APOD: 2026 September 18 – Messier 33: The Triangulum Galaxy

On September 18, 2026, NASA's Astronomy Picture of the Day (APOD) showcased the Triangulum Galaxy, also known as Messier 33, which is an impressive face-on spiral galaxy located in the northern constellation Triangulum. Measuring over 50,000 light-years in diameter, M33 ranks third among the Local Group of galaxies after the Andromeda Galaxy (M31) and our own Milky Way.

As the third-largest member of the Local Group, M33 is believed to be a satellite of the Andromeda Galaxy. Astronomers residing in these two neighboring galaxies would have the privilege of observing each other's magnificent spiral star systems, likely providing spectacular celestial views. From the perspective of the Milky Way, the telescopic image of M33 reveals its blue star clusters and pinkish star-forming regions along the loosely wound spiral arms.

At about the 5 o'clock position relative to the center of the galaxy, the cavernous NGC 604 stands out as the brightest star-forming region, as captured in the sharp telescopic image. Similar to the Andromeda Galaxy, M33's population of well-measured variable stars has contributed to its role as a cosmic yardstick for establishing the distance scale of the Universe.

This APOD was shared by George Chatzifrantzis, with contributions from Jerry Bonnell, Cecilia Chirenti, Robert Nemiroff, and Keighley Rockcliffe, and is a service of the NASA Scientific Activation and Michigan Tech. University.