Constellations for May 2023 –

Ursa Major, Canes Venatici and Coma Berenices

The ancient constellation of the Great Bear (strictly speaking, a female bear) lies with its cub near the north pole, as the ancient Greeks associated bears with the north; hence the word ‘arctic’ from the Greek for ‘bear’: ‘arktos’. It is being chased by two hunting dogs, Canes Venatici in Latin, which were introduced by Hevelius in 1687. Coma Berenices is an ancient constellation which represented the tresses of hair given as a votive offering by Queen Berenices II of Egypt.   

The constellation of Ursa Major is much larger than the Plough, which is simply an asterism. While it does contain some bright stars outside the Plough, it is easiest just to consider the Plough as representing Ursa Major. Just above Dubhe towards Polaris and to its west are two bright galaxies [1] which can be seen in a light-polluted area like Havering. M81 is a spiral galaxy 12 million light years away, which is tilted; making it easier to observe ...

... M82 is an irregular galaxy (probably a spiral galaxy edge-on) which has an energetic star-making region at its centre. Called the Cigar Galaxy because of its shape, it is the same distance as M81, and the two galaxies are interacting. There was a supernova in M82 in 2014 (SN2014J) which was visible in a small telescope from Havering.  Ursa Major also contains M101 [2] above the star at the end of the Plough handle, Alkaid; but this large galaxy is seen face-on and, like M33, difficult to observe.

Below Alkaid and technically in Canes Venatici is the famous Whirlpool Galaxy, M51 [3]. It is easier to see, but is only ever a faint smudge in a small telescope; it is 31 million light years distant and hence much further away than M81 or M82. Ursa Major also has the bright double star Mizar in the Plough’s handle. Mizar forms a very wide double with Alcor, and some people can separate them with the naked eye. In a telescope, Mizar is a fairly close double with two bright white stars.

Between the bowl of the Plough and the rear of Leo there is a pair of fairly bright stars [4], Alula Borealis (Nu Ursae Majoris) and Alula Australis (Xi Ursae Majoris). The latter is a double, the two white stars are roughly the same brightness (4.3 and 4.8), but are only 2.3 arcseconds apart. Five degrees to the north-west of Alula Australis lies Lalande 21185 [5], the fourth closest star to us (8.3 light years away) and the brightest red dwarf, but it is still only magnitude 7.5.

There is only one bright star in Canes Venatici, Cor Caroli (named in honour of King Charles I), which is a nice double in a telescope with a bright white star and a dimmer blue star. Between Cor Caroli and Megrez in the Plough is the red carbon star and variable, La Superba [6] or Y Canum Venaticorum, which varies between 4.9 and 7.3. Between Cor Caroli and Phecda in the Plough lies M106 [7] – see above – which is a tilted spiral galaxy like M81 or M31. This makes it easier to observe but, as it is 24 million light years away, it is dimmer than M81. Finally, between Cor Caroli and Arcturus one finds the bright globular cluster M3 [8]. which is 34,000 light years distant.

There are no bright stars in Coma Berenices, but if you sweep down from the handle of the Plough (or Cor Caroli) and the horizon – or between the bowl of the Plough and Arcturus – with binoculars, you should see a huge star cluster which is the legendary tresses of Berenices – or Melotte 111 [9], to give its scientific name. It is about 450 million years old (so between the Pleiades and the Hyades in age), and 280 light years away.