3) Types of clusters: Know the difference between galactic and globular clusters

Question

3) Types of clusters: Know the difference between galactic and globular clusters. Know which ones are older, have more heavy material, and which ones are involved in multiple star forming generations. Know which ones are easier to observe and why.

4) Finding the galactic center: Know which type of cluster is used. Know what wavelength of light is used and why. Know which star is observed and how its orbit is used to determine the maximum possible volume and minimum possible density for the SMBH at the center of our galaxy.

Explanation / Answer

3)

Galactic clusters are found only in spiral and irregular galaxies, in which active star formation may be occurring. They stars are young, usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters. As a result, typical star density in the center of a cluster is only about 1.5 stars per cubic light year (the stellar density around the solar system is about 0.003 star per cubic light year)

Globular clusters, on the other hand, are densely packed collections of stars, roughly spherical in shape, containing hundreds of thousands, and sometimes even millions of old stars. The density of stars is quite high, like 30 stars per cubic light year at the core, making it appear to us as a single object. The typical distance between stars in a globular cluster is about 1 light year, but at its core, the separation drops to just a few billion kilometers. The size of globular clusters can be 100 light years and above. Globular clusters are usually found orbiting the galactic cores as 'satellites'.

The main differences tend to be in age, number of stars, and general size.

Galactic clusters are made up of young stars, and tend to disassociate over time. Our own sun was likely part of an galactic cluster at one point. galactic clusters tend to consist of no more than a few hundred stars, and are relatively spread out.

On the other hand, globular clusters are composed of very old, metal poor stars on the whole, and are gravitationally bound, meaning that they will not disassociate over time. They usually contain several thousands of stars if not tens of thousands of stars, and are very densely packed.

The general working theory is that galactic clusters are the remnants of bursts of star formation, while globular clusters are the remains of the cores of small galaxies eaten by the Milky Way. Some credence for this recently is research that indicates there may well be intermediate-mass black holes anchoring globular clusters, similar to how a supermassive black hole anchors our own galaxy. Many galactic clusters have some degree of nebulosity to them, such as the Pleiades, that lends evidence to the theory that they are the remnants of a burst of formation in stellar nurseries.

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