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Proper motion. Measure of observed changes in the apparent locations of stars. Relation between proper motion and velocity components of an object. A year ago the object was d units of distance from the Sun, and its light moved in a year by angle μ radian/s. If there has been no distortion by gravitational lensing or otherwise then μ = where ...
proper motion, in astronomy, the apparent motion of a star across the celestial sphere at right angles to the observer’s line of sight; any radial motion (toward or away from the Sun) is not included. It is observed with respect to a framework of very distant background stars or galaxies. Proper motion is generally measured in seconds of arc ...
- The Editors of Encyclopaedia Britannica
Proper Motion is the apparent angular motion of a star across the sky with respect to more distant stars. Typical proper motion is ~0.1 arcsec/year. Largest: 10.25 arcsec/yr (Barnard's Star). This is the projection onto the sky of the star's true motions through space relative to the Sun.
5 The Motion of Stars. In addition to the daily (diurnal) motion of the sky as a whole, the stars move (very slowly) on the celestial sphere with respect to each other. This motion is called proper motion, and is commonly denoted by the symbol . The motion of stars can be decomposed into a radial component (along the line of sight) and a ...
Studies of stellar spectra have shown that hydrogen makes up about three-quarters of the mass of most stars. Helium is the second-most abundant element, making up almost a quarter of a star’s mass. Together, hydrogen and helium make up from 96 to 99% of the mass; in some stars, they amount to more than 99.9%.
The proper motion (μ) has a magnitude and a direction, and is often broken down into the components of right ascension (μ RA) and declination (μ Dec) where. μ 2 =μ RA2 + μ Dec2. The product of a star’s proper motion μ and distance D yield the transverse velocity VT = μD (ie the velocity perpendicular to our line of sight).
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The star with the largest known proper motion is called Barnard’s Star. It moves at a speed of 10.3 arcseconds per year, which is the equivalent of it moving across the face of the Moon in 180 years (where the Moon’s angular diameter on the sky is about half a degree). All other stars have much smaller proper motions.
