The cluster was given its Bayer designation ξ Tucanae in Johann Bayer's Uranometria. It was recorded as a cluster in 1751-2 by Nicolas-Louis de Lacaille, who initially thought it was the nucleus of a bright comet. Lacaille then listed it as "Lac I-1", the first object listed in his deep-sky catalogue. Its designated number "47" was assigned in Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss, compiled by Johann Elert Bode and published in Berlin in 1801. Bode did not observe this cluster himself, but had reordered Lacaille's catalogued stars by constellation in order of Right Ascension.
Characteristics
47 Tucanae is the second brightest globular cluster in the sky, and is noted for having a small very bright and dense core. It is one of the most massive globular clusters in the Galaxy, containing millions of stars. The cluster appears roughly the size of the full moon in the sky under ideal conditions. Though it appears adjacent to the Small Magellanic Cloud, the latter is some distant, being over fifteen times farther than 47 Tuc. The core of 47 Tuc was the subject of a major survey for planets, using the Hubble Space Telescope to look for partial eclipses of stars by their planets. No planets were found, though ten to fifteen were expected based on the rate of planet discoveries around stars near the Sun. This indicates that planets are relatively rare in globular clusters. A later ground-based survey in the uncrowded outer regions of the cluster also failed to detect planets when several were expected. This strongly indicates that the low metallicity of the environment, rather than the crowding, is responsible. s in 47 Tucanae 47 Tucanae contains at least two stellar populations of stars, of different ages or metallicities. The dense core contains a number of exotic stars of scientific interest, including at least 21 blue stragglers. Globular clusters efficiently sort stars by mass, with the most massive stars falling to the center. 47 Tucanae contains hundreds of X-ray sources, including stars with enhanced chromospheric activity due to their presence in binary star systems, cataclysmic variable stars containing white dwarfs accreting from companion stars, and low-mass X-ray binaries containing neutron stars that are not currently accreting, but can be observed by the X-rays emitted from the hot surface of the neutron star. 47 Tucanae has 25 known millisecond pulsars, the second largest population of pulsars in any globular cluster. These pulsars are thought to be spun up by the accretion of material from binary companion stars, in a previous X-ray binary phase. The companion of one pulsar in 47 Tucanae, 47 Tuc W, seems to still be transferring mass towards the neutron star, indicating that this system is completing a transition from being an accreting low-mass X-ray binary to a millisecond pulsar. X-ray emission has been individually detected from most millisecond pulsars in 47 Tucanae with the Chandra X-ray Observatory, likely emission from the neutron star surface, and gamma-ray emission has been detected with the Fermi Gamma-ray Space Telescope from its millisecond pulsar population.
Central black hole?
It is not yet clear whether 47 Tucanae hosts a central black hole. Hubble Space Telescope data constrain the mass of any possible black hole at the cluster's center to be less than approximately 1,500 solar masses. However, in February, 2017, astronomers announced that a black hole of some 2,200 solar masses may be located in the cluster; the researchers detected the black hole's signature from the motions and distributions of pulsars in the cluster. However, a recent analysis of an updated and more extensive timing data set on these pulsars provides no solid evidence in favor of the existence of a black hole.
