APOD: 2021 September 30 - The Hydrogen Clouds of M33

Good morning, Rallypoint. The Astronomy Picture of the Day (APOD) is titled "The Hydrogen Clouds of M33." The small, northern constellation Triangulum is home to Messier Object 33 (M33, aka the Pinwheel Galaxy or Triangulum Galaxy). M33 is over 50,000 light-years in diameter, and lies 3 million light years from the Milky Way. The galaxy is itself thought to be a satellite of the Andromeda Galaxy (M31), and its concentration of variable stars make M33 a cosmic yardstick for establishing the distance scale of the Universe.

By using a hydrogen-alpha (Hα) filter, an extensive network of hydrogen gas clouds are annotated in today's APOD. The thumbnail images for each are located along the left and right of the main image. Intense radiation from the numerous stellar nurseries in M33 ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow.