Galaxies like the Milky Way are thought to have been built through a series of mergers, attracting smaller galaxies and groups of stars and making these foreign stars their own. In some cases, the mergers were recent enough that we can still detect the previously independent body as a group of stars orbiting the Milky Way together. But, over time, interactions with the rest of the stars in the Milky Way will slowly disrupt any structures the cluster houses.
So it’s surprising that researchers have found what appears to be the remnants of a globular cluster of some of the oldest stars around. The discovery is consistent with the “growth through fusion” model of galaxy building, but it raises questions about how the mass will remain intact as long as it does.
Gaia data mining
The results began with an analysis of data from the European Space Agency Mission Gaia, which aims to make at least a map of the Milky Way in three dimensions. Gaia has photographed nearly a billion objects dozens of times, which is enough to estimate their location and motion around the core of the Milky Way. This map helped scientists identify structures within our galaxy based on the fact that there are some star clusters that are not only physically close to each other, but all move in the same direction.
Gaia data mining for these types of structures is so useful that there is a software algorithm called STREAMFINDER that defines them. This program led to the discovery of the C-19 stellar stream, a group of stars moving together through the Milky Way’s halo.
One way to check if these groups of stars really started as part of a single group is to check their age. Clusters often consist of stars of similar ages. One way to find out if stars are forming at the same time is to check for the content of heavier elements. There were few elements heavier than helium during the Big Bang, so most of the heavier elements in existence now came from earlier stars. The later in the history of the universe the star formed, the more likely the star would contain these heavier elements.
(Astronomers call any element heavier than helium a metal and refer to the star’s heavy element content as a metal. But this is likely to confuse most non-astronomers, so we’ll avoid it.)
So, the astronomers behind the new work measured the levels of heavy elements in stars that were thought to belong to the C-19 stream. With the exception of one limb, they were all exactly the same, indicating that the stream is in fact the disruptive remnant of a group. But the results also contained a surprise: a remarkably low amount of heavy elements.
The typical way to record heavy elements is through the ratio of iron (which forms only late in a massive star’s life) to hydrogen. Hydrogen has always been the most abundant element in the universe, while iron levels have slowly built up over time. So the higher the iron to hydrogen ratio, the more recently the star formed.
In the case of the C-19 stream, the ratio was very low. So low that C-19 stars formed 3 billion years ago after the Big Bang, or when the universe was only about a quarter of its current age. It is possible that they formed a little earlier.
Within the Milky Way, a few hundred stars with similarly low levels of heavy elements have been identified. But no cluster has ever been seen in which every star is at such a low level. In fact, prior to this discovery, the clusters in the Milky Way were thought to contain Earth-heavy elements—all of them having levels higher than those seen in the C-19 stream. This was true despite the fact that based on the distribution of known groups, we would expect about five levels with heavy elements similar to those in the C-19 stream.
The lack of other clusters suggests that most early clusters like this stream were already so disrupted that they faded into the background of the Milky Way’s stars. Which raises the question why there is no C-19 stream. This is particularly unexpected since the current’s orbit around the galactic core takes it deep into the Milky Way, giving it plenty of opportunities to engage in interactions with other features that should disable it.
One possibility that could explain this is that the cluster originally entered the Milky Way as part of a dwarf galaxy that was swallowed up. The dwarf galaxy’s structure could provide a degree of protection until it crashes and its stars are scattered across the Milky Way. And if true, the mass that gave rise to the C-19 stream contained a large portion of the stars in the dwarf galaxy at that time.
No matter how we explain it, the presence of the C-19 stream tells us things about the history of the universe. The authors conclude, “The presence of C-19 by itself proves that globular clusters should have been able to form in the low-mineral environments where the first galactic structures were to aggregate.”