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Radial velocity
 
 
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Radial velocity is the velocity
Velocity

In physics, velocity is defined as the Derivative of Position vector. It is a vector physical quantity; both speed and direction are required to define it....
 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
Doppler effect

The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency and wavelength of a wave for an observer moving relative to the source of the waves....
, so the frequency of the light decreases for receding objects (redshift
Redshift

In physics and astronomy, redshift occurs when electromagnetic radiation?usually visible light?emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum due to the Doppler effect....
) and increases for approaching objects (blueshift).

The radial velocity of a star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
 or other luminous but distant objects can be measured accurately by taking a high-resolution spectrum
Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
 and comparing the measured wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
s of known spectral line
Spectral line

A spectral line is a dark or bright line in an otherwise uniform and continuous optical spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies....
s to wavelengths from laboratory measurements.






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Encyclopedia


Radial velocity is the velocity
Velocity

In physics, velocity is defined as the Derivative of Position vector. It is a vector physical quantity; both speed and direction are required to define it....
 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
Doppler effect

The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency and wavelength of a wave for an observer moving relative to the source of the waves....
, so the frequency of the light decreases for receding objects (redshift
Redshift

In physics and astronomy, redshift occurs when electromagnetic radiation?usually visible light?emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum due to the Doppler effect....
) and increases for approaching objects (blueshift).

The radial velocity of a star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
 or other luminous but distant objects can be measured accurately by taking a high-resolution spectrum
Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
 and comparing the measured wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
s of known spectral line
Spectral line

A spectral line is a dark or bright line in an otherwise uniform and continuous optical spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies....
s to wavelengths from laboratory measurements. By convention, a positive radial velocity indicates the object is receding; if the sign is negative, then the object is approaching.

In many binary star
Binary star

A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star or secondary....
s, the orbit
ORBit

ORBit is a Common Object Request Broker Architecture 2.4 compliant Object Request Broker . It features mature C , C++ and Python bindings, and less developed bindings for Perl, Lisp , Pascal , Ruby , and Tcl....
al motion usually causes radial velocity variations of several kilometers per second. As the spectra of these stars vary due to the Doppler effect, they are called spectroscopic binaries. Radial velocity studies can be used to estimate the masses of the stars, and some orbital elements, such as eccentricity and semimajor axis. The same method has also been used to detect planet
Planet

A planet , as 2006 definition of planet by the International Astronomical Union , is a celestial body orbiting a star or Stellar evolution#Stellar remnants that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared the neighbourhood of planetesimals....
s around stars, in the way that the movement's measurement determines the planet's orbital period, while the resulting size of the displacement allows the calculation of the lower bound on a planet's mass
Mass

In physical science, mass refers to the degree of acceleration a body acquires when subject to a force: bodies with greater mass are accelerated less by the same force....
. Radial velocity methods alone may only reveal a lower bound, since a large planet orbiting at a very high angle to the line of sight will perturb its star radially as much as a much smaller planet with an orbital plane on the line of sight. It has been suggested that planets with high eccentricities calculated by this method may be mimicking 2 planet systems of circular or near-circular resonant orbit.

See also

  • Extrasolar planet
    Extrasolar planet

    An extrasolar planet, or exoplanet, is a planet beyond the Solar System, orbiting a star other than the Sun. As of February 2009, 342 exoplanets are listed in the Extrasolar Planets Encyclopaedia....
  • Proper motion
    Proper motion

    The proper motion of a star is its angular change in position over time as seen from the Sun, as inferred after improper motions are accounted for....
  • Doppler spectroscopy
    Doppler spectroscopy

    Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopy method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....