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.
Stellar
heavyweights
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.
Blowing
bubbles
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.
Particle
accelerators
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.
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