47 Tucanae (or NGC 104) is a globular cluster located in the constellation Tucana. It is about 16,700 light years away from Earth, and 120 light years across. It can be seen with the naked eye, with a visual apparent magnitude of 4.9. Its number comes not from the Flamsteed catalogue, but the more obscure 1801 "Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss" compiled by Johann Elert Bode.
In February 2017, indirect evidence for an intermediate-mass black hole in 47 Tucanae was announced.
47 Tucanae is the second brightest globular cluster in the sky (after Omega Centauri), and is noted for having a 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 210,000 light-years distant, over 10 times farther away.
The core of 47 Tucanae 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 10-15 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.
47 Tucanae's dense core contains a number of exotic stars of scientific interest. Globular clusters efficiently sort stars by mass, with the most massive stars falling to the center. 47 Tucanae contains at least 21 blue stragglers near its core. It also 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 Tucanae 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 (making 47 Tucanae the first globular cluster to be detected in gamma-rays).
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 in the motions of pulsars in the cluster.
47 Tucanae was discovered by Nicolas Louis de Lacaille in 1751, who thought it was the nucleus of a bright comet. Its southern location had hidden it from European observers until then.
In December 2008, Ragbir Bhathal of the University of Western Sydney claimed the detection of a strong laser-like signal from the direction of 47 Tucanae.
In May 2015, the first observations of the process of mass segregation in this globular cluster were announced.
In February 2017, the discovery of the first intermediate-mass black hole, about 2,200 times the mass of the Sun, was announced. Within the globular cluster, it was found by its actions on surrounding stars.