| 47 Tucanae | |
|---|---|
 After Omega Centauri, 47 Tucanae is the brightest globular cluster in the night sky.[1] | |
| Observation data (J2000epoch) | |
| Class | III[2] |
| Constellation | Tucana |
| Right ascension | 00h 24m 05.539s[3] |
| Declination | −72° 04′ 53.20″[3] |
| Distance | 4.45 ± 0.01 kpc (14,500 ± 32.6 ly)[4][5] |
| Apparent magnitude(V) | +4.09[6] |
| Apparent dimensions(V) | 43.8′ |
| Physical characteristics | |
| Mass | 7.00×105[7] M☉ |
| Radius | 60 ly[8] |
| VHB | 14.2 |
| Metallicity | = −0.78[9]dex |
| Estimated age | 13.06 Gyr[9] |
| Notable features | 2nd brightest globular cluster afterOmega Centauri |
| Other designations | ξ Tuc,NGC 104,Caldwell 106,Mel 1,[10] GCl 1,1RXS J002404.6-720456 |
| See also:Globular cluster,List of globular clusters | |
47 Tucanae or 47 Tuc (also designated as NGC 104 and Caldwell 106) is aglobular cluster located in theconstellationTucana. It is about 4.45 ± 0.01 kpc (14,500 ± 32.6 ly) fromEarth, and 120 light years in diameter. 47 Tuc can be seen with thenaked eye, with anapparent magnitude of 4.1.[10] It appears about 44arcminutes across including its far outreaches. Due to its far southern location, 18° from thesouth celestial pole, it was not catalogued by European astronomers until the 1750s, when the cluster was first identified byNicolas-Louis de Lacaille from South Africa.
47 Tucanae is thesecond brightest globular cluster afterOmega Centauri, and telescopically reveals about ten thousand stars, many appearing within a small dense central core. The cluster may contain anintermediate-mass black hole.[11][12]
The cluster was recorded in 1751-2 byNicolas-Louis de Lacaille, who initially thought it was the nucleus of a brightcomet.[13] Lacaille then listed it as "Lac I-1", the first object listed in his deep-sky catalogue. The number "47" was assigned inAllgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss ("General description and verification of the stars and indexes"), compiled byJohann Elert Bode and published in Berlin in 1801.[14] Bode did not observe this cluster himself, but had reordered Lacaille's catalogued stars by constellation in order ofright ascension.
In the 19th century,Benjamin Apthorp Gould assigned the Greek letter ξ (Xi) to the cluster to designate it ξ Tucanae, but this was not widely adopted and it is almost universally referred to as 47 Tucanae.[15]
47 Tucanae is the second brightest globular cluster in the sky (afterOmega 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 fullmoon in the sky under ideal conditions. Though it appears adjacent to theSmall Magellanic Cloud, the latter is some 60.6 ± 1.0 kpc (200,000 ± 3,300 ly) distant,[16] being over fifteen times farther than 47 Tuc.
A blue giant star with aspectral class of B8III is the brightest star in visible andultraviolet light, with aluminosity of about 1,100 times that of the Sun, and is aptly known as the "Bright Star". It is apost-AGB star, having passed theasymptotic giant branch phase of its life, and is currently fusinghelium. It has aneffective temperature of about 10,850 K, and is about 54% the mass of the Sun.[17]
The core of 47 Tuc was the subject of a major survey forplanets, using theHubble Space Telescope to look for partialeclipses of stars by their planets. No planets were found, though ten to fifteen were expected based on the rate of planet discoveries aroundstars near the Sun. This indicates that planets are relatively rare in globular clusters.[18] 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 lowmetallicity of the environment, rather than the crowding, is responsible.
47 Tucanae contains at least twostellar populations of stars, of different ages ormetallicities.[19] The dense core contains a number ofexotic stars of scientific interest, including at least 21blue stragglers.[20] Globular clusters efficiently sort stars by mass, with the most massive stars falling to the center.[21]
47 Tucanae contains hundreds of X-ray sources, including stars with enhancedchromospheric activity due to their presence inbinary star systems,cataclysmic variable stars containingwhite dwarfs accreting from companion stars andlow-mass X-ray binaries containingneutron stars that are not currently accreting, but can be observed by theX-rays emitted from the hot surface of the neutron star.[22]47 Tucanae has 35 known[23]millisecond pulsars, the second largest population ofpulsars in any globular cluster, afterTerzan 5.[24]These pulsars are thought to be spun up by theaccretion of material from binary companion stars, in a previousX-ray binary phase. The companion of one pulsar in 47 Tucanae, 47 Tuc W, seems to still be transferring mass towards its companion, indicating that this system is completing a transition from being an accreting low-mass X-ray binary to amillisecond pulsar.[25] X-ray emission has been individually detected from most millisecond pulsars in 47 Tucanae with theChandra X-ray Observatory, likely emission from the neutron star surface,[26] and gamma-ray emission has been detected with theFermi Gamma-ray Space Telescope from its millisecond pulsar population (making 47 Tucanae the first globular cluster to be detected in gamma-rays).[27]
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 1,500solar masses.[28] 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.[11] Despite this, 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.[12]
In December 2008,Ragbir Bhathal of theUniversity of Western Sydney claimed the detection of a strong laser-like signal from the direction of 47 Tucanae.[29]
In May 2015, the first evidence of the process ofmass segregation in this globular cluster was announced.[30] The cluster'sHertzsprung–Russell diagram suggests stars approximately 13 billion years old, which is unusually old.[31]
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