more credit. Feigelson found that very young, midsized stars in the Orion Nebula -- in the same stellar class as our Sun except they are only a million years old -- produce powerful flares visible in X-rays. His team spotted these X-ray flares with the Chandra X-Ray Observatory.
These baby-tantrum flares are indeed energetic enough to forge heavy isotopes, Feigelson says. If the infant stars in Orion can do it now, then our Sun could have done the same when the solar system was forming about 4.5 billion years ago, when the Sun itself was only a few million years old.
The rare isotopes are found in surprisingly high concentration in ancient meteorites called carbonaceous chondrites. While elements such as silver and gold on Earth were created long before the creation of the solar system, these rare isotopes were made during the formation of the solar system. Scientists know this because the radioactive half-life of the isotopes date the asteroids to 4.5 billion years of age, much like the familiar carbon-14 isotope dates organic material on Earth.
The perplexing presence of these isotopic anomalies led to the theory that a supernova occurred very close to the solar system's progenitor gas cloud, simultaneously triggering its collapse and seeding it with short-lived isotopes. Supernovae are powerful enough to forge all types of elements and isotopes. Solar flares could produce such isotopes, but the flares would have to be hundreds of thousands of times more powerful and hundreds of times more frequent than those that our Sun generates today. Could the Sun have been so violent early on?
According to the latest observations in Orion, yes. Feigelson and his Penn State colleagues didn't actually observe the presence of heavy isotopes. They merely have identified a mechanism of how a young star naturally produces the high energies needed to make the said isotopes. Also, the heat from all the X-ray radiation can melt rocks, making them more likely to stick together and form planets. "This is an excellent example of how apparently distant scientific fields, like X-ray astronomy and the origins of solar systems, can in fact be closely linked," says Feigelson.
Not all scientists are convinced. According to Alastair Cameron, one of the scientists who originated the supernova theory, the early Sun still wasn't energetic enough to synthesize these isotopes. Cameron, a retired theorist from Harvard University now working at the University of Arizona, says "I don't believe even for a microsecond" that any living star could make these isotopes. It would take more energy than even the young Sun possessed. Also, if solar flares were making isotopes, we wouldn't see a greater abundance of isotopes as we move farther away from the Sun. After all, solar flares send out bullets, in the form of protons and electrons, that smack into gas and solids to create isotopes from larger nuclei, a process called spallation. The Sun's particles would have either whimpered out before reaching the asteroid belt or smashed into matter far closer to the Sun.
Donald Clayton of Clemson University, an expert on nucleosynthesis, remains excited, however, saying that Feigelson "has shown that stellar flare acceleration produces radioactive nuclei whether we want them or not." The scientific debate will now concentrate on whether solar flares can explain everything or whether we still need contamination from a local explosion, he says. All this debate, and Feigelson was just minding his own business observing the pretty Orion Nebula.
Last Updated: 21 February 2011