NGC 2420

Lore & Background

NGC 2420 lies about 6.5 degrees south-southeast of the star Pollux and a bit over two degrees east-northeast of the Eskimo Nebula. Through a small telescope at low magnification it appears as a uniform ghostly light that gets brighter to the centre, similar to a tail-less comet; at ×33 magnification some individual stars can be glimpsed with averted vision. At higher magnification the cluster is resolved into a rich field of individual stars. It is included in the Herschel 400 Catalogue.

The cluster is located at a greater distance from the galactic centre than the Sun, lying in the direction of the galactic anti-centre, and lies 19° above the galactic plane, which corresponds to a distance of 3,000 light years. It has a Trumpler classification of I1r, indicating a rich detached cluster with a central concentration made out of stars of similar apparent magnitude. Photographic photometry down to magnitude 19 indicates that the cluster has over 500 members, with the brightest being of 11th magnitude. The total number of members is estimated to be around 1,000 within a diameter of 30 light years.

It is estimated that 41% of the stars of the cluster are binaries. The cluster has been found to contain two binary systems made out of two blue stragglers, while a third binary system is made out of a blue straggler and an extremely low mass white dwarf. These binary stars shine bright in far ultraviolet. Using data from the Hubble Space Telescope, von Hippel and Gilmore detected eight white dwarf candidates and estimated a cooling age of about 2 billion years. The metallicity of the cluster has been a subject of many studies and has been found to be a bit sub-solar, with a metallicity of -0.26 according to WEBDA. The cluster was found to be a bit metal poor, with a [Fe/H] of -0.7 to -0.6 from spectrographic studies performed in the 1980s while subsequent CCD photometry indicates a higher metallicity of about -0.30. Pancino et al. indicated a [Fe/H]= -0.05 ± 0.03, which is close to the trend of metallicity decreasing with increasing distance from the galactic centre. The red giants of the cluster have a mean metallicity of −0.16 ± 0.04. Based on spectrographic data of the Gaia-ESO survey, the stars around the turnoff point have sub-solar metallicities but they increase for stars of smaller mass. The stars of the cluster have homogenous CN and CH molecular band strengths. Similar low dispersion was observed in other elements.

Reader's Guide

NGC 2420 is notable as an old open cluster located far from the galactic centre, in the direction of the galactic anti-centre. Its age of about two billion years and its position 3,000 light years above the galactic plane make it a valuable object for studying stellar evolution and galactic structure. The cluster's rich population of over 500 confirmed members, with an estimated total of 1,000 stars within a 30-light-year diameter, provides a substantial sample for photometric and spectroscopic studies. The high binary fraction of 41%, including rare systems with blue stragglers and an extremely low mass white dwarf, offers insights into binary star evolution and the formation of blue stragglers. The detection of eight white dwarf candidates with the Hubble Space Telescope has allowed a cooling age estimate consistent with the cluster's overall age. The ongoing debate over its metallicity—ranging from [Fe/H] values of -0.7 to -0.05—reflects the challenges in measuring abundances in old clusters and highlights the cluster's role in understanding the metallicity gradient of the Milky Way. Its inclusion in the Herschel 400 Catalogue makes it a target for amateur astronomers, who can observe it as a ghostly glow at low power and resolve it into stars at higher magnification.

Did You Know?

• NGC 2420 was discovered by William Herschel on 19 November 1783.
• An estimated 41% of its stars are binaries.
• It contains two binary systems of two blue stragglers and one binary of a blue straggler and an extremely low mass white dwarf.