Proper motion
The
proper motion of a
star is the measurement of its change in position in the sky over time after
improper motions are accounted for. This contrasts with
radial velocity which is the measurement of the change in distance toward or away from the viewer over time.
From day to day, stars seem to be in fixed positions with respect to each other, meaning they always form the same
constellations, and (for example)
Ursa Major looks the same now as forty years ago. Long-term observation shows that the constellations change shape, albeit very slowly, and that each star has an independent
motion.
This motion is caused by the true movement of the stars relative to the
Sun and
solar system through space. It is measured by two quantities: the
proper motion angle and the
proper motion itself. The first quantity indicates the direction of the proper motion on the celestial sphere (clockwise from due North, East being left), and the second quantity gives the motion's magnitude, in seconds of arc per year.
Barnard's star has the largest proper motion of all stars, moving at 10.3
seconds of arc per year. Large proper motion is usually a strong indication that a star is relatively close to the
Sun. This is indeed the case for Barnard's Star which, at a distance of about 6
light years, is, after the Sun and the
Alpha Centauri system, the
nearest known star to Earth (yet, being a
red dwarf, too faint to see without a
telescope or powerful binoculars, with an
apparent magnitude of 9.54).
A proper motion of 1 arcsec per year at a distance of 1 light year corresponds to a relative transverse speed of 1.45 km/s. For Barnard's star this works out to 90 km/s; adding in the radial velocity of 111 km/s gives a true motion of 142 km/s. True or absolute motion is more difficult to measure than the proper motion, as the true transverse velocity is the product of the proper motion times the distance; that is, true velocity measurements depend on distance measurements, which are difficult in general. Currently, the nearby star with the largest true velocity (relative to the Sun) is
Wolf 424 which moves at 555 km/s. Only a little over half of the nearby star catalogue have measured true velocities.
Proper motion was discovered in
1718 by
Edmund Halley, who noticed that
Sirius,
Arcturus and
Aldebaran were over half a degree away from the positions charted by the ancient Greek astronomer
Hipparchus roughly 1850 years earlier.
In research published in
2005, the first measurement of the proper motion of a galaxy (the
Triangulum Galaxy) was made. [
1]
The following are the stars with highest proper motion from the
Hipparcos catalog. It does not include stars such as
Teegarden's star which are too faint for that catalog.
#
Barnard's star#
Kapteyn's star #
Groombridge 1830 #
Lacaille 9352#
CD -37 15492 (Gliese 1)#
HIP 67593#
61 Cygni A & B #
Lalande 21185#
Epsilon Indi*
Aberration of light*
Double star*
Epoch (astronomy)*
Henderson, Thomas *
Kapteyn, Jacobus*
List of astronomical topics*
Parallax*
Radial velocity*
Star catalogue*
Superluminal motion*
Timeline of stellar astronomy*
Hipparcos: High Proper Motion Stars*
The 150 Stars in the Hipparcos Catalogue with Largest Proper Motion
