Messier 5

M5 is a Class V bright globular cluster located in the northern constellation Serpens. The cluster lies at a distance of 24,500 light-years from Earth, in the galactic halo of the Milky Way. It has the designation NGC 5904 in the New General Catalogue. It is also classified as GCl 34, C 1516+022. With an apparent magnitude of 6.65, Messier 5 can be seen, but only under extremely dark skies and it only appears as a faint star near the star 5 Serpentis. The best time to observe M5 is in the months of March, April and May. The cluster can be found at right ascension 15h 18m 33.22s and declination +02°04’51.7”; about a fist-width to the north of Zubeneschamali, the brightest star in the constellation Libra. It can also be found about two fist-widths to the southeast of Arcturus, the brightest star in Boötes constellation and fourth brightest star in the sky, or three fist-widths east of Spica, the brightest star in Virgo and the 15th brightest of all stars. Arcturus and Spica can easily be located by following the line formed by the three bright stars that mark the handle of the Big Dipper. The imaginary line leads first to Arcturus and then to Spica.

Messier 5 was discovered by the German astronomer Gottfried Kirch on May 5, 1702, while observing a comet. He believed it was a star with nebulosity. Charles Messier found the object on May 23, 1764, and described it as a nebula without stars. The cluster was not resolved into individual stars until 1791, when William Herschel observed it in a larger telescope, a 40-foot focal length reflector, and counted about 200 stars.

Messier 5 is one of the larger globular clusters known, spanning about 165 light-years in diameter. The cluster has a tidal radius of 202 light-years. Member stars are gravitationally bound to it within this space and can’t be torn away from the cluster by the gravitational pull of the Milky Way. The cluster has an ellipsoidal rather than spherical shape. It is receding from us at about 52 km/s.  The compact core region is about 6 light-years in diameter, corresponding to an angular size of 0.84′. Messier 5 is home to more than 100,000 stars, as many as 500,000 according to some sources. The cluster’s overall spectral type is F7.  The brightest stars in M5 are of magnitude 12.2. The estimated age of M5 is 13 billion years, making it one of the older known globular clusters associated with our galaxy.

The cluster contains at least 105 variable stars, of which 97 are of the RR Lyrae type and can be used to measure distances in outer space. The first variables in M5 were discovered by the English amateur astronomer Andrew Ainslie Common in 1890. American astronomer Solon Irving Bailey later found 85 RR Lyrae variables in the cluster. The brightest and most easily observed variable star in M5 – Cepheid Variable 42 – changes from magnitude 10.6 to 12.1 in a period of just under 26.5 days. M5 also has a significant number of blue stragglers, old stars that appear younger and bluer than they should be, likely as a result of interaction with other stars.

Astronomers have also observed a dwarf nova in M5, a type of cataclysmic variable star that consists of a close binary system in which one of the stars is a white dwarf that accretes matter from its binary companion. As a result, the white dwarf exhibits periodic outbursts. In 1997, two millisecond pulsars were discovered in the cluster. Credits: Messier Objects, Universe Today, Wikipedia.