Globular Clusters Evolve in Interesting Ways Over Time

Globular clusters are among the oldest objects in the Universe. The early Universe was filled with dwarf galaxies and its just possible that globular clusters are the remains of these ancient relics. Analysis of the stars in the clusters reveals ages in the region of 12-13 billion years old. A new paper just published shows that the globular clusters are home to two distinct types of stars; the primordial ones with normal chemical composition and those with unusual heavy amounts of heavier elements. 

Globular clusters are dense, spherical collections of stars that orbit the outer regions of galaxies, usually in the galactic halo. They contain hundreds of thousands, sometimes millions of stars bound together by gravity. They differ from open clusters, which are younger and less tightly bound and found in the main body of a galaxy. Globular clusters in contrast, are ancient with ages typically in the regions of 10 to 13 billion years old. 

M13 – Credit: R. Jay GaBany

There stellar components are mostly composed of low-mass, metal-poor stars, suggesting they formed early in the history of the universe before the heavier elements appeared. Studying globular clusters can reveal lots about stellar evolution, the formation of galaxies and even dark matter. Our own Galaxy the Milky Way is home to over 150 known globular clusters like well known M13 in the northern hemisphere or Omega Centauri in the southern hemisphere.

Omega Centauri is the brightest globular cluster in the night sky. It holds about 10 million stars and is the most massive globular cluster in the Milky Way. It's possible that globulars and nuclear star clusters are related in some way as a galaxy evolves. Image Credit: ESO.
Omega Centauri is the brightest globular cluster in the night sky. It holds about 10 million stars and is the most massive globular cluster in the Milky Way. It’s possible that globulars and nuclear star clusters are related in some way as a galaxy evolves. Image Credit: ESO.

In a paper recently published in Astronomy and Astrophysics, a team of researchers have advanced our understanding of these clusters by revealing more about their formation and dynamical evolution. The team led by Emanuele Dalessandro from the National Institute for Astrophysics (INAF) explored multiple populations of stars in the clusters. They studied the change in positions of the stars and their velocity in the first 3D kinematic analysis of 16 globular clusters. 

The team used data from ESA’s Gaia telescope the European Southern Observatory Very Large Telescope and Multi Instrument Kinematic Survey to measure the 3D velocity of stars within the clusters. This was a combination of proper motion (motion across the sky) and radial velocity (motion towards and away from us.) To gather the measurements, spectroscopic survey data was used.

Artist’s impression of the Gaia spacecraft detecting artificial signals from a distant star system. In this synchronization scheme, the star system’s inhabitants send the signal shortly after witnessing a supernova, which is also seen by telescopes on Earth. (Credit: Danielle Futselaar / Breakthrough Listen)

The formation and evolution of globular clusters has been one of the most hotly debated questions for the last few decades. The significance of understanding them is huge explains Dalessandro,’because they not only help us to test cosmological models of the formation of the Universe due to their age but also provide natural laboratories for studying the formation, evolution, and chemical enrichment of galaxies.’ Understanding the physical processes behind their formation was key to understanding how they evolve. This was the goal of their study which revealed for the first time that globular cluster form through multiple star formation events. 

Source : The first 3D view of the formation and evolution of globular clusters

3 Replies to “Globular Clusters Evolve in Interesting Ways Over Time”

  1. Do they say which type of GC they are? It matters because we’ve known for some time that captured/alien GCs host multiple populations of main-sequence stars, but native GCs do not.

  2. By “nuclear star clusters,” do you mean clusters in the bulge of galaxies? Or “clusters” of stars inside Globular Clusters? I understand some have gone through a “core collapse” and some have not. Does this study have anything to do with that? Or are we talking about lurking black holes causing local phenomena inside a Globular? (perhaps a kind of “cluster”?)

  3. It is interesting that their analysis support a scenario where the original star population is isotropic and eject gases that sinks to the cluster center to initially form an anisotropic, rapid corotating secondary (or later) generation of stars. With age these distributions all tend toward isotropy.

    @James: It is unfortunate that you don’t give a reference, as I can’t find any such knowledge.

    This paper purposefully chose globular clusters which were representative for Milky Way:
    “The selected clusters are representative of the overall Galactic GC population as they properly encompass the cluster’s dynamically-sensitive parameter space, spanning a wide range of central densities and concentrations, different stages of dynamical evolution, and different environmental conditions.” Their findings, based on 16 clusters, tries to capture the putative difference you propose.

    On the contrary, a 2022 review of globular clusters in the galactic neighborhood see no major differences apart from possibly dwarf galaxy low mass globular cluster tending towards not generating secondary star populations:
    “Another approach to investigate the role of the parent galaxy on the multiple population phenomenon consists of comparing the multiple populations of GCs formed in situ, those that are the products of a merging process and the GCs that are not associated with any parent stellar stream and are characterized by either high or low energy (see [224], for accurate links of most Galactic GCs with a variety of progenitor galaxies)7 . In these three different groups of GCs, the fractions of 1P stars and the mass of the host GCs follow similar trends, thus indicating that there is no evidence of any dependence of the present-day population ratio in GCs on the progenitor system [106]. It is worth noting that most candidate simple population GCs are either high-energy clusters, where the progenitor galaxy is not known, or have been associated with the progenitor of the Helmi stream [227] and the Sagittarius dwarf spheroidal. Based on these results, it is tempting to speculate that simple population GCs are low-mass clusters that formed in the environment of dwarf galaxies [106].” [Multiple Populations in Star Clusters, Antonino P. Milone, Anna F. Marino, arXiv:2206.10564.]

    @Steven: The article here discuss a matter which the published paper do not (what I can see from browsing).

    To somewhat answer your questions, Wikipedia:

    “A nuclear star cluster (NSC) or compact stellar nucleus (sometimes called young stellar nucleus) is a star cluster with high density and high luminosity near the center of mass of most galaxies.[1] NSCs are the central massive objects of fainter, low-mass galaxies where supermassive black holes (SMBHs) are not present or are of negligible mass. In the most massive galaxies, NSCs are entirely absent. Some galaxies, including the Milky Way, are known to contain both a NSC and a SMBH of comparable mass.[2] ”

    “Although the mechanisms behind their formation are not entirely known, hypotheses provide four possibilities: …”

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