M33 is a spiral galaxy in the direction of the constellation Triangulum. It is a convenient galaxy to investigate the spiral structure because it is one of the “face-on” type galaxies where the spiral shape can be seen from the polar direction. M33 together with M31 (the Andromeda Galaxy) in the direction of the constellation Andromeda, M32, and our Milky Way Galaxy form the Local Group. In the night sky, M33 and M31 are located symmetrically around the star β (beta) Andromeda. You can see the blurry shape of M33 even with binoculars.

Still Active Star Formation

Many spiral galaxies have a round, bright structure protruding in the polar direction at the center. This is called a “bulge.” The presence of a bulge is not clear in M33. The existence of the interstellar dust, seen as the dark band in the center of the galaxy, and the hydrogen ionization regions, seen as the red shining nebulae, tell us that star formation is still active in these areas. Also, there are parts in the disk where the red nebulae, as well as the shining blue young stars and the open clusters formed by them dominate. You can see that the star formation stretches continuously from the center of the galaxy to the outside of the spiral.

The galaxy doesn’t have a bright bulge nor clear spiral arms. However, in the photo, the contrast of the blue spiral arms and the red nebulae is remarkable. M33 is a spiral galaxy with unique eye-catching beauty.

Text: Tomoko Ono (Public Relations Center, NAOJ)
Translation by: Hiroko Tsuzuki and Ramsey Lundock (NAOJ)

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Related Links

• Subaru Reveals History of Star Formation in M33 (Subaru Telescope)