Please answer the following question: Information: - Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a "moving group". - A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek "", literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral and irregular. Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang. - An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. - A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin "globulus"a small sphere. A globular cluster is sometimes known more simply as a "globular". - A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful telescopes. - The Arches Cluster is the densest known star cluster in the Milky Way located about 100 light years from its center , in the constellation Sagittarius ( The Archer ) , 25,000 light - years from Earth . The discovery of this cluster was reported by Nagata et al . in 1995 , and independently by Cotera et al . in 1996 . Due to extremely heavy optical extinction by dust in this region , the Arches Cluster is obscured in the visual bands , and is observed in the X-ray , infrared , and radio bands . It contains approximately 135 young , very hot stars that are many times larger and more massive than the Sun , plus many thousands of less massive stars . This star cluster is estimated to be around two and a half million years old . Although larger and denser than the nearby Quintuplet Cluster , it appears to be slightly younger . The most evolved stars are barely edging away from the main sequence while the Quintuplet Cluster includes a number of hot supergiants as well as a red supergiant and three Luminous Blue Variables . Work by Donald Figer , an astronomer at the Rochester Institute of Technology suggests that 150 solar masses ( M ) is the upper limit of stellar mass in the current era of the universe . He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster and found no stars over that limit despite a statistical expectation that there should be several . However , later research demonstrated a very high sensitivity of the calculated star masses upon the extinction laws used for mass derivation , which can affect the upper mass limit by about 30 % using different extinction laws ( possibly from 150 M to about 100 M ) . 'arches cluster' is related to which object entity through the relation of 'instance of'? Choices: - 1 - age - cloud - cluster - collection - dark matter - direction - distance - formation - four - galaxy - giant - globular cluster - group - hundred - march - matter - molecular cloud - morphology - open cluster - reference - size - sky - space - sphere - star - star cluster - system - time - two
A:
open cluster