For many years, scientists have been baffled about why the Milky Way has a lot of new stars at its center but lost most of its old ones, until now. It turns out that these red giant stars could still there but are too dim to be spotted, speculates the team of scientists from Georgia Institute of Technology.

The mass of red giants, stars in the last phase of its life, were taken away in an unspecified but huge amount millions of years ago as a result of repeated collisions with gaseous disk that filled out the galaxy’s center. The study was published on June 1 in The Astrophysical Journal.

This is just a theory, however. It was introduced in 2014 with the use of computer simulations of red giant stars, recreating all these stars’ week long collisions with the gaseous disk complete with the objects’ velocities and densities.

“Red giants could have lost a significant portion of their mass only if the disk was very massive and dense,” explains co-lead author Tamara Bogdanovic, an assistant professor at Georgia Tech. “So dense, that gravity would have already fragmented the disk on its own, helping to form massive clumps that became the building blocks of a new generation of stars.”

Thomas Forrest Kieffer, also from Georgia Tech, said that this event possibly happened four to eight million years ago, which is about the age as the young stars currently residing in our galaxy. The collisions stripped at least 20 to 30 percent of the red giant stars’ kinetic energy, consequently decreasing their orbits and pushing them toward a black hole.

red giant stars

A sequence of snapshots based on a simulation of a red giant entering and exiting a clump in a fragmenting accretion disk. Credit: Georgia Tech

“We don’t know very much about the conditions that led to the most recent episode of star formation in the galactic center or whether this region of the galaxy could have contained so much gas,” says Bogdanovic. “If it did, we expect that it would presently house under-luminous red giants with smaller orbits, spinning more rapidly than expected.”

More studies are still needed, the team asserts. Nevertheless, if their theory turns out correct, we would be one step closer into understanding the origin of the Milky Way.