Also commonly known as Praesepe (Latin: manger), M44 is a large open cluster in the constellation Cancer, and among the closest star clusters to Earth. As far back as the 2nd century AD, Ptolemy referred to it as “[the] nebulous mass in the breast of Cancer”, no doubt due to the fact that to the naked eye under dark skies, the cluster appears as a large fuzzy object. Praesepe is also among the first objects Galileo is known to have studied, with his primitive instruments having managed to resolve as many as 40 stars in the cluster.
• Constellation: Cancer
• Coordinates: RA 08h 40.4m|Dec. 19° 59″
• Distance: 577 light years (Estimated)
• Cluster Type: Open cluster
• Mass: 500-600 solar masses
• Radius: 11.4 light years (Inner core radius)
• Apparent Magnitude: +3.7
• Age: 600-700 million years
• Other designations: Praesepe, NGC 2632
While Cancer is not a particularly conspicuous constellation, finding M44 close to the centre of the constellation is not difficult. Simply look for the cluster roughly halfway between the bright stars Regulus in Leo to its east, and Pollux in Gemini to its west. M44 is best viewed from northern latitudes during February and May, when it is high above the horizon. At low power, M44 easily fits within the FOV of almost any telescope, being 95 minutes of arc in diameter.
The facts that M44 and the Hyades cluster in Taurus share a common proper motion and are of a similar age, suggests that both clusters share a common, or at least similar origin. Both star clusters are also alike in composition, and contain white dwarfs, red giants, and main sequence stars of the A, F, G, K, and M classes, which further suggest a common evolutionary history.
All told, M44 contains at least 1,000 confirmed members with a combined mass of between 500 and 600 times that of the Sun. In terms of its actual make-up, M44 consists of about 680 M-type dwarf stars, about 300 or so Sun-like types F, G, and K stars, with the rest being A-class stars. The cluster also contains five giant stars, four being type-A stars, and the fifth being a G0II-class star. To date, eleven white dwarfs have been identified within the cluster, all of which are thought to have had massive, B-type stars as progenitors. However, there are not many brown dwarfs in M44, probably because many that were present have been stripped from the halo by tidal effects.
Like most star clusters of all types, M44 has also experienced a process of mass segregation. In this process, more massive stars “sink” to the centre, or core, of a cluster, while lighter, less massive stars tend to accumulate around the fringes, or halo surrounding the core. This process has produced a core that spans a distance of 22.8 light years, and with a tidal radius of 39 light years. However, the tidal radius includes stars that are not proper members of the cluster, in the sense that they are merely “passing through” the clusters’ neighborhood.
Recent studies by astronomers at Leicester University and Queen’s University in Belfast suggest that M44 is actually composed of the stars of two separate clusters that are in the process of colliding, which would explain the high star count of the cluster that is significantly greater than in most other open clusters. The studies also suggest that M44 will break up and fly apart within the next 10 million years or so, due to the energy of the collision that is now causing both clusters to break up and dissipate.
During 2012, two stars hosting one planet each were discovered within M44, and while it has long been known that planets do occur within star clusters, these two examples were the first planets to be found orbiting Sun-like stars within such structures. Both planets, designated Pr0201b and Pr0211b, are “Hot Jupiters”, or massive gas planets that orbit their host stars in very tight orbits. However, follow-up investigations in 2016 have revealed that the Pr0211 system in fact consists of two planets, the second planet having the designation Pr0211-c.
Much had been written about the exact reason why Charles Messier included this cluster in his catalogue in 1769, since most of Messiers’ catalogue at that time listed much fainter objects. One theory holds that Messiers’ simply wanted a more comprehensive catalogue than the 42-object list that his scientific arch-rival, Nicolas Louis de La Caille, had published in 1755.
Before this rivalry, though, the ancient Greeks and Romans saw in the cluster a manger from which the two donkeys that carried Dionysos and Silenus into battle against the Titans, are feeding. The two donkeys are represented by the adjacent stars Asellus Borealis and Asellus Australis. Prior to this, Hipparchus (c.130 BC) described the cluster as “[the] Little Cloud”, while more than a hundred years earlier, Aratus referred to it as “Little Mist” (Achlus) in his famous poem, Phainomaina.
To ancient Chinese astrologers, M44 was a part of [the] Ghost, which was the 23rd lunar mansion. To these ancient astrologers, M44 was a demon or ghost travelling in a carriage that resembled a “cloud of pollen blown from willow catkins.” Less flatteringly though, M44 was also known to ancient Chinese observers as Jishi qi, or sometimes as Tseih She Ke, which translates into “Exhalation of Piled-up Corpses”. A similar name for M44 in Chinese, Jishi, simply means “cumulative corpses”.