Trumpler 5 - Open Star Cluster
Trumpler 5 is an open star cluster in the constellation "Monoceros" at a distance of nearly 10,000 light-years. It has an apparent diameter of 6' (arcminutes)
and a very low surface brightness.
The mass of Trumpler 5 is estimated to be over 3,000 solar masses, which is quite a large number for an open star cluster. The inner region has a size of 18
light-years, including the outskirts you get a diameter of 45 light-years. The the cluster is moving away from us with a speed of v= +51km/s.
Due to the large distance the brightest stars are only about 14.5mag bright, which corresponds to an absolute magnitude of M= +2.1mag or a luminosity of just
12 suns. About 30-40 stars are brighter than 16mag, but the majority is even fainter than 17mag. The stars are reddened and slightly dimmed in brightnes by dust
masses in front of the cluster. The reddening is about E(B-V)= 0.66mag.
Since very luminous stars are missing, Trumpler 5 is quite old. Its age is given with 2.9...4.0 billion years. The turnoff point is 1.25 solar masses.
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In my 20" f/3.2 telescope at a magnification of 120x "Trumpler 5" appears as a very faint, diffuse patch of light with a very low surface brightness. It can only
be seen with averted vision. But then it stands out quite well from the surrounding star field.
Things get interesting at 270x. The diffuse background almost disappears and in moments of very good seeing about 20-30 extremely faint stars flash out and than
vanish into the darkness the very next moment. Probably more aperture will be needed to see the single stars permanently.
