Collinder 154

Collinder 154 is an old, rich open star cluster in the constellation Gemini, also known as Cr 154, NGC 2420 and Melotte 69, about 2° 15′ east of the Eskimo Nebula (NGC 2392). It is about 8,300 light-years away (Gaia DR2). It contains about 685 members within a 20 arcminute radius (about 12 parsecs), with a core radius of only 1.3 arcminutes, and shows very little interstellar reddening. It stands out for its age and chemistry. At about 2 to 2.5 billion years (2.0 ± 0.2 billion years from its white dwarf cooling sequence, and 2.5 to 2.7 billion years from Gaia-ESO) it is an old open cluster, and it is metal-deficient ([Fe/H] about −0.2 to −0.45), which made it a classic “transition object” between solar-metallicity open clusters and metal-rich globulars. It lies high above the Galactic plane, in the outer disc. At magnitude 8.3 it is a modest telescope target, and William Herschel discovered it on 19 November 1783.