Proper motion

Information about 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.

Introduction

Over the course of centuries, stars appear to maintain nearly fixed positions with respect to each other, so that they form the same constellations over historical time. Ursa Major, for example, looks nearly the same now as it did hundreds of years ago. However, precise long-term observations show 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 (with 0 degrees meaning the motion due north, 90 degrees due east, and so on), and the second quantity gives the motion's magnitude, in seconds of arc per year.

Proper motion may also be given by the angular components in the right ascension (μα) and declination (μδ). The net proper motion (μ) is given by:[1]

where δ is the declination. The proper motion angle (θ) is related to these components by:<ref name="majewski" />


Enlarge picture
Barnard's Star, showing position every 5 years 1985–2005.
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; including the radial velocity of 111 km/s (which is at right angles to the transverse velocity) 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 involves 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.

Usefulness in Astronomy

Stars with large proper motions tend to be nearby; most stars are far enough away that their proper motions are very small, of order a few thousandths of an arcsecond per year. It is possible to construct nearly complete samples of high proper motion stars by comparing photographic sky survey images taken many years apart. The Palomar Sky Survey is one source of such images. In the past, searches for high proper motion objects were undertaken using blink comparators to examine the images by eye, but modern efforts use techniques such as image differencing to automatically search through digitized image data. Because the selection biases of the resulting high proper motion samples are well-understood and well-quantified, it is possible to use them to construct an unbiased census of the nearby stellar population -- how many stars exist of each true brightness, for example. Studies of this kind show that the local population of stars consists largely of intrinsically faint, inconspicuous stars such as red dwarfs.

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.[2]

Stars with high proper motion

The following are the stars with highest proper motion from the Hipparcos catalog.[3] It does not include stars such as Teegarden's star which are too faint for that catalog.

Highest proper motion stars[4]
# Star Proper motion Radial
velocity
(km/s)		 Parallax
(mas)
μα
(mas/yr)		 μδ
(mas/yr)
1Barnard's star-798.7110337.77-106.8549.30
2Kapteyn's star6500.34-5723.17+245.5255.12
3Groombridge 18304003.69-5814.64-98.0109.22
4Lacaille 93526766.631327.99+9.7303.89
5CD -37 15492 (GJ 1)5633.95-2336.69+23.6229.32
6HIP 675932282.155369.33—76.20
761 Cygni A & B4133.053201.78-64.3287.18
8Lalande 21185-580.46-4769.95-85.0392.52
9Epsilon Indi3961.41-2538.33-40.4275.79

References

1. ^ Majewski, Steven R. (2006). Stellar Motions. University of Virginia. Retrieved on 2007-05-14.
2. ^ McKee, Maggie. "Distant galaxy's subtle sidling measured", New Scientist, March 3, 2005. Retrieved on 2007-07-21. 
3. ^ Staff (September 15, 2003). The 150 Stars in the Hipparcos Catalogue with Largest Proper Motion. ESA. Retrieved on 2007-07-21.
4. ^ SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved on 2007-07-21.

External links

STAR is an acronym for:

Organizations:
  • Society for Telescopy, Astronomy, and Radio, a non-profit astronomy club in New Jersey
  • Special Tasks and Rescue or Special Tactics and Response, synonyms for SWAT

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The improper motion of a star refers to the change of its coordinates on the sky not originating from the motion of the star itself, as opposed to proper motion.

Sources of improper motion

  • Aberration of light
  • Diurnal motion
  • Nutation
  • Parallax

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Radial velocity is the velocity of an object in the direction of the line of sight (i.e. its speed straight towards you, or away from you). The light of an object with a substantial radial velocity will be subject to Doppler effect, so the wavelength of the light decreases for
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constellation of Orion is the area outlined in the dashed yellow line. Orion contains a striking and well-known star pattern that has the form of a hunter.]] A constellation is any one of the 88 areas into which the sky — or the celestial sphere — is divided.
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Ursa major

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List of stars in Ursa major
Abbreviation: UMa
Genitive: Ursae Majoris
Symbology: the Great Bear
Right ascension: 10.67 h
Declination: +55.38
Area: 1280 sq. deg.
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cite any references or sources. Please help improve this article by citing reliable sources.
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The Sun

Observation data
Mean distance
from Earth 1.4961011 m
(8.31 min at light speed)
Visual brightness (V) −26.74m [1]
Absolute magnitude 4.
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Solar System or solar system[a] consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons,[1]
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celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere.
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A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60) of one degree. [1] Since one degree is defined as one three hundred sixtieth (1/360) of a circle, 1 MOA is 1/21600 of the amount of arc in a closed circle, or
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Right ascension (abbrev. RA; symbol α) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.
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In astronomy, declination (abbrev. dec or δ) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle.
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Barnard's Star (or Barnard's Runaway Star[4]) is a very low-mass red dwarf star in the constellation Ophiuchus. In 1916, American astronomer E. E. Barnard measured its proper motion as 10.

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Arcsec is an abbreviation for either:
  • arcsecond or
  • arcsecant.
In the latter case it would normally have a lower-case initial 'a' except when an author follows a convention whereby a capital A denote a "principal value" or the like.
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The Sun

Observation data
Mean distance
from Earth 1.4961011 m
(8.31 min at light speed)
Visual brightness (V) −26.74m [1]
Absolute magnitude 4.
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1 light-year =
SI units
01015 m 01012 km
Astronomical units
0103 AU 0 pc
US customary / Imperial units
01015 ft 01012 mi
A light-year or lightyear (symbol:
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See Alpha Centauri (disambiguation) for other uses.
Alpha Centauri (α Cen / α Centauri, also known as Rigil Kentaurus), is the brightest star system in the southern constellation of Centaurus.
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This list of stars nearest to Earth is ordered by increasing distance out to a maximum of 5 parsecs (16.308 light-years), as measurements for greater distances introduce margins of error routinely greater than 2%.
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red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0.
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A telescope is an instrument designed for the observation of remote objects and the collection of electromagnetic radiation. The earliest known telescopes are credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of
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The apparent magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere.
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Wolf 424 is a binary star system comprising two red dwarf stars at a distance of approximately 14.2 light years from the Sun. It is located in the constellation Virgo, between the stars ε Virginis and δ Virginis.

The Wolf 424 system has a semi-major axis of 4.
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blink comparator was a viewing apparatus used by astronomers to find differences between photographic plates taken of the same area of the sky at different times. It was also sometimes known as a blink microscope.
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Image differencing is an image processing technique used to determine changes between images. The difference between two images is calculated by finding the difference between each pixel in each image, and generating an image based on the result.
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Selection bias is a distortion of evidence or data that arises from the way that the data are collected. It is sometimes referred to as the selection effect. The term selection bias
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red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0.
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Edmond Halley FRS (sometimes "Edmund"; IPA: /ˈɛdmənd ˈhɔːlɪ/) (November 8, 1656 – January 14, 1742) was an English astronomer, geophysicist, mathematician, meteorologist, and physicist.
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Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) (IPA: /ˈsɪriəs/) is the brightest star in the night-time sky with a visual apparent magnitude of −1.47.
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Arcturus (α Boo / α Boötis / Alpha Boötis) (IPA: /ɑrkˈtjʊərəs/) is the brightest star in the constellation Boötes, and the third brightest star in the night sky, with a visual magnitude of
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Aldebaran is the brightest star in the constellation Taurus and one of the brightest stars in the nighttime sky. It has the Bayer designation Alpha Tauri. Because of its location in the head of Taurus, it has historically been called the Bull's Eye.
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