Proper motion

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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.

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[edit] Introduction

From day to day, stars seem to be in fixed positions with respect to each other, meaning they always form the same constellations. Ursa Major, for example, 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.

[edit] History

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]

[edit] Stars with high proper motion

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.

  1. Barnard's star
  2. Kapteyn's star
  3. Groombridge 1830
  4. Lacaille 9352
  5. CD -37 15492 (Gliese 1)
  6. HIP 67593
  7. 61 Cygni A & B
  8. Lalande 21185
  9. Epsilon Indi

[edit] See also

[edit] References

[edit] External links

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