Astrometry literally means measuring the stars. This type of measurement determines a specific star's location in the sky with great precision. In order to establish a star's location, it is necessary to first establish a coordinate system in which the location can be specified. Traditionally, very distant stars, which show very little motion as viewed from Earth, have been used to establish that coordinate system. However, the accuracy of the coordinate system is dependent on the accuracy of the positions of defining stars, effort has been made to use the extremely distant point-like objects known as quasars to establish an improved standard coordinate system. Because quasars give off radio waves, their positions can be determined with extreme accuracy, but the implementation of this system has yet to be accomplished.
Astrometry is of fundamental importance to the study of the stars. Astronomers can use the distance of the star to help determine its other properties. The annual motion of the Earth about the Sun causes nearby stars to appear to move about in the sky with respect to distant background stars. The amplitude of this apparent motion determines the distance of the star from our sun, which is known as its trigonometric parallax. The angular rate of change of the star's position is called its proper motion. If the distance to the star is also known, the proper motion can be converted into a transverse velocity relative to the Sun, which is the apparent speed of the star across the line of sight. For most stars this motion is extremely small and may require positional determinations 50 years or longer for accurate measurement. The transverse velocity may be combined with the radial velocity determined from the star's spectra to yield the true space velocity with respect to the Sun.
Occasionally the proper motion will be found to vary in a periodic manner, suggesting that the target star is orbiting another object in addition to its steady motion across the sky. Such stars are called astrometric binary stars. Stars that are orbited by planets, which are too faint to be directly observed, show this motion. However, the motion is liable to be extremely small unless the star is quite small and the planet rather large.
Often, astronomers cannot determine the distance of the star directly from the coordinate system. In this case, a method called statistical parallax is used. For example, if one independently knew the transverse velocity of a star, one could use the proper motion to obtain a distance. While it is impossible to determine the transverse velocity of a specific star without knowledge of its distance and proper motion an estimation can be obtained by using the transverse velocity for a collection of similar stars. In addition, the radial velocity of the star can be obtained directly from its spectra without knowledge of its distance or proper motion. These can then be combined with the observed proper motions to yield distances to the similar stars of a particular type and average values for their intrinsic properties.
A similar trick can be used on a group of stars which move together through space on more-or-less parallel tracks. Such groups of stars are called galactic, or open, clusters. Just as the parallel tracks of a railroad appear to converge to a point in the distance, so the stellar motions will appear to point to a distant convergent point in the sky. The location of this point with respect to each star specifies the angle between the radial velocity and the space velocity for that star. Knowledge of that angle allows the tangential velocity of the star to be obtained from the directly measured radial velocity. Again, knowledge of the individual proper motion and tangential velocity allows for a determination of the distance to each star of the cluster. This scheme is known as the moving cluster method.
Adaptive optics has greatly improved the accuracy of stellar positions made from the ground. Since the mid-twentieth century, astronomers have known that it was possible in principle to undo the distortions of astronomical images generated by the atmosphere. First one had to measure those distortions, then construct a "lens" with characteristics that could be changed as fast as the atmosphere itself changed. Theoretically such a lens could then "undo" the distortions of the atmosphere, leaving the astronomer with the steady image of the star beyond the atmosphere. This seemed impossible to accomplish until very recently. Powerful lasers have now been used to produce artificial stars high in the atmosphere, enabling astronomers to measure the atmospheric distortions. Remarkable increases in computer speed have allowed the analysis of those distortions to be completed in milliseconds so that a thin mirror can be adjusted to correct for atmospheric distortions. Such systems are generally referred to as adaptive optics and in principle they allow observations of stellar positions to be made from the ground.
However, while adaptive optics systems were being developed, several satellites and satellite programs addressed the fundamental problems of astrometry. The pointing accuracy of the Hubble Space Telescope required a greatly increased "catalogue" of stars and their positions so that guide stars could be found for all potential targets of the telescope. A number of ground surveys which provided positions of several million stars were undertaken expressly to provide those guide stars. Partly in response to these surveys, machines were developed that could automatically measure star positions on the thousands of photographic glass plates that were taken for the projects. Now the determination of stellar positions can be accomplished directly by an electronic detector much like those found in a video camera, thereby replacing the photographic plate. This development also allowed the design of satellites dedicated to the determination of stellar positions. The must notable of these is Hipparcos, developed by the European Space Agency (ESA). Hipparcos was designed to measure the positions of more than 100,000 stars with an accuracy of between two and four milliarc seconds. This is easily more than ten times the accuracy readily achievable from the ground. After nearly a decade of development, Hipparcos was launched aboard the Ariane spacecraft in 1989. Unfortunately, due to a failure of the final stage of the rocket, the satellite never achieved the geo-stationary orbit approximately 25,000 mi (40,250 km) above the earth. Instead, its orbit is highly elliptical with its furthest point near the desired distance, but dipping down close to the earth's atmosphere at its low point. This greatly reduced the efficiency of the satellite.
Eventually satellites like Hipparcos, along with improved ground observations, will significantly enhance the number of stars for which we have good positions and other astrometrical data. Not only will this clarify our view of the local stellar neighborhood within our galaxy, it will also increase our fundamental knowledge of stars themselves.
See also Celestial coordinates.
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