Terzan 5 | |
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![]() Hubble image of Terzan 5 | |
Observation data (J2000 epoch) | |
Class | G2 [1] |
Constellation | Sagittarius |
Right ascension | 17h 48m 05s [2] |
Declination | −24° 46′ 48″ [2] |
Distance | 18.8 ± 1.6 kly [note 1] (5.9 ± 0.5 kpc [4] ) |
Apparent magnitude (V) | 12.8 [1] |
Apparent dimensions (V) | 1′02″ (half-mass diameter) [4] |
Physical characteristics | |
Mass | ~2×106 [4] M☉ (4 × 1036 kg) |
Radius | 2.7 ly [note 2] |
VHB | 22.5 [3] |
Metallicity | = −0.21 dex |
Estimated age | 12 Gyr [6] |
Notable features | Possibly the core of a disrupted dwarf galaxy |
Other designations | Ter 5, IRC–20385 |
Terzan 5 is a heavily obscured globular cluster belonging to the bulge (the central star concentration) of the Milky Way galaxy. [3] It was one of six globulars discovered by French [7] astronomer Agop Terzan in 1968 [8] and was initially labeled Terzan 11. The cluster was cataloged by the Two-Micron Sky Survey as IRC–20385. [9] It is situated in the Sagittarius constellation in the direction of the Milky Way's center. Terzan 5 probably follows an unknown complicated orbit around the center of the galaxy, but currently it is moving towards the Sun with a speed of around 90 km/s. [6]
The absolute magnitude of Terzan 5 is at least MV=−7.5. [1] Its bolometric luminosity is about 800,000 times that of the Sun, while its mass is about 2 million solar masses. [4] The small core of Terzan 5—about 0.5 pc in size [4] —has one of the highest star densities in the galaxy. Its volume mass density exceeds 106M☉/pc3, [10] while its volume luminosity density exceeds 105.5L☉/pc3, where M☉ and L☉ are the Sun's mass and luminosity, respectively. The cluster also has one of the highest metallicities among the Milky Way's globular clusters—[Fe/H]=−0.21. [5]
In 2009 it was discovered that Terzan 5 consists of at least two generations of stars with ages of 12 and 4.5 billion years and slightly different metallicities, possibly indicating that it is the core of a disrupted dwarf galaxy, not a true globular cluster. [6] There are only a few other globular clusters in the Milky Way that contain stars with different ages. Among them are M54, Omega Centauri and Liller 1. The latter, like Terzan 5, is thought to be a fossil fragment from the assembly of the galactic bulge. [11] The cluster also contains around 1300 core helium burning horizontal branch (HB) stars, [6] including at least one RR Lyrae variable star. [10]
Terzan 5 is known to contain 49 millisecond radio pulsars as of December 2023, [12] the largest MSP population among all globular clusters in the Galaxy; their true number may be as high as 200. [13] The first such object, PSR B1744-24A, discovered in 1990, has the period of 11.56 ms. The population of pulsars inside Terzan 5 includes PSR J1748–2446ad, the fastest known millisecond pulsar, which is spinning at 716 Hz (the rotation period is 1.40 ms). [14]
Terzan 5 also contains an X-ray burster, discovered in 1980, known as Terzan 5 or XB 1745-25. [15] It also contains around 50 weaker X-ray sources, many of which are likely Low-mass X-ray binaries (LMXB) or cataclysmic variables. [16]
The large number of X-ray sources and millisecond pulsars may be a direct consequence of the high density of the cluster's core, which leads to a high rate of star collisions, and to formation of close binaries, including binary systems which contain a neutron star. [16]
In addition to discrete X-ray sources Terzan 5 produces a diffuse non-thermal X-ray emission and high (a few GeV) and ultra-high (0.5–24 TeV) energy gamma-rays. The high energy gamma rays probably originate in the magnetosphere of abundant millisecond pulsars, while ultra-high energy gamma rays likely result from the inverse Compton scattering by the relativistic electron emitted by the pulsars off the cosmic microwave background radiation. [13]
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Robert Michael Rich is an American astrophysicist. He obtained his B.A. at Pomona College in 1979 and earned his Ph.D. from the California Institute of Technology in 1986 under thesis supervisor Jeremy R. Mould. He was a Carnegie Fellow at Carnegie/DTM until 1988 when he joined the faculty of Columbia University where he was the doctoral thesis adviser to Neil deGrasse Tyson, and is on the faculty of the University of California, Los Angeles.