Messier 44, NGC 2632 - Beehive, Praesepe

Messier 44, also known as the Beehive or Praesepe, is an open cluster in Cancer known since ancient times. Visible to the naked eye, is one of the nearest open clusters to our Solar System. The cluster's age and proper motion coincide with those of the Hyades open cluster, suggesting that both share a similar origin.

History and Mythology

The Greek poet Aratos mentioned this object as "Little Mist" in 260 B.C., and Hipparchus included it in his star catalog as the "Little Cloud" or "Cloudy Star" in 130 B.C. The classical Greek astronomer Ptolemy described it in his Almagest as the "Nebulous Mass in the Breast of Cancer"; to the Greeks, it was known as Phatne.

The Latin translation for Phatne is Praesepe, which means "manger". The ancient Greeks and Romans saw this cluster as a manger from which two donkeys, the adjacent stars Asellus Borealis (γ Cnc) and Asellus Australis (δ Cnc) are eating. Eratosthenes reported that these two "Aselli" (asses) are the donkeys which the gods Dionysos and Silenus rode into battle against the Titans. Their braying frightened the Titans, enabling the gods to win; as a reward, they were placed in sky, along with their Phatne.

Ancient Chinese skywatchers saw this nebulous object as a ghost or demon riding in a carriage, and likened its appearance to a "cloud of pollen blown from willows." It is the main celestial object in the 23rd lunar mansion (Hsiu Kuei or Xiu Gui) of ancient Chinese astrology.

The Praesepe appeared on Johann Bayer's charts as "Nubilum" ("Cloudy") around 1600. But its true nature remained a mystery until Galileo looked at the cluster through his newly-invented telescope, in 1609. He reported: "The nebula called Praesepe is not one star only, but a mass of more than 40 small stars."

Charles Messier added the Praesepe as the 44th object in his catalog on March 4, 1769. Messier's inclusion of the Praesepe is curious, as most of Messier's objects were much more easily confused with comets. Messier may simply have wanted a larger catalog than his scientific rival Lacaille.

The origin of the cluster's other popular name, the Beehive, is uncertain.

Amateur Observation

The Beehive is most easily observed during evenings from February to May, when Cancer is high in the sky. To the naked eye it looks like a fuzzy spot. M 44 cannot be resolved with the unaided eye, but with binoculars or a small telescope, one can count at least 75 stars. At 95' across, it fits well into the field of view of such instruments.

The cluster has a visual magnitude of 3.7. Its brightest members are yellow-white stars of magnitude 6 to 6.5; they include 42 Cancri, the eclipsing binary TX Cancri, the metal-line star Epsilon Cancri, several Delta Scuti-class variables of magnitudes 7 to 8, and one peculiar blue star.

Larger telescopes reveal that more than 200 of the 350 stars in the region of sky covered by the Praesepe are actually members, confirmed by their common motion. Others are foreground or background stars, or may not yet have had their membership determined.

Properties and Evolution

The cluster's distance, as measured by the Hipparcos satellite, is 577 light years - well within the recent consensus of 520 - 610 light years. The cluster's core radius is estimated at 11.4 light years; its half-mass radius is 12.7 light years, and its tidal radius about 39 light years. However, the tidal radius also includes many stars that are merely passing through, and not true cluster members.

Altogether, the Beehive probably contains at least 1000 members, with a total mass of 500 - 600 Suns. Of these, 68% are M dwarfs, 30% are sunlike F, G, and K stars, and about 2% are bright class A stars. Also present are five giants (four of spectral class K0 III, and a fifth of class G0 III). So far, eleven white dwarfs have been identified. These represent the final evolutionary phase of the cluster's most massive members, which originally belonged to spectral class B. Brown dwarfs, however, are extremely rare in this cluster, probably lost by tidal stripping from its halo.

The age of the Praesepe is estimated at 600 to 730 million years. This is very similar to the age of another famous naked-eye cluster, the Hyades in Taurus (estimated at 625 to 790 million years old). Interestingly, these two clusters also have a very similar direction of proper motion. Although they are now separated by hundreds of light-years, they probably share a common origin in some great diffuse gaseous nebula. This would also explain the similarity of their stellar populations.