Stellar kinematics
Encyclopedia
Stellar kinematics is the study of the movement of star
s without needing to understand how they acquired their motion. This differs from stellar dynamics
, which takes into account gravitational effects. The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy
.
In astronomy
, it is widely accepted that most stars are born within molecular clouds known as stellar nurseries
. The stars formed within such a cloud compose open clusters containing dozens to thousands of members. These clusters dissociate with time. Stars that separate themselves from the cluster's core are designated as members of the cluster's stellar association. If the remnant later drifts through the galaxy as a somewhat coherent assemblage, then it is termed a moving group.
, 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. 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. 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. The peculiar motion of the Sun with respect to the LSR is (U, V, W) = (10.00 ± 0.36, 5.23 ± 0.62, 7.17 ± 0.38) km/s.
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, higher metallicity stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy. 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.
Within the Milky Way
galaxy, there are three primary components of stellar kinematics: the disk, halo and bulge or bar. These kinematic groups are closely related to the stellar populations in the galaxy, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. The halo may be further sub-divided into an inner and outer halo, with the inner halo having a net prograde rotation with respect to the galaxy and the outer a net retrograde
movement.
relative to the surrounding interstellar medium. The three types of high-velocity stars are: runaway stars, halo stars and hypervelocity stars.
relative to the surrounding interstellar medium
. The proper motion
of a runaway star often points exactly away from a stellar association, whose member it therefore once must have been before it was hurled out.
Two possible mechanisms may give rise to a runaway star:
While both mechanisms are theoretically possible, astronomers generally favour the supernova hypothesis as more likely in practice.
One 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 galaxy 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
.
Typical examples are the halo stars passing through the disk of the galaxy 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.
the gravitational pull of the galaxy. Ordinary stars in the galaxy have velocities on the order of 100 km/s, while hypervelocity stars (especially those near the center of the galaxy, which is where most are thought to be "produced"), have velocities on the order of 1000 km/s.
The existence of HVSs was first predicted by J. Hills in 1988. The first HVS was discovered in 2005 by Warren Brown and collaborators at the Harvard-Smithsonian Center for Astrophysics
. Currently, sixteen are known, one of which is believed to originate from the Large Magellanic Cloud
rather than the Milky Way
. All of the currently known HVSs are over 50,000 parsecs away and are unbound from the galaxy.
It is believed that about 1000 HVSs exist in our Galaxy. Considering that there are around 100 billion stars in the Milky Way
, this is a minuscule fraction (~0.000001%).
in the centre of the Milky Way
. One of the two partners is captured by the black hole, while the other escapes with high velocity. Also, it is worth noting that "captured" does not necessarily mean "swallowed", for in all likelihood the companion to the HVS will never fall into the black hole.
Known HVSs are main-sequence stars with masses a few times that of the Sun.
A team at Argentina's Cordoba Observatory believe that our HVS are a result of a merging with a collision between the Milky Way, and an orbiting dwarf galaxy
. A dwarf galaxy that has been orbiting the Milky Way, passed through the centre of the Milky Way. When the dwarf galaxy made its closest approach to the centre of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space.
Some neutron stars are inferred to be traveling with similar speeds, however they are unrelated to both 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. The neutron star
RX J0822-4300
, which was measured to move at a record speed of over 1500 km/s (0.5% c) in 2007 by the Chandra X-ray Observatory
, is thought to have been produced this way.
, the remains of a star forming region, or collections of overlapping star formation bursts at differing time periods in adjacent regions. Most stars are born within molecular cloud
s 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.
Stellar associations were first discovered by the Armenia
n astronomer Viktor Ambartsumian in 1947. 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.
for associations that illuminate reflection nebula
e. 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. Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.
Young associations will contain 10–100 massive stars of spectral class
O and B, and are known as OB associations. These 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. It is believed that the majority of all stars in the Milky Way were formed in OB associations. O class stars are short-lived, and will expire as supernova
e after roughly a 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
10 million years. (Compare this to the current age of the Sun
at about 5 billion years.)
The Hipparcos
satellite provided measurements that located a dozen OB associations within 650 parsec
s of the Sun. The nearest OB association is the Scorpius-Centaurus Association
, located about 400 light years from the Sun
.
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.
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 parsec
s from the Sun. 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. To summarize the characteristics of Moving groups members: they have the same age and origin, the same chemical composition and they have the same amplitude and direction in their vector of velocity.
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. These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. 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. An example of an R-association is Monoceros R2, located 830 ± 50 parsec
s from the Sun.
moving group at 2 billion years, or young, such as the AB Doradus moving group
at only 50 million.
Moving groups were studied intensely by Olin Eggen in the 1960s A list of the nearest young moving groups has been compiled by López-Santiago et al. 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, while members are concentrated at declination
s near 60° N, some outliers are as far away across the sky as Triangulum Australe
at 70° S.
s 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.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s without needing to understand how they acquired their motion. This differs from stellar dynamics
Stellar dynamics
Stellar dynamics is the branch of astrophysics which describes in a statistical way the collective motions of stars subject to their mutual gravity. The long range of gravity and the slow "relaxation" of stellar systems prevent the use of the methods of statistical physics...
, which takes into account gravitational effects. The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
.
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, it is widely accepted that most stars are born within molecular clouds known as stellar nurseries
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
. The stars formed within such a cloud compose open clusters containing dozens to thousands of members. These clusters dissociate with time. Stars that separate themselves from the cluster's core are designated as members of the cluster's stellar association. If the remnant later drifts through the galaxy as a somewhat coherent assemblage, then it is termed a moving group.
Space velocity
The component of stellar motion toward or away from the Sun, known as radial velocityRadial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
, can be measured from the spectrum shift caused by the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
. The transverse, or proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
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
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
, the space velocity can be computed. This is the star's actual motion relative to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
or the local standard of rest
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...
(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. The Sun's motion with respect to the LSR is called the peculiar solar motion.
The components of space velocity in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
's Galactic coordinate system
Galactic coordinate system
The galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...
are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...
, V positive in the direction of galactic rotation, and W positive in the direction of the North Galactic Pole. The peculiar motion of the Sun with respect to the LSR is (U, V, W) = (10.00 ± 0.36, 5.23 ± 0.62, 7.17 ± 0.38) km/s.
The stars in the Milky Way can be subdivided into two general populations, based on their metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that population I, higher metallicity stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy. 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.
Within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy, there are three primary components of stellar kinematics: the disk, halo and bulge or bar. These kinematic groups are closely related to the stellar populations in the galaxy, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. The halo may be further sub-divided into an inner and outer halo, with the inner halo having a net prograde rotation with respect to the galaxy and the outer a net retrograde
Retrograde
-Retrograde:* Retrograde motion, in astronomy, describes retrograde motions of celestial bodies relative to a gravitationally central object* Apparent retrograde motion, in astronomy, is the apparent motion of planets as observed from a particular vantage point...
movement.
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 neighbourhood. The velocity is also sometimes defined as supersonicSupersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
relative to the surrounding interstellar medium. The three types of high-velocity stars are: runaway stars, halo stars and hypervelocity stars.
Runaway stars
A runaway star is one which is moving through space with an abnormally high velocityVelocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
relative to the surrounding interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. The proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
of a runaway star often points exactly away from a stellar association, whose member it therefore once must have been before it was hurled out.
Two possible mechanisms may give rise to a runaway star:
- In the first scenario, a close encounter between two binary systemBinary system (astronomy)A binary system is an astronomical term referring to two objects in space which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions A binary system is an astronomical term referring to two objects in space (usually stars, but also...
s may result in the disruption of both systems, with some of the stars being ejected at high velocities. - In the second scenario, a supernovaSupernovaA supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
explosion in a multiple starMultiple starA multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...
system can result in the remaining components moving away at high speed.
While both mechanisms are theoretically possible, astronomers generally favour the supernova hypothesis as more likely in practice.
One example of a related set of runaway stars is the case of AE Aurigae
AE Aurigae
AE Aurigae is a runaway star in the constellation Auriga; it lights the Flaming Star Nebula.AE Aurigae is a blue O-type main sequence dwarf with a mean apparent magnitude of +5.99. It is classified as an Orion type variable star and its brightness varies irregularly between magnitudes +5.78 and...
, 53 Arietis
53 Arietis
53 Arietis is a variable star of Beta Cephei type, alternatively denoted UW Arietis, in the constellation Aries. Its mean apparent magnitude is 6.13 and its spectral type is O or early B. It is a runaway star, thought to have been produced as a consequence of one of a pair of binary stars colliding...
and Mu Columbae
Mu Columbae
Mu Columbae is a star in the constellation of Columba. It is one of the few O-class stars that is visible to the unaided eye. The star is known to lie approximately 1,300 light years from our solar system .This is a relatively fast rotating star that completes a full revolution approximately every...
, all of which are moving away from each other at velocities of over 100 km/s (for comparison, the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
moves through the galaxy at about 20 km/s faster than the local average). Tracing their motions back, their paths intersect near to the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
about 2 million years ago. Barnard's Loop
Barnard's Loop
Barnard's Loop is an emission nebula in the constellation of Orion. It is part of a Orion Molecular Cloud Complex which also contains the bright Horsehead and Orion nebulae. The loop takes the form of a large arc centred approximately on the Orion Nebula...
is believed to be the remnant of the supernova that launched the other stars.
Another example is the X-ray object Vela X-1
Vela X-1
Vela X-1 is a pulsing, eclipsing high-mass X-ray binary system, associated with the Uhuru source 4U 0900-40 and the supergiant star HD 77581....
, where photodigital techniques reveal the presence of a typical supersonic bow shock hyperbola
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
.
Halo stars
High-velocity 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 centre of the Galaxy. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Galaxy. Although their orbital velocities in the Galaxy 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 galaxy at steep angles. One of the nearest 45 stars, called Kapteyn's star
Kapteyn's Star
Kapteyn's Star is a class M1 red dwarf star about 13 light years from Earth in the southern constellation of Pictor. With a magnitude of nearly 9 it is visible through binoculars or a telescope.-History:...
, 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 (HVSs), or, less formally, Exiled Stars, are stars with a velocity so great that they are able to escapeEscape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
the gravitational pull of the galaxy. Ordinary stars in the galaxy have velocities on the order of 100 km/s, while hypervelocity stars (especially those near the center of the galaxy, which is where most are thought to be "produced"), have velocities on the order of 1000 km/s.
The existence of HVSs was first predicted by J. Hills in 1988. The first HVS was discovered in 2005 by Warren Brown and collaborators at the Harvard-Smithsonian Center for Astrophysics
Harvard-Smithsonian Center for Astrophysics
The Harvard–Smithsonian Center for Astrophysics is one of the largest and most diverse astrophysical institutions in the world, where scientists carry out a broad program of research in astronomy, astrophysics, earth and space sciences, and science education...
. Currently, sixteen are known, one of which is believed to originate from the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
rather than the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. All of the currently known HVSs are over 50,000 parsecs away and are unbound from the galaxy.
It is believed that about 1000 HVSs exist in our Galaxy. Considering that there are around 100 billion stars in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
, this is a minuscule fraction (~0.000001%).
Production methods
The main production method for HVSs is summarized as thus: they are believed to originate by close encounters of binary stars with the supermassive black holeSupermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
in the centre of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. One of the two partners is captured by the black hole, while the other escapes with high velocity. Also, it is worth noting that "captured" does not necessarily mean "swallowed", for in all likelihood the companion to the HVS will never fall into the black hole.
Known HVSs are main-sequence stars with masses a few times that of the Sun.
A team at Argentina's Cordoba Observatory believe that our HVS are a result of a merging with a collision between the Milky Way, and an orbiting dwarf galaxy
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
. A dwarf galaxy that has been orbiting the Milky Way, passed through the centre of the Milky Way. When the dwarf galaxy made its closest approach to the centre of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space.
Some neutron stars are inferred to be traveling with similar speeds, however they are unrelated to both HVSs and the HVS ejection mechanism. Neutron stars are the remnants of supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
explosions, and their extreme speeds are very likely the result of an asymmetric supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
explosion. The neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
RX J0822-4300
RX J0822-4300
|- style="vertical-align: top;"| Distance | 7.000 Ly|- style="vertical-align: top;"| Galactic coordinates | 260.3841 -03.4718...
, which was measured to move at a record speed of over 1500 km/s (0.5% c) in 2007 by the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
, is thought to have been produced this way.
List of HVSs
- HVS 1 - (SDSS J090744.99+024506.8SDSS J090745.0+24507SDSS J090744.99+024506.8 , is a star that is leaving the Milky Way galaxy at twice the galactic escape velocity...
) (a.k.a. The Outcast Star) - the first hypervelocity star to be discovered - HVS 2 - (SDSS J093320.86+441705.4) or (US 708)
- HVS 3 - (HE 0437-5439HE 0437-5439HE 0437-5439 is a massive, unbound hypervelocity star , also called HVS3. It is a main sequence B-type star located in the Dorado constellation. It was discovered in 2005 with the Kueyen 8.2 meter telescope, which is part of the European Southern Observatory's Very Large Telescope array. HE...
) - possibly from the Large Magellanic Cloud - 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)
Kinematic groups
A set of stars with similar space motion and ages is known as a kinematic group. These are stars that could share a common origin, such as the evaporation of an open clusterOpen cluster
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...
, the remains of a star forming region, or collections of overlapping star formation bursts at differing time periods in adjacent regions. Most stars are born within molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s known as stellar nurseries
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
. 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
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
, and kinematics (radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
and proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
). 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.
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 Armenia
Armenia
Armenia , officially the Republic of Armenia , is a landlocked mountainous country in the Caucasus region of Eurasia...
n astronomer Viktor Ambartsumian in 1947. The conventional name for an association uses the names or abbreviations of the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
(or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.
Types
Viktor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars. A third category, R, was later suggested by Sidney van den BerghSidney van den Bergh
Sidney Van den Bergh, OC, FRS is a retired Canadian astronomer.Born in the Netherlands, son of businessman and politician Sidney James van den Bergh and grandson of Unilever co-founder Samuel van den Bergh, he showed an interest in science from an early age, learning to read with books on astronomy...
for associations that illuminate reflection nebula
Reflection nebula
In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
e. 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. Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.
Young associations will contain 10–100 massive stars of spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
O and B, and are known as OB associations. These 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. It is believed that the majority of all stars in the Milky Way were formed in OB associations. O class stars are short-lived, and will expire as supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e after roughly a 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
10 million years. (Compare this to the current age of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
at about 5 billion years.)
The Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite provided measurements that located a dozen OB associations within 650 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s of the Sun. The nearest OB association is the Scorpius-Centaurus Association
Scorpius-Centaurus Association
The Scorpius-Centaurus Association is the nearestOB Association to the Sun. This stellar association is composed of three subgroups The Scorpius-Centaurus Association (sometimes called Sco-Cen or Sco OB2) is the nearestOB Association to the Sun. This stellar association is composed of three...
, located about 400 light years from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
OB associations have also been found in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
and the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
. These associations can be quite sparse, spanning 1,500 light years in diameter.
Young stellar groups can contain a number of infant T Tauri
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
stars that are still in the process of entering the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
. 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 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s from the Sun. 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. To summarize the characteristics of Moving groups members: they have the same age and origin, the same chemical composition and they have the same amplitude and direction in their vector of velocity.
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. These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. 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. An example of an R-association is Monoceros R2, located 830 ± 50 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s from the Sun.
Moving groups
If the remnants of a stellar association drift through the galaxy as a somewhat coherent assemblage, then they are termed a moving group. Moving groups can be old, such as the HR 1614HR 1614
HR 1614 is a spectroscopic binary star system in the constellation Eridanus....
moving group at 2 billion years, or young, such as the AB Doradus moving group
AB Doradus moving group
AB Doradus Moving Group is a group of about 30 associated stars that are moving through space together with the star AB Doradus. A moving group is distinguished by its members having about the same age, composition and motion through space...
at only 50 million.
Moving groups were studied intensely by Olin Eggen in the 1960s A list of the nearest young moving groups has been compiled by López-Santiago et al. The closest is the Ursa Major Moving Group
Ursa Major Moving Group
The Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80 light years away...
which includes all of the stars in the Plough/Big Dipper
Big Dipper
The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...
asterism
Asterism (astronomy)
In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...
except for α Ursae Majoris and η Ursae Majoris
Eta Ursae Majoris
Eta Ursae Majoris is a star in the constellation Ursa Major. It has the traditional names Alkaid and Benetnash ....
. This is sufficiently close that the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
lies in its outer fringes, without being part of the group. Hence, while members are concentrated at declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
s near 60° N, some outliers are as far away across the sky as Triangulum Australe
Triangulum Australe
Triangulum Australe is a small constellation in the southern sky, created in the sixteenth century. Its name is Latin for 'the southern triangle', which distinguishes it from Triangulum in the northern sky...
at 70° S.
Stellar streams
A stellar stream is an association of starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s orbiting a galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
that was once a globular cluster
Globular cluster
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...
or dwarf galaxy
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
that has now been torn apart and stretched out along its orbit by tidal forces.
Known kinematic groups
Some kinematic groups include:- Local Association (PleiadesPleiades (star cluster)In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
moving group) - AB Doradus moving groupAB Doradus moving groupAB Doradus Moving Group is a group of about 30 associated stars that are moving through space together with the star AB Doradus. A moving group is distinguished by its members having about the same age, composition and motion through space...
- Alpha Persei moving cluster
- Beta Pictoris moving groupBeta Pictoris Moving GroupThe Beta Pictoris Moving Group is a young moving group of stars located relatively near Earth. A moving group, in astronomy, is a group of stars that share a common motion through space as well as a common origin...
- Castor moving groupCastor Moving GroupThe Castor Moving Group is a moving group, that is, a set of stars with common velocities in space, thought to have a common origin. The stars that have been identified as part of the group include Castor, Fomalhaut, Vega, α Cephei and α Librae. They are all of similar age.-Discovery and...
- Corona Australis association
- Eta Chamaeleontis cluster
- Hercules streamHercules streamHercules stream is a large moving group of stars that are trailing behind the local rate of galactic rotation and are heading further out in the disk. Members of this stream may sometimes be referred to as Hercules stars. This stream was first hypothesized to be the remnants of a cluster ofstars...
- Hyades StreamHyades StreamThe Hyades Stream is a large collection of scattered stars that also share a similar trajectory with the Hyades Cluster. In 1869, Richard A. Proctor observed that numerous stars at large distances from the Hyades share a similar motion through space...
- IC 2391IC 2391IC 2391 is an open cluster in the constellation Vela. It has been purported to be first described by the Persian astronomer Al Sufi in about 964. It was also found by Abbe Lacaille and has been cataloged as Lac II 5. The cluster is at a distance of about 500 light-years away from Earth and can be...
supercluster - MBM 12 association
- Tucana-Horoglium Association
- TW Hydrae associationTW Hydrae associationThe TW Hydrae association is a group of approximately thirty very young stars located 50 parsecs from Earth that share a common motion and appear to all be roughly the same age, 5-10 million years old...
- Ursa Major Moving GroupUrsa Major Moving GroupThe Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80 light years away...
- Zeta Herculis moving groupZeta Herculis Moving GroupZeta Herculis Moving Group is a set of stars that share a common motion through space.The existence of this moving group was first published in 1958 by Olin J. Eggen. Based upon the high velocity motion of the star Zeta Herculis through space, he searched for stars that were following a similar...
Further reading
- Blaauw A., Morgan W.W. (1954), The Space Motions of AE Aurigae and mu Columbae with Respect to the Orion Nebula, Astrophysical Journal, v.119, p. 625
- Hoogerwerf R., de Bruijne J.H.J., de Zeeuw P.T. (2000), The Origin of Runaway Stars, Astrophysical Journal, v. 544, p. L133
External links
- ESO press release about runaway stars
- Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
- Two Exiled Stars Are Leaving Our Galaxy Forever
- Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
- Hypervelocity stars. I. The spectroscopic survey information on HV 6 & HV 7
- Young stellar kinematic groups, David Montes, Departamento de Astrofísica, Universidad Complutense de Madrid.