Oldest Known Objects
May Be Surprisingly Immature
(28 April 2008) Some of the oldest
objects in the Universe may still have a long way to go, according to a new
study using NASA's Chandra X-ray Observatory.
These new results
indicate that globular clusters might be surprisingly less mature in their
development than previously thought.
Globular clusters, dense bunches of
up to millions of stars found in all galaxies, are among the oldest known
objects in the Universe, with most estimates of their ages ranging from 9 to 13
billions of years old. As such they contain some of the first stars to form in
a galaxy and understanding their evolution is critical to understanding the
evolution of galaxies.
"For many years, globular clusters have been used
as wonderful natural laboratories to study the evolution and interaction of
stars," said John Fregeau of Northwestern University, who conducted the study.
"So, it's exciting to discover something that may be new and fundamental about
the way they evolve."
Conventional wisdom is that globular clusters pass
through three phases of evolution or development of their structure,
corresponding to adolescence, middle age, and old age. These "ages" refer to
the evolutionary state of the cluster, not the physical ages of the individual
stars.
In the adolescent phase, the stars near the centre of the cluster
collapse inward. Middle age refers to a phase when the interactions of double
stars near the centre of the cluster prevents it from further collapse.
Finally, old age describes when binaries in the centre of the cluster are
disrupted or ejected, and the centre of the cluster collapses
inwards.
For years, it has been thought that most globular clusters are
middle- aged with a few being toward the end of their evolution. However,
Chandra data along with theoretical work suggest this may not be the
case.
When single and double stars interact in the crowded centres of
globular clusters, double stars can form that transfer mass and give off
X-rays. Since such double stars are expected to mostly be formed in the middle
of a globular cluster's evolution and then lost in old age, the relative number
of X-ray sources gives clues about the stage of evolution the cluster is
in.
A new study by Fregeau of 13 globular clusters in the Milky Way
shows that three of them have unusually large number of X-ray sources, or X-
ray binaries, suggesting the clusters are middle-aged. Previously, these
globular clusters had been classified as being in old age because they had very
tight concentrations of stars in their centres, another litmus test of age used
by astronomers.
The implication is that most globular clusters,
including the other ten studied by Fregeau, are not in the middle age of their
evolution, as previously thought, but are actually in adolescence.
"It's
remarkable that these objects, which are thought to be some of the oldest in
the Universe, may really be very immature in their development," said Fregeau
whose paper appears in The Astrophysical Journal. "This would represent a major
change in thinking about the current evolutionary status of globular
clusters."
If confirmed, this result would help reconcile other
observations with recent theoretical work that suggest the tightness of the
central concentration of stars in the most evolved globular clusters is
consistent with them being in a middle, rather than an advanced phase of
evolution. Other theoretical studies have suggested it can take longer than the
current age of the Universe for globular clusters to reach old
age.
Besides improving the understanding of the basic evolution of
globular clusters, this result has implications for understanding stellar
interactions in dense environments. It also removes the need for exotic
mechanisms - some involving black holes - which were thought to be needed to
prevent the many middle-aged clusters from collapsing.
"Some exotic
scenarios, including some of my own, have been invoked to try to make sense of
the observations and save the old theory," said Fregeau. "If this result holds
up, we don't have to worry about the exotic scenarios any more."
NASA's
Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for
the agency's Science Mission Directorate. The Smithsonian Astrophysical
Observatory controls science and flight operations from the Chandra X-ray
Center in Cambridge, Mass.
(source: NASA Marshall Space Flight
Center)
