We’ve said it before, and we’re going to say it over and over again in the future: The Universe is trying to kill us. Destruction exists at every size scale we can observe, and it is out of the carnage we see that we try to recreate an understanding of our universe’s past.
One of the more commonly destroyed structures in our galaxy are families of stars. From shredded dwarf galaxies to shredded globular clusters, every new star survey has found new streams of stars that were formed when tidal effects from our galaxy’s gravity stretched a system out into a thread of stars of similar compositions and motions.
A new paper in Nature, with lead author Zhen Wan, describes a newly discovered star stream. Found by the Southern Stellar Stream Spectroscopic Survey, or S5, this stream was identified through the careful study of stellar motions. Careful follow-up found that these stars also shared the same extremely low heavy element content. As a reminder, in astronomy, we have hydrogen, helium, and everything else, and in this case, these stars are mostly hydrogen and helium, indicating that the material in the stream had been enriched by a minimal number of supernovae before these stars formed. This stream is located in the direction of the Phoenix constellation, and when I say these stars have minimal heavy elements, I mean we have never seen a globular cluster this metal-poor.
It’s impossible to know if this stream of stars was originally a globular cluster or a dwarf spheroidal galaxy. These two kinds of systems can contain similar numbers of stars, and in rare cases, dwarf galaxies have a single epoch of star formation just like a globular cluster. In the paper, this team argues it is more likely that the progenitor to this stream was a globular cluster because they more often have the single star formation burst and metallicity seen in this stream and more consistently orbit in the direction of these stars.
If this was a globular cluster, it means we need to rethink the history of globular clusters, which we thought could only form after the universe had undergone a certain amount of enrichment and required more heavy elements than are seen here. That said, it could be something more like the Ursa Minor dwarf spheroidal galaxy, with a single epoch of star formation that pushed out all the gas and dust in the system, preventing future generations of star formation.
Given the choice between this being a rare object that was destroyed or a more common object that had to break the rules as we know them to form, I personally would have gone with the destroyed dwarf galaxy. Either way, this is a stream of stars with a less-complex makeup than we’ve ever seen before, and that is kind of awesome.
“The Tidal Remnant of an Unusually Metal-poor Globular Cluster,” Zhen Wan et al., 2020 July 29, Nature