Moving cluster method
Encyclopedia
In astrometry
, the moving cluster method and the closely related convergent point method are means, primarily of historical interest, for determining the distance to star clusters. They were used on several nearby clusters in the first half of the 1900s to determine distance. The method is now largely superseded by other, usually more accurate distance measures.
s and doppler shift of each member of a group of stars known to form a cluster. The idea is that since all the stars are close together, they will tend to move towards a common convergence point on the sky. This is essentially a perspective
effect.
D(pc)=v(km/s)/4.74u("/yr)
s), and also be fairly tightly bound so it can be made out on the sky. Also, the method is quite difficult to work with compared with more straightforward methods like trigonometric parallax. Finally, the uncertainty in the final distance values are in general fairly large compared those obtained with precision measurements like those from Hipparcos
.
Of the clusters it has been used with, certainly the most famous are the Hyades
and the Pleiades
. The moving cluster method was in fact the only way astronomers had to measure the distance to these clusters with any precision for some time in the early 20th century.
Because of the problems outlined above, this method has not been used practically for stars for several decades in astronomical research.
However recently, it has been used to estimate the distance between the brown dwarf
2M1207
and its observed exoplanet 2M1207b
. In December 2005, American astronomer Eric Mamajek reported a distance (53 ± 6 parsecs) to 2M1207b using the moving cluster method.
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
, the moving cluster method and the closely related convergent point method are means, primarily of historical interest, for determining the distance to star clusters. They were used on several nearby clusters in the first half of the 1900s to determine distance. The method is now largely superseded by other, usually more accurate distance measures.
Introduction
The moving cluster method relies on observing the proper motionProper 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...
s and doppler shift of each member of a group of stars known to form a cluster. The idea is that since all the stars are close together, they will tend to move towards a common convergence point on the sky. This is essentially a perspective
Perspective (visual)
Perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...
effect.
D(pc)=v(km/s)/4.74u("/yr)
Historical and Current Usage
The method has only ever been used for a small number of clusters. This is because for the method to work, the cluster must be quite close to Earth (within a few hundred parsecParsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s), and also be fairly tightly bound so it can be made out on the sky. Also, the method is quite difficult to work with compared with more straightforward methods like trigonometric parallax. Finally, the uncertainty in the final distance values are in general fairly large compared those obtained with precision measurements like those from 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...
.
Of the clusters it has been used with, certainly the most famous are the Hyades
Hyades
Hyades may refer to:*Hyades *Hyades , an open star cluster in the constellation Taurus...
and the Pleiades
Pleiades (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...
. The moving cluster method was in fact the only way astronomers had to measure the distance to these clusters with any precision for some time in the early 20th century.
Because of the problems outlined above, this method has not been used practically for stars for several decades in astronomical research.
However recently, it has been used to estimate the distance between the brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
2M1207
2M1207
2M1207, 2M1207A or 2MASS J12073346-3932539 is a brown dwarf located in the constellation Centaurus; a companion object, 2M1207b, may be the first extrasolar planetary mass companion to be directly imaged, and is the first discovered orbiting a brown dwarf.2M1207 was discovered during the course of...
and its observed exoplanet 2M1207b
2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...
. In December 2005, American astronomer Eric Mamajek reported a distance (53 ± 6 parsecs) to 2M1207b using the moving cluster method.
See also
- AstrometryAstrometryAstrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
- ParallaxParallaxParallax 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"...
- Stellar parallaxStellar parallaxStellar parallax is the effect of parallax on distant stars in astronomy. It is parallax on an interstellar scale, and it can be used to determine the distance of Earth to another star directly with accurate astrometry...
- Cepheid Stars
- RR Lyrae StarsRR LyraeRR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variable class of stars. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...