Messier 101: Pinwheel Galaxy

The Pinwheel Galaxy is a big, face-on spiral just off the end of the Big Dipper's handle. On paper it looks easy: its total brightness is close to magnitude 8. But that light is spread over an area almost as wide as the full Moon, so it is one of the most light-pollution-sensitive objects in the Messier list. Under a dark sky, though, it is a fine sight.

What it is

M101 is a large spiral galaxy that faces us almost squarely, so we look straight down onto its arms. It lies about 25 million light-years away, and its 22′ extent works out to roughly 160,000 light-years, considerably larger than the Milky Way. The arms are loose and slightly lopsided, probably pulled by gravitational encounters with smaller companion galaxies. They are dotted with giant star-forming regions, several bright enough to have their own NGC numbers. M101 has hosted two notable recent supernovae: SN 2011fe, a Type Ia explosion caught within hours of its start, and SN 2023ixf, a core-collapse supernova that became one of the closest of its kind in years. For the debate over whether M102 is a duplicate of M101, see that page.

When to see it

M101 is best in spring, highest around midnight in late April and high in the evening sky from April into June. From about 40°N it climbs to roughly 76° and never sets, though it is low in the north on autumn and winter evenings. From the Southern Hemisphere it barely clears the northern horizon from around 30°S and is not visible farther south.

How to find it

Use the last two stars of the Big Dipper's handle, Mizar and Alkaid. M101 forms a nearly equilateral triangle with them, on the northeast side of the line joining them: it is about 5.7° due east of Mizar and 5.6° north-northeast of Alkaid. Sweep slowly with binoculars or a finder; you are looking for a large, faint glow rather than a point.

What you will see

In binoculars under a dark sky, M101 shows as a large, round, very faint glow with a slightly brighter center. From a suburban site it can disappear completely, and many observers have searched for it and missed it because they expected something compact. A 70–100 mm telescope at low power shows the bright core and a broad, faint halo, with hints of mottling on a good night. A 200 mm or larger telescope under a dark sky begins to show the arms as patchy curves and picks out the brightest star-forming knots, such as NGC 5461 and NGC 5462, as small hazy spots. Filters do not help with the galaxy itself.

History

Pierre Méchain discovered M101 on March 27, 1781, and Messier added it to his catalog soon after. In the 1840s Lord Rosse saw its spiral form through his large telescope in Ireland, making it one of the earliest known "spiral nebulae."