Map showing the location of M44 in the constellation of Cancer
TheBeehive Cluster (also known asPraesepe (Latin for "manger", "cot" or "crib"),M44,NGC 2632, orCr 189), is anopen cluster in the constellationCancer. One of the nearest open clusters toEarth, it contains a larger population of stars than other nearby bright open clusters holding around 1,000stars. Under dark skies, the Beehive Cluster looks like a smallnebulous object to the naked eye, and has been known since ancient times. Classical astronomerPtolemy described it as a "nebulous mass in the breast of Cancer". It was among the first objects thatGalileo studied with histelescope.[3]
Age and proper motion coincide with those of theHyades, suggesting they may share similar origins.[4][5] Both clusters also containred giants andwhite dwarfs, which represent later stages of stellar evolution, along with manymain sequence stars.
Distance to M44 is often cited to be between 160 and 187parsecs (520–610light years),[6][7][8] but the revisedHipparcos parallaxes (2009) for Praesepe members and the latest infraredcolor-magnitude diagram favors an analogous distance of 182 pc.[9][10] There are better age estimates of around 600 million years[5][7][11] (compared to about 625 million years for the Hyades).[12] The diameter of the bright inner cluster core is about 7.0 parsecs (23 light years).[11]
At 1.5° across, the cluster easily fits within the field of view of binoculars or low-powered small telescopes.Regulus,Castor, andPollux areguide stars.
In 1609,Galileo first telescopically observed the Beehive and was able to resolve it into 40 stars.Charles Messier added it tohis famous catalog in 1769 after precisely measuring its position in the sky. Along with theOrion Nebula and thePleiades cluster, Messier's inclusion of the Beehive has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets. Another possibility is that Messier simply wanted to have a larger catalog than his scientific rivalLacaille, whose 1755 catalog contained 42 objects, and so he added some well-known bright objects to boost his list.[13]Wilhelm Schur, as director of theGöttingen Observatory, drew a map of the cluster in 1894.
Ancient Greeks and Romans saw this object as a manger from which two donkeys, the adjacent starsAsellus Borealis andAsellus Australis, are eating; these are the donkeys thatDionysos andSilenus rode into battle against theTitans.[14]
Hipparchus (c.130 BC) refers to the cluster asNephelion ("Little Cloud") in his star catalog.[15]Claudius Ptolemy'sAlmagest includes the Beehive Cluster as one of seven "nebulae" (four of which are real[16]), describing it as "The Nebulous Mass in the Breast (of Cancer)".[17]Aratus (c.260–270 BC) calls the clusterAchlus or "Little Mist" in his poemPhainomena.[15]
Johann Bayer showed the cluster as a nebulous star on hisUranometria atlas of 1603, and labeled it Epsilon. The letter is now applied specifically to the brightest star of the clusterEpsilon Cancri, of magnitude 6.29.[18]
This perceived nebulous object is in theGhost (Gui Xiu), the 23rdlunar mansion of ancient Chinese astrology. Ancient Chinese skywatchers saw this as a ghost or demon riding in a carriage and likened its appearance to a "cloud of pollen blown from willow catkins". It was also known by the somewhat less romantic name ofJishi qi (積屍氣, also transliteratedTseih She Ke), the "Exhalation of Piled-up Corpses".[15] It is also known simply as Jishi (積屍), "cumulative corpses".
Like manystar clusters of all kinds, Praesepe has experiencedmass segregation.[7][11][19] This means that bright massive stars are concentrated in the cluster's core, while dimmer and less massive stars populate its halo (sometimes called thecorona). The cluster's core radius is estimated at 3.5 parsecs (11.4 light years); its half-mass radius is about 3.9 parsecs (12.7 light years); and itstidal radius is about 12 parsecs (39 light years).[7][11] However, the tidal radius also includes many stars that are merely "passing through" and notbona fide cluster members.
Widefield image of the Beehive Cluster
Altogether, the cluster contains at least 1000 gravitationally bound stars, for a total mass of about 500–600 Solar masses.[7][11] A recent survey counts 1010 high-probability members, of which 68% areM dwarfs, 30% are Sun-like stars ofspectral classes F, G, and K, and about 2% are bright stars of spectral class A.[7] Also present are five giant stars, four of which have spectral class K0 III and the fifth G0 III.[4][7][20]
So far, elevenwhite dwarfs have been identified, representing the final evolutionary phase of the cluster's most massive stars, which originally belonged to spectral type B.[5]Brown dwarfs, however, are rare in this cluster,[21] probably because they have been lost by tidal stripping from the halo.[7] A brown dwarf has been found in theeclipsing binary system AD 3116.[22]
The cluster has a visual brightness of magnitude 3.7. Its brightest stars are blue-white and of magnitude 6 to 6.5.42 Cancri is a confirmed member.
In September 2012, two planets which orbit separate stars were discovered in the Beehive Cluster. The finding was significant for being the first planets detected orbiting stars likeEarth'sSun that were situated in stellar clusters. Planets had previously been detected in such clusters, but not orbiting stars like the Sun.[23]
The planets have been designatedPr0201 b andPr0211 b. The 'b' at the end of their names indicates that the bodies are planets. The discoveries are what have been termedhot Jupiters, massivegas giants that, unlike the planetJupiter, orbit very close to their parent stars.[23]
In 2016 additional observations found a second planet in thePr0211 system, Pr0211 c. This made Pr0211 the first multi-planet system to be discovered in an open cluster.[25]
TheKepler space telescope, in itsK2 mission, discovered planets around several more stars in the Beehive Cluster. The stars K2-95,[26] K2-100, K2-101, K2-102, K2-103, and K2-104[27] host a single planet each, and K2-264 has a two-planet system.[28]
^abKlein-Wassink, W.J. (1927). "The proper motion and the distance of the Praesepe cluster".Publications of the Kapteyn Astronomical Laboratory Groningen.41:1–48.Bibcode:1927PGro...41....1K.
