Physics Photo of the Week

May 9, 2025

Open Star Clusters - Astrophotos by Donald Collins, College View Observatory

 

Throughout the Milky Way Galaxy there are thousands of open star clusters.  One of closest open star clusters is the Pleiades - or "Seven Sisters" - featured in the lower left photo.  The Pleiades cluster is the closest cluster from the Earth - 440 light years distant.  The bright stars of the cluster form a small "dipper" asterism.  Star clusters are groups of stars within the spiral arms of a galaxy and have formed from relatively recent condensations from clouds of gas and dust within the galaxy.  The Pleiades are about 100 million years old, which is fairly young for star clusters.  The stars were formed from the gravitational condensation of gas and dust clouds into several hundred stars.  The Pleiades is easily seen with naked eyes throughout late fall through mid spring.  They are visible now (late April) in the western sky about halfway between the horizon and zenith.  They are rather faint and show up better if you use averted vision where your retinal cells are most sensitive to dim light.  

The upper left image above shows the cluster called M 36 - the 36th entry in Messier's catalog of faint objects compiled in the late 1700's.  It is the youngest of the four major clusters represented here - 25 million years.  However M 36 is the most distant of the four clusters - 4000 light years.

M 44 - the upper right photo - is moderately close and moderately old - 610 light years distant and 550 million years old.  It is called the "Beehive Cluster" and is almost visible to naked eyes with dark skies.

The most interesting photo - lower right and reproduced here - contains the largest cluster, M35.  It appears to contain the largest number of stars and is about the same age as the Pleiades.  Even more interesting about this photo is that it contains a small second cluster in the lower right part of the photo.  This small apparent cluster size is due to its great distance - 11,000 light years - 25 times the distance to the Pleiades, or about 3 times further distant than M35, the dominant cluster in the photo.  This distant cluster is shown as a zoomed-in version below.

 

Astronomers estimate the ages of the various clusters by studying the colors of the stars and the brightnesses.  Young clusters have bright stars (big stars) that are fusing hydrogen into helium (like the Sun).  The large bright hydrogen-fusing stars consume their initial hydrogen more quickly by fusing at a higher temperatures than smaller H-burning stars.  These bright stars then start burning the helium that was generated by the H-burning phase.  The physics of the helium fusing is quite different.  Helium needs more pressure to ignite, so the helium fusion into heavier elements occurs in a much smaller core.  The outer envelope of the helium stars then expands to a gigantic size and is much cooler than the big stars of the hydrogen fusing stage (3000 deg K vs. 10000 deg K).  The giant cool stars are red whereas the giant young hot stars are blue.  Essentially the number of red stars compared to the number of blue stars is an indicator of the cluster's age.  Older clusters have a larger proportion of red stars.

The images above do not show the colors of the bright stars very well.  The digital SLR camera that I used overexposed all the bright stars in the clusters that the pixels for all the colors were exposed to the maximum, that the bright stars appeared white.

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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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This is the final Physics Photo of the Week for the 2024-2025 academic year.  We'll see you in the fall semester, 2025!  Meanwhile, send me your interesting physics photos!  DFC