Messier 52 (NGC 7654) is an open cluster located in a rich Milky Way field near the far western edge of Cassiopeia. This cluster was discovered by Charles Messier in 1774.
Appearance
With a total magnitude of 7.3, amateurs can see M 52 as a nebulous patch in good binoculars or finder scopes. In 4-inch telescopes, it appears as a fine, rich, compressed cluster, containing about 200 stars of 7th magnitude and fainter, in a 13'-diameter area. M 52 is often described as fan- or V-shaped. Many very faint stars spread NE and SE of the main group. A yellow 8th magnitude star stands out on the SW side, and a group of four stars is detached at the north end.
Open cluster Czernik 43 is visible just to the south. Also situated about 35' SW of M52 is the Bubble Nebula, NGC 7635.
Properties
The distance of this cluster is not very well known; estimates range from about 3,900 to 5,000 light-years. The uncertainty is due to high interstellar absorption of its light by galactic dust. A distance of 5,000 light years would imply a true diameter of nearly 15 light-years, and a central density of 3 stars per cubic parsec.
The cluster's brightest stars are yellow giants: the brightest is of spectral type F9 and magnitude 7.77, the second-brightest of type G8 and magnitude 8.22. However, M 52 also contains a number of blue-white class B main-sequence stars; the brightest is of magnitude 11.0 and spectral type B7, which implies a cluster age of around 35 million years. The cluster also contains one peculiar Of star, i.e. an extremely hot star with peculiar spectral lines of ionized helium and nitrogen.