What's the best way to get old in this universe of ours? We're not talking 80 or 90 years here—try 13 billion. New research from the Max Planck Institute for Astronomy (MPA) suggests that some of the oldest objects in existence—globular star clusters, groups of stars that orbit around a galaxy like planets orbit a star—somehow managed to keep it together even when galaxies collided around them.
So how did some star clusters become, as an MPA statement called them, "the only survivors of a 13-billion-year-old massacre?"
The had to be big.
The MPA team, led by Dr. Diederik Kruijssen, ran computer simulations of galaxies colliding and found that only the largest and brightest clusters made it out. Smaller ones were destroyed or scattered in crashes during the 2-billion-year period at the beginning of the universe when galaxy collisions were common. In the video above, included in the paper, you can see two simulated galaxies collide, scatter, reform, crash again and finally meld into one. The colored dots are star clusters; watch how most of them don't stand a chance.
The statement notes, "According to the simulations, most of the star clusters were destroyed shortly after their formation, when the galactic environment was still very hostile to the young clusters. After this episode ended, the surviving globular clusters have lived quietly until the present day."
The research explains a well-known conundrum among globular clusters: they all have about the same number of stars, unlike other clusters, which vary greatly in size. Dr. Kruijssen comments, "In the early Universe, starbursts were commonplace –it therefore makes perfect sense that all globular clusters have approximately the same large number of stars. Their smaller brothers and sisters that didn’t contain as many stars were doomed to be destroyed.”
The fact that all globular clusters are similar would likely mean that they formed in more or less the same environment, so the researchers hope to use them as "fossils" of the early universe.
Their paper is forthcoming in Monthly Notices of the Royal Astronomical Society.