Messier 33: The Triangulum Galaxy

Messier 33, the Triangulum Galaxy, is one of the nearest spiral galaxies and one of the few that can be seen without a telescope. It is also one of the most misleading objects for beginners: its magnitude suggests an easy target, but its light is spread over an area larger than the full Moon, so it needs a dark sky.

What it is

M33 is a spiral galaxy seen nearly face-on, about 3 million light-years away. It is the third-largest member of the Local Group, the small family of galaxies that also includes our Milky Way and the Andromeda Galaxy, M31. Its 73′ by 45′ apparent size works out to roughly 60,000 light-years across the long axis, a bit over half the size usually given for the Milky Way's disk. Its loose spiral arms are full of star-forming regions. The largest of them, NGC 604, is one of the biggest known stellar nurseries in the Local Group and can be seen in amateur telescopes as a separate knot of light.

When to see it

M33 is an autumn object. From about 40°N it is highest in the south around midnight in mid-October and around 10 p.m. in mid-November, climbing to about 81° above the horizon, nearly overhead. It stays in the evening sky into January. From the Southern Hemisphere it is low in the northern sky and much harder to see well.

How to find it

Start at Mirach (Beta Andromedae), the 2nd-magnitude star you also use to find M31. Then find Alpha Trianguli, the 3.4-magnitude star at the narrow tip of the small triangle of Triangulum, about 11° southeast of Mirach. M33 lies roughly between the two, about 7° southeast of Mirach and about 4° west-northwest of Alpha Trianguli, a little south of the midpoint. Use the lowest power you have, because the galaxy is too big and faint for a narrow field.

What you will see

Under a truly dark sky, keen-eyed observers can glimpse M33 with the naked eye using averted vision; it is a classic test of sky quality. In binoculars from a dark site it is a large, pale, oval glow, often easier to see than in a telescope. A 70–100 mm telescope at low power shows a soft glow with a slightly brighter center; from suburban skies it may not show at all. A 200 mm or larger telescope under dark skies shows hints of the spiral arms and the bright knot of NGC 604 near the edge of the galaxy. No filter helps the galaxy itself, though a UHC filter can make NGC 604 stand out.

History

The Sicilian astronomer Giovanni Battista Hodierna recorded M33 before 1654. Charles Messier found it independently in 1764. In the 1920s, Edwin Hubble identified Cepheid variable stars in M33 as well as in M31, which helped show that these "spiral nebulae" are separate galaxies far beyond the Milky Way.