Star clusters are held together by a combination of their own gravity and the gravity of the gas from which they form. At the end of the star formation process, the gas is dispersed, and the gravity of the stars is usually not enough to hold the cluster together any longer. Thus, only a small minority of clusters manage to survive the dispersion of their parent gas cloud. Some of them are later separated by tidal forces exerted by other clouds nearby. For others, over a very long period of time, individual stars are driven out of the cluster due to random encounters between the stars. And all of these processes are one-sided: once stars leave a cluster, they move too fast to be recaptured.
Each type of star cluster undergoes these processes to some extent, but not in the same way. Globular clusters, for example, are massive old clusters that form differently from smaller, younger, open clusters. In globular clusters, random encounters between stars have eliminated only a tiny fraction of their population depending on the age of the universe. Small, open clusters, like the Sun’s parent cluster, are much more vulnerable.
We still cannot say exactly what process was responsible for the dissolution of the Sun’s parent cluster, or even exactly where in the galaxy the cluster formed. The Sun is 4.6 billion years old. During this period, the galactic disc has completed more than 20 rotations, so the Sun and the stars it was born with have had a long time to roam and disperse. However, astronomers are looking for our star’s siblings by looking for chemical compositions that match our Sun – almost like a stellar fingerprint. They made the first discovery of a solar sibling, HD 162826, in 2014 and a probable second sibling, HD 186302, in 2018. There will likely be a few thousand additional siblings hidden throughout the Milky Way. – just find them. .
Astrophysicist, Australian National University, Canberra, Australia