Stellar kinematics

In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. The subject of stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as the measurement of the internal kinematics of more distant galaxies. Measurement of the kinematics of stars in different subcomponents of the Milky Way including the thin disk, the thick disk, the bulge, and the stellar halo provides important information about the formation and evolutionary history of our Galaxy. Kinematic measurements can also identify exotic phenomena such as hypervelocity stars escaping from the Milky Way, which are interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center.

Stellar kinematics is related to but distinct from the subject of stellar dynamics, which involves the theoretical study or modeling of the motions of stars under the influence of gravity. Stellar-dynamical models of systems such as galaxies or star clusters are often compared with or tested against stellar-kinematic data to study their evolutionary history and mass distributions, and to detect the presence of dark matter or supermassive black holes through their gravitational influence on stellar orbits.

Space velocity

Proper motion
Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = vt / d with vt = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity vt.)

The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed.[1] This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion.[2] The Sun's motion with respect to the LSR is called the "peculiar solar motion".

The components of space velocity in the Milky Way's Galactic coordinate system are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic Center, V positive in the direction of galactic rotation, and W positive in the direction of the North Galactic Pole.[3] The peculiar motion of the Sun with respect to the LSR is[4]

(U, V, W) = (11.1, 12.24, 7.25) km/s,

with statistical uncertainty (+0.69−0.75, +0.47−0.47, +0.37−0.36) km/s and systematic uncertainty (1, 2, 0.5) km/s. (Note that V is 7 km/s larger than estimated in 1999 by Dehnen et al.[5])

Use of kinematic measurements

Stellar kinematics yields important astrophysical information about stars, and the galaxies in which they reside. Stellar kinematics data combined with astrophysical modeling produces important information about the galactic system as a whole. Measured stellar velocities in the innermost regions of galaxies including the Milky Way have provided evidence that many galaxies host supermassive black holes at their center. In farther out regions of galaxies such as within the galactic halo, velocity measurements of globular clusters orbiting in these halo regions of galaxies provides evidence for dark matter. Both of these cases derive from the key fact that stellar kinematics can be related to the overall potential in which the stars are bound. This means that if accurate stellar kinematics measurements are made for a star or group of stars orbiting in a certain region of a galaxy, the gravitational potential and mass distribution can be inferred given that the gravitational potential in which the star is bound produces its orbit and serves as the impetus for its stellar motion. Examples of using kinematics combined with modeling to construct an astrophysical system include:

  • Rotation of the Milky Way's Disc From the proper motions and radial velocities of stars within the Milky way disc one can show that there is differential rotation. When combining these measurements of stars' proper motions and their radial velocities, along with careful modeling, it is possible to obtain a picture of the rotation of the Milky Way disc. The local character of galactic rotation in the solar neighborhood is encapsulated in the Oort constants.
  • Structural Components of The Milky Way Using stellar kinematics, astronomers construct models which seek to explain the overall galactic structure in terms of distinct kinematic populations of stars. This is possible because these distinct populations are often located in specific regions of galaxies. For example, within the Milky Way, there are three primary components, each with its own distinct stellar kinematics: the disc, halo and bulge or bar. These kinematic groups are closely related to the stellar populations in the Milky Way, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. For the Milky Way, the speed of disk stars is and an RMS velocity relative to this speed of . For bulge population stars, the velocities are randomly oriented with a larger relative RMS velocity of and no net circular velocity.[6] The Galactic stellar halo consists of stars with orbits that extend to the outer regions of the galaxy. Some of these stars will continually orbit far from the galactic center, while others are on trajectories which bring them to various distances from the galactic center. These stars have little to no average rotation. Many stars in this group belong to globular clusters which formed long ago and thus have a distinct formation history, which can be inferred from their kinematics and poor metallicities. The halo may be further subdivided into an inner and outer halo, with the inner halo having a net prograde motion with respect to the Milky Way and the outer a net retrograde motion.[7]
  • External Galaxies Spectroscopic observations of external galaxies make it possible to characterize the bulk motions of the stars they contain. While these stellar populations in external galaxies are generally not resolved to the level where one can track the motion of individual stars (except for the very nearest galaxies) measurements of the kinematics of the integrated stellar population along the line of sight provides information including the mean velocity and the velocity dispersion which can then be used to infer the distribution of mass within the galaxy. Measurement of the mean velocity as a function of position gives information on the galaxy's rotation, with distinct regions of the galaxy that are redshifted / blueshifted in relation to the galaxy's systemic velocity.
  • Mass distributions Through measurement of the kinematics of tracer objects such as globular clusters and the orbits of nearby satellite dwarf galaxies, we can determine the mass distribution of the Milky Way or other galaxies. This is accomplished by combining kinematic measurements with dynamical modeling.

Recent advancements due to Gaia

In 2018 the Gaia data release 2 has yielded an unprecedented number of high quality stellar kinematic measurements as well as stellar parallax measurements which will greatly increase our understanding of the structure of the Milky Way. The Gaia data has also made it possible to determine the proper motions of many objects whose proper motions were previously unknown, including the absolute proper motions of 75 globular clusters orbiting at distances as far as 21 kpc.[8] In addition, the absolute proper motions of nearby dwarf spheroidal galaxies have also been measured, providing multiple tracers of mass for the Milky Way.[9] This increase in accurate measurement of absolute proper motion at such large distances is a major improvement over past surveys, such as those conducted with the Hubble Space Telescope.

Stellar kinematic types

Stars within galaxies may be classified based on their kinematics. For example, the stars in the Milky Way can be subdivided into two general populations, based on their metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that population I stars with higher metallicity are generally located in the stellar disk while older population II stars are in random orbits with little net rotation.[10] The latter have elliptical orbits that are inclined to the plane of the Milky Way.[10] Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds.[11]

There are many additional ways to classify stars based on their measured velocity components, and this provides detailed information about the nature of the star's formation time, its present location, and the general structure of the galaxy. As a star moves in a galaxy, the smoothed out gravitational potential of all the other stars and other mass within the galaxy plays a dominant role in determining the stellar motion.[12] Stellar kinematics can provide insights into the location of where the star formed within the galaxy. Measurements of an individual star's kinematics can identify stars that are peculiar outliers such as a high-velocity star moving much faster than its nearby neighbors.

High-velocity stars

Depending on the definition, a high-velocity star is a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighborhood. The velocity is also sometimes defined as supersonic relative to the surrounding interstellar medium. The three types of high-velocity stars are: runaway stars, halo stars and hypervelocity stars. High-velocity stars were studied by Jan Oort, who used their kinematic data to predict that high-velocity stars have very little tangential velocity.[13]

Runaway stars

Hs-2009-03-a-web print
Four runaway stars plowing through regions of dense interstellar gas and creating bright bow waves and trailing tails of glowing gas. The stars in these NASA Hubble Space Telescope images are among 14 young runaway stars spotted by the Advanced Camera for Surveys between October 2005 and July 2006

A runaway star is one that is moving through space with an abnormally high velocity relative to the surrounding interstellar medium. The proper motion of a runaway star often points exactly away from a stellar association, of which the star was formerly a member, before it was hurled out.

Mechanisms that may give rise to a runaway star include:

  • Gravitational interactions between stars in a stellar system can result in large accelerations of one or more of the involved stars. In some cases, stars may even be ejected.[14] This can occur in seemingly stable star systems of only three stars, as described in studies of the three-body problem in gravitational theory.[15]
  • A collision or close encounter between stellar systems, including galaxies, may result in the disruption of both systems, with some of the stars being accelerated to high velocities, or even ejected. A large-scale example is the gravitational interaction between the Milky Way Galaxy and the Large Magellanic Cloud.[16]
  • A supernova explosion in a multiple star system can accelerate both the supernova remnant and/or remaining stars to high velocities.[17][18]

Multiple mechanisms may accelerate the same runaway star. For example, a massive star that was originally ejected due to gravitational interactions with its stellar neighbors may itself go supernova, producing a remnant with a velocity modulated by the supernova kick. If this supernova occurs in the very nearby vicinity of other stars, it is possible that it may produce more runaways in the process.

An example of a related set of runaway stars is the case of AE Aurigae, 53 Arietis and Mu Columbae, all of which are moving away from each other at velocities of over 100 km/s (for comparison, the Sun moves through the Milky Way at about 20 km/s faster than the local average). Tracing their motions back, their paths intersect near to the Orion Nebula about 2 million years ago. Barnard's Loop is believed to be the remnant of the supernova that launched the other stars.

Another example is the X-ray object Vela X-1, where photodigital techniques reveal the presence of a typical supersonic bow shock hyperbola.

Halo stars

Halo stars are very old stars that do not share the motion of the Sun or most other stars in the solar neighbourhood which are in similar circular orbits around the center of the Milky Way. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Milky Way. Although their orbital velocities in the Milky Way may be no faster than the Sun's, their different paths result in the high relative velocities.

Typical examples are the halo stars passing through the disk of the Milky Way at steep angles. One of the nearest 45 stars, called Kapteyn's Star, is an example of the high-velocity stars that lie near the Sun. Its observed radial velocity is -245 km/s, and the components of its space velocity are U = 19 km/s, V = -288 km/s, and W = -52 km/s.

Hypervelocity stars

Hypervelocity stars (designated as HVS or HV in stellar catalogues) are stars with velocities that deviate substantially from the normal velocity of stars in a galaxy. Such stars may have velocities so great that they exceed the escape velocity of the galaxy.[19] In the Milky Way, stars usually have velocities on the order of 100 km/s, whereas hypervelocity stars, have velocities on the order of 1000 km/s. These fast moving stars are more numerous near the center of the Milky Way, which is also where most are thought to be produced. One of the fastest known stars in our Galaxy is the O-class sub-dwarf US 708, moving away from the Milky Way with a total velocity of around 1200 km/s.

The existence of HVSs was first predicted by Jack G. Hills in 1988,[20] and their existence confirmed by Warren Brown, Margaret Geller, Scott Kenyon, and Michael Kurtz in 2005.[21] As of 2008, 10 unbound HVSs were known, one of which was believed to have originated from the Large Magellanic Cloud rather than the Milky Way.[22] Further measurements placed its origin within the Milky Way.[23] Due to uncertainty about the mass distribution within the Milky Way, determining whether a HVS is unbound is difficult; five additional known high-velocity stars may be unbound from the Milky Way and 16 HVSs are thought to be bound. The nearest currently known HVS (HVS2) is about 19 kpc from the Sun.

As of 1 September 2017, there have been roughly 20 observed hypervelocity stars. Though most of these were observed in the Northern Hemisphere, the possibility remains that there are HVSs only observable from the Southern Hemisphere.[24]

It is believed that about 1,000 HVSs exist in the Milky Way.[25] Considering that there are around 100 billion stars in the Milky Way, this is a minuscule fraction (~0.000001%). Since the second data release of Gaia (DR2), most high-velocity late-type stars are found to have a high probability of being bound to the Milky Way.[26]

In March 2019, LAMOST-HVS1 was reported to be a confirmed hypervelocity star ejected from the stellar disk of the Milky Way galaxy.[27]

Origin of hypervelocity stars

HVSs are believed to predominately originate by close encounters of binary stars with the supermassive black hole in the center of the Milky Way. One of the two partners is gravitationally captured by the black hole (in the sense of entering orbit around it), while the other escapes with high velocity, becoming an HVS. Such maneuvers are analogous to the capture and ejection of interstellar objects by a star.

Supernova-induced HVSs may also be possible, although they are presumably rare. In this scenario, a HVS is ejected from a close binary system as a result of the companion star undergoing a supernova explosion. Ejection velocities up to 770 km/s, as measured from the galactic rest frame, are possible for late-type B-stars.[28] This mechanism can explain the origin of HVSs which are ejected from the galactic disk.

Known HVSs are main-sequence stars with masses a few times that of the Sun. HVSs with smaller masses are also expected and G/K-dwarf HVS candidates have been found.

HVSs that have come into the Milky Way came from the dwarf galaxy Large Magellanic Cloud. When the dwarf galaxy made its closest approach to the center of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some of its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space, due to the slingshot-like effect of the boost.[29]

Some neutron stars are inferred to be traveling with similar speeds. This could be related to HVSs and the HVS ejection mechanism. Neutron stars are the remnants of supernova explosions, and their extreme speeds are very likely the result of an asymmetric supernova explosion or the loss of their near partner during the supernova explosions that forms them. The neutron star RX J0822-4300, which was measured to move at a record speed of over 1,500 km/s (0.5% of the speed of light) in 2007 by the Chandra X-ray Observatory, is thought to have been produced the first way.[30]

Some kind of supernovas are expected to happen if a white dwarf collides with its nearby partner and consumes the outer matter of this partner. The white dwarf and its nearby partner have very high orbital velocities at this time. The huge mass lost of the white dwarf during the supernova causes the nearby partner to leave at its previous huge orbital speed of several hundred kilometers per second as a hypervelocity star. The supernova remnant of the exploding white dwarf leaves because of its own high orbital speed as a new fast-traveling neutron star. This seems to be the most likely origin of most HVSs and fast-traveling neutron stars.

Partial list of HVSs

As of 2014, twenty HVS were known.[31][25]

  • HVS 1 – (SDSS J090744.99+024506.8) (a.k.a. The Outcast Star) – the first hypervelocity star to be discovered[21]
  • HVS 2 – (SDSS J093320.86+441705.4 or US 708)
  • HVS 3 – (HE 0437-5439) – possibly from the Large Magellanic Cloud[22]
  • HVS 4 – (SDSS J091301.00+305120.0)
  • HVS 5 – (SDSS J091759.42+672238.7)
  • HVS 6 – (SDSS J110557.45+093439.5)
  • HVS 7 – (SDSS J113312.12+010824.9)
  • HVS 8 – (SDSS J094214.04+200322.1)
  • HVS 9 – (SDSS J102137.08-005234.8)
  • HVS 10 – (SDSS J120337.85+180250.4)
  • TYC 8840-1782-1

Kinematic groups

A set of stars with similar space motion and ages is known as a kinematic group.[32] These are stars that could share a common origin, such as the evaporation of an open cluster, the remains of a star forming region, or collections of overlapping star formation bursts at differing time periods in adjacent regions.[33] Most stars are born within molecular clouds known as stellar nurseries. The stars formed within such a cloud compose gravitationally bound open clusters containing dozens to thousands of members with similar ages and compositions. These clusters dissociate with time. Groups of young stars that escape a cluster, or are no longer bound to each other, form stellar associations. As these stars age and disperse, their association is no longer readily apparent and they become moving groups of stars.

Astronomers are able to determine if stars are members of a kinematic group because they share the same age, metallicity, and kinematics (radial velocity and proper motion). As the stars in a moving group formed in proximity and at nearly the same time from the same gas cloud, although later disrupted by tidal forces, they share similar characteristics.[34]

Stellar associations

A stellar association is a very loose star cluster, whose stars share a common origin, but have become gravitationally unbound and are still moving together through space. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.

Stellar associations were first discovered by the Armenian astronomer Viktor Ambartsumian in 1947.[35] The conventional name for an association uses the names or abbreviations of the constellation (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.

Types

Infrared VISTA view of a nearby star formation in Monoceros
Infrared ESO's VISTA view of a stellar nursery in Monoceros.

Viktor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars.[35] A third category, R, was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae.[36] The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work.[37] Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.

OB associations

Carina Nebula
Carina OB1, a large OB association.

Young associations will contain 10 to 100 massive stars of spectral class O and B, and are known as OB associations. In addition, these associations also contain hundreds or thousands of low- and intermediate-mass stars. Association members are believed to form within the same small volume inside a giant molecular cloud. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart.[38] It is believed that the majority of all stars in the Milky Way were formed in OB associations.[38] O-class stars are short-lived, and will expire as supernovae after roughly one million years. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within ten million years. (Compare this to the current age of the Sun at about five billion years.)

The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun.[39] The nearest OB association is the Scorpius–Centaurus Association, located about 400 light-years from the Sun.[40]

OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy. These associations can be quite sparse, spanning 1,500 light-years in diameter.[11]

T associations

Young stellar groups can contain a number of infant T Tauri stars that are still in the process of entering the main sequence. These sparse populations of up to a thousand T Tauri stars are known as T associations. The nearest example is the Taurus-Auriga T association (Tau-Aur T association), located at a distance of 140 parsecs from the Sun.[41] Other examples of T associations include the R Corona Australis T association, the Lupus T association, the Chamaeleon T association and the Velorum T association. T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. Group members have the same age and origin, the same chemical composition, and the same amplitude and direction in their vector of velocity.

R associations

Associations of stars that illuminate reflection nebulae are called R associations, a name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution.[36] These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed.[37] This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms.[42] An example of an R association is Monoceros R2, located 830 ± 50 parsecs from the Sun.[37]

Moving groups

If the remnants of a stellar association drift through the Milky Way as a somewhat coherent assemblage, then they are termed a moving group or kinematic group. Moving groups can be old, such as the HR 1614 moving group at two billion years, or young, such as the AB Dor Moving Group at only 120 million years.

Moving groups were studied intensely by Olin Eggen in the 1960s.[43] A list of the nearest young moving groups has been compiled by López-Santiago et al.[32] The closest is the Ursa Major Moving Group which includes all of the stars in the Plough/Big Dipper asterism except for α Ursae Majoris and η Ursae Majoris. This is sufficiently close that the Sun lies in its outer fringes, without being part of the group. Hence, although members are concentrated at declinations near 60° N, some outliers are as far away across the sky as Triangulum Australe at 70° S.

Stellar streams

A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces.

Known kinematic groups

Some nearby kinematic groups include:[32]

See also

References

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Further reading

External links

Abell 1201 BCG

Abell 1201 BGC is a massive elliptical galaxy

residing as the brightest cluster galaxy of the Abell 1201 galaxy cluster.

At a redshift of 0.169, this system is around 2.7 billion light years from Earth,

and offset about 11 kiloparsecs from the X-ray peak of the intracluster gas.

With an ellipticity of 0.32±0.02, the stellar distribution is far from spherical.

In solar units, the total stellar luminosity is

4×1011 L☉ in SDSS r-band,

and 1.6×1012 L☉ in 2MASS K-band.

Half the stars orbit within an effective radius of 15 kpc,

and their central velocity dispersion is about 285 km s−1 within 5 kpc

rising to 360 km s−1 at 20 kpc distance.The BCG also acts as a gravitational lens,

bending the light of a more distant background galaxy (at redshift 0.451)

into an apparent tangential arc about 6 kpc to one side.

This makes the galaxy an important case in investigations of the intrinsic properties of dark matter.

Detailed models of the lens mass distribution, starlight and stellar kinematics

indicates that the galaxy cluster's dark halo has a shallow inner density gradient and perhaps a soft dark matter core.

At face value, this is incompatible with the dark matter cusp predicted by collisionless Cold dark matter theories,

and adds to evidence that dark matter experiences additional non-gravitational forces.

Years later, a faint smaller counterimage to the arc was discovered at a closer radius.

Explaining the position and brightness of this counterimage requires a dark central concentration of unseen mass.

Based on lens modelling, it could be a supermassive black hole equivalent to 13 billion suns:

(1.3±0.6)×1010 M☉.

At the time of measurement, this is one of the most massive black hole candidates

(without relying on assumptions about quasar luminosities and efficiencies).

This UMBH may be ten times larger than expected from

the usual scaling relations between black holes and host galaxies.However, alternative methods of modelling the stellar velocity dispersion maps

(accounting for an aggregate constraint on the lens mass)

reveals an ambiguity between the UMBH mass and the dark halo profile.

In solutions where the UMBH is more massive, the dark matter is more cuspy.

In solutions where the UMBH is smaller or absent, the dark matter is more cored.

The dark halo's ellipticity and the mass-to-light ratio of stars also enter the ambiguity.

Thus, if the standard computational models are robust,

then Abell 1201 BCG presents a dilemma and a challenge to either

the conventional ideas of black hole growth

or the simplest theories about dark matter, or both.

Association (astronomy)

An association (astronomy) is a combined or co-added group of astronomical exposures from which cosmic rays have been removed. WFPC2 associations constitute one type of association and are tools in the Hubble Space Telescope (HST) archive for using data from the Wide Field and Planetary Camera 2 (WFPC2). Associations were introduced in the HST archive at the beginning of 1998. Since then, astronomers have been able to retrieve on-the-fly re-calibrated co-added WFPC2 images that have already been cleaned of cosmic rays from the Space Telescope European Coordinating Facility (ST-ECF), the Canadian Astronomy Data Centre (CADC) and Space Telescope Science Institute (STScI) archives.

Atlas 3d survey

ATLAS3D is an astronomical survey that considers every galaxy in the deep sky within the local (42 Mpc) volume (1.16×105 Mpc3). This project utilizes multi-wavelength filters of a sample of 260 early-type galaxies. The survey use of numerical simulations and semi-analytic modeling to consider every galaxy and deep sky within the local (42 Mpc) volume (1.16×105 Mpc3). The first goal of this project is to quantify the global stellar kinematics and dynamics of a statistically significant sample of objects. This will permits catalog and characterize the class of early-type galaxies, as well as, to relate them to their formation and evolution.

The project use probe the mass-assembly epochs and timescales, in order to the ATLAS3D derive the star formation history. Another interesting feature is that the project will help in characterize the different phases of the interstellar medium. This research process will link the kinematics of molecular, atomic and ionized gas with the dynamical structure, star formation and environment of the host galaxies. Other important contribution is the change of Hubble’s classification of galaxies.

Elliptical galaxy

An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies.

Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of globular clusters. Elliptical galaxies are believed to make up approximately 10%–15% of galaxies in the Virgo Supercluster, and they are not the dominant type of galaxy in the universe overall. They are preferentially found close to the centers of galaxy clusters.Elliptical galaxies range in size from tens of millions to over one hundred trillion stars. Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false, although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure.

Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies.

Hipparcos

Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky. This permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars. The resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, Gaia, was launched in 2013.

The word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.

IC3PEAK

IC3PEAK is a Russian experimental electronic band from Moscow, formed in October 2013. The band initially recorded songs in English, but their latest album is in Russian. In 2018, a series of the band's concerts were cancelled or disrupted by law enforcement.

Intergalactic star

An intergalactic star, also known as an intracluster star or a rogue star, is a star not gravitationally bound to any galaxy. Although a source of much discussion in the scientific community during the late 1990s, intergalactic stars are now generally thought to have originated in galaxies, like other stars, but later expelled as the result of either colliding galaxies or of a multiple star system travelling too close to a supermassive black hole, which are found at the center of many galaxies.

Collectively, intergalactic stars are referred to as the intracluster stellar population, or IC population for short, in the scientific literature.

José Gabriel Funes

Fr. José Gabriel Funes, S.J. (born January 31, 1963 in Córdoba) is an Argentine Jesuit priest and astronomer. He was the Director of the Vatican Observatory from August 19, 2006 until September 18, 2015, when he was succeeded by Pope Francis's appointment of the Reverend Brother Guy Consolmagno, S.J. Funes serves on the Advisory Council of METI (Messaging Extraterrestrial Intelligence).

Kriging

In statistics, originally in geostatistics, kriging or Gaussian process regression is a method of interpolation for which the interpolated values are modeled by a Gaussian process governed by prior covariances. Under suitable assumptions on the priors, kriging gives the best linear unbiased prediction of the intermediate values. Interpolating methods based on other criteria such as smoothness (e.g., smoothing spline) need not yield the most likely intermediate values. The method is widely used in the domain of spatial analysis and computer experiments. The technique is also known as Wiener–Kolmogorov prediction, after Norbert Wiener and Andrey Kolmogorov.

The theoretical basis for the method was developed by the French mathematician Georges Matheron in 1960, based on the Master's thesis of Danie G. Krige, the pioneering plotter of distance-weighted average gold grades at the Witwatersrand reef complex in South Africa. Krige sought to estimate the most likely distribution of gold based on samples from a few boreholes. The English verb is to krige and the most common noun is kriging; both are often pronounced with a hard "g", following the pronunciation of the name "Krige". The word is sometimes capitalized as Kriging in the literature.

Leo II (dwarf galaxy)

Leo II (or Leo B) is a dwarf spheroidal galaxy about 690,000 light-years away in the constellation Leo. It is one of 24 known satellite galaxies of the Milky Way.

Leo II is thought to have a core radius of 178 ± 13 pc and a tidal radius of 632 ± 32 pc.

It was discovered in 1950 by Robert George Harrington and Albert George Wilson, from the Mount Wilson and Palomar Observatories in California.

List of stellar properties

Pages Related to Stellar properties, Pages using the word stellar in a physics context.

Stellar aberration

Stellar aberration (derivation from Lorentz transformation)

Stellar age estimation

Stellar archaeology

Stellar astronomy

Stellar atmosphere

Stellar birthline

Stellar black hole

Stellar cartography

Stellar chemistry

Stellar chonography

Stellar classification

Stellar cluster

Stellar collision

Stellar core

Stellar coronae

Stellar density

Stellar disk

Stellar distance

Stellar drift

Stellar dynamics

Stellar engine

Stellar engineering

Stellar envelope see stellar atmosphere

Stellar evolution

Stellar flare

Stellar flux

Stellar fog

Stellar halo

Stellar interferometer

Stellar isochrone

Stellar kinematics

Stellar limb-darkening

Stellar luminosity

Stellar magnetic field

Stellar magnitude

Stellar mass

Stellar mass black hole

Stellar mass loss

Stellar molecule

Stellar navigation

Stellar near-collision

Stellar neighborhood

Stellar nucleosynthesis

Stellar nursery

Stellar occultation

Stellar parallax

Stellar physics

Stellar planetary

Stellar population

Stellar precession

Stellar pulsations

Stellar quake

Stellar radius

Stellar remnant

Stellar rotation

Stellar scintillation

Stellar seismology

Stellar spectra

Stellar spheroid

Stellar spin-down

Stellar structure

Stellar surface fusion

Stellar system

Stellar triangulation

Stellar uplift

Stellar variation

Stellar vault

Stellar wind

Stellar wind (disambiguation)

Stellar wobble

Stellar X-ray astronomy

Stellar-wind bubble

Other

Catalog of Stellar Identifications

Fossil stellar magnetic field

General Catalogue of Stellar Radial Velocities

General Catalogue of Trigonometric Stellar Parallaxes

Interstellar cloud

Inter-stellar clouds

Interstellar medium

List of stellar angular diameters

List of stellar streams

Low-dimensional chaos in stellar pulsations

Mark III Stellar Interferometer

Michelson stellar interferometer

NEMO (Stellar Dynamics Toolbox)

Non-stellar astronomical object

Quasi-stellar object

Substellar object

Sub-stellar object

Sydney University Stellar Interferometer

TD1 Catalog of Stellar Ultraviolet Fluxes

Timeline of stellar astronomy

Utah state stellar cluster

Young stellar object

List of stellar streams

This is a list of stellar streams. A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces. A notable exception in the list about Milky Way streams given below is the Magellanic Stream, composed of gas (mostly hydrogen).

Local standard of rest

In astronomy, the local standard of rest or LSR follows the mean motion of material in the Milky Way in the neighborhood of the Sun. The path of this material is not precisely circular. The Sun follows the solar circle (eccentricity e < 0.1 ) at a speed of about 255 km/s in a clockwise direction when viewed from the galactic north pole at a radius of ≈ 8.34 kpc about the center of the galaxy near Sgr A*, and has only a slight motion, towards the solar apex, relative to the LSR.The LSR velocity is anywhere from 202–241 km/s. In 2014, very-long-baseline interferometry observations of maser emission in high mass star forming regions placed tight constraints on combinations of kinematic parameters such as the circular orbit speed of the Sun (Θ0 + V☉ = 255.2 ± 5.1 km/s). There is significant correlation between the circular motion of the solar circle, the solar peculiar motion, and the predicted counterrotation of star-forming regions. Additionally, local estimates of the velocity of the LSR based on stars in the vicinity of the Sun may potentially yield different results than global estimates derived from motions relative to the Galactic center.

NGC 1291

NGC 1291, also known as NGC 1269, is a ring galaxy with an unusual inner bar and outer ring structure located about 33 million light-years away in the constellation Eridanus. It was discovered by James Dunlop in 1826 and subsequently entered into the New General Catalogue as NGC 1291 by Johan Ludvig Emil Dreyer. John Herschel then observed the same object in 1836 and entered it into the catalog as NGC 1269 without realizing that it was a duplicate. This galaxy was cited as an example of a "transitional galaxy" by NASA's Galaxy Evolution Explorer team in 2007.

NGC 2964

NGC 2964 is an intermediate spiral galaxy located in the constellation Leo. It is located at a distance of circa 60 million light years from Earth, which, given its apparent dimensions, means that NGC 2964 is about 60,000 light years across. It was discovered by William Herschel οn December 7, 1785.There is evidence that the galaxy has a weak bar running across the minor axis of the galaxy. The galaxy has four spiral arms, two emerging from ansae at each end of the bar and the others emerging near the centre of the bar, with the arms emerging from the ends of the bar being of higher surface brightness. All but the west arm have knotty appearance. The outer parts of the galaxy present twisted stellar kinematics, but otherwise the rotation is quite regular.Spiral dust lanes have been observed running towards the centre, where star formation possibly takes place. A circumnuclear ring observed in [OIII]/H-beta could the be site of active star formation. Another indication of a nuclear star forming ring is the detection of double peaked carbon monoxide emission lines. The central region of the galaxy has a stellar population of young-intermediate age, probably created by constant starburst activity that lasted for about 5 billion years. The total star formation rate of NGC 2964 is estimated to be about 4 M☉ per year.The nucleus of NGC 2964 appears to feature HII region activity. The spectographic study of the nucleus revealed double peaked emission lines, which can be attributed to an ionisation cone created by an active galactic nucleus. In the centre of NGC 2964 is believed to lie a supermassive black hole whose upper mass limit is estimated to be between 1.4 and 24 million M☉.NGC 2964 is the brightest galaxy in a galaxy group known as the NGC 2964 group. Other members of the group include NGC 2968, NGC 2970, NGC 3003, NGC 3011, NGC 3021. Other nearby galaxies include NGC 3118, NGC 3067, NGC 3032, NGC 3026, and their galaxy groups. NGC 2964 forms a non-interacting pair with NGC 2968, which lies 5.8 arcminutes away. A hydrogen bridge has been found to connect the two galaxies, while a tail extends towards NGC 2970.

NGC 4580

NGC 4580 is an unbarred spiral galaxy located about 70 million light-years away in the constellation Virgo. NGC 4580 is also classified as a LINER galaxy. It was discovered by astronomer William Herschel on February 2, 1786 and is a member of the Virgo Cluster.

NGC 708

NGC 708 is an elliptical galaxy located 240 million light-years away in the constellation Andromeda and was discovered by astronomer William Herschel on September 21, 1786. It is classified as a cD galaxy and is the brightest member of Abell 262. NGC 708 is a weak FR I radio galaxy and is also classfied as a type 2 seyfert galaxy.NGC 708 is surrounded by 4700 globular clusters.

OB star

OB stars are hot, massive stars of spectral types O or early-type B that form in loosely organized groups called OB associations. They are short lived, and thus do not move very far from where they formed within their life. During their lifetime, they will emit much ultraviolet radiation. This radiation rapidly ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region or Strömgren sphere.

In lists of spectra the "spectrum of OB" refers to "unknown, but belonging to an OB association so thus of early type".

SAGES Legacy Unifying Globulars and GalaxieS Survey

The SLUGGS (SAGES Legacy Unifying Globulars and GalaxieS) survey is an astronomical survey of 25 nearby early-type (E and S0) galaxies. This survey uses a combination of imaging from Subaru/Suprime-Cam and spectroscopy from Keck/DEIMOS to investigate the chemo-dynamical properties of both the diffuse starlight and the globular cluster systems of the target galaxies.

Pilot data for the survey was obtained in 2006 and data acquisition is going to be completed in 2015.

Formation
Evolution
Luminosity class
Spectral
classification
Remnants
Hypothetical stars
Nucleosynthesis
Structure
Properties
Star systems
Earth-centric
observations
Lists
Related articles
Bound
Unbound
Visual grouping

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