Messier 103
A small open cluster of faint stars in Cassiopeia.
Messier 103 (M103, or NGC 581) is a compact open cluster of dim stars located in the constellation Cassiopeia. Pierre Méchain first spotted it on March 27, 1781, and Charles Messier later included it as the final entry in his famous catalog. The cluster lies about 9,400 light-years from the Sun and spans roughly 15 light-years across. Its two most noticeable stars include a magnitude 10.5 star and a magnitude 10.8 red giant at the cluster’s center. A bright foreground double star, Struve 131, appears nearby but is not gravitationally bound to the cluster. Based on a greater than 50% probability of gravitational attachment, the cluster contains about 172 stars. Its age is estimated between 12.6 and 25 million years.
After Méchain discovered Messier 101 through 103, Messier added this cluster to his own list. In 1783, William Herschel described the area as containing 14 to 16 fairly large stars along with many extremely faint ones. Åke Wallenquist first counted 40 stars in M103, while Antonín Bečvář later raised that number to 60. Archinal and Hynes proposed the current figure of 172 stars. Admiral William Henry Smyth noted the red giant, calling it a double star on Cassiopeia’s knee, about one degree northeast of Delta Cassiopeiae, also known as Ruchbah or Rukhbah.
M103 is easy to locate and can be seen with binoculars or a small telescope. It appears as a hazy, fan-shaped patch roughly one-fifth the apparent diameter of the Moon—about 6 arcminutes or 0.1 degrees across. To find it, center on Ruchbah, the lowest star in Cassiopeia’s distinctive “W” asterism. The cluster shows up as a fuzzy spot about one-third of the way along an imaginary line toward Epsilon Cassiopeiae, the northern endpoint of the “W,” on the outer side of that shape.
Quick Facts
- Epoch
- J2000.0
- Ra
- 01 · 33.2
- Dec
- +60 · 42
- Dist Ly
- 9,400 light-years
- Dist Pc
- 2.884 ± 0.313 kpc
- Appmag V
- 7.4
- Age
- 12.6 ± 0.2 million years
- Names
- NGC 581, Cr 14
Facts from the source article.
Lore & Background
Messier 103 was discovered on 27 March 1781 by Pierre Méchain, and later added by Charles Messier as the last deep-sky object in his catalogue. In 1783, William Herschel described the region as containing 14 to 16 pretty large stars and a great many extremely faint ones. Åke Wallenquist first identified 40 stars in M103, while Antonín Bečvář raised this to 60; Archinal and Hynes later suggested the cluster has 172 stars. Admiral William Henry Smyth pointed out the cluster's 10.8-magnitude red giant, noting it was a double star on Cassiopeia's knee, about 1° northeast of Delta Cassiopeiae (also called Ruchbah or Rukhbah).
The cluster holds two prominent stars, of which the brightest is magnitude 10.5, and in the center a magnitude 10.8 red giant. Another bright foreground object is the double star Struve 131, but it is not a member of the cluster. Cluster membership is about 172 stars based on a greater than 50% probability of gravitational attachment that binds the cluster together. M103 is between 12.6 to 25 million years in age.
Messier 103 is located 9,400 light-years from the Sun and is about 15 light years across. It is an easy object to find and is visible in binoculars or a small telescope, appearing as a nebulous fan-shaped patch about a fifth the apparent diameter of the Moon (6 arcminutes or 0.1° across). To find M103, observers are advised to center on Ruchbah (the lowest star of the 'W' asterism of Cassiopeia) and look for a hazy patch about 1/3 the length of an imaginary line toward Epsilon Cassiopeiae.
Reader's Guide
Messier 103 holds significance as the last deep-sky object added by Charles Messier to his famous catalogue, following its discovery by Pierre Méchain. The cluster's membership count has been a subject of refinement over time, with early counts by Åke Wallenquist (40 stars) and Antonín Bečvář (60 stars) later updated to 172 stars by Archinal and Hynes, based on a greater than 50% probability of gravitational attachment. Its age is given as a range of 12.6 to 25 million years, reflecting uncertainty in stellar evolution models. The cluster's prominent red giant and the foreground double star Struve 131 (not a member) add to its observational interest. Admiral William Henry Smyth's historical description of the red giant as a double star on Cassiopeia's knee provides a link to 19th-century astronomy. M103 remains a popular target for amateur astronomers due to its visibility in binoculars and small telescopes, and its location near the distinctive 'W' asterism of Cassiopeia makes it relatively easy to locate.
Did You Know?
- It is Charles Messier's last deep-sky object in his catalogue.
- The cluster contains a central red giant of magnitude 10.8.
- The double star Struve 131 appears in the same field but is not a member of the cluster.
More in Messier Objects, Part 3 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced