Early spring’s slogan is ‘bring on the galaxies’, but with so many to choose from where should you start to look? You could do much worse than to begin with magnificent Messier 81 (NGC 3031) in Ursa Major, occasionally referred to a ‘Bode’s Galaxy’, named for the eighteenth-century German astronomer Johann Elert Bode who discovered it in 1774.
Messier 81 is a classic, grand-design spiral galaxy that, shining with an integrated magnitude of +6.8, is bright enough for a pair of binoculars to entrap, and can be studied through a medium-aperture telescope. Furthermore, Messier 81 has the close company of Messier 82, an interacting edge-on galaxy that provides a picturesque pairing that’s the most sought after in the entire sky.
Messier 81 is not an especially large galaxy, having a physical diameter of 92,000 light years. However, it offers observers a very generous apparent diameter of 26’ x 14’, tilted at an oblique angle to our line of sight. This makes it one of the largest galaxies in the sky, owing to its close proximity to us: cosmologically speaking, a mere 12 million light years away.
Messier 81’s morphological classification (Hubble/de Vaucouleurs) is SA(s)ab, indicating that no central bar is present and that its spiral arms are not that tightly wound. Deep images, including many superb examples shot by amateur astronomers, show M81 as beautiful and graceful, consisting of quite a large central region dominated by older stars, which give it a tell-tale yellow cast. Well seen too are its two bluish, well-defined but thin spiral arms, which are dotted with numerous pinky-red H-II regions and host over 100 star clusters, home to many young, hot stars that have formed in the past few million years.
Messier 81 gives its name to the M81 Group, a gathering of over 30 galaxies, including M82, which form the nearest galaxy group to our Local Group.
Messier 81 and 82 are strongly interacting and astronomers believe that every hundred million years or so M81 makes a run at M82. Messier 81 is much the more massive of the pair, ten-times more astronomer’s think, and causes destructive havoc by triggering strong episodes of star formation in M82. The larger galaxy doesn’t come away unscathed from these encounters though, as there’s possible evidence for accelerated star formation in M81 following a coming together some 300 million years ago. Its eastern spiral arm has been deformed by an encounter only 10 million years ago.
Messier 81 is an observational gem, but it’s a little tricky to find. From Phecda (gamma [γ] Ursae Majoris [UMa]) and Dubhe (alpha [α] UMa), two of the Plough’s ‘pan’ stars, imagine a diagonal line drawn between these two stars and extend it the same distance again away from Dubhe; you should land close to M81. In early March, M81 culminates at about 11pm GMT at the dizzy height of 72 degrees altitude, placing it advantageously high overhead. In fact, M81 never sets from UK shores (is circumpolar), so it can be observed for long periods.
Under good conditions on a moonless night, a telescope in the 80–100mm (~three- to four-inch) class working at low powers will reveal the galaxy central core engulfed in an oval-shaped haze some 15’ x 7’ in apparent diameter. Its low surface brightness, an affliction common to the majority of galaxies, can make M81 tough to spot in a suburban sky or in hazy conditions. Nearby M82 can be seen as pencil-shaped, offering a very condensed core at high power. You’ll need to upgrade to the light grasp of perhaps a 400mm (16-inch) telescope, working at low power, to catch a glimpse of M81’s smooth and subtle spiral arms.
Imagers should carefully scrutinise their work in case M81 (or M82 for that matter) produces a stunning supernova. Nearly 30 years ago, Messier 81 was the centre of attention owing to the sudden appearance of supernova 1993J, a type-IIb supernova that peaked at magnitude +10.5 and thrilled astronomers across the globe.
Images show M81 with a yellowish central region, dominated by old stars, and two bluish, sweeping and well-defined spiral arms dotted by numerous pinky-red HII regions, sites of ongoing star formation of young hot stars.