Globular clusters are large balls of stars that orbit the core of a galaxy, just like a satellite. It has traditionally been assumed that older stars within the cluster are ejected out into the galaxy, but new observations using the Hubble Space Telescope and a smaller galaxy have found that the old stars haven’t been ejected out; they’re completely missing. This challenges current theories regarding globular cluster formation. Søren Larsen of Radboud University in the Netherlands is lead author of the paper, which has been published in The Astrophysical Journal.
It used to be believed that the stars in globular clusters all formed around the same time. However, research has since revealed that there are stars in the clusters of the Milky Way that belong to two distinct age groups. The older stars polluted the younger stars with nitrogen and other elements, leaving the second generation with levels 50-100 times higher than the first. The pollution hints that the clusters could have been up to ten times larger in the past, but the older stars just aren’t there in the amounts that they are expected to be. Where did they go?
The size of the Milky Way made it plausible that the older stars were ejected out of the clusters, and simply blended in with other stars in the galaxy that are of a similar age. The Fornax Dwarf Spheroidal galaxy located about 62 million light-years away has six globular clusters, four of which were analyzed by the Hubble’s Wide Field Camera 3. They found that even in this smaller galaxy, the clusters had stars belonging to different generations.
"We knew that the Milky Way's clusters were more complex than was originally thought, and there are theories to explain why. But to really test our theories about how these clusters form we needed to know what happened in other environments," Larsen explained in a press release. "Before now we didn’t know whether globular clusters in smaller galaxies had multiple generations or not, but our observations show clearly that they do!"
Just like the Milky Way, the newer stars in the globular clusters of Fornax have pollution levels that suggest a larger amount of older stars than have actually been observed. It would stand to reason that those older stars have also been ejected out of the cluster, into the galaxy. However, the galaxy is small enough that if that were to happen, astronomers would be able to spot them. There aren’t enough old stars in the galaxy to conceal any that might have been ejected from the cluster. The old stars seem to be completely missing.
"If these kicked-out stars were there, we would see them — but we don't!" added co-author Frank Grundahl of Aarhus University in Denmark. "Our leading formation theory just can't be right. There's nowhere that Fornax could have hidden these ejected stars, so it appears that the clusters couldn't have been so much larger in the past."
These missing stars throw a wrench in the theories of how globular clusters formed and evolved, but don’t offer up many clues about what actually did happen. Understanding this process will need to take the theory back to square one in order to come up with a new model that better accounts for the chemical pollution and the amount of stars in each generation.