January 07, 2009
Some stars go ballistic, racing through interstellar space like
bullets and tearing through clouds of gas.
Images from NASA's Hubble Space Telescope, taken by Raghvendra Sahai
of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and colleagues
reveal 14 of these young, runaway stars.
The stars are plowing through regions of dense interstellar gas,
creating brilliant arrowhead structures and trailing tails of glowing
gas. These arrowheads, or bow shocks, form when the stars' powerful
stellar winds, streams of matter flowing from the stars, slam into
surrounding dense gas. The phenomenon is similar to that seen when
a speeding boat pushes through water on a lake.
"We think we have found a new class of bright, high-velocity stellar
interlopers," said Sahai. "Finding these stars is a complete surprise
because we were not looking for them. When I first saw the images, I
said, 'Wow. This is like a bullet speeding through the interstellar medium.'
Hubble's sharp 'eye' reveals the structure and shape of these bow shocks."
The astronomers can only estimate the ages, masses and velocities of these
renegade stars. The stars appear to be young -- just millions of years old.
Their ages are based partly on their strong stellar winds.
Most stars produce powerful winds either when they are very young or very
old. Only very massive stars greater than 10 times the sun's mass have
stellar winds throughout their lifetimes.
But the objects observed by Hubble are not very massive because they do
not have glowing clouds of ionized gas around them. They are medium-sized
stars that are a few to eight times more massive than the sun. The stars
are not old because the shapes of the nebulae around aging, dying stars
are very different, and old stars are almost never found near dense interstellar clouds.
Depending on their distance from Earth, the bullet-nosed bow shocks could
be 100 billion to a trillion miles wide (the equivalent of 17 to 170 solar
system diameters, measured out to Neptune's orbit). The bow shocks indicate
that the stars are traveling fast, more than 180,000 kilometers an hour
(more than 112,000 miles an hour) with respect to the dense gas they are
plowing through, which is roughly five times faster than typical young stars.
"The high-speed stars were likely kicked out of their homes, which were
probably massive star clusters," Sahai said.
There are two possible ways this stellar expulsion could have happened.
One way is if one star in a binary system exploded as a supernova and the
partner got kicked out. Another scenario is a collision between two binary-star
systems or a binary system and a third star. One or more of these stars could
have picked up energy from the interaction and escaped the cluster.
Assuming their youthful phase lasts only a million years and they are moving
at roughly 180,000 kilometers an hour (about 112,000 mph), the stars have
traveled about 160 light-years.
Runaway stars have been seen before. The joint European-NASA Infrared Astronomical
Satellite, which performed an all-sky infrared survey in 1983, spied a few
similar-looking objects. The first observation of these objects was in the
late 1980s. But those stars produced much larger bow shocks than the stars
in the Hubble study, suggesting that they are more massive stars with more
powerful stellar winds.
"The stars in our study are likely the lower-mass and/or lower-speed counterparts
to the massive stars with bow shocks detected by the Infrared Astronomical Satellite,"
Sahai explained. "We think the massive runaway stars observed before were just the
tip of the iceberg. The stars seen with Hubble may represent the bulk of the population,
both because many more lower-mass stars inhabit the universe than higher-mass stars,
and because a much larger number are subject to modest speed kicks."
Sahai presented his results at the American Astronomical Society meeting in Long Beach,
Calif. The science team also includes M. Morris of the University of California, Los Angeles;
M. Claussen of the National Radio Astronomy Observatory in Socorro, N.M.; and R. Ainsworth
of the University of Tennessee in Knoxville.
JPL is managed by the California Institute of Technology, Pasadena, for NASA. More
information is at http://www.nasa.gov/hubble and www.jpl.nasa.gov .
Media contacts: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory
whitney.clavin@jpl.nasa.gov
Donna Weaver /Ray Villard 410-338-4493 / 410-338-4514
Space Telescope Science Institute, Baltimore, Md.
dweaver@stsci.edu / villard@stsci.edu
2009-002