High-energy radiation, in the form of gamma rays or X-rays, is produced by the most extreme and enigmatic objects in the universe: black holes, neutron stars, and the violent collisions created when a supernovae toss their shells into the interstellar me On a clear night under dark skies, you can look overhead and pick out a band of the brightest stars a cradle of star formation whose violent shock wave angles oddly through the main disk of the Milky Way and, at its fringes, brushes our solar system.
You would then know just about as much as any astronomer does about the Gould belt.
But unlike Neil Gehrels of NASA's Goddard Space Flight Center, you might not have been studying the Compton Gamma Ray Observatory's catalogue of gamma-ray emissions, exploring the question of whether the Gould belt had its own distinct set of objects emitting high-energy gamma rays.
Today, armed with results, Gehrels and his colleagues announced that there is in fact a distinct batch of gamma-ray emitters in the Gould belt, confirming an expectation that goes back more than a decade.
White dots represent massive stars in the Gould belt. Yellow dots are unidentified gamma-ray sources in the belt.
The results will appear in the March 23 issue of the journal Nature.
Gehrels told SPACE.com that the objects -- which are probably massive stars, pulsars, giant clouds of gas or remnants of exploded stars -- are closer to our solar system and fainter than other sources of gamma rays.
The objects are all less than about 1,000 light-years away. The entire Milky Way galaxy is more than 100,000 light-years across.
"The Goddard group has shown in a rather convincing way that the current best catalogue of medium- to high-energy gamma-ray sources contains a significant number of objects that are very likely associated with massive stars and regions of star formation relatively close to us," said Josh Grindlay of the Harvard-Smithsonian Center for Astrophysics.
Grindlay, who was not involved in the study, said the findings will help explain how an area of massive star formation, made up of giant clouds of gas and stars 20 to 30 times the size of our sun, contributes to the overall gamma-ray emission in the galaxy.
"There are other galaxies, called star-formation galaxies, where enormous bursts of star formation are going on," Grindlay said in a telephone interview. "The hope would be that one could apply these results from right close by to understand what an entire galaxy would look like when it's got whole bursts of these massive stars being formed."
Gamma ray sources in the Gould belt as imaged by the Compton observatory.
Intense energy
| Gamma-ray sources |
| Pulsars are small spinning stars that send out radio waves, which sweep through the cosmos like a ballerina's skirt. Near the end of their stellar life cycles, pulsars have a mass similar to our sun, all squeezed into a ball just a few miles across. |
Black holes are thought to exist throughout the universe. One is believed to hide at the center of our galaxy, and there may be many small ones scattered through the Milky Way. |
Supernovas are stars that have spent their nuclear fuel. In a giant explosion, they fling their outer shells into space, creating a violent stellar wind that crashes in and through the surrounding gas and dust of space. |
High-energy radiation, in the form of gamma rays or X-rays, is thought to be produced by clouds of gas, black holes, neutron stars and pulsars, as well as the violent collisions created when supernovas cough up their shells into the interstellar medium.
Gamma rays are mostly absorbed by Earth's atmosphere, so they typically need to be studied from space. Over the past decade, the Compton observatory has found roughly 170 points of gamma-ray emission, all nestled within the pancake-like disk that makes up the bulk of our Milky Way galaxy.
The objects comprise what is known as the EGRET catalog. But scientists can't say for sure what any of these sources are. Also at issue has been whether the Gould belt might harbor similar objects.
The Gould belt?
The origin of the Gould belt, named after Benjamin A. Gould (who in the mid-1800s became the first American to obtain a Ph.D. in astronomy) is not known for sure. It may have been created when a massive cloud slammed into the galaxy more than 30 million years ago, or it could have been born by an internal explosion.
Either way, it is now a stellar nursery.
"Theorists speculate that it was caused by a cluster of massive stars that exploded as supernovae and produced a compression wave in the interstellar medium," Gehrels said. "This then stimulated star formation."
By chance, Gehrels explained, the Gould belt is tilted by about 20 degrees with respect to the main disk of the galaxy. Again, we don't know why.
Lively drama
The findings are important because they show that there is more than one source of gamma-ray emissions in the galaxy, said Isabelle Grenier of the Université Paris. In an analysis of the findings, Grenier writes in Nature that the study, along with other information, provides "compelling evidence" that the Gould belt is an important birthplace of gamma-ray sources.
"As possible relics of supernova explosions in the Gould belt," Grenier writes, "the gamma-ray sources highlight how dynamic the cycle of matter through star formation and death is in our neighborhood -- a lively drama that eludes us because of the stillness of the optical sky to our naked eyes.
It will likely be five years or more before astronomers have better gamma-ray data to work with, when NASA's Gamma Ray Large-Area Space Telescope (GLAST) is scheduled to launch.
advertisement
