Stellar Fossils from Milky Way's Past Revealed
This near-infrared image shows the star cluster called Terzan 5 within the Milky Way's bulge.
Credit: F.R.Ferraro, University of Bologna.

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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