A cluster
of ancient stars is likely the relic of a dwarf galaxy that merged with the
Milky Way during its early days, scientists now find.
Called
Terzan 5, the globular star cluster lies within the Milky Way's central bulge
of stars that protrudes up and down from the galaxy's flattened disk. A
globular cluster can host a collection of 10,000 to 1 million-plus stars bound
together by gravity. The traditional view of the 150 or so clusters in our
galaxy has been that the stars of each one, typically ancient in cosmic terms,
were born at about the same time from one cloud of gas and dust.
New
observations with the European Southern Observatory's Very Large Telescope show
that's not the case for Terzan 5, whose stars break out into two populations
with different ages and chemical compositions, which can indicate where the
star's material originated.
The
findings will be detailed in the Nov. 26 issue of the journal Nature.
Scientists had already known about another
anomalous cluster, Omega
Centauri, which has two distinct star groups in its grasp,
suggesting the cluster was a remnant of a disrupted dwarf galaxy that merged
with the Milky Way. (Omega Centauri is our galaxy's largest globular cluster
known, thought to contain some 10 million stars.)
In another
paper published this week in Nature, a team of scientists led by Jae-Woo Lee of
Sejong University in Korea finds various other globular clusters contain two
star populations and could be relics of more massive dwarf galaxies.
Both new
findings will help astronomers piece together our galaxy's past, including how
it formed some 13.7 billion
years ago and then evolved at least in part by gobbling up other galaxies.
"The
history of the Milky Way is encoded in its oldest fragments, globular clusters
and other systems of stars that have witnessed the entire evolution of our
galaxy," said University of Bologna researcher Francesco Ferraro, who led
the team studying Terzan 5.
Odd
cluster
Specifically,
when looking at data collected on Terzan 5, Ferraro's team found one group of
stars dating back 12 billion years, while the youngest stars were born some 6
billion years ago. The two populations have different abundances of iron, with
the younger group having three times more of the heavy element.
Iron and
other heavy elements are mostly produced in supernovae
explosions, the guts of which form seeds for new star formation. Each
generation of stars, theory holds, have more heavy elements than their
predecessors.
"The
supernovae explosions are so violent that they tend to eject the gas outside
the entire stellar system," Ferraro told SPACE.com. "Then in order to
retain the gas the stellar system must be very massive." Ferraro added a
typical globular cluster doesn't have enough mass, or gravity, to retain the
supernova's ejected material.
As such,
the team says their findings indicate Terzan 5 is not a genuine globular
cluster.
Piecing
together the past
Here's what
Ferraro and colleagues think must have happened: Some 12 billion years ago,
Terzan 5's first clump of stars formed, then some exploded. Then, about 6
billion years ago, the iron-enriched gas from the explosions helped to generate
a second group of stars.
Since
supernovae explosions tend to kick gas out from the clusters in which they
originate, the researchers think Terzan 5 must have been much more massive, by
a factor of 500 to 1,000, in the past compared with today.
The team
also found the oldest stellar population in Terzan 5 has the same chemical
composition as the Milky Way's central bulge.
Taken
together, the results provide insights as to how the bulge, though not the
entire galaxy, formed.
"Hence
the bulge may have formed through the early aggregation of individually
pre-formed and somewhat internally evolved stellar systems," Ferraro said.
"Thus suggesting that the inner bulges of spiral galaxies can originate
by accretion of small sub-structures."
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