Physics Photo of the Week
December 8, 2023
M13 - A Globular Cluster - Photos by Don Collins - College View Observatory
There are about 100 of these large clusters of stars orbiting the Milky Way Galaxy. Similar clusters are found in other galaxies. These assemblages of stars contain several hundred thousand stars and up to a million stars. These clusters are also very old - about 10 billion years - older than the Sun and the Solar System. Unlike open clusters (such as the Pleiades or "Seven Sisters") the globular clusters have so many stars that they are gravitationally stable. All the stars are whizzing around a common center of mass in random orbits similar to a swarm of insects. This globular cluster is about 20,000 light years from Earth and about 140 light years in diameter.
If our Solar System were in the midst of this cluster of close to 1 million stars, the sky would be ablaze with starlight! However, in spite of the incredible age of the cluster, a Solar System would probably not last very long. The near collisions of so many stars with random orbits would disrupt any planetary orbits and disperse the planets in all directions with similar random orbits and would eventually be "eaten" by colliding with other stars in the dense population.
Populations of the types of stars in globular clusters yields a paradox. Blue, high luminosity stars we know last only a few 10's of millions of years. The more massive the star the hotter the temperature - thus massive hot stars are much bluer than solar type stars. As a result the massive stars consume their hydrogen fuel much more quickly and die in violent deaths by going supernova. Within 10 billion years all the hot blue stars should be gone. However the color photo of M13 shows plenty of blue stars. Why would there be so many young looking hot blue stars in a cluster of stars that is older than the age of most blue stars? The answer to this puzzle appears to be that these "blue stragglers" are actually new stars that have been formed from collisions of former stars in the cluster. This hypothesis makes sense because the number of blue stragglers is rather small - the hot stars formed from collisions of smaller stars live shorter lives than the smaller stars making up most of the cluster. Once formed, these new hot stars die out soon after they were formed - in 10 million years rather than 10 billion years.
Globular clusters are also believed to harbor intermediate black holes in their core. Evidence from measuring stellar velocities close to the core with the Hubble Telescope published about 20 years ago has verified that the orbits of the inner stars (using infrared light with its greater penetrating power) are governed by a black hole of several hundred solar masses in the core of one of these clusters. The photo at right is the Globular Cluster M15 (only in monochrome). M15 at right is the first globular cluster where the evidence for a central black hole has been found. The new James Webb telescope is bound to detect more globular black hole evidence in a few years. Stay tuned...
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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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