M14, globular cluster in Ophiuchus

Image by Jim Dixon
M14 is a slightly elliptically shaped stellar swarm, about 100 light years across and about 30,000 light years away; older determinations have given values between 64,000 ly (Shapley) and 23,000 (Mallas/Kreimer) to 24,000 ly (Glyn Jones, Kinman, Becvar); the Sky Catalogue 2000.0 had 38,000 ly. Shapley assigned it an ellipticity of 9, extended in position angle 110 deg. While its bright main body about only about 3 arc minutes in angular diameter, the cluster's outlayers reach out to a total apparent diameter of 11.7 arc min. It lacks a dense central condensation (Burnham), as its concentration class VIII indicates. Its apparent visual brightness of 7.6 visual magnitudes corresponds to an absolute magnitude of -9.12, or to a luminosity about 400,000 times that of our sun - so while, because of its greater distance, it is apparently dimmer than the two other great Ophiuchus clusters, M10 and M12, it is intrinsically much more luminous.

The brightest stars of M14 are of about visual mag 14.0, and its horizontal branch giant stars at mag 17.2. Helen B. Sawyer Hogg gives the average magnitude of the 25 brightest member stars as 15.44 and its overall spectral type as G0; modern determinations have put it at F4. A color-magnitude (or Hertzsprung-Russell) diagram of this cluster is found in Smith Kogan et.al. (1974).

M14 contains the considerably large number of over 70 variables, many of them W Virginis stars (Population II Cepheids; Demers and Wehlau 1971).

In 1938, a nova appeared in M14, which however was not discovered before 1964, when Amelia Wehlau of the University of Western Ontario surveyed a collection of photographic plates taken by Helen Sawyer Hogg between 1932 and 1963 (Hogg and Wehlau, 1964). This nova was visible on 8 plates, taken between June 21-28, 1938, as a 16th mag star - this faintness explains, at least in part, why it had not been discovered earlier. Mrs. Hogg has estimated that this corresponds to an absolute magnitude of -1.5 (a modern check yields -0.7), but thought that at its maximum, it should have been as bright as mag 9.2, or absolute magnitude -7.5 (modern check), or almost 5 magnitudes brighter than the brightest cluster members! This was the second known nova in a globular cluster after that of 1860 in M80, T Scorpii, and the first one ever photographed. In 1983, the 4-m telescope of CTIO and the 3.9-m Anglo-Australian Telescope were used in attempt to look for a remnant of the nova (Shara et.al. 1986). In 1991, astronomers used the Hubble Space Telescope to observe the field around this nova in M14, but could not find the star or a nebulous remnant (Margon et.al. 1991).

In 1997, a carbon star (a star with strong carbon lines in its spectrum) was discovered in M14 (Cote et.al., 1997); this star has probably lost its outer layers in close encounters with other cluster members so that the carbon-enriched former core reaches up to the surface.