M33, the Triangulum Galaxy, is a perennial favorite of amateur and professional astronomers alike, due to its orientation and relative proximity to us. It is the second nearest spiral galaxy to our Milky Way (after M31, the Andromeda Galaxy) and a prominent member of the "local group" of galaxies. From our Milky Way perspective, M33's stellar disk appears at moderate inclination, allowing us to see its internal structure clearly, whereas M31 is oriented nearly edge-on.
The Galaxy Evolution Explorer imaged M33 as it appears in ultraviolet wavelengths. Ultraviolet imaging primarily traces emission from the atmospheres of hot stars, most of which formed in the past few hundred million years. These data provide a reference point as to the internal composition of a typical star-forming galaxy and will help scientists understand the origin of ultraviolet emission in more distant galaxies.
These observations of M33 allow astronomers to compare the population of young, massive stars with other components of the galaxy, such as interstellar dust and gas, on the scale of individual giant molecular clouds. The clouds contain the raw material from which stars form. This presents direct insight into the star formation process as it occurs throughout an entire spiral galaxy and constitutes a unique resource for broader studies of galaxy evolution.