Shallow Sky Object of the Month: M44

Original article appears in GuideStar February, 2012.

Beehive Cluster Object: M44—The Beehive
Class: Open Cluster
Constellation: Cancer
Magnitude: 3.1
R.A.: 8 h, 39 m, 57 s
Dec: 19 degrees, 40 m, 21 s
Optics needed: Visible with the unaided eye on a dark night at a dark site. Easily seen with binoculars or a small wide-field telescope

If I’m out with my binoculars, and the constellation Cancer is up (although not always visible), I look for the Beehive. It’s an easy to find open cluster, not in the Milky Way, so it’s not is a busy star field. The cluster was observed as early as 260 BC and later by Galileo in 1609. It was Galileo who identified the object as a collection of stars and he was said to have identified 40 stars in the cluster.

My visual impression of the cluster is that it is a collection of reasonably bright stars with several stars arranged in small triangles. Once you’ve seen this cluster you won’t confuse it with any other.

This cluster is 577 light years away from us and is middle aged, as these objects go. Various estimates put the age of the cluster at about 600 million years. This is far to young to have any solar mass end-of-life stars included in the cluster.

Some of the information I found indicates that there are red giants in this cluster, and some photographs show red stars in the cluster. (Can you see these?) The cluster is old enough that some red giant stars, arising from high mass (live fast, die young) stars would exist. This seems consistent with commonly understood stellar evolution.

Other information indicates that white dwarf stars exist in the cluster. White dwarf stars are the near-final state of low mass (under 8 solar mass) stars. A one-solar-mass star, like our Sun, with a 10 billion year lifetime would not have had enough time to become a white dwarf in a 600 million year old cluster.

I asked Dr. Mike Inglis, the author of several books for amateur astronomers including Observer’s Guide to Stellar Evolution if he could explain the presence of white dwarfs in M44.

The answer from Mike is that only more-massive low-mass stars (approximately 3.2 to 8, or maybe 10 solar masses) could have evolved to white dwarfs in the lifetime of the cluster. Stars over 10 solar masses do not end their lives as a white dwarf.

Mike provided me with the following equation:

Stellar lifetime = 1010/M2.5 years

Where: M=the mass of the star in solar masses

I had never seen this little equation before. Not surprisingly, the lifetime a one-solar-mass star (the Sun) is 10 billion years, the generally accepted number.

A 4 solar mass star, easily small enough to be called low-mass, has a lifetime of only 312 million years and could have evolved to a white dwarf in M44.