When: throughout the night all month
What’s special: Messier 81 (NGC 3031) in Ursa Major is one of the finest galaxies in the entire sky. Outside of the Local Group galaxies M31 and M33, it’s the brightest galaxy in Messier’s catalogue, shining with an integrated-magnitude of +6.8, easily bright enough for a pair of binoculars to track it down. Known also as Bode’s Galaxy, for its discoverer the German astronomer Johann Elert Bode, it lies a mere 12 million light years away, so it spans a substantial 27 ×14 arcminutes across at its fullest extent
If all this positivity wasn’t enough, the presence of Messier 82, a very close and interacting edgeon companion placed just over half a degree to the north, makes the picturesque pairing the most sought after in the entire sky.
How to observe: M81 is easy to see but a little tricky to find. You could try using two of the stars that make up the ‘pan’ of the Plough, Phad (gamma [γ] Ursae Majoris), which marks the south-eastern (bottom left) corner, and Dubhe (alpha [α] Ursae Majoris), the Plough’s brightest star, to the north-west. Imagine a diagonal line drawn from Phad to Dubhe and extending the same distance away from Dubhe, and you should land in the vicinity of M81.
Common to all spiral galaxies that are presented to us close to or fully face-on, a low surface brightness can make M81 tough to spot in a suburban sky or in hazy conditions. Under good conditions on a moonless night, a telescope in the 75–100mm (~three- to four-inch) class working at a low power will reveal the central core of the galaxy engulfed in an oval-shaped haze. Nearby M82 can be seen as pencil-shaped and offering a very condensed core at high power. You’ll need the light grasp of perhaps a 400mm (16-inch) telescope, working at low power, to begin to trace M81’s smooth and subtle spiral arms.
On April evening, M81 culminates just as darkness really takes hold, lying high overhead. From UK shores, it’s circumpolar (never sets) and is on show at a favourable altitude throughout the hours of darkness.
M81 and M82 are strongly interacting, the ten-times more massive M81 causing strong episodes of star formation in M82. There’s possible evidence for accelerated star formation in M81 too, following an encounter some 300 million years ago. M81 was in the news over 25 years ago with the sudden appearance of supernova 1993J, which peaked at magnitude +10.5.