A Cluster in the Sky with Diamonds
Picture yourself in a boat on a river
With tangerine trees and marmalade skies
Somebody calls you, you answer quite slowly
A girl with kaleidoscope eyes
Lucy in the Sky with Diamonds
Lennon/McCarthy
Released June 1, 1967
In the 18th century, while searching for comets, French astronomer Charles Messier compiled a list of approximately 100 stationary, diffuse, spherical objects in the night sky. The list was intended to inform other astronomers, so that they would not mistake these objects for genuine comets, which move against the background of the fixed stars. Messier 55 (M55) is one of these objects, now known to be a Globular Cluster located in the constellation of Sagittarius some 17,500 light years from Earth. A light year is not an interval of time, it is the distance light travels in one year at its velocity of 300,00 kilometers per second (186,000 miles per second).
This European Space Organization (ESO) image of M55, obtained with the ground–based VISTA Infrared Survey Telescope, shows (almost) the entirety of the cluster.
Globular clusters are gravitationally–bound satellites of our own Milky Way Galaxy, indeed, of all types of galaxies. They form quite early in the history of their host galaxies, and because they are extremely tightly bound, they have notably long lifetimes before eventually dissolving into their surrounding environments. They harbor some of the very oldest stars in the Galaxy, almost as old as the Milky Way itself at about 13 billion years of age. There are some 150 known Milky Way globular clusters, and more are likely to exist hidden behind the thick disk of our galaxy, obscured by clouds of gas and dust. Our own Sun is a relative newcomer on the scene, having formed only about 4.5 billion years ago, as these things go. Astronomers take an unusually long view on most matters.
A March 17, 2023 NASA press release featured this striking Hubble Space Telescope (HST) image of the central portion of M55 (click here). It was obtained with the Advanced Camera for Surveys (ACS).
There are three types of stars visible: 1) The main sequence stars, those which like our Sun are still in the hydrogen–burning stage of their lives. These are the faintest objects in the image and by far the most numerous. 2) The red supergiant stars, those which have exhausted most of their nuclear fuel and are approaching the end of their lives. These are the very bright, far less numerous orange objects in the image. 3) So–called “blue straggler” stars, a population which we shall see are anomalous and which will be discussed in some detail later. These are the bright blue objects in the image.
Main Sequence Stars
As previously mentioned, Main Sequence stars like our Sun are in a stable, long–lived stage of their evolution. They derive their luminosity from nuclear fusion in their cores, transforming hydrogen into helium and releasing energy in the process. I have discussed this in some detail in a previous blog post Starry, Starry Night. The more massive the star, the more rapidly it burns its limited fuel supply, and the shorter its lifetime. Stars with the same mass as our Sun live for about 9 billion years before transitioning into the late stages of their evolution. They spend about 1 billion more years as a very large, very luminous Red Giant star before ending their lives as small, compact remnants called White Dwarf Stars.
M55 is a very old globular cluster, with an age of about 12.5 billion years, almost as old as the Milky Way itself. This is so old that main sequence stars with the same mass as our Sun have already died, and the most massive main sequence star in M55 has a mass of only 0.9 Solar masses. All the rest are less massive and less luminous.
Red Giant Stars
When a main sequence star exhausts the hydrogen it has been fusing to helium in its core, which contains the central 10% of the stellar mass, the heat and pressure from hydrogen fusion that was holding the core up against gravity can no longer be sustained. The core begins to shrink and become hotter, and while it does, hydrogen fusion continues in a thin layer just outside the core. This takes place at the substantially higher temperature of the core, and so the luminosity of the star goes up dramatically. The outer layers of the star expand and become cooler and redder, and the star shines enormously more brightly with an orange hue. It has become a Red Giant.
Blue Stragglers
The colors of stars depend on their masses and luminosities. Stars up to twice as massive as the Sun shine very brightly with a blue hue. Our Sun shines less brightly in the yellow, and less massive stars shine even less brightly with redder colors. So how can there be bright, blue stars in a globular cluster with an age of 12.5 billion years? All the stars with masses between about 1.5 and 2 times the mass of the Sun have lifetimes very much shorter than this, and they should have died long ago. A popular theory suggests that these are the result of stellar collisions in the densely packed globular cluster core. Two stars with masses less than the Sun collide and fuse, forming a star with a combined mass of up to 1.8 Solar masses, and these are the “blue stragglers” described above. Your very own author, cher moi, once studied these stars early in his research career. The results were published in the October 3, 1997 issue of the peer–reviewed publication Astrophysical Journal Letters. Read… All… About… It… here (the published research paper), and here (from NASA’s Hubble Space Telescope archive).
I’ll close with a fascinating simulation of the movements of stars in globular clusters. Unlike our common notions of astronomical objects in nearly circular orbits around a central object, like the planets around the Sun in our own Solar System, stellar orbits in globular clusters are largely radial. This means they plunge almost entirely directly into and back out of the cluster’s dense central regions, and this is the motivation for the collisional theory of blue straggler formation, as collisions occur with much higher probability in the center. This video is an accurate representation of the nature of the motions. As we have noted above, globular clusters have very long lifetimes due to being very tightly bound by their mutual, self–generated gravity. But very long is not forever, and the simulation shows the eventual dissolution of the cluster into its surrounding environment.
All the best,
From Broomall, PA
Sunday, March 19 at 2:10 PM,
Rex
