Magnificent Messier 81 spiral galaxy overhead in April
BY MARK ARMSTRONG
Posted: 12 April 2013
Ursa Major, the Great Bear, wheels high overhead on April evenings and is ripe for deep-sky enthusiasts to pick some juicy stuff. The great galaxy Messier 81 (NGC 3031) is probably the best deep-sky object amongst a plethora of possibilities, perhaps only rivaled by M101.
Bode is also credited with the discovery of M82, M53, and M92. M81's spiral structure eluded both William Herschel and Lord Rosse, taking the era of photography to provide sufficient resolution to reveal the 'nebula's' true nature.
M81 is big, bright and a great object for beginners looking for a chance to spot their first galaxy. Shining at mag. +7.9, some people have claimed naked-eye sightings at very dark sites butfrom our shores try big binoculars mounted on a tripod. Its apparent diameter is whopping 27 x 14 arcminutes, the whole extent of which is only really apparent in deep CCD images. This translates to a physical diameter of 92,000 light-years at its distance of 11.8 million light years, making it only slightly smaller than our own Milky Way galaxy but it's thought to be considerably less massive, weighing in at an estimated 50,000 million solar masses.
M81 is a good example of a 'grand design' spiral, with open, well-defined spiral arms; it's classified as a type-SA(s)ab. It gives its name to what is probably the nearest galaxy group to our own Local Group, with at least ten confirmed members which include its close physical companion M82, NGC 2976, NGC 3077 and NGC 2403 in Camelopardalis. M82, a starburst galaxy, is only 38 arcminutes north of M81 and together form the finest pairing in the Northern Hemisphere. Physically, they are only 125,000 light years apart, and the strongly enhanced star formation going on in M82 is undoubtably the result of its numerous close encounters every hundred million years or so with its much more massive companion.
Although it's an easy object for binoculars and small telescopes, a noticeably oval-shaped nebula with a bright, stellar-like core, to see any trace of the spiral arms will probably take at least an aperture in the 250-300-mm class. Visually, it extends to 20 arcminutes along its major-axis, orientated north-west by south-east. Amateurs have produced some magnificent images of M81 and even a moderate aperture telescope, coupled with a DSLR, will give good results. Be sure to check any digital images you take very carefully as this face-on monster is a prime candidate to host a supernova. In 1993, Spanish amateur Francisco Garcia Diaz discovered the magnificent type-II supernova designated 1993J. This great supernova peaked at mag. +10.5 and provided astronomers with valuable data. An early detection of another would be a great coup.
Dubhe (alpha UMa, mag. +1.8), the north-western, or top-right, star of the 'bowl' of the Big Dipper or Plough asterism, is a good starting point from which to locate M81. From Dubhe move seven degrees north in declination and decrease in Right Ascension by one hour and eight minutes towards Camelopardalis (its co-ordinates are R.A. 09h 55m 35.7s, Decl.+69° 03' 49").
From the UK, M81 has the benefit of being circumpolar and culminating close to the zenith at an extremely favourable 72 degrees altitude. On April nights, it can be observed as soon as it is dark and is still around 40 degrees up in the north-west when the morning twilight kills the night.
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