Messier object 67 is an open star cluster in the constellation of Cancer. A star cluster is a group of several hundred to a thousand or more stars of around the same age that are gravitationally bound to each other, however loosely, and formed from the same cloud of molecular gases. In the case of M67, unlike a globular cluster, the stars are spread over many light years, hence the ‘open’ cluster classification.

M67 was discovered in 1779 by German astronomer Johann Gottfried Koehler, when he described it as a “rather conspicuous nebula in an elongated figure, near Alpha of Cancer”. It was independently rediscovered in 1780 by Charles Messier, who resolved it into stars and catalogued it, describing it as a ‘cluster of small stars with nebulosity, below the southern claw of Cancer’.

The cluster is about 2900 light years away, with recent data from the Gaia Mission giving a very accurate distance measurement of 2879 light years. It is one of the most ancient clusters known with an estimated age of around 4 billion years. Most open clusters are found along or near the galactic plane, but M67 is so old that it has migrated some 1500 light years away from the plane. It now resides out on the fringes of the Milky Way where there is little obscuring gas and dust. Consequently the cluster has been much studied, with particular reference to the area of stellar evolution.

M67 has over 500 noticeable stars, but over 1500 solar masses, resulting in an average density of around 0.2 solar masses per cubic light year which is about 50 times greater than the average density in our part of the Milky Way. Around 100 of the stars are like our Sun, there are many red giants and there are no main sequence stars bluer than spectral type F. Among the stars are another 150 white dwarf stars and around 30 anomalous 'blue straggler' stars that do not fit within the main correlation of stars. Messier 67 has undergone mass segregation, which means that its heavier stars have moved toward the cluster’s centre and less massive ones have migrated toward the outer region over time. As a result, the cluster’s stars are much more gravitationally bound than those in younger open clusters. The ages and ubiquity of stars like that of the Sun have led many astronomers to believe that our own star originated from M67. However after extensive computer simulations, this has been re-evaluated as unlikely.

Research Assistant: Finlay Stuart