M33, ngc598, the Triangulum Galaxy, the Pinwheel Galaxy and various other names are used for this nice looking spiral galaxy in the constellation of Triangulum. Part of the Local Group of Galaxies, it lies a little further on from us than M31, at a distance of 2.73 million light years (840Kpc). Some 60000 light years (18Kpc) across and containing 40 billion stars, M33 is the third largest member of the Local Group. Weighing in at 50 billion solar masses, it is considerably less massive than either M31 or the Milky Way and roughly 1/4-1/3 their diameters, respectively. Shining with an apparent magnitude of 5.72, it's visible from a dark site and is the furthest object that can be seen by the naked eye (a title once given to M31).

Due to their relatively close proximity, M31 and M33 are most likely gravitationally bound to one another, with M33 probably a satellite of M31. In 2004, a clumpy stream of hydrogen gas was found to stretch between the two galaxies, possibly due to a past interaction between both galaxies. The results of a study on water masers in M33 (in 2005) showed that its velocity relative to the Milky Way was around -190Km/s, also pointing to an interaction with M31 in the past, probably within 2-8Ga. Latest figures put the galactocentric velocity of M33 at -44km/s, with a heliocentric radial velocity of -179km/s. Recent results of astrometric measurements made by the GAIA probe point to M33 being in its first approach towards M31 and not in orbit about the larger galaxy.

M33 appears to contain around 54 globular clusters, although the actual number is probably up to 122. Unlike the Milky Way globulars, the clusters of M33 appear to be several billion years younger than ours and there has been a burst of star formation creating more clusters in the last 100Ma. This appears to be due to an infall of gas from the galaxy's halo regions surrounding the galaxy. It was found that the UV emissions of the massive stars forming in this starburst match the similar emissions found in the LMC. Within the galaxy itself, there are several prominent HII (ionised hydrogen) regions of relatively large sizes, along with many smaller regions. The prominent regions consist of four separate nebula: ngc588, ngc592, ngc595 and ngc604. ngc604 is one of the largest HII regions within the Local Group, some 1520 light years across and containing some 100000 solar masses of gas that is being ionised by at least 200 massive O and WR stars. Unlike the central cluster of the Tarantula Nebula (R136) in the LMC, the central cluster of ngc604 is more like a large stellar association than a mega open cluster/nascent globular cluster as is found in the Tarantula. Compared to the Orion Nebula, ngc604 is some 46 times the lineal diameter and 6300 times more luminous than M42. Placed at the same distance as M42, ngc604 would outshine Venus in the night sky.

The Hubble Classification of M33 is SA(s)cd, meaning that it is an unbarred spiral galaxy, with no nuclear ring and relatively loosely wound spiral arms. The galaxy appears to have at least four major arms which spiral out of the nucleus directly from the core of the galaxy. However, unlike its larger companions, there is no nuclear bulge and the central regions are an extended HII region. It appears that there is no supermassive black hole at the centre of M33, with no evidence for a concentrated mass higher than 3000 solar masses being present in the centre, as is found by the velocity of stars about the centre. However, the centre is home to an ultraluminous X-ray source with a luminosity of around 1.2x10^39 ergs/sec, making it the most luminous X-ray source in the Local Group. The galaxy itself is tilted at 54 degrees to our line of sight, making study of the galaxy relatively easy as much of the structure is not being obscured by dust and gas. The disk of the galaxy also appears to be warped out to 8Kpc, with the lack of tidal tails and dwarf spheroidals about the galaxy pointing to relatively little interaction with any other galaxies in the more recent past.

As to the future of M33, w.r.t. the eventual collision between the Milky Way and M31, it's unsure as to what fate awaits M33. That it will eventually orbit and become absorbed by the much larger result of the collision is without much doubt, however there is a possibility that the galaxy maybe flung out of the local group. Though, the nature of M33's interactions with both the Milky Way and M31 in that far flung time is one of some speculation. It may just collide with M31 before that galaxy collides with us. It may even become slingshot around M31 and collide with us first, before the main collision occurs. Or (more likely), it could go into orbit about "Milkomeda" and then eventually fall into the larger galaxy, fueling star formation for a time.

Pic was taken with t20 (iTelescope, NMSO), 1hour LRGB combination (10x3min L, 5x2min each RGB).