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2. The Universe / Space, Stars and Galaxies / Constellations / Constellations: Cancer 'the Crab'
The Beehive Star Cluster (Praesepe)
The nebula called Praesepe, which is not one star only, but a mass of more than 40 small stars.
The Beehive Cluster is the modern name for Praesepe1, which is Latin for 'manger' or 'crib'. It is an open cluster of stars in the constellation of Cancer 'the crab', but it was once considered a nebula before Galileo turned his telescope upon it and identified the individual stars. Historically the Beehive Cluster has been recognised by ancient cultures such as the Chinese, who called it Tseih She Ke (Exhalation of Piled-up Corpses). The Beehive Cluster was also registered by many eminent astronomers, who all either described its appearance, gave it a name or assigned it some history, including:
The ancient Greeks used the Beehive Cluster and its attendants as a weather forecaster – recorded by Aratos of Soli in the Prognostica2 – 'A murky manger with both stars [gamma and delta] shining unaltered is a sign of rain.' What this means is when condensation (rain clouds) begins gathering in the sky, then the two stars are still visible but the cluster is not.
Some cultures claim the Beehive Cluster honours the place where the child of the Virgin Mary was born in a stable in Bethlehem. The cluster is externally flanked by gamma Cancri (Asellus Borealis - 'little donkey of the north') and delta Cancri (Asellus Australis - 'little donkey of the south'), which represent two animals of the Biblical Nativity story.
French astronomer Charles Messier listed the Beehive Cluster in his famous catalogue of 'non-comets' as Messier 44 in March 1769. It also appears in the New General Catalogue compiled by John Louis Emil Dreyer, director of the Armagh Observatory from 1882 to 1916, as NGC 2632.
The US state of Utah has a beehive3 for its emblem; one appears in the middle of the State Flag and it also features on the State Seal, earning Utah the popular, though unofficial, nickname 'The Beehive State'. So in 1996 the Beehive Cluster was chosen to be the State Astronomical Symbol.
This symbol, composed of a hive of stars, transposes our beehive symbol to a new and grand level as we enter our second century as a group of people living in a place where we can still see, with our own eyes, the beautiful and dim features of the starry universe.
Stars of the Beehive Cluster
The stars of the cluster were created approximately 700 million years ago, before even the dinosaurs ruled here on Earth. This makes the stars younger than our own Sun but the Beehive Cluster itself one of the oldest star clusters in our galaxy. The brightest stars range in magnitude from +6 to +6.5. Not all of the components of the Beehive Cluster are solo stars; a bright multiple system with an apparent magnitude of +6.39 is 39 Cancri4, a collection of six individual stars at various stages of their evolution, ranging from white dwarfs to orange giants.
Some of the stars have variable magnitudes and have been given variable star catalogue designations5, the brightest of which are BT Cancri (+6.66v), BU Cancri (+7.67v), BN Cancri (+7.80v) and BX Cancri (+7.96v). Only one member of the Beehive Cluster has a Bayer designation6, the +6.3 magnitude epsilon Cancri (which also has a Flamsteed number of 41 Cancri). The significance of having a Bayer designation is that they are the main (brightest) stars of a constellation listed from alpha through omega. So with only one in the Beehive Cluster you know it's rather dimmer than the constellation. There is a white giant star member of the Beehive Cluster which has the symbolic Flamsteed number 42 Cancri.
Viewing the Beehive Cluster
At magnitude +3.7 the Beehive Cluster is easily identifiable on a clear night if you have rudimentary knowledge of the constellations — it lies within the heart of Cancer 'the crab'. Because Cancer is one of the dimmer constellations, the Beehive Cluster is actually more conspicuous than the stars of the constellation itself. Th best time to view is late evening in the winter months; locate the more striking constellations Gemini and Leo, and Cancer lies between them. Even if you can't make out the pattern, your eyes will be drawn to the smudge that is the Beehive Cluster.
When you're looking for a smudge in the sky, you can try using your peripheral vision. This is because the rods of the eye (which operate well in low level light) are predominant in the periphery, while the cones are predominant in the centre of the eye. You can try it — find some faint thing in the sky and stare directly at it and it will all but disappear. Now turn your head and it will show up like a beacon.
For the amateur astronomer wishing to see the Beehive Cluster in more detail, a low-magnification telescope will distinguish over 40 stars, but a larger optical aid will reveal over 200. Still, it's a magnificent sight in binoculars, especially when one of the planets happens to be passing through or at least providing a close encounter for us denizens of Earth. This is, of course, only a line-of-sight encounter; the planets traversing the cluster are much closer to us than the stars within the Beehive Cluster, which are around 540-580 light years7 distant. Between and beyond those stars it's possible to locate far away galaxies, millions of light years beyond our own. This kind of deep-sky viewing would require a grand scale telescope though, as the brightest of the galaxies beyond M44 are only magnitude +13.8 and dimmer. Some of the galaxies are on the New General Catalogue8, so have individual NGC numbers, eg: NGC 2647, NGC 2643, NGC 2637, NGC 2625 and NGC 2624. Later discoveries include: IC 2388, PGC 24284, PGC 24335 and UGC 4526.
The constellation of Cancer is on the ecliptic plane9, therefore some of its stars and features such as the Beehive Cluster can form alignments and occultations with the Moon. This is one time when the Moon is not your friend, as the brightness of the Moon would render the remaining viewable stars of the Beehive Cluster all-but invisible. However, astrophotographers can use equipment and techniques to lower the glare and provide spectacular shots which are then shared with mere mortals via the Astronomy Picture of the Day website.
A Comet Spears the Beehive
In 2004 the Beehive Cluster was speared by a comet, providing astrophotographers with an opportunity to capture a once-in-a-lifetime spectacular image. Comet NEAT (also known as Comet Q4 and its full designation C/2001 Q4 NEAT) is a non-periodic visitor to the inner Solar System, which means they are unpredictable10. Comet NEAT was discovered in 2002 and imaged in February 2003 by the Solar and Heliospheric Observatory (SOHO) spacecraft11. The comet was actually filmed being struck by a solar eruption known as a CME (coronal mass ejection) on 18 February, 2003. It then swung back around the Sun and from our perspective it appeared to shoot through the Beehive Cluster in May 2004 before heading back to the frigid depths of the outer Solar System, where it will spend 37,000 years out of sight of human eyes.
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