Despite it being our galactic home, many open questions remain about the origin and evolution of the Milky Way. To answer such questions, astronomers study individual stars and clusters of stars within our galaxy as well as those in others. Using data from the European Space Agency’s Gaia satellite, which is undertaking the largest and most precise 3D map of our galaxy by surveying an unprecedented one per cent of the Milky Way’s 100 billion or so stars, an international group has discovered a stream of stars spread across the night sky with peculiar characteristics. The stars appear not only to be very old, but also very similar to one another, indicating a common origin.
The discovered stream of stars, called C-19, are spread over tens of thousands of light years, and appear to be the remnant of a globular cluster. A globular cluster is a very dense clump of stars with a total typical mass of 104 or 105 solar masses, the centre of which can be so dense that stable planetary systems cannot form due to gravitational disruptions from neighbouring stars. Additionally, the clusters are typically very old. Estimates based on the luminosity of dead cooling remnants (white dwarfs) reveal some to be up to 12.8 billion years old, in stark contrast to neighbouring stars in their host galaxies. The origin, formation and reason for clusters to end up in these galaxies remains poorly understood.
The stars appear not only to be very old, but also very similar to one another, indicating a common origin
One way to discern the age of globular clusters is to study the elemental composition of the stars within them. This is often expressed as the metallicity, which is the ratio of all elements heavier than hydrogen and helium (confusingly referred to as metals in the astronomical community) to these two light elements. Hydrogen and helium were produced during the Big Bang, while anything heavier was produced in the first generation of stars, implying that the first generation of stars had zero metallicity and that the metallicity increases with each generation. Until recently the lowest metallicities of stars in globular clusters were 0.2% that of the Sun. This “lower floor” in metallicity was thought to put constraints on their maximum age and size, with lower-metallicity clusters thought to be unable to survive to this day. The newly discovered stream, however, has metallicities lower than 0.05% that of the Sun, changing this perception.
Despite it being our galactic home, many open questions remain about the origin and evolution of the Milky Way. To answer such questions, astronomers study individuals stars and clusters of stars within our galaxy as well as those in others. Using data from the European Space Agency’s Gaia satellite, which is undertaking the largest and most precise 3D map of our galaxy by surveying an unprecedented one per cent of the Milky Way’s 100 billion or so starts, an international group has discovered a stream of stars spread across the night sky with peculiar characteristics. The stars appear not only to be very old, but also very similar to one another, indication a common origin.
The discovered stream of stars, called C-19, are spread over tens of thousands of light years, and appear to be the remnant of a globular cluster. A globular cluster is a very dense clump of stars with a total typical mass of 104 or 105 solar masses, the centre of which can be so dense that stable planetary systems cannot form due to gravitational disruptions from neighbouring stars. Additionally, the clusters are typically very old. Estimates based on the luminosity of dead cooling remnants (white dwarfs) reveal some to be up to 12.8 billion years old, in stark contrast to neighbouring stars in their host galaxies. The origin, formation and reason for clusters to end up in these galaxies remains poorly understood.
Captured clusters
The stars in the recently observed C-19 stream are no longer a dense cluster. Rather, they all appear to follow the same orbit within our galaxy, the plane of which is almost perpendicular to the galactic disk in which we orbit its centre. This similarity in orbit, as well as their very similar metallicity and general chemical content, indicate that they once formed a globular cluster which was absorbed by the Milky Way. The orbit dynamics further indicate it was captured at a time when the potential well of the Milky Way was significantly smaller than it is now, implying that the capture of this cluster by our galaxy occurred long ago. Since then, the once dense cluster heated up and got smeared out as it orbited the galactic centre through interactions with the disk, as well as with the potential dark-matter halo.
The discovery, published in Nature, does not directly answer the question of where and how globular clusters were formed. It does however provide us with a nearby laboratory to study issues like cluster and galaxy formation, the merging of such objects and the subsequent destruction of the cluster through interactions with both baryonic as well as potential dark matter. This particular cluster furthermore consists of some of the oldest stars found, and could have been formed before the re-ionisation of the universe, which is thought to have taken place between 150 million and a billion years after the Big Bang. Further information about such ancient objects can be expected soon thanks to the recently launched James Webb Space Telescope. This instrument will be able to see some of the earliest formed galaxies, and can thereby provide additional clues on the origin of the fossils now found within our own galaxy.
Further reading
N Martin et al. 2022 Nature 601 45.
