The Quintuplet Cluster is a dense open cluster located at a distance of 26,000 light years from Earth in the direction of Sagittarius constellation and only about 100 light years from the Galactic centre.
Like the nearby Arches Cluster, it cannot be observed visually because it is obscured by dust clouds, but must be studied in infrared, radio and X-ray wavelengths. The cluster is located 12 arc minutes to the northwest of Sagittarius A*.
The Quintuplet Cluster was named for the five prominent infrared sources that it contains. The cluster was first detected in 1983 as a pair of infrared sources, which were designated GCS-3 and GCS-4 (Galactic Centre Source 3 and 4). GCS-3 was later resolved into four separate sources that, together with GCS-4, formed a quintuplet. The cluster itself was discovered in 1990, when infrared observations allowed researchers to see it through the obscuring dust near the Galactic centre. The objects were believed to be hot, luminous young stars. The first five stars discovered, still believed to be protostars, were labelled Q1, Q2, Q3, Q4 and Q9. The stars Q5 and Q6 were later identified as members of the same cluster.
Observations in 1994 indicated that the stars were more evolved than originally believed. Researchers discovered broad helium emission lines in the stars’ spectra. Two of the stars were classified as Wolf-Rayet stars, one as a luminous blue variable (which was thought to be one of the most luminous stars in the Milky Way at the time), and several were identified as red supergiants. The supergiants are some of the largest stars in the Milky Way and are expected to go through their supply of fuel quickly and have a very short lifetime.
A study of almost 600 stars in the cluster conducted in 1999 revealed that the Quintuplet had another luminous blue variable, as well as more Wolf-Rayet stars than any other cluster discovered to date.
The Quintuplet Cluster is less dense than the Arches Cluster and contains fewer extremely massive and luminous stars. It is also slightly older than its neighbour, as indicated by the number of red supergiants in the cluster. The Quintuplet has an estimated age of 4 million years.
Stars
The most luminous star in the cluster is WR 102hb, a Wolf-Rayet star with a magnitude of 9.6, temperature of 25,100 K, and luminosity 2,600,000 times that of the Sun. WR 102ea, the second most luminous star, has the same surface temperature, a luminosity 2,500,000 times solar and an apparent magnitude of 8.8.
The Quintuplet Cluster is home to several luminous blue variables (LBV), including the famous Pistol Star and the less well-known V4650 Sagittarii (qF362). These stars are expected to end their lives in supernova explosions within the next 3 million years. They are among the most luminous stars known.
The Pistol Star has a luminosity of 1,600,000 Suns and V4650 Sgr, of 1,800,000 Suns. The runaway star V4998 Sagittarii is also a luminous blue variable, positioned about 23 light years away from the cluster. The star is surrounded by an ejection nebula more than 0.8 parsecs in diameter and has a luminosity about 4,000,000 times that of the Sun.
The clusters, Arches (upper right), Quintuplet (upper center), and the GC cluster (bottom center), contain massive stars that appear as very bright, point-like X-ray sources when winds from their surfaces collide with those from an orbiting companion. Vast amounts of energy are also released when these stars explode as supernovas, heating the surrounding material.
Named for its arc-like appearance, the Sickle Nebula contains fingers of ionized gas that are illuminated by the extremely hot stars in the cluster. The fingers are shaped by the strong stellar winds and radiation from the massive stars.
The Quintuplet Cluster is believed to be physically associated with the Galactic centre because the stars and other objects, including the Pistol Nebula, have large radial velocities, likely as a result of orbiting so close to the Milky Way centre.
The cluster will be torn apart by the gravitational tidal forces in the galaxy’s core in a few million years.
The Quintuplet Cluster
Constellation: Sagittarius
Right ascension: 17h 46m 13.9s
Declination: −28° 49′ 48″
Apparent size: 50”
Distance: 26,000 light years (8,000 parsecs)
Mass: 10,000 solar masses
Age: 4 million years
Designations: Quintuplet Cluster, IRAS 17430-2848, G000.16-00.06
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