The Westerlund 2 stellar cluster contains the most massive binary star system, WR 20.
Credit: NASA/E. Churchill (Univ. of Wisconsin)
Enveloped by charged bubbles, a cluster of young, massive stars has revealed a new source of gamma-ray energy in our galaxy, astronomers reported today at the first Gamma Ray Large-Area Space Telescope (GLAST) Symposium in Palo Alto, Calif.
Gamma rays have the smallest wavelengths and have more energy than any other light wave along the electromagnetic spectrum.
Until now, supernovae remnants were the dominant known source of gamma-ray energy, which requires a sort of celestial particle accelerator to ramp particles up to such high energies.
Called Westerlund 2, the cluster contains an ensemble of young stars and extremely massive stars called Wolf-Rayet stars, one of which--WR 20a--takes the trophy for the most massive binary known in the Milky Way. Each of its stars weighs 80 solar masses.
Wolf-Rayet stars, named for their discoverers, begin life as cosmic Goliaths, boasting at least 20 times the mass of the Sun. Near the end of their lives, the giants rapidly lose mass as supersonic stellar winds jet from their surfaces. Intense heat and radiation at their cores can trigger stellar winds that reach speeds that range from 2.2 million to 5.4 million miles per hour (3.6 million to 9 million kilometers per hour).
Their quick, tumultuous lives end when they explode as supernova and blast massive amounts of heavy elements out into space.
The international team of astrophysicists spotted the star bubbles with the High Energy Stereoscopic System (HESS) gamma-ray telescope array located in Namibia.
The astronomers said the gamma radiation appears to extend beyond the stellar cluster and shows a constant emission rate over time, suggesting the acceleration is the result of stellar winds.
In the Westerlund 2 cluster, the whipping stellar winds have actually blown bubbles around the stars. The scientists found that the wind energy is nearly equivalent to that released in supernova explosions.
What drives the explosive winds? "[The stars] provide really massive particles which are driven out with the wind. But that is for sure not energetic enough yet to power up to the energies we are talking about here," said co-researcher Olaf Reimer, a senior research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University.
When the streams of energetic particles encounter the shock-wave boundary at the edge of the bubble, they get a boost of energy. "The particles encounter a density gradient, and they always pick up a tiny bit of energy," Reimer told SPACE.com.
The scientists think the energetic boost is sufficient to match the atom-smashing power of a particle accelerator and launch particles to gamma-ray energies.
"Shocks and turbulent motion inside a bubble can efficiently transfer energy to cosmic rays, providing a plausible mechanism for particle acceleration," said Luke Drury of the Dublin Institute for Advanced Studies.