Probably discovered by Hodierna before 1654. Independently discovered by Charles Messier 1764.

The Triangulum galaxy M33 is another prominent member of the Local Group of galaxies. This galaxy is small compared to its big apparent neighbor, the Andromeda galaxy M31, and to our Milky Way galaxy, but by this more of average size for spiral galaxies in the universe. One of the small Local Group member galaxies, LGS 3, is possibly a satellite of M33, which itself may be a remote but gravitationally bound companion of the Andromeda galaxy M31.

M33 is approaching us (our Solar System) at 182 km/s according to R. Brent Tully, or at 179 +/-3 km/s according to NED. Corrected for our motion around the Milky Way's Galactic Center, it is approaching our Galaxy at 24 km/sec.

M33 was probably first found by Hodierna before 1654 (perhaps together with open cluster NGC 752). It was independently rediscovered by Charles Messier, and cataloged by him on August 25, 1764. Nevertheless, William Herschel, who otherwise carefully avoided to number Messier's objects in his survey, assigned it the number H V.17, on the ground of an observation dated September 11, 1784. Also because of the cataloging of Herschel, the brightest and largest HII region (diffuse emission nebula containing ionized hydrogene) has obtained a NGC number of its own: NGC 604 (William Herschel's H III.150); it is situated in the northeastern part of the galaxy; apparently the bright knot near the top of our image. This is one of the largest H II regions known at all: it has a diameter of nearly 1500 light-years, and a spectrum similar to the Orion nebula M42. Hui Yang (University of Illinois) and Jeff J. Hester (Arizona State University) have taken a photograph of NGC 604 with the Hubble Space Tepescope, resolving over 200 young hot massive stars (of 15 to 60 solar masses) which have recently formed here.

M33 was among the first "spiral Nebulae" identified as such by William Parsons, the Third Earl of Rosse; see his drawing. It was also among the first "nebulae" identified as galaxies, in which Cepheid variable stars were found; Edwin Hubble published a fundamental study in 1926 (Hubble 1926).

Several other knots in the spiral arms of M33 have been assigned their own NGC catalog numbers: NGCs 588, 592, 595, and NGC 603 (the latter is listed as nonexistent in the RNGC though, although they mention it was listed by Zwicky), as well as ICs 131, 132, 133, 134, 135, 136, 137, 139-40, 142, and 143 (NGC 2000.0 lists IC 134 and 139-40 as stellar, while the Webb Society Deep-Sky Observer's Handbook, Vol. 4 [Galaxies] shows IC 139-40 on the chart on p. 215, which is credited to Ronald J. Buta of McDonald Observatory, University of Texas). Some of them are identified in our map also. Kenneth Glyn Jones notes that they should be visible in 12.5-inch telescopes. The giant emission nebula NGC 595 was investigated by William H. Waller with the HST (e.g. Astronomy, June 1995, p. 16-18); with Hubble he resolved the hot massive stars that excite that nebula's gas to shine.

Our image, which was obtained by David Malin from photographic plates made with the Isaac Newton Telescope on La Palma, shows many of these objects in the spiral arms of this beautiful Sc spiral (NGC 604, for example, is the prominent red patch near the left edge in the upper half of our photo). Interested readers can obtain more detailed information on this image. By different processing, David Malin has enhanced various features in alternative images from this INT photo of M33.

The results of the Hipparcos satellite have lead to a revision of the cosmic distance scale, therefore also of our distance to M33: The current value is about 3.0 million light-years. Most sources give a distance of 2.3 to 2.4 million light-years, but the Sky Catalogue 2000.0 has more than 2.9 million light-years (900 kpc), which by chance may be closer after the new Cepheid distance recalibration, due to 1997 Hipparcos satellite results. Investigations of Cepheids in M33 of 1991 (Freedman et.al., 1991) have revealed that M33 is at a slightly greater distance from us than the Andromeda Galaxy M31. With our distance values, the distance of M33 from M31 is about 750,000 light-years. Assuming the former value, its angular dimension of 73 arc minutes in major axis (about 2.5 times the Moon's diameter) corresponds to about 50,000 light-years, half the diameter of the Milky Way. However, the faintest outlayers seem to reach more far out, so that the true diameter may be at least 60,000 light-years. The mass of the Triangulum Galaxy has been estimated between 10 and 40 billion solar masses.

The Triangulum galaxy M33 is of type Sc, and even a "late" representative of that type so that Tully classifies it as Scd (in the Nearby Galaxies Catalog). The pronounced arms exhibit numerous reddish HII regions (including NGC 604), as well as blueish clouds of young stars. Baade has also discovered Population II stars, and globular clusters have been found. Although no supernovae have yet been detected in the Triangulum galaxy, several supernova remnants have, and were cartographed by radio astronomers with high acuracy. At least 112 variables have been discovered in M33, including 4 novae and about 25 Cepheids. A strong X-ray source is also situated in this galaxy.

For the observer, this galaxy can be glanced with the naked eye under exceptionally good conditions; for most people, it is the most distant object visible to the naked eye (there are rare reports that some eagle-eyed stargazers managed to see M81 under exceptional conditions, but this is exceptional with all respects). It is outstanding in good binoculars, but as its considerable total brightness is distributed quite evenly over an area of nearly four times that covered by the full Moon, its surface brightness is extremely low. Therefore, it is difficult to impossible to view this galaxy in telescopes which do not allow low magnification - lowest is best for this object ! The best view of M33 the present author had was with a 6-inch refractor at magnification 25. M33 is also a most rewarding target for the astrophotographer, who can track down its spiral arms and brighter nebulae with considerably inexpensive equipment.

More ambitious observers with large telescopes (> 40 cm aperture) may try to track down some of M33's globular clusters; Rich Jakiel has tracked down 5 globular clusters in M33 with a 50-cm telescope.