The stellar
residents of the jam-packed celestial cities known as globular clusters employ
a traffic system that causes lightweight stars to zoom to the city edges while keeping
giants centrally located, astronomers have concluded.
The study,
detailed in the September issue of the Astrophysical Journal Supplement
Series, gives the first direct evidence of such sorting, called "mass segregation,"
a process long suspected to occur in globular
clusters but never observed.
A globular
cluster is a dense collection of 10,000 to more than a million stars in a
region spanning just 10 to 30 light-years. The nearest
star to our Sun, for comparison, is more than 4 light-years away.
Researchers
analyzed 47
Tucanae, the second largest cluster in the Milky Way's galactic
neighborhood, and determined the cluster sorts out stars according to their
masses. Due to the associated gravitational pull, heavier stars slow down and
sink to the cluster's core, while lighter stars pick up speed and zip out to
the cluster's periphery [video].
Tight
squeeze
Just as
bumps and jostles are much more likely in a packed mall or city bus, encounters
in a densely populated cluster are more likely than in the quiet stellar
backwater. These stellar nudges can be dramatic and over time are thought to
result in mass segregation.
The crowded
conditions, however, have made it difficult for astronomers to pinpoint with
any accuracy individual stars, let alone their velocities. Plus, the blurring
effect of Earth's
atmosphere can make the stars' tiny movements impossible to view, unless you
have the Hubble Space Telescope's sharp eyes.
Sharp
eyes
The
research team led by Georges Meylan of the École Polytechnique Federale de
Lausanne in Sauverny, Switzerland, used Hubble to peer into the
heart of 47 Tucanae, which contains a million stars and is located 12,000
light-years away in the southern hemisphere.
They
snapped a series of photos of the cluster over regular intervals for seven years. By placing a "picture frame" around the
same cluster regions in a time sequence, they could track the positions of as
many as 130,000 stars in each revealing the motions of the stars across the sky.
The
astronomers analyzed the data in a way that yielded star speeds corresponding
to changes in their positions at the level of about 1/100th of a pixel on
Hubble's digital cameras.
The method
provided accurate speeds for almost 15,000 stars at the cluster's center,
including 23 unusually hot and bright stars called blue
stragglers thought to be the product of stellar collisions. The blue
stragglers, which boast twice the mass of the average star, moved more slowly
than typical stars, a finding that agrees with predictions for mass
segregation.
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