M5 is a 5th magnitude globular star cluster in Serpens Caput located about 8 degrees southwest of Alpha Serpentis. It is one of the finest globulars for viewing in the sky, and a great show object in the summer sky. This star cluster can be seen with binoculars under a dark sky, but puts on a much better display when viewed with a moderate size telescope. It is one of the three best globular star clusters in the northern hemisphere of the sky, sharing that honor with M13 in Hercules and M3 in Canes Venatici. M5 is estimated to have a total population of stars in excess of half a million, although many of them are too faint to be seen from our vantage point. If our Sun was placed in M5 it would appear as a very dim star of magnitude 19.4, and would not show up in this image. M5 is about 26,000 light-years from Earth, and has a radial velocity of about 30 miles per second in recession. This globular cluster contains an unusually large number of variable stars. It is believed to be one of the most ancient clusters known.
M5 was originally discovered by Gottfried Kirch on May 5, 1702, while he was looking for a comet. Charles Messier rediscovered it in May 1764, and thought it was a nebula. In 1791, Sir William Herschel is believed to be the first person to resolve M5 and realize it was a massive cluster of stars.
Recently, a neutron star of about 1.9 solar masses was discovered in M5. It is a millisecond pulsar and part of a binary system. The measured mass is greater than what astronomers' models had previously predicted was possible for a neutron star. The existence of such a heavy neutron star suggests that the matter at the interior of the star does not get squeezed into a sea of exotic "quark matter" as previously hypothesized by some. Instead, the nuclear material must remain as discrete neutrons which are more incompressible. This neutron star in M5 and another one discovered recently in NGC 6440 by Paulo Freire are the only ones known with masses that exceed the Chandrasekhar limit of 1.44 Suns, the point at which it was previously thought that a black hole would be produced.
This is a composite LRGB CCD image taken with a Takahashi FCT-150 at prime focus. The CCD camera was an SBIG ST-8E using a CFW-8 color filter wheel. This image was taken from my backyard in Scottsdale, Arizona.
M5 (NGC 5904)
Constellation: Serpens Caput
RA: 15h 18m 48.8s Dec: +02d 03' 54"
March 8, 2004
Image by Sid Leach
Scottsdale, Arizona
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