Star cluster

star clustersclusterCclustersstar cloudcluster of starsstellar clusterclusters of starsstellar clustersassociations of stars
Star clusters are very large groups of stars.wikipedia
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Globular cluster

globular clustersglobulartidal radius
Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of 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.
The name of this category of star cluster is derived from the Latin, globulus—a small sphere.

Open cluster

open star clusterstar clusteropen clusters
Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of 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.
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.

Star

starsstellarmassive star
Star clusters are very large groups of stars.
Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

Stellar association

OB associationassociationstellar associations
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 stellar association is a very loose star cluster, looser than both open clusters and globular clusters.

Pleiades

Pleiades star clusterThe PleiadesPleiades cluster
Star clusters visible to the naked eye include the Pleiades (M45), Hyades, and the Beehive Cluster (M44). The most prominent open clusters are the Pleiades and Hyades in Taurus.
It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky.

Messier object

MessierMessier CatalogueList of Messier objects
Star clusters visible to the naked eye include the Pleiades (M45), Hyades, and the Beehive Cluster (M44).
The catalogue consists of a diverse range of astronomical objects, from star clusters and nebulae to galaxies.

Hyades (star cluster)

HyadesHyades clusterHyades star cluster
Star clusters visible to the naked eye include the Pleiades (M45), Hyades, and the Beehive Cluster (M44). The most prominent open clusters are the Pleiades and Hyades in Taurus.
The Hyades (Greek Ὑάδες, also known as Caldwell 41, Melotte 25, or Collinder 50) is the nearest open cluster and one of the best-studied star clusters.

Galaxy

galaxiesgalacticgalactic nuclei
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.
Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery.

Solar mass

mass of the SunSun's masssolar masses
They commonly consist of very old Population II stars—just a few hundred million years younger than the universe itself—which are mostly yellow and red, with masses less than two solar masses.
It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies.

Stellar evolution

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Until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe.
Observations from the Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous Galactic protostars and their parent star clusters.

H II region

HII regionHII regionsH II
They form H II regions such as the Orion Nebula.
In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.

Beehive Cluster

PraesepeM44NGC 2632
Star clusters visible to the naked eye include the Pleiades (M45), Hyades, and the Beehive Cluster (M44).
Like many star clusters of all kinds, Praesepe has experienced mass segregation.

Naked eye

naked-eyeunaided eyenaked human eye
A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age.
The visibility of diffuse objects such as star clusters and galaxies is much more strongly affected by light pollution than is that of planets and stars.

Star formation

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Embedded clusters form in molecular clouds, when the clouds begin to collapse and form stars.
Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

Taurus (constellation)

TaurusTaurus constellationToro
The most prominent open clusters are the Pleiades and Hyades in Taurus.
This outline is created by prominent members of the Hyades, the nearest distinct open star cluster after the Ursa Major Moving Group.

Classical Cepheid variable

classical Cepheidδ Cep variableclassical Cepheids
Indeed, the open cluster NGC 7790 hosts three classical Cepheids which are critical for such efforts.
Delta Cephei is also of particular importance as a calibrator for the period-luminosity relation since its distance is among the most precisely established for a Cepheid, thanks in part to its membership in a star cluster

Galactic Center

Galactic Centregalactic corecenter
In our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the Galactic Centre, orbiting the centre in highly elliptical orbits.
The scenarios invoked to explain this formation involve either star formation in a massive star cluster offset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gas accretion disk around the central black-hole.

Orion Nebula

1976M42Messier 42
They form H II regions such as the Orion Nebula.
The entirety of the Orion Nebula extends across a 1° region of the sky, and includes neutral clouds of gas and dust, associations of stars, ionized volumes of gas, and reflection nebulae.

Hypercompact stellar system

hyper-compact stellar systemHypercompact
A hypercompact stellar system (HCSS) is a dense cluster of stars around a supermassive black hole that has been ejected from the center of its host galaxy.

Robust associations of massive baryonic objects

RAMBORAMBOsRobust associations of massive baryonic objects (RAMBOs)
In astronomy, a RAMBO or robust association of massive baryonic objects is a dark cluster made of brown dwarfs or white dwarfs.

Cepheid variable

CepheidCepheidsCepheid variables
Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are then used as standard candles.
Delta Cephei is also of particular importance as a calibrator of the Cepheid period-luminosity relation since its distance is among the most precisely established for a Cepheid, partly because it is a member of a star cluster

Main sequence

main-sequencemain-sequence starmain sequence dwarf
Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated.
The following year he began studying star clusters; large groupings of stars that are co-located at approximately the same distance.

Radiation pressure

solar radiation pressurelight pressurepressure of light
The embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through radiation pressure, stellar winds and outflows, or supernova explosions.
Stars predominantly form in regions of large clouds of dust and gases, giving rise to star clusters.

Cosmic distance ladder

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Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are then used as standard candles.
In a gravitationally-bound star cluster such as the Hyades, the stars formed at approximately the same age and lie at the same distance.

Hertzsprung–Russell diagram

Hertzsprung-Russell diagramHR diagramcolor-magnitude diagram
A Hertzsprung–Russell diagram can be plotted for these clusters which has absolute values known on the luminosity axis.
The H-R diagram can be used by scientists to roughly measure how far away a star cluster or galaxy is from Earth.