NASA's Astronomy Picture of the Day for September 18, 2026 featured Messier 33, also called the Pinwheel or Triangulum Galaxy, located in the small northern constellation Triangulum. The image was credited to George Chatzifrantzis.

According to the accompanying explanation, M33 is over 50,000 light-years in diameter, making it the third largest galaxy in the Local Group after the Andromeda Galaxy and the Milky Way. It lies about 3 million light-years from the Milky Way and is thought to be a satellite of Andromeda.

The telescopic image shows blue star clusters and pinkish star-forming regions along loosely wound spiral arms. The explanation notes that NGC 604 is the brightest star-forming region, appearing near the 5 o'clock position from the galaxy center. Well-measured variable stars in M33, like those in Andromeda, have helped make it a cosmic yardstick for the Universe's distance scale.

The page also stated that APOD's main NASA site was moving from apod.nasa.gov to science.nasa.gov/apod, and that the following day's image would be titled "A Zodiacal Night."