M33 is the next nearest spiral galaxy to the Milky Way after the Andromeda Galaxy. The field of view of this image, which was taken by the Suprime-Cam attached to the Subaru Telescope, is about eight times of the size of a single Suprime-Cam shot. It covers an area of sky about eight times the size of the full Moon. Individual stars, star associations, star-forming regions, and dark nebulae can easily be seen in the image.
Distribution of stars in M33 is sometimes sparse and the universe behind can be observed through "windows" in the stellar population. This galaxy is likely one of the outlying members of the Local Group and its physical size is about the same as the Andromeda Galaxy.
This is the most active star-forming region in M33; it is a hydrogen gas nebula ionized by photons from newborn stars, and shining in a remarkable red color. More than 200 massive young stars are shining at the center of NGC 604. This nebula is a good example of places where associations of massive stars and the open clusters form in late-type galaxies.
Intense bursts of star formation have occurred at the center of M33, where bands of dark lanes (interstellar dust) and HII (ionized hydrogen gas) regions are scattered around. M33's busy central region is quite different from the Milky Way's, which has a central bulge with a black hole and comparatively little star formation.
4Outer Edge of the Disk
Numerous stars are found even in the outer regions of M33. Most of them are old, red stars formed more than 10 billion years ago during an early epoch of M33's formation history.
5Chain Reaction of Star Formation
In spiral galaxies such as M33, stars form in regions that lie along their hydrogen gas-rich arms. Photons from newborn stars ionize surrounding clouds of gas, while supernova explosions blast their material out to the interstellar medium, heating and compressing it. Both of these processes can trigger new star formation. A sequence of dark lanes, HII regions, and young blue stars can be seen everywhere across the spiral arms in M33, and demonstrates how star formation propagates in a spiral galaxy.
6Extended Globular Cluster
The NAOJ-led research group discovered this extended globular cluster. It is a suspected remnant of a dwarf galaxy that merged to become part of M33. Finding such a cluster in the M33 (which is a late-type dwarf spiral) implies a complex merger history in this galaxy's distant past.