Star Clusters and their stars Open clusters and globular clusters General characteristics of globular clusters Globular cluster stars in the H-R diagram Stellar interactions in globular clusters Exotic binaries in globular clusters

Star clusters Virtually all stars, Sun included, are born in star clusters. At first, with lots of gas and dust in the vicinity, they look like they are just a part of a molecular cloud. However, as the stars start to shine, they blow away the gas and dust that helped create them (through pressure radiation).

Star clusters As the gas clears, we observe an open cluster. These clusters always have young stars, many of which are blue. The reason is that the stars in open clusters are not bound to the cluster. Therefore, they disperse while they are still young. What if the cluster is massive (and dense) enough to keep its stars in its vicinity?

Globular clusters If the cluster is massive and dense enough to hold its stars together, then they will stay together – and age together. We have therefore a globular cluster. In our Galaxy, all globular clusters (i.e., the clusters where the stars are bound) have very old stars. You can tell by their color – all blue stars have already left the main sequence, leaving only long-lived, cooler stars. No open clusters are ever observed with old stars in them.

Globular clusters Are there any young globular clusters? Not in our Galaxy. But in the Large Magellanic clouds, there is an ongoing burst of star formation, caused by the tidal interaction with the Milky Way. Many of the new open clusters are very massive. Could one of them be dense and massive enough to stay bound?

Globular clusters Yes! NGC 1818, in the LMC, is one example of a young globular cluster.

General characteristics of GCs There are almost 200 globular clusters orbiting our Galaxy. They typically have 105 – 106 stars. They are spherically distributed around the center of our Galaxy. They tend to concentrate towards the center of the Galaxy, with many in the constellations Sagittarius, Scorpio and Ophiunchus It was by studying the distribution of globular clusters that astronomers first suspected that the Sun was not at the center of our Galaxy, that it instead was somewhere in Sagittatius.

General characteristics of GCs M87 is a giant elliptical galaxy in Virgo. It has an estimated 15,000 globular clusters.

Stellar evolution in GCs In the Galaxy, particularly where there is ongoing star formation (in the spiral arms), we see young and old stars. The Hertzprung-Russell diagram is therefore ”filled”.

Stellar evolution in GCs In globular clusters, no new stars have been formed after the original episode of star formation – the gas was blown out of the cluster because of star light. Therefore, the brightest (shorter- lived) stars have left the main sequence. Cluster age can be determined from the brightest stars still in the main sequence. These are generally similar to the Sun, except for the lower metallicity (the Universe was much younger when these stars formed). Therefore cluster age is about 1010 yr or a bit older.

Stellar evolution in GCs Because all stars in a globular cluster were formed at a very similar time, the curve they form in the H-R diagram is called an isochrone. These curves are extremely useful in finding out what happens to stars after they leave the main sequence. Because most globular clusters have ages similar to what the Sun will have when it leaves the main sequence, globular clusters show us directly what will happen at the end of the Sun's life.

Stellar interactions in GCs Stellar densities in GCs can me much higher than in the Solar vicinity – normally 1000 stars per cubic pc in the cores of most GCs, but 106 stars per cubic pc in clusters with collapsed cores – example: M15 Such high densities lead to stellar interactions that produce objects that we can never find in the Galaxy!

Stellar interactions in GCs Blue stragglers have been considered a problem for stellar evolution theory. They appear as MS stars that look brighter and bluer than the vast majority of stars in the cluster. It looks as if they might be the result of stellar mergers – once two stars merge, they will be more massive, and therefore hotter and fusing hydrogen at a faster rate

Stellar interactions in GCs Some evidence of this can be obtained from studying the distribution of blue stragglers – they are preferentially formed in the cores of dense globular clusters, precisely where the rates of stellar collisions are higher. Such stars are an example of the “exotic” objects formed only in GCs and never in the Galaxy.

Stellar interactions in GCs Even more exotic objects can be found in X-rays. Globular clusters have an anomalous number of X-ray binaries for their mass. The excess of X-ray binaries compared to the Galaxy is created by the disruption of main- sequence binary systems by compact objects (white dwarfs and neutron stars).

Stellar interactions in GCs If a massive compact star approaches a binary system to a distance smaller than 3-4 orbital separations, the binary might be disrupted. The most likely outcome is the ejection of the lighter star in the previous binary system and the formation of a binary consisting of the massive compact star and the intruder. Such binary systems will either disrupt or become more compact as they interact with other stars in the cluster.

Exotic binaries in GCs Many of these newly- formed binaries with compact objects will become X-ray binaries. As the MS star leaves the MS, it will grow to fill its Roche lobe. At this stage, matter will be pulled into an accretion disk orbiting the compact star, causing X-ray emission.

Exotic binaries in GCs The compact star is a previously existing dead neutron star, the accretion of material and angular momentum from the disk will spin it up – this stage is called a Low-Mass X-ray Binary. An extreme case is MXB 1820-30, in the globular cluster NGC 6624. This system has the shortest orbital period known for any astronomical object – only 11.4 minutes. Relative to their mass, globular clusters have a disproportionately high number of LMXBs.

Exotic binaries in GCs Once accretion into the NS stops, we observe a millisecond pulsar. GCs are known to be excellent breeding grounds for millisecond pulsars – more MSP are known there than in the disk of the Galaxy.