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M 9
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M 9

February 18, 2005
Globular Cluster M9 (NGC 6333), class VIII, in Ophiuchus

Right Ascension: 17 : 19.2 (h:m)
Declination: -18 : 31 (deg:m)
Distance: 25.8 (kly)
Visual Brightness: 7.7 (mag)
Apparent Dimension: 12.0 (arc min)

Discovered 1764 by Charles Messier.

M9 is one of the nearer globular clusters to the nucleus of our Galaxy, with a computed distance of 5500 light-years from the Galactic Center (Burnham gives 7500, a slightly too high value). Its 12.0 arc minutes angular diameter corresponds to a linear extension of 90 light years at its distance of about 25,800 light years from our Solar system. However, it appears somewhat smaller visually, about 3 or 4', and on conventional photos can be traced to about 9.3 arc minutes. To the north and west, its light is significantly dimmed by interstellar dust, as it lies at the edge of a patch of dark nebula (Barnard 64); its light is probably weakened by at least one magnitude (a factor of about 2.5). Taking these facts into account, the apparent visual brightness of this cluster of 7.7 magnitudes corresponds to an absolute brightness of -8.04 Mag, or a luminosity of roughly 120,000 times that of our sun. Visually it appeared oval to Mallas, and the ellipticity of 9 mentioned by Shapley can be seen in our photograph also. As its concentration class VIII indicates, the stars of M9 are about intermediately compressed toward its center.

M9 is receding from us at the very high velocity of 224 km/sec. 13 variables have been found in this cluster, of which Walter Baade found 10. Its brightest stars are about 13.5 mag so that intermediate-sized amateur telescopes (about 6 inch) are needed to show them; its horizontal-branch giants are of about mag 16.2. Its overall spectral type has been given as F2, its color index +0.06.

Globular cluster M9 is one of the original discoveries of Charles Messier, who cataloged it on May 28, 1764, and described it as "Nebula without star" of 3' diameter. It was first resolved into stars by William Herschel about 20 years later.

This globular cluster can just be glimpsed as a dim small and round nebula in 10x50 binoculars under good conditions. 4-inch Telescopes show the central part of M9 of about 3' diameter, in slightly oval shape and fading to the edges, but such an instrument can show even the brightest stars only under exceptional conditions; a 6-inch shows them clearly. 8-10 inch instruments show it as a globular cluster of 7' or 8' diameter, with a more compact central region of 5'. Large amateur telescopes (12" up) fully resolve it to the core.

M9 is found best from the 2.43-mag star Sabik (35 Eta Ophiuchi, spectrum A2 V); M9 is about 3 deg SE (2.1deg E and 2.8deg S). A star of 6th mag is within about half a deg to the N, one of about 7th mag to the NW, another 6th-mag star within a degree to the E.

Nearby and only about 80' to the NE lies the slightly smaller (8 arc min) and slightly fainter (mag 8.25) globular cluster NGC 6356, at roughly double the distance of M9 (about 50,000 light years). About at the same separation to the SE is the much fainter (mag 9.7) and smaller (3 arc min) globular NGC 6342. The dust cloud Barnard 64 has its core about 25' W of M9 but extends almost to the cluster.


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