![]() After several decades, the baseline can be orders of magnitude greater than the Earth–Sun baseline used for traditional parallax. For stars in the Milky Way disk, this corresponds to a mean baseline of 4 AU per year, while for halo stars the baseline is 40 AU per year. The motion of the Sun through space provides a longer baseline that will increase the accuracy of parallax measurements, known as secular parallax. Hubble Space Telescope precision stellar distance measurement has been extended 10 times further into the Milky Way. Parallax measurements may be an important clue to understanding three of the universe's most elusive components: dark matter, dark energy and neutrinos. This statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables. ![]() For a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities. The former is determined by plotting the changing position of the stars over many years, while the latter comes from measuring the Doppler shift of the star's spectrum caused by motion along the line of sight. Stars have a velocity relative to the Sun that causes proper motion (transverse across the sky) and radial velocity (motion toward or away from the Sun). Distances can be measured within 10% as far as the Galactic Center, about 30,000 light years away. The Gaia space mission provided similarly accurate distances to most stars brighter than 15th magnitude. The Hubble Space Telescope's Wide Field Camera 3 has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs (16,000 ly) for small numbers of stars. In the 1990s, for example, the Hipparcos mission obtained parallaxes for over a hundred thousand stars with a precision of about a milliarcsecond, providing useful distances for stars out to a few hundred parsecs. Astronomers usually express distances in units of parsecs (parallax arcseconds) light-years are used in popular media.īecause parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars which are near enough to have a parallax larger than a few times the precision of the measurement. The amount of shift is quite small, even for the nearest stars, measuring 1 arcsecond for an object at 1 parsec's distance (3.26 light-years), and thereafter decreasing in angular amount as the distance increases. ![]() These shifts are angles in an isosceles triangle, with 2 AU (the distance between the extreme positions of Earth's orbit around the Sun) making the base leg of the triangle and the distance to the star being the long equal-length legs. ![]() As the Earth orbits the Sun, the position of nearby stars will appear to shift slightly against the more distant background. The most important fundamental distance measurements in astronomy come from trigonometric parallax, as applied in the stellar parallax method. A similar diagram can be drawn for a star except that the angle of parallax would be minuscule. The lower diagram shows an equal angle swept by the Sun in a geostatic model. In the upper diagram, the Earth in its orbit sweeps the parallax angle subtended on the Sun. Parallax is an angle subtended by a line on a point. Half the apex angle is the parallax angle. Stellar parallax motion from annual parallax. ![]()
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