M33

The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, about 2.73 million light-years from Earth. It is a loosely wound spiral galaxy spanning about 60,000 light-years across, containing an estimated 40 billion stars. M33 hosts NGC 604, one of the largest known HII regions — a massive star-forming complex about 40 times the size of the Orion Nebula.

Located in the constellation Triangulum, about 4 degrees west-northwest of Alpha Trianguli. Despite being large (over 1 degree across) and reasonably bright, M33 has very low surface brightness, making it one of the most challenging Messier objects from light-polluted sites. Dark skies are essential. Binoculars show a large, faint, diffuse glow. A wide-field telescope at low power (30-50x) is best. Averted vision helps. NGC 604 is visible as a bright knot in the northeast spiral arm. Best observed from October through January.

Probably first recorded by Giovanni Battista Hodierna before 1654. Independently discovered by Charles Messier in 1764. It was one of the 'spiral nebulae' at the center of the Great Debate in 1920 about whether such objects were within or beyond the Milky Way. Edwin Hubble proved its extragalactic nature in the 1920s.