M5 – Globular cluster

M5 – Globular cluster in the Snake

Globular cluster M5 is the last in this series of globular clusters at the beginning of Messier’s list is referred to as a relatively large, but fairly standard. It contains blue stragglers, variable stars, white dwarfs and a single neutron star. There is plenty to write about the M5 thanks to the interesting material close at hand. M5 is pretty hard to find because of a lack of surrounding bright stars. The easiest way to find it is by searching the mid-line between the stars Omega Serpentis and 109 Virginis. M5 can be seen with binoculars.


Facts and figures about M5 Globular Cluster

  • Sign: Snake
  • Other designations: NGC 5904
  • Position: right ascension 15h18m33s.22; declination + 2d04m51.7s
  • Type: Globular cluster
  • Density Class: V (slightly more concentrated towards the center)
  • Magnitude: 6.7
  • Diameter: 165 light years
  • Distance: 24500 light years

M5 was discovered in 1702 by German astronomer Gottfried Kirch (1639-1710), while it was passed by a bright comet. Sixty years later it was rediscovered by Messier and went into his catalog at number 5. M5 was described by him as, “…beautiful mist between Libra and the Snake near the fifth star of the horse according to the catalog of Flamsteed, it contains no star…one can see him well in a beautiful sky.” William Herschel who first saw that M5 was a cluster. The first variable stars were found in 1890. The number has now increased to 105. Just like the M2,  M5 is elliptical in shape.

The dwarf nova M5

M5 contains about 150,000 stars, although there are higher estimates. The exact mass is unknown; a 2008 analysis gave results of 875,000 times the Sun. At 13 billion years of age, M5 is very old and even stars with the mass of the Sun will have already burned to white dwarfs. One white dwarf is still regularly seen. It forms a very close binary with a small main sequence star or red giant. The white dwarf steals gas from another star and while spiraling inwards the star forms a disk (called accretion disk). The temperature in the disk is so high that the hydrogen begins to spontaneously merge into heavier elements with a huge explosion. This is seen from Earth as a strong but short-lived spike in brightness typically one week after the initial eruption.

M5 and Palomar 5

Just south of M5 is a globular cluster. However, according to the list of Messier it is considered far from spectacular. This is Palomar 5; discovered in 1950. Palomar 5 is one of the finest examples of a globular cluster associated with the weaker brethren. It is a thin (density class XII) globular cluster with tens of thousands of stars. Only seen a handful can be seen with a medium-sized telescope. The total brightness is about 1% of that of M5. Further analysis led to the discovery that Palomar 5 had a tail of stars 13,000 light-years long which are pulled by the gravity of the Milky Way.