After Omega Centauri, 47 Tucanae is the brightest globular cluster in the night sky.
|Observation data (J2000 epoch)|
|Right ascension||00h 24m 05.67s|
|Declination||–72° 04′ 52.6″|
|Distance||4.0 ± 0.35 kpc (13,000 ± 1,100 ly)|
|Apparent magnitude (V)||+4.09|
|Apparent dimensions (V)||30′.9|
|Metallicity||= –0.78 dex|
|Estimated age||13.06 Gyr|
|Notable features||2nd brightest globular cluster after Omega Centauri|
|Other designations||ξ Tuc, NGC 104, GCl 1, Mel 1  Caldwell 106 1RXS J002404.6-720456|
47 Tucanae, 47 Tuc (or NGC 104) is a globular cluster located in the constellation Tucana. It is about 4.0 ± 0.35 kpc (13,000 ± 1,100 ly) away from Earth, and 120 light years across. 47 Tuc can be seen with the naked eye, with the apparent magnitude of 4.1v, and whose apparent size subtends about 50′ or 0.9° across. Due to its far southern location just 18° from the south celestial pole, it was not discovered until the 1750s, when the cluster was first identified by Nicolas-Louis de Lacaille from South Africa.
47 Tucanae is the second brightest globular cluster (following Omega Centauri), and telescopically reveals about ten thousand stars, many appearing within a small dense central core. In February 2017, indirect evidence for a likely intermediate-mass black hole in 47 Tucanae was announced based on the analysis of data on the millisecond pulsars in the cluster , however this is still open to debate .
It was first given its Bayer designation ξ Tucanae by Johann Bayer in 1603 within the published Uranometria Omnium Asterismorum ("Uranometry of all the asterisms"), or now simply as "Uranometria" , but was discovered as a cluster by Nicolas-Louis de Lacaille in 1751-2, who initially thought it was the nucleus of a bright comet. It then become the first object listed in his deep-sky catalogue as 'Lac I-1'. Its designated number '47' was later assigned in "Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss" (General description and verification of the stars and indexes) compiled by Johann Elert Bode and was published in Berlin during 1801. Bode did not observe this cluster himself, but had reordered Lacaille's catalogued stars by constellation by order of Right Ascension.
47 Tucanae is the second brightest globular cluster in the sky (after Omega Centauri), 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 60.6 ± 1.0 kpc (200,000 ± 3,300 ly) 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.
47 Tuc'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 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 (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 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 .
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