Observing Project 13A Open Star Clusters

Let's start by taking a gaze at some of the sky's most attractive open clusters and what they offer to the stargazer as we look around the sky.

The sky's most beautiful open cluster and the brightest entry in Messier's catalogue (+1.6) is the Pleadies cluster (M45). In Chapter 1, we listed out in detail each of the seven brightest naked-eye stars. The Pleadies offer far more to see through the telescope. By some estimates, the cluster contains more than 500 stars. Using a low-power eyepiece, preferably a wide field type, see how many members of the Pleaides you can see with your telescope. Also as you look at the cluster, can you make out the bluish reflection nebula that surrounds the star cluster? The cluster's nebulosity remember is not part of the cluster's birth cloud but rather is a chance crossing between the cluster and a passing gas cloud, each of which exhibits a different proper motion. The cluster is Trumpler-type II, 3, r. The designation means detached (stands out from the background), a wide variety of stellar brightness and richly populated. Do you agree?

High in the sky as darkness falls in March is the open cluster M35, near the feet of Gemini. The cluster is just bright enough to be visible to the unaided eye under very dark skies. This cluster is approximately 2,800 light years distant and shines at magnitude +5.8. The cluster spans about 25 arc minutes. M35 is listed as Trumpler type III, 3, r. The cluster has a wide variety of star brightness and note the color of the stars. Many are yellow and orange, indicating that the cluster is of roughly average age for an open cluster, maybe about a hundred million years. It does not stand out as clearly from the background as do some of the others open clusters that we've discussed thus far. Use the widest field eyepiece you have available because M35 offers more treats if you're prepared and know what to look for and where it is. Just 15 minutes to the southwest of M35 is the small open cluster NGC 2158. This tiny cluster is only 5 arc minutes across and shines at magnitude +8.6. I estimate it as being Trumpler-type II, 1, r. It is so compact that at one point it was actually considered to be a globular. The cluster is also far distant in the background at approximately 16,000 light years distant. The open cluster of young stars makes a fascinating contrast with the much older cluster of cooler stars in the distance. And there is yet more for owners of larger telescopes. More to the west of M35, by about 50 arc minutes is the open cluster IC 2157. The cluster is about the same size and brightness as NGC 2158 but is very star-poor and loosely associated. With a field of view of about 1.5 degrees and sufficient aperture, you should be able to see all three.

Rising nearly half-way to the zenith during the months of June and July in Serpens is the Eagle Nebula and forming from it is the open cluster M16. The nebula and cluster lie about 7,000 light years away in the next arm inward in the

Milky Way. M16 has a Trumpler classification of II, 3, m, n and with an angular diameter of about 7 minutes spans a linear distance of about 15 light years. The much larger nebula spans some 70 by 55 light years. The cluster is extremely young at an estimated age of less than six million years old. Star formation is still on going in the cluster and the nebula. This is one of the youngest formations you will ever lay eyes on in the sky. In a 4-inch telescope, you should be able to make out about twenty members of the cluster in a dark sky. The members of the cluster are extremely hot and bright, most of spectral type O6. This makes the cluster shine at an absolute magnitude of -8.6; perhaps the brightest cluster in the sky. The Eagle Nebula and M16 together is one of the heaven's most spectacular examples of babies leaving the womb and beginning their travels through the universe.

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