The Triangulum galaxy (M33 / NGC 589) is a spiral of type Sc at a distance of 3.0 million light years. This is an improved estimate after the Hipparcos satellite survey of Cepheid variable stars in 1997 - the old estimate was 2.3 m.l.y. It is a member of our local group of galaxies.
Our local group of galaxies contains three galaxies that are significantly larger than the rest. The largest is the Andromeda galaxy which contains 1000 million stars. Our own galaxy comes second with about 200 million stars. The Triangulum galaxy has 25 million stars and is half the diameter of our galaxy. The size of the Triangulum galaxy is actually about average for spiral galaxies in the universe. The Triangulum galaxy is relatively close to the Andromeda galaxy (M31) and may be gravitationally bound to it. This galaxy is moving towards our solar system at 180 km/sec and therefore has a blue shift instead of the more normal red shift. After correcting for the solar systems orbital speed around our galaxy, M33 is approaching our galaxy at 24 km/sec.
The spiral arms contain many red HII regions. Indeed, this galaxy contains one of the largest HII emission nebula ever observed (NGC 604) with an enormous diameter of 1500 light years. Despite its size, it is a normal emission nebula - its spectrum is very similar to that of the Orion Nebula. The HII region contains over 200 massive young stars (15 to 60 solar masses) and it is these stars that are responsible for illuminating the whole nebula. This HII region is visible at the top left of this picture (north east of the galaxy). Several of the other HII regions are also visible in this photo.
This galaxy does not have a massive black hole at it's center. This is thought to be the case for one in six spiral galaxies. These galaxies also have a far less prominent central bulge.
The Triangulum galaxy was one of the first galaxies to be recognized to be a spiral nebula. It was also one of the first spiral nebula to be identified as a galaxy after its Cepheid variables were used to measure its distance.
At magnitude 5.7 this galaxy is probably the most distant object that is visible to the naked eye. It can be a difficult object to observe with a telescope because the surface brightness is very low due to its large size. It's best seen at a magnification of 25x, so a small telescope actually provides the best views (the ideal aperture is 175mm). If we could see our own galaxy, its spiral arms would look very similar to the Triangulum galaxy.