Find the Beehive Cluster

An Introduction to Open Clusters

A finder chart for the Beehive Cluster in Cancer, the Crab. Made using Stellarium open source charting software. A larger view can be found at Carolyn Collins Petersen

Cancer: Home of the Beehive Cluster

Stargazing is part observation and part planning. No matter what time of year it is, you always have something cool to look at or you are planning out your future observations. Amateurs are always plotting their next conquest of a difficult-to-spot nebula or the first view of an old favorite star cluster.

Take the Beehive Cluster, for example. It's in the constellation Cancer, the Crab, which is a zodiac constellation that lies along the ecliptic, which is the apparent path of the Sun across the sky throughout the year.

 This means that Cancer is visible for most observers in both the northern and southern hemispheres in the evening sky from late winter from about January through May. Then it disappears in the glare of the Sun for a few months before showing up in the early morning sky beginning in September. 

Beehive Specs

The Beehive is a little star cluster with the formal Latin name "Praesepe", which means "the manger". It's just barely a naked-eye object, and looks like a fluffy little cloud. You need a really good dark-sky site and reasonably low humidity to see it without using binoculars. Any good pair of 7×50 or 10×50 binoculars will work, and will show you a dozen or two stars in the cluster. When you look at the Beehive, you see stars that are about 600 light-years away from us. 

There are about a thousand stars in the Beehive, some similar to the Sun. Many are red giants and white dwarfs, which are older than the rest of the stars in the cluster.

The cluster itself is about 600 million years old.

One of the interesting things about the Beehive is that it has very few massive, hot, bright stars. We know that the brightest, hottest and most massive stars usually last anywhere from ten to several hundred million years before they explode as supernovae.

Since the stars we DO see in the cluster are older than this, either it lost all its massive members already, or perhaps it didn't start with many (or any).

Open Clusters

Open clusters are found throughout our galaxy. They usually contain up to a few thousand stars that were all born in the same cloud of gas and dust, which makes most of the stars in a given cluster roughly the same age. The stars in an open cluster are mutually gravitationally attracted to other when they first form, but as they travel through the galaxy, that attraction can be disrupted by passing stars and clusters. Eventually, an open cluster's stars move so far apart that it disintegrates and its stars are scattered to the galaxy. There are several known "moving associations" of stars that used to be open clusters. These stars are moving at roughly the same velocity but are not gravitationally bound in any way. Eventually they, too, will wander on their own paths through the galaxy. The best examples of other open clusters are the Pleiades and the Hyades, in the constellation Taurus.