New Hubble Space Telescope observations tell us we could be wrong about how globular clusters of stars form in the cosmos.
Although globular clusters in another galaxy show a similarity to those in the Milky Way, this galaxy's clusters also pose a mystery that cannot be explained by current formation theories.
Globular clusters are large stars that orbit their galaxies' centers, although they often sit far away from them. Once, astronomers thought that all stars in such a cluster were about the same age, formed around the same time. However, after studying clusters in the Milky Way, astronomers believe that they actually contain two generations of stars: about half that are part of a single generation of older stars, as well as younger stars, a second generation that is "polluted" with more elements, such as nitrogen.
Astronomers studied clusters in the Milky Way and discovered that there are higher concentrations of these polluted stars than initially thought. This suggests that there are large numbers of the first-generation stars that are missing from the clusters. The leading theory suggests this is because, at some point, those stars were kicked out from their clusters.
Observations made in the Milky Way support this theory, because we've found evidence of those older stars around, but away, from the clusters.
However, after looking at globular clusters in the Fornax Dwarf Spheroidal galaxy, astronomers realize that their theory is now being challenged, not by other astronomers, but by Fornax's clusters themselves.
The Fornax clusters showed multiple generations of stars, similar to clusters in our galaxy: there are the first-generation of stars, plus the polluted stars. This suggests that these clusters formed in the way we've theorized about the Milky Way's clusters.
That isn't the case, though. After looking at the number of polluted stars in the Fornax clusters, astronomers believe that these stars would have needed 10 times more mass in their past to kick out a lot of their first-generation stars, before they shrunk down to their current size. However, there is no sign of any older stars around these clusters.
"If these kicked-out stars were there, we would see them— but we don't!" says 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."
This means that what we thought we knew about how globular clusters of stars form is probably incorrect and requires further tweaking of our theories.
[Photo Credit: NASA, ESA, S. Larsen (Radboud University, the Netherlands)
