Astronomers have discovered a type of ancient star at the center of the Milky Way that suggests the core grew through the merging of globular star clusters.
The international team of researchers used the European Southern Observatory's (ESO) infrared telescope VISTA in Chile to look at the nuclear stellar bulge, a 400-light-year region at the very heart of our galaxy. They discovered, for the first time, a dozen RR Lyrae at least 10 billion years old. These are variable stars usually found in globular clusters, compact groups of thousands or maybe millions of stars.
"This discovery of RR Lyrae stars in the center of the Milky Way has important implications for the formation of galactic nuclei," said co-author Rodrigo Contreras Ramos, from the Millennium Institute of Astrophysics, in a statement. "The evidence supports the scenario in which the nuclear bulge was originally made out of a few globular clusters that merged."
The research, which will appear in the Astrophysical Journal Letters, strongly supports the idea that large groups of stars migrated towards the center of our galaxy during the early days of its formation, more than 10 billion years ago.
Astronomers only have a general idea of how galaxies form, so studying our own Milky Way can provide important insights into the galaxy formation process.
We know that matter started accumulating in the gravity wells formed after the Big Bang. Gas fell towards the center of these space-time distortions, forming supermassive black holes as well as millions of stars.
It is not clear if the stars arrived at the core already formed or if the gas there was compressed into the multitude of stars we see today. Finding these RR Lyrae stars suggests that globular clusters were actually already established before they got to the core, where they eventually merged into what we see today.
The discovery was possible thanks to the powerful VISTA. By using infrared light, the telescope has been able to see through the dust that often obscures optical observation campaigns. This, combined with the fact that RR Lyrae are usually much dimmer than other stars found in the galactic center, made the observation campaign very difficult.
RR Lyrae are important stars because their brightness changes periodically, and the frequency of these changes depends on their true luminosity. Using this information, astronomers can work out the distance of these stars. This is very important. By knowing where these stars are and how they are moving around the core, we can find out a lot more about our own Milky Way.