NGC 104 - 47 Tucanae

NGC 104, better known as 47 Tucanae, is the second brightest and largest globular cluster in the sky (after Omega Centauri). It is included in the Caldwell catalogue as C 106.

Although a conspicuous naked-eye object, 47 Tucanae lies very far south, at declination -72°. Thus, it was not discovered by European observers until 1751, when Nicholas de Lacaille catalogued it in his list of southern nebulous objects. Next to observe and catalog it were James Dunlop in 1826, and John Herschel in 1834.

At magnitude 4.0, this cluster looks like a misty star to the unaided eye. This has earned it the Flamsteed designation 47 Tucanae. Under ideal conditions, the cluster appears 30' across in the sky - roughly the size of the full Moon. Binoculars clearly show an increase in brightness toward the center, and a telescope of at least 4" aperture resolves some of its roughly one million member stars. The cluster is noted for having a very bright and dense core.

The stars of 47 Tucanae are about 13,400 light years away, and spread over a volume nearly 120 light years across. Globular cluster 47 Tucanae is approaching us at roughly 19 km/sec, and is home to a number of exotic x-ray binary star systems.

A search for Jupiter-sized planets in 47 Tuc, carried out by a team of astronomers using the Hubble Space Telescope, came up empty-handed. The work involved checking 34,000 stars in the cluster for signs of large transiting planets. The absence of any positive results strengthens the argument that planets are rare or nonexistent in globular clusters because of their very low concentration of heavy elements.