Messier 73 is a pattern (called an asterism) made of four stars in the constellation Aquarius. and is best viewed in autumn. While M73 is not an open star cluster, it’s still a nice target for backyard astronomers. Most observers agree M73 makes a “Y” pattern. In my opinion, it looks like the starship from the classic arcade game Asteroids, with the fourth star being a bullet coming out of its nose!
Since M73’s discovery in 1780, astronomers debated if the object was an open star cluster or an asterism. This controversy continued until the 2000s when a team of astronomers settled the debate. A 2002 study documented in this article on SYFY, showed that M73’s stars are not related. This is so since they are at different distances and move at various velocities and directions.
To learn more, watch British astrophysicist Dr. Rebecca Smethurst’s summary of M73 on YouTube:
My Observations
| Date | October 11, 2021 |
| Time | 9:00 p.m. |
| Location | Seattle, WA |
| Magnification | 203x |
| Scope | Meade 8″ SCT |
| Eyepiece | 10mm |
| Seeing | Good |
| Transparency | Good |
Since there is a bright moon out tonight, I was unsure if I was supposed to be seeing more stars in this cluster. After a look at the star app, it appears I’m viewing M73 in its entirety. The primary star has a yellowish hue, while the other three stars look white By the way, this asterism looks like the triangle starship in the classic Asteroids video game.
Key Stats
| Constellation | Aquarius |
| Best Viewing | Autumn |
| Visual Magnitude | +8.9 |
| Absolute Magnitude | -.07 |
| Distance from Earth | 2,000 ly |
| Diameter | 5.3 ly |
| Apparent Size | 9 arcmin |
| Milky Way Location | Orion Spur |
| My Viewing Grade | B |
Historical Observations
October 4, 1780, observation by Charles Messier
“Cluster of three or four small stars, which resembles a nebula at first glance, containing very little nebulosity; this cluster is located on the parallel [of declination] of the preceding [M 72]; its position has been determined from the same star [Nu Aquarii].”
Sources and Notes
Banner photo of M73 provided by the Digital Sky Survey. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions.
Charles Messier Observations: SEDS Messier Database
The Garden Astronomer 