Globular cluster 47 Tucanae Astronomy Picture of the Day, 2015 May 19 (Click to embiggen)
Globular star cluster 47 Tucanae is a jewel box of the southern sky. 47 Tuc lies about 17,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Recent observations have shown that 47 Tuc's white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass. Other colorful low mass stars including yellowish red giant stars are easy to pick out on the outskirts of the cluster in this recently released sharp telescopic portrait by the Hubble Space Telescope.
NASA (National Aeronautics and Space Administration) run a website called Astronomy Picture of the Day, APOD for short. A different astronomy and space science related image is featured each day, along with a brief explanation.
It is well worth taking a few minutes of your time every day for some beautiful and thought-provoking images.
When I read the caption on this picture at Left, I was intrigued by the statement that "47 Tuc's white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass."
Now, I have always understood gravity to be an ATTRACTIVE force, so what is this "gravitationally expelled" business going on? It can't be a case of heavier stars sinking to the bottom (cluster centre) and pushing the lighter ones out, as stars only attract each other, even sometimes merging.
Helpfully, NASA have provided a link attached to the word "gravitationally", which leads to a Wikipedia page entitled "Mass segregation" and an interesting description of what is happening.
Essentially, when two stars interact gravitationally, lighter stars come away with the higher velocity, moving to higher orbits about the cluster's centre of mass, while the more massive stars tend to sink into lower orbits (that is, orbits closer to the center of the cluster).
This process was observed for the first time by looking at globular cluster 47 Tucanae in 2015, in a study of the orbits of some of its white dwarfs, so I guess that is what led to the picture being posted.
I find it an awesome concept, that a similar process to what leads to that dry brown ring in the bottom of a finished coffee, is happening to stars in a cluster, and even to galaxies in a cluster of galaxies.
Ref. the page on Mass Segregation at Wikipedia for a fuller description of this phenomenon.