M33, also known as the Triangulum Galaxy or Pinwheel Galaxy, is a spiral galaxy in the constellation Triangulum, that's located at a distance of approximately 2.81 million light-years. It's the third largest member of the Local Group, which also includes our Milky Way galaxy, the Andromeda Galaxy (M31) and at least 50 other smaller galaxies.

A magnitude +5.7, M33 can be viewed with the naked eye from a dark site and is widely regarded as the most distant permanent object that can be seen without optical aid. There are rare reports that some eagle-eyed stargazers have managed, under exceptional conditions, to spot more distance M81 but that's incredible viewing by all respects.

M33 was probably discovered by Italian astronomer Giovanni Battista Hodierna before 1654. He listed it in his work regarding cometary orbits and admirable objects of the sky. Charles Messier independently re-discovered the galaxy on the night of August 25, 1764.

A good starting point to locate M33 is the Great Square of Pegasus. Start by focusing on its northeastern star Alpheratz (α And - mag. +2.1). Although it lies at one corner of the square, Alpheratz is officially designated as belonging to Andromeda. Next move 15 degrees northeast to red-giant star Mirach (β And). At mag. +2.1, Mirach is the same brightness as Alpheratz. Then imagine a line from Mirach moving in a southeasterly direction for 11 degrees until you reach alpha Tri (α Tri - mag. +3.4). This is the southernmost star of the Triangulum triangle and just over halfway along this line is M33.

The galaxy is best seen from Northern Hemisphere locations during the months of October, November and December.

M33 The Triangulum Galaxy (credit:- www.astronomie.be)

Finder Chart for M33 (credit:- freestarcharts)

Finder Chart for M33 - pdf format (credit:- freestarcharts)

Finder Chart for M74 (also shown M33) (credit:- freestarcharts)

Finder Chart for M74 (also shown M33) - pdf format (credit:- freestarcharts)

Despite being visible to the naked eye from a dark site, the Triangulum Galaxy is often regarded as an elusive object. It appears face-on from our perspective and although large and covering more than a degree of sky (71 x 42 arc minutes), it suffers from a low surface brightness. As a result, even a small amount of light pollution can render it invisible.

However, it's a different story under dark skies. Binoculars reveal a large mist without detail. An 80mm (3.1-inch) scope shows the galaxy as a very large diffuse patch of light, covering much of the field of view with a slightly brighter centre that's best viewed using low powers. A medium size 150mm (6-inch) or 200mm (8-inch) scope enhances the core and brings out some mottling. A larger telescope of at least 300mm (12-inch) aperture is required to reveal the spiral structure, along with dark dust lanes, knotty patches of nebulosity and other subtle features. For the budding astrophotographer, M33 is a rewarding target with the spiral arms and brighter nebulae relatively easy to capture.

Overall, M33 has a diameter of about 60,000 light-years and is estimated to contain 40 billion stars. For comparison, the Milky Way contains 400 billion stars and M31 about 1 trillion (1,000 billion).

M33 Data Table

Messier33
NGC598
NameTriangulum galaxy
Object TypeSpiral galaxy
ClassificationSA(s)cd
ConstellationTriangulum
Distance (light-years)2.81 Million
Apparent Mag.+5.7
RA (J2000)01h 33m 51s
DEC (J2000)+30d 39m 37s
Apparent Size (arc mins)71 x 42
Radius (light-years)30,000
Number of Stars40 Billion
Other NamePinwheel galaxy

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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