M73 (NGC 6994)

Messier 73 (NGC 6994) is an asterism located in the constellation Aquarius, in the Orion Arm of the Milky Way Galaxy in the Local Group of galaxies. M73 is 2500 light years away from Earth.

M73 is best viewed during early spring, is magnitude 9, and can be viewed with binoculars. M73 is 2.8' in apparent size. For reference, the full moon is 30'.

Observing difficulty: Easy

1. Name:

1. Type:
2. asterism

1. Constellation:
2. Aquarius

1. NGC or IC:
2. NGC 6994

1. Magnitude:
2. 9

1. Viewing:
2. binoculars

1. Size:
2. 2.8'

1. Distance (light years):
2. 2500 LY

1. RA:
2. 20h 59.0m

1. Dec:
2. -12 3'

1. Season:
2. early spring

1. Milky Way location:
2. Orion Arm

1. Galaxy group:
2. Local Group

1. Messier Marathon #:
2. 109

* The naked eye can see up to magnitude ~7-8 objects under ideal dark sky conditions.

The Unique Asterism in Aquarius

Messier 73 (M73), also known as NGC 6994, is a fascinating object in the Messier Catalogue due to its unique nature. Unlike most other entries which are deep sky objects like galaxies, nebulae, or star clusters, M73 is an asterism - a pattern of stars rather than a physically bound cluster. This grouping of stars is found within the constellation Aquarius and was first catalogued by French astronomer Charles Messier in October 1780.

Characteristics of M73

M73 consists of a pattern of four stars arranged in a 'Y' shape. These stars range in magnitude from 10.5 to 12 and share a similar location in the sky, giving the appearance of a cluster from our vantage point. However, studies indicate that these stars are not gravitationally bound and are at varied distances from us, meaning this grouping is likely a line-of-sight coincidence.

The spectral types of the stars in the M73 asterism are diverse, ranging from G5 to K0 for the two brightest stars, which are yellow and orange giants respectively, to B9 for the faintest star, a hot blue-white star. Their spread-out nature and disparate characteristics serve to underline M73's status as an asterism rather than a true star cluster.

Magnitude and Size

The overall visual magnitude of M73 is approximately 9.0, putting it just out of reach of the naked eye but within the capabilities of a small telescope or good binoculars. The asterism spans about 2.8 arc minutes in the sky, which is roughly one-tenth the apparent diameter of the full moon.

Finding and Viewing M73

Locating M73 in the sky involves finding its home constellation, Aquarius. This constellation is best seen in the autumn months for observers in the Northern Hemisphere. M73 lies near the border with Capricornus, to the southeast of the star Delta Aquarii. Once Delta Aquarii is identified, look roughly 3 degrees to the southeast to find the asterism.

Due to its faint magnitude, M73 is a challenging object for visual observers, particularly in areas with significant light pollution. Small telescopes or binoculars will reveal a small, fuzzy patch of light, while larger telescopes will resolve the individual stars.

In conclusion, while M73 may not offer the grandeur of a galaxy or the spectacle of a nebula, its unique status as an asterism within the Messier catalogue makes it an intriguing target for amateur astronomers. Its modest brightness and small size offer a rewarding challenge for those with the patience to seek it out.