NASA'S Hubble Finds Rare 'Blue Straggler' Stars In Milky Way's Hub

WASHINGTON -- NASA's Hubble Space Telescope has found a rare class of
oddball stars called blue stragglers in the hub of our Milky Way, the
first detected within our galaxy's bulge.

Blue stragglers are so named because they seemingly lag behind in the
aging process, appearing younger than the population from which they
formed. While they have been detected in many distant star clusters,
and among nearby stars, they never have been seen inside the core of
our galaxy.

It is not clear how blue stragglers form. A common theory is that they
emerge from binary pairs. As the more massive star evolves and
expands, the smaller star gains material from its companion. This
stirs up hydrogen fuel and causes the growing star to undergo nuclear
fusion at a faster rate. It burns hotter and bluer, like a massive young star.

The findings support the idea that the Milky Way's central bulge
stopped making stars billions of years ago. It now is home to aging
sun-like stars and cooler red dwarfs. Giant blue stars that once
lived there have long since exploded as supernovae. The results have
been accepted for publication in an upcoming issue of The
Astrophysical Journal. Lead author Will Clarkson of Indiana
University in Bloomington, will discuss them today at the American
Astronomical Society meeting in Boston.

"Although the Milky Way bulge is by far the closest galaxy bulge,
several key aspects of its formation and subsequent evolution remain
poorly understood," Clarkson said. "Many details of its
star-formation history remain controversial. The extent of the blue
straggler population detected provides two new constraints for models
of the star-formation history of the bulge."

The discovery followed a seven-day survey in 2006 called the
Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS).
Hubble peered at 180,000 stars in the crowded central bulge of our
galaxy, 26,000 light-years away. The survey was intended to find hot
Jupiter-class planets that orbit very close to their stars. In doing
so, the SWEEPS team also uncovered 42 oddball blue stars with
brightness and temperatures typical for stars much younger than
ordinary bulge stars.

The observations clearly indicate that if there is a young star
population in the bulge, it is very small. It was not detected in the
SWEEPS program. Blue stragglers long have been suspected to be living
in the bulge, but had not been observed because younger stars in the
disk of our galaxy lie along the line-of-sight to the core, confusing
and contaminating the view.

Astronomers used Hubble to distinguish the motion of the core
population from foreground stars in the Milky Way. Bulge stars orbit
the galactic center at a different speed than foreground stars.
Plotting their motion required returning to the SWEEPS target region
with Hubble two years after the first observations were made. The
blue stragglers were identified as moving along with the other stars
in the bulge.

"The size of the field of view on the sky is roughly that of the
thickness of a human fingernail held at arm's length, and within this
region, Hubble sees about a quarter million stars toward the bulge,"
Clarkson said. "Only the superb image quality and stability of Hubble
allowed us to make this measurement in such a crowded field."

From the 42 candidate blue stragglers, the investigators estimate 18
to 37 are likely genuine. The remainder could be a mix of foreground
objects and, at most, a small population of genuinely young bulge stars.

"The SWEEPS program was designed to detect transiting planets through
small light variations" said Kailash Sahu, the principal investigator
of the SWEEPS program. "Therefore the program could easily detect the
variability of binary pairs, which was crucial in confirming these
are indeed blue stragglers."

Hubble is a project of international cooperation between NASA and the
European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Md., manages the telescope. The Space Telescope Science
Institute (STScI) conducts Hubble science operations. STScI is
operated for NASA by the Association of Universities for Research in
Astronomy in Washington. For images and more information about the
findings, visit:

http://www.nasa.gov/hubble

and

http://hubblesite.org/news/2011/16

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