Galaxy M 33 (NGC 598) in Triangulum

NGC 598 (M 33) is the nearest Sc to our own galaxy. It is a member of the Local Group. The distance modulus is probably close to m-M = 24.5 (Sandage, Ap.J., 127 513, 1958). The limiting magnitude of the 200-inch is about m_pg = 23.0. This means that the stellar content of M33 can be studied in detail down to stars of absolute magnitude Mpg = -1.5. Bright O and B stars, cepheid variables, globular andopen starclusters, novae,irregular variables, and HII regions are known in M 33. The first extensive study of the stellar content was made by Hubble in 1926 (Ap. J., 63, 236, 1926). No comparable study has been made in recent times.
Spiral structure is easy to trace in M 33. The arms are "massive" in the sense used by Reynolds and by Hubble (Observatory, 50, 1927). The arms are completely resolved into bright stars. Most of these are blue supergiants, but there are also at least 3000 red supergiants of Mv = -5 which are similar to those found in h and X Persei.
The brightest HII region is NGC 604, in the northeast quadrant. This has a linear diameter of about 320 parsecs. Many smaller HII regions abound in the spiral arms. Detailed, large-scale photographs of parts of M 33 are given as negative prints in Ap.J., 127 513, 1958.
The integrated color index ofthe system is CI (International) = 0.40 (Holmberg).

RA: 01 33.9 - DEC: +30 40 - MAG: 7
U2000: 91 - TIRION: 4 - SIZE: 73'x45'