 | This article needs additional citations for verification .(December 2009) |
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 The moving-cluster method is now largely superseded by other, usually more accurate distance measures.
The moving-cluster method relies on observing the proper motions and Doppler shift of each member of a group of stars known to form a cluster. The idea is that since all the stars share a common space velocity, they will appear to move towards a point of common convergence ("vanishing point") on the sky. This is essentially a perspective effect.
Using the moving-cluster method, the distance to a given star cluster (in parsecs) can be determined using the following equation:
where "θ" is the angle between the star and the cluster's apparent convergence point, "μ" is the proper motion of the cluster (in arcsec/year), and "v" is the star's radial velocity (in AU/year).
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 parsecs), 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. [1]
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
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 half-angle of inclination between those two lines. Due to foreshortening, nearby objects show a larger parallax than farther objects, so parallax can be used to determine distances.
The parsec is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (au), i.e. 30.9 trillion kilometres. The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 au subtends an angle of one arcsecond. This corresponds to 648000/π astronomical units, i.e. 1 pc = 1 au/tan(1 arcsec). The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand.
Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars.
The Hyades is the nearest open cluster and one of the best-studied star clusters. Located about 153 light-years away from the Sun, it consists of a roughly spherical group of hundreds of stars sharing the same age, place of origin, chemical characteristics, and motion through space. From the perspective of observers on Earth, the Hyades Cluster appears in the constellation Taurus, where its brightest stars form a "V" shape along with the still-brighter Aldebaran. However, Aldebaran is unrelated to the Hyades, as it is located much closer to Earth and merely happens to lie along the same line of sight.
Stellar parallax is the apparent shift of position (parallax) of any nearby star against the background of distant stars. By extension, it is a method for determining the distance to the star through trigonometry, the stellar parallax method. Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).
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 first high-precision measurements of the intrinsic brightnesses, proper motions, and parallaxes of stars, enabling better calculations of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, astrophysicists were able to finally measure 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 cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.
Gaia is a space observatory of the European Space Agency (ESA), launched in 2013 and expected to operate until 2025. The spacecraft is designed for astrometry: measuring the positions, distances and motions of stars with unprecedented precision, and the positions of exoplanets by measuring attributes about the stars they orbit such as their apparent magnitude and color. The mission aims to construct by far the largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars, among others.
Mu Arae, often designated HD 160691, officially named Cervantessur-VAN-teez, is a main sequence G-type star approximately 50 light-years away from the Sun in the constellation of Ara. The star has a planetary system with four known extrasolar planets, three of them with masses comparable with that of Jupiter. Mu Arae c, the innermost, was the first hot Neptune or super-Earth discovered.
2M1207, 2M1207A or 2MASSW 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.
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth. It is one of the first candidate exoplanets to be directly observed. It was discovered in April 2004 by the Very Large Telescope (VLT) at the Paranal Observatory in Chile by a team from the European Southern Observatory led by Gaël Chauvin. It is believed to be from 3 to 10 times the mass of Jupiter and may orbit 2M1207 at a distance roughly as far from the brown dwarf as Pluto is from the Sun.
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light. These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light. Gravitational microlensing was first theorised by Refstal (1964) and first discovered by Irwin et al (1988). The first object in the sky where it was discovered was the Einstein cross or Huchra lens 2237 +0305. The initial lightcurve of the object was published by Corrigan et al (1991). In Corrigan et al (1991) they calculated that the object causing the microlensing was a Jupiter sized object. This was the first discovery of a planet in another galaxy.
Doppler spectroscopy is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star. As of November 2022, about 19.5% of known extrasolar planets have been discovered using Doppler spectroscopy.
In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.
Gliese 3634 b is a super-Earth exoplanet in the orbit of the nearby red dwarf Gliese 3634 at approximately 64.5 light-years in constellation Hydra. The planet is approximately eight times the mass of Earth, and orbits its star every two and a half days at a distance of 0.0287 AU. The planet was the first to be discovered by a group of astronomers searching for exoplanets in the orbit of very-low-mass stars after the team reorganized their strategy, choosing to search for targets that they could also confirm using the transit method. However, a transit event associated with Gliese 3634 b was not detected. The planet's discovery was published in Astronomy and Astrophysics on February 8, 2011.
Eta Chamaeleontis, Latinized from η Chamaeleontis, is a star in the constellation Chamaeleon. It has an apparent magnitude of about 5.5, meaning that it is just barely visible to the naked eye. Based upon parallax measurements, this star is located some 325 light-years light years away from the Sun.
Proxima Centauri c is a controversial exoplanet candidate claimed to be orbiting the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of a triple star system. It is located approximately 4.2 light-years from Earth in the constellation of Centaurus, making it, Proxima b, and Proxima d the closest known exoplanets to the Solar System.
The most important fundamental distance measurements in astronomy come from trigonometric parallax, as applied in the stellar parallax method. As the Earth orbits the Sun, the position of nearby stars will appear to shift slightly against the more distant background. These shifts are angles in an isosceles triangle, with 2 AU making the base leg of the triangle and the distance to the star being the long equal length legs. The amount of shift is quite small, even for the nearest stars, measuring 1 arcsecond for an object at 1 parsec's distance, and thereafter decreasing in angular amount as the distance increases. Astronomers usually express distances in units of parsecs ; light-years are used in popular media.
