47 Tucanae

Early history

The cluster was recorded in 1751-2 by Nicolas-Louis de Lacaille, who initially thought it was the nucleus of a bright comet.{{cite book Lacaille then listed it as "Lac I-1", the first object listed in his deep-sky catalogue. The number "47" was 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 published in Berlin in 1801.{{cite book Bode did not observe this cluster himself, but had reordered Lacaille's catalogued stars by constellation in order of right 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.

Characteristics

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 ± distant,{{cite journal

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.{{cite web |access-date = 16 November 2010 |archive-url = https://web.archive.org/web/20100820190933/http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=192 |archive-date = 2010-08-20 |url-status = dead 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.{{cite news |access-date=5 May 2006}} Globular clusters efficiently sort stars by mass, with the most massive stars falling to the center.{{cite web |access-date=14 November 2010}}

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.{{cite journal 47 Tucanae has 35 known{{cite web |access-date= 2016-04-27 |archive-date= 2021-03-18 |archive-url= https://web.archive.org/web/20210318161142/http://www.naic.edu/~pfreire/GCpsr.html |url-status= dead 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 its companion, indicating that this system is completing a transition from being an accreting low-mass X-ray binary to a millisecond pulsar.{{cite journal |display-authors=etal |archive-url=https://web.archive.org/web/20240215163621/https://www.openaccessrepository.it/record/140999 |url-status=dead |archive-date=February 15, 2024 |url-access=subscription

The cluster's brightest star in visible and ultraviolet light is a blue giant star with a spectral class of B8III and with a luminosity of about 1,100 times that of the Sun. Aptly known as the "Bright Star", it is a post-AGB star, having passed the asymptotic giant branch phase of its life, and is currently fusing helium. It has an effective temperature of about 10,850 K, and is about 54% the mass of the Sun.

Possible 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.{{cite journal |display-authors=etal

Modern discoveries

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.{{cite web

In May 2015, the first evidence of the process of mass segregation in this globular cluster was announced.{{cite news

References

References

1. "Retirement in the suburbs".
2. (2006). "The Two Micron All Sky Survey (2MASS)". The Astronomical Journal.
3. (2012). "Ultraviolet Properties of Galactic Globular Clusters with Galex. II. Integrated Colors". The Astronomical Journal.
4. distance × sin( diameter_angle / 2 ) = 60 ly. radius
5. (2009). "Mixed Populations in Globular Clusters: Et Tu, 47 Tuc?". The Astrophysical Journal Letters.
6. (2021). "Observations of the Bright Star in the Globular Cluster 47 Tucanae (NGC 104)". The Astronomical Journal.
7. "The Astrophysics Spectator: The Hertzsprung-Russell Diagrams of Star Clusters".
8. (May 2010). "Accreted versus in situ Milky Way globular clusters". [[Monthly Notices of the Royal Astronomical Society]].
9. (August 2010). "Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way". Monthly Notices of the Royal Astronomical Society.
10. "NGC 104".