47 Tucanae is a bright globular cluster located 14,500 light-years away in the southern constellation Tucana. With an apparent magnitude of 4.09, the cluster is visible to the unaided eye on a clear night. It is the second brightest globular cluster in the sky, after Omega Centauri in the constellation Centaurus. 47 Tucanae is catalogued as NGC 104 in the New General Catalogue and as Caldwell 106 in Sir Patrick Moore’s Caldwell catalogue of deep sky objects visible in amateur telescopes.
47 Tucanae has a diameter of 120 light-years and an apparent size of 43.8 arcminutes, comparable to the size of the full Moon in good conditions. The cluster appears next to the Small Magellanic Cloud (SMC), a satellite galaxy of the Milky Way. Even though they appear close in the sky, the cluster is over 15 times closer to us than the dwarf galaxy and the two are not physically related.
NGC 104 contains millions of stars and is one of the most massive globular clusters in the Milky Way. It has an estimated mass of 700,000 Suns. The most massive stars lie in the cluster’s small, dense, bright core.
The cluster has the Shapley-Sawyer Concentration Class III, indicating a strong inner core of stars. Telescopes resolve about 10,000 stars in the cluster. Many of these stars are in the cluster’s centre.
The brightest star in 47 Tucanae is commonly referred to as the “Bright Star.” It is a hot blue giant of the spectral type B8III that has evolved past the asymptotic giant branch phase. The star has an effective temperature of 10,850 K and a luminosity 1,100 times that of the Sun. Like other post-AGB stars, it has lost a lot of its mass through a strong stellar wind and now has a mass about 54% that of the Sun.
47 Tucanae hosts at least two populations of stars with different metallicities and ages. Like other massive globular clusters, it contains a complex stellar population. The cluster’s core is home to a number of exotic stars, including blue stragglers. These are stars that are hotter, bluer, more massive and more luminous than expected for their evolutionary stage. They are believed to be the products of interactions and collisions between two or more stars in a tightly packed environment.
The cluster is also home to low-mass X-ray binaries containing neutron stars, black widow pulsars, and accreting white dwarfs. A 2021 study identified candidate symbiotic stars and RS CVn contact binaries.
In 2015, observations with NASA’s Chandra X-ray Observatory, the NuSTAR telescope and the Australia Telescope Compact Array revealed a star in the closest orbit known around a black hole. Designated 47 Tucanae X9, the system is composed of a black hole that is pulling gas from a white dwarf. The white dwarf orbits the black hole with a period of only 28 minutes at a separation of about 2.5 times the distance between the Earth and the Moon.
NGC 104 contains at least 25 millisecond pulsars. X-ray emission from 19 of these pulsars was detected by the Chandra X-ray Observatory in 2006. The radiation is believed to originate from the pulsars’ heated polar caps.
These stellar remnants were likely part of X-ray binary systems and accreted material from their companions. One of these companions, designated 47 Tucanae W, appears to still be transferring material onto its companion neutron star. This indicates that the system is evolving from a low-mass X-ray binary to a millisecond pulsar. The neutron star’s spin rate was increased by the mass transfer from the companion star.
47 Tucanae was the first globular cluster to be detected in gamma-rays. The gamma-ray emission from the cluster was detected by the Fermi Gamma-ray Space Telescope in 2009. It originates from the millisecond pulsars in the cluster. The gamma rays were believed to be produced in the outer magnetosphere of the neutron stars.
In dense clusters like 47 Tucanae, heavier members tend to move towards the centre while the less massive ones move toward the outskirts. The process is known as mass segregation. The first evidence of mass segregation in the cluster was reported in 2015, when a team led by Jeremy Heyl of the University of British Columbia (UBC), Vancouver, captured white dwarfs in the process of moving outward. The observations were made with NASA’s Hubble Space Telescope.
The cluster may host an intermediate-mass black hole in its centre. However, the existence of the black hole is uncertain. Data obtained with the Hubble Space Telescope in 2006 allowed astronomers to study the radial velocities of stars in the cluster’s core and constrain the mass of any central black hole to 1,000 – 1,500 solar masses.
A 2017 study led by Bülent Kızıltan and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, and Holger Baumgardt of the University of Queensland, Brisbane, Australia, provided evidence for the existence of a gas-starved black hole that was not accreting at a sufficient rate. Based on the distribution and motions of the pulsars in NGC 104, the researchers estimated that the black hole has a mass of about 2,300 solar masses. The study was published in the journal Nature. However, later long-term observations of the pulsars in the cluster yielded no evidence of a black hole.
47 Tucanae was the target of a survey for planets, conducted using the Hubble Space Telescope (HST) in 2000. The survey was conducted by an international team of astronomers led by Ronald Gilliland of the Space Telescope Science Institute (STScI) in Baltimore, Maryland.
Even though astronomers expected to find 10-15 planets in the cluster’s core, they detected none, which suggests that planets are not as common in globular clusters as previously believed. A survey of the less crowded outer regions of 47 Tucanae did not find any planets either, which indicates that the crowding in the cluster’s core is not as responsible for the absence of planets as the low metallicity of the cluster.
47 Tucanae has a similar apparent size to Omega Centauri. At its longest extension, it stretches across 43.8 arcminutes, while Omega Centauri has apparent dimensions of 36.3 arcminutes across. Omega Centauri lies further away, at a distance of 15,800 light-years from the solar system. It is much more loosely concentrated towards the centre and has the concentration class VIII. With an apparent magnitude of 3.9, it is the brightest globular cluster in the sky.
The third and fourth brightest globular clusters, the Sagittarius Cluster (Messier 22) in the constellation Sagittarius and the Great Peacock Globular (NGC 6752) in Pavo are smaller and a full magnitude fainter. M22 has an apparent magnitude of 5.1 and an apparent size of 32 arcminutes, while NGC 6752 shines at magnitude 5.4 and occupies 20.4 arcminutes. All four clusters lie in the southern sky and are easier to observe from the southern hemisphere.
Despite its brightness and size, 47 Tucanae is largely unfamiliar to northern observers. It only appears low above the horizon from northern locations that are closest to the equator.
47 Tucanae was not known to European astronomers before the 1750s, when French astronomer Nicolas-Louis de Lacaille spotted it from South Africa.
Lacaille spent four years (1750 – 1754) studying the southern sky from the Cape of Good Hope and is credited for creating 14 southern constellations. Tucana was not one of them. (It was created by Dutch astronomer Petrus Plancius in the late 16th century based on observations of Dutch explorers Pieter Dirkszoon Keyser and Frederick de Houtman.) However, when he created the new constellations, Lacaille reorganized this part of the southern sky, transferring 47 Tucanae and a part of the Small Magellanic Cloud to Tucana. The cluster and the galaxy had previously been assigned to the constellation Hydrus.
Lacaille observed the cluster in 1751-1752 and initially believed it was the nucleus of a comet. He catalogued it as Lac I-1 in his catalogue of deep sky objects, labelling it as “neb.”
Both 47 Tucanae and the brighter Omega Centauri have stellar designations. While Omega Centauri was given the Bayer designation by Johann Bayer himself based on Ptolemy’s description of the cluster as a star on the centaur’s back, 47 Tucanae was not assigned to the cluster by John Flamsteed (the English astronomer who assigned numbers to stars visible from southern England). The number “47” first appeared in German astronomer Johann Elert Bode’s Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss (“General description and verification of the stars and indexes”), published in 1801.
American astronomer Benjamin Apthorp Gould, creator of The Astronomical Journal, designated the cluster ξ Tucanae (Xi Tucanae), but the designation was not widely adopted. The cluster is still commonly referred to as 47 Tucanae.
In December 2008, Australian astronomer Ragbir Bhathal of the University of Western Sydney reported the detection of a strong laser-like signal emanating from 47 Tucanae as part of the Australian Optical SETI Project. However, subsequent searches in the same area did not find the signal again.
47 Tucanae is located in the constellation Tucana (the Toucan). It lies in the far southern sky and stays below the horizon for most observers in the northern hemisphere. At declination -72°, it is only 18 degrees from the south celestial pole and can only be seen from locations south of the latitude 18° N.
The cluster appears next to the Small Magellanic Cloud (SMC) in the sky. The Small Magellanic Cloud is a dwarf galaxy in the Milky Way’s neighbourhood, located approximately 200,000 light-years away. The galaxy is visible to the unaided eye and appears as a faint fuzzy patch on a clear, moonless night. It lies on the imaginary line drawn from Rigil Kentaurus (Alpha Centauri) to Achernar (Alpha Eridani). 47 Tucanae appears only 2.5 degrees west of the SMC.
The best time of the year to observe 47 Tucanae and other deep sky objects in Tucana is during the month of November, when the constellation climbs higher above the horizon in the evening.
47 Tucanae – NGC 104
|Right ascension||00h 24m 05.359s|
|Declination||−72° 04′ 53.20″|
|Distance||14,500 ± 32.6 light-years (4.45 ± 0.01 kiloparsecs)|
|Age||13.06 billion years|
|Names and designations||47 Tucanae, 47 Tuc, NGC 104, Caldwell 106, ξ Tucanae, Melotte 1, GCl 1, 1RXS J002404.6-720456|