Battle of Giant Windy Stars

Uploaded by SpaceRip on 12.10.2012

NASA's Swift and ESA's XMM-Newton satellites have used their X-ray telescopes to watch
the interaction between a pair of gigantic stars. This is a stellar grouping called Cygnus
OB2, one of the richest collections of hot, massive O-type stars in the galaxy. Unfortunately
for optical astronomers, this spectacular sight is mostly hidden by dust clouds. One
of these stars is a radio source simply called Cygnus OB2 number 9.
In 2008, astronomers concluded that the changes in radio emission only made sense if the star
were actually a binary, with two massive stars in a tight orbit. Massive stars run so hot
that they actually drive away some of their gas in powerful outflows called stellar winds.
In binaries with two large young stars orbiting closely, these hot outflows collide and produce
X-rays. Only a few of these colliding wind systems are known, and they aren't especially
well behaved for astronomers.
The collision may become turbulent, and X-rays may appear only at some times in the orbit.
Which brings us back to Cygnus OB2 number nine. It should produce X-rays where the stars'
winds collide, but there was no evidence for it. Perhaps it only occurred when the stars
were closest in their 2.4-year orbit. The first opportunity to test this came during
the close approach on June 28, 2011. So astronomers enlisted Swift and XMM-Newton, as well as
ground-based observatories to monitor the system at intervals throughout the year.
As the stars approach each other, their fierce winds crash together at several million miles
an hour, reaching temperatures of millions of degrees and creating X-rays. As they separate,
the collision strength decreases. Even so, these powerful winds will influence star formation
in the binary's neighborhood. Cygnus OB2 number nine has turned out to be a model colliding
wind binary, one of the most reliable and best behaved now known. It will be a key to
better understanding how these systems help shape the galaxy.