NGC 2129, Open Star Cluster
NGC 2129 is an open star cluster in the constellation Gemini at the border of the constellation Taurus. It was discovered in 1784 by the
famous astronomer F.W. Herschel.
The cluster is located at a distance of 7,200 light-years and lies in the same spiral arm as our Sun. From the apparent size of 6 arcminutes
the true diameter can be determined to 10 lightyears. With an age of only 10 million years NGC 2129 is very young, so it contains some
hot stars of spectral type B.
Most prominent are the two very bright stars HD 250290 and 250289 with magnitudes of 7.4mag and 8.3mag, corresponding to an absolute
magnitude of M=-4.4mag and a luminosity of 4,500 Suns. The temperature at their surface is 26,000 Kelvin (spectral type B2/B3). The
parallax measurement by GAIA gives values of 0.46mas and 0.49mas, so both stars can be counted to NGC 2129. From a distance of 5 light
years the two stars would be glistening bright with -8.5mag and therefore even be visible in the daytime sky.
----------------------------------------------------------------------------------------------------------------------------------------------
At a magnification of 120x the star cluster already appears wonderfully resolved in my 20 inch Dobsonian.
But NGC 2129 is most beautiful at 210x, because the sky background is very dark and the many stars sparkle like bright diamonds on black
velvet. There are only about 15 brighter stars, but the cluster itself appears quite compact and stands out well against the star-poor surroundings.
The stars are evenly distributed.
If you are just at Messier 35, a detour to NGC 2129 is definitely worthwhile.
