Anne’s Picture of the Day: Globular Cluster Messier 56

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February 27, 2013

Messier 56, a globular cluster in Lyra

Image Credit: NASA & ESA, Acknowledgement: Gilles Chapdelaine

Messier 56 (also known as NGC 6779) is a globular cluster of roughly 84 light-years across that lies about 32,900 light years away from the Earth, about 31,500 light-years from the Galactic Center and 4,800 light-years above the galactic plane in the constellation of Lyra. It is following a retrograde orbit through the Milky Way and is approaching us at 145 kilometers per second.

The cluster has a mass some 230,000 times that of the Sun and is composed of a large number of stars, tightly bound to each other by gravity, although it lacks the bright core possessed by most globulars. The brightest stars in Messier 56 are of 13th magnitude, while it contains only about a dozen known variable stars, like V6 (RV Tauri star; period: 90 days) or V1 (Cepheid: 1.510 days).

Messier 56’s age is estimated to be between 13 and 13.7 (!) billion years old and its metallicity (the abundance of elements other than hydrogen and helium) is very low, a sign of stars that were born early in the Universe’s history, before many of the elements in existence today were formed in significant quantities. This abundance is equivalent to 1% of that in the Sun.

The majority of clusters with this particular type of chemical makeup lie along a plane in the Milky Way’s halo. This suggests such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way’s globular cluster system as had been previously hypothesized. In this case, Messier 56 may have been acquired during the merger of a dwarf galaxy, of which Omega Centauri forms the surviving nucleus.

In 2000, a diffuse X-ray emission was identified coming from the vicinity of the cluster. This is most likely interstellar medium that has been heated by the passage of the cluster through the galactic halo. The relative velocity of the cluster is about 177 km s−1, which is sufficient to heat the interstellar medium in its wake to a temperature of 940,000 K.

Located about half-way between Beta Cygni (Albireo) and Gamma Lyrae, Messier 56 is a great object for both binoculars and small telescopes, but because it isn’t particularly large or bright, it does require dark skies. Visually, only about the inner third of this cluster is visible.

This image consists of visible and near-infrared exposures taken with the Advanced Camera for Surveys on the Hubble Space Telescope. A version of this image was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Gilles Chapdelaine. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public.

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