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Messier 33, also known as the Triangulum Galaxy, is a face-on spiral galaxy located approximately 3 million light-years from Earth in the constellation Triangulum. With a diameter exceeding 50,000 light-years, it is the third-largest member of the Local Group of galaxies, after the Andromeda Galaxy and the Milky Way, and is believed to be a satellite galaxy of Andromeda. The galaxy features prominent blue star clusters and pink star-forming regions along its spiral arms, with NGC 604 being its brightest star-forming region, and its well-measured variable stars have been instrumental in establishing cosmic distance measurements.
Why it matters
M33's population of variable stars serves as a crucial "cosmic yardstick" for determining astronomical distances and establishing the distance scale of the Universe. As a nearby spiral galaxy with clearly observable features, it provides astronomers with valuable data for understanding galactic structure, star formation processes, and the dynamics of galaxy groups.
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Messier 33: The Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, Messier 33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other’s grand spiral star systems. As for the view from the Milky Way, this sharp telescopic image shows off M33’s blue star clusters and pinkish star forming regions along the galaxy’s loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 5 o’clock position from the galaxy center. Like M31, M33’s population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.
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Tomorrow’s picture: star party weekend
| Date | September 18, 2026 |
|---|---|
| Credit & Copyright: | George Chatzifrantzis |
| Authors & editors: | Jerry Bonnell, Cecilia Chirenti, Robert Nemiroff, Keighley Rockcliffe |
| A service of: |
ASD at NASA / GSFC, NASA Science Activation & Michigan Tech. U. |
Source: APOD: 2026 September 18 – Messier 33: The Triangulum Galaxy