Open Star Clusters

A fellow stargazer once mentioned in a meeting of our local astronomy club that she avoided observing open star clusters because they were just, well – stars. But that is exactly why I enjoy them and why, as a nighttime guide at the Kitt Peak Visitor Center, I like showing them to our guests in our Nightly Observing and Dark Sky Discovery Programs. The trick to enjoying these clusters is letting your imagination, as well as your eyes, play over them.

Deep sky objects often elicit fanciful names because humans are prone to see familiar shapes in the randomness of nature. That is why we can look at billowy clouds in the sky and see everything from Teddy bears to dinosaurs. Open star clusters are no different. Consider some of the clusters we observe regularly in our programs: the Butterfly Cluster, the Salt and Pepper Cluster, the Wild Duck Cluster, the Beehive Cluster, and the Owl Cluster, sometimes also known as the ET Cluster. When looking at these celestial gems, look carefully. Give your imagination free rein to play over the stars and see what images come to mind. You might see a pattern uniquely your own. It will be that image that abides long after you step away from the telescope or put away the binoculars.

The Beehive Open Star Cluster, M44, in Cancer

The Beehive Cluster, NOIRLab/AURA/NSF

What Makes Them Open?

Open star clusters are called open because the stars in the cluster are not held in the cluster by gravity. Instead they formed together in a large cloud of hydrogen gas such as the one we see in the constellation Orion. When you look at these clusters, you see something akin to a stellar family unit. Imagine that many millions of years ago, there existed a large cloud of hydrogen gas where the cluster now resides. That cloud slowly collapsed to start forming all those stars. Knowing that the cluster stars are different in age and composition from the other random stars in the field of view makes them seem special. Over hundreds of millions of years, the stars in the cluster will begin to disperse as the cluster moves through the galaxy.

All of that, then, brings to mind our star, the Sun. If stars form in clusters, so must the Sun have had other stars around it at birth – stellar siblings. What happened to the Sun’s family? That’s tough to determine because the stars that formed with the Sun have had about 4.5 billion years to wander off. The good news is that astronomers have, with considerable certainty, identified a long lost solar relative. It goes by the name of HD162826 and resides in the constellation Hercules. It isn’t a very bright star, but you can spot it with a pair of binoculars. As you look at it you’ll have to wonder how many of the other innumerable stars in the night sky might be related to our own.

Finding Open Star Clusters

When it comes to finding open star clusters in the night sky, you won’t have to work very hard. Many can be seen with the naked eye under suitably dark conditions. In fact, you have likely seen some already. In the fall, the Seven Sisters are easily visible in the constellation Taurus. A bright collection of young stars not that far from Earth, they stirred the imaginations of the ancients centuries ago. In the spring, the easiest way to find the constellation Cancer is to look for the hazy patch between Leo and Gemini. That haze viewed in a pair of binoculars becomes the Beehive Cluster. It, too, was known in antiquity. The cluster contains about 200 stars, roughly 80 of which make a fine sight in a pair of binoculars.

Bright bluish young stars standout against a background of fainter more distant stars. A nebulous haze glows faintly around them, suggesting this is a very young cluster of stars.

The Pleiades open star cluster, imaged during a Kitt Peak Astrophotography Workshop, with Casey Good

There are many other open clusters to be found by just following the Milky Way across the sky. It is in the arms of the galaxy that these clusters abound, because that is where the star-forming material is located. As you seek them out, think about what you are seeing. All those stars clustered together for hundreds of millions of years, yet destined to go their own ways, as did our Sun. In the meantime, these stellar gems offer splendid viewing in binoculars or telescopes, and fertile ground for your imagination.

—Robert Wilson, Kitt Peak Visitor Center Guide

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To help prevent the spread of the virus causing Covid-19, we have closed the observatory and the mountain road to the public, and cancelled all upcoming public programs. Please do not travel up the mountain road for any reason, or by any means. If you are scheduled for programs in the next few months, please check back before traveling to Arizona, or traveling up the mountain.

Headlights Are Not Allowed
on Kitt Peak After Dark

Kitt Peak is first and foremost a research facility. To avoid interfering with research during nighttime programs, the Visitor Center adheres to strict lighting control procedures. 

Your vehicle’s headlights will be covered. When you leave after any evening program, you will be guided by an observatory vehicle for one mile down the mountain road.  After that first mile, staff will remove the headlight covers for you. 

Guests may not leave the programs early except in cases of genuine emergency.  By making a reservation in any of our nighttime programs, you indicate your understanding of and willingness to abide by these procedures. 

No one may visit the observatory after 4:00 P.M. unless registered for a nighttime program.