M33 (NGC 598)

M33 is generally thought to be one of the notable discoveries of Messier, found in August 1764, and described as a "whitish light of almost even brightness".

The present author thinks, indeed, it is, together with NGC752, an original discovery of Giovan Battista Hodierna: "quae iuxta Triangulum, vel hinc inde duplex", a nebula mentioned three times in his "De Admirandis Coeli Characteribus", printed in Palermo in 1654.

The Latin expression must be translated as "the nebula close to the Triangle, which goes from one side to the other (of the constellation), or is double". Remark: both M33 and NGC752 are visible to the naked eye, almost under a clear and dark sky. Burnham writes (Burnham's Celestial Handbook, p. 1899): "From a truly dark sky site, it [M33] may be glimpsed faintly without optical aid; this has been independently confirmed by Walter Scott Houston, Leslie C. Peltier, and other experienced observers". The visual magnitude of NGC752 is, according to Sky Catalogue 2000.0, 5.7.

Look at the map of the constellation: M33 and NGC752 lie in symmetric position as regards the Triangulum; like a "double nebula", or a nebula which goes "hinc inde", from one side to the other.

M33 lies on the extreme western region of the constellation: just S of the line a Trianguli-b Andromedae, about one-third of the distance from a Trianguli.

It is one of the bright members of our Local Group of Galaxies. Although it is visible, in a very dark and clear sky, to the naked eye, M33 is a challenging object for the small telescope: its great size and low surface brightness need ideal sky conditions and low powers. Good binoculars (for example my 11x70) could give a better sight of this large but faint object.

Although disappointing to the visual observations, M33 is a most rewarding target for the astrophotographer, who can track down its spiral arms and brighter nebulae with considerably inexpensive equipment.

Also because of the cataloging of Herschel, the brightest and largest diffuse emission nebula of this galaxy has obtained a NGC number of its own: NGC 604. This is one of the largest H II regions known at all: it has a diameter of nearly 1500 light years, and shows a spectrum resembling that of 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.

The great spiral arms show several other 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 M33, 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.

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.