09 December 2015 by Brian Jones
Observers at mid-northern latitudes will find the distinctive form of Cassiopeia at or near the overhead point during autumn and early-winter evenings. Her husband Cepheus can be seen immediately to her west with the prominent line of bright stars forming Andromeda found a little way to the south of Cassiopeia. According to legend, Andromeda was the daughter of King Cepheus and Queen Cassiopeia of Ethiopia.
The westernmost star in Andromeda is Sirrah, which also marks the North Eastern corner of the adjoining and very conspicuous quadrilateral of stars forming the Square of Pegasus (the Winged Horse). Pegasus is one of the largest constellations in the entire sky and is a striking feature of the night sky at this time of year.
Keep an eye out in December
December evenings see Andromeda high in the southern sky, not far from the overhead point and stretching out from the north eastern corner of the Square of Pegasus. The three main stars in the group – Sirrah, Mirach and Almach – are all of roughly the same apparent brightness. Sirrah, whose name is derived from the Arabic ‘surrat al-farras’ meaning ‘the Horse’s Navel’, shines from a distance of around 100 light years and measurements reveal that its true luminosity is roughly 160 times that of our own Sun. The origins of its name, which allude to a particular part of equine anatomy, hark back to a time when Sirrah was considered as being a member of the adjoining constellation Pegasus.
A little to the east of Sirrah, and somewhat further away from us in space, is the red giant star Mirach, which lies at a distance of just under 200 light years. The name of this star is loosely derived from the Arabic ‘al-mi’zar’ meaning ‘the Girdle’ or ‘the Waist Cloth’. Lying near the eastern border of Andromeda is Almach, which forms one of the most beautiful double stars in the night sky. Shining from a distance of a little over 350 light years, the yellowish and greenish-blue components of Almach are resolvable through small telescopes.
A celestial showpiece
Our main focus for now, however, is one of the true celestial showpieces. Located near the northern border of Andromeda is the Great Andromeda Galaxy, which can be tracked down by following a line from Mirach northwards through the two fainter stars Mu and Nu as shown here. Providing the sky is really dark, clear and moonless, this object can be seen as a faint and extended misty patch of light just to the west of Nu. When viewed carefully through good binoculars, M31 can be seen to span a section of sky several times wider than a full Moon.
Both the Andromeda Galaxy and our own Milky Way Galaxy are spiral in shape, although the Andromeda Galaxy is roughly half as big again as our own. To the naked eye this object reveals itself as a faint, misty patch of light and it has often been described as being cloud-like in appearance. Indeed, as long ago as the 10th century Persian astronomers referred to it as ‘the Little Cloud’.
The Andromeda Galaxy is also known as Messier 31, or M31, this being the number it has in the catalogue of star clusters, nebulae and galaxies compiled during the 18th century by the French astronomer Charles Messier who, believing it to be a nebula, described its visual appearance as a ‘. . . beautiful nebula, shaped like a spindle . . . ’.
Photo: Adam Evans
The only galaxy visible to the naked eye
This huge island universe is separated from us by a vast gulf of intergalactic space. Located at a distance of around 2.5 million light years, M31 is the only galaxy visible in northern skies without optical aid and is the most distant object visible to the unaided eye. Modern telescopes can detect millions of galaxies, the vast majority of which are either merely referred to by catalogue numbers or remain nameless altogether. Only a few possess proper names, that of the Great Andromeda Galaxy being one of them. The fact that this galaxy is named after a beautiful mythological princess adds to its overall appeal, and its position high in the sky during the autumn and early-winter months ensures its easy visual accessibility, confirming its role as a definite and eagerly-anticipated target for the backyard astronomer.
Prior to the 20th century the Great Andromeda Galaxy, along with other objects of its type, had been thought of as nothing more than huge gas clouds located within the confines of the Milky Way. In the 1920s, however, the American astronomer Edwin Powell Hubble proved that M31 was a galaxy in its own right and was situated well outside our Milky Way. By proving the existence of galaxies beyond our own, Hubble’s work dramatically altered our understanding and concept of the universe.
Why not seek out the Andromeda Galaxy for yourself? A dark and moonless night should help you in your search for this wonderful object.
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