Editing Radial velocity (section)

===Spectroscopic radial velocity===

Light from an object with a substantial relative radial velocity at emission will be subject to the [[Doppler effect]], so the frequency of the light decreases for objects that were receding ([[redshift]]) and increases for objects that were approaching ([[blueshift]]).

The radial velocity of a [[star]] or other luminous distant objects can be measured accurately by taking a high-resolution [[Electromagnetic spectrum|spectrum]] and comparing the measured [[wavelength]]s of known [[spectral line]]s to wavelengths from laboratory measurements. A positive radial velocity indicates the distance between the objects is or was increasing; a negative radial velocity indicates the distance between the source and observer is or was decreasing.

[[William Huggins]] ventured in 1868 to estimate the radial velocity of [[Sirius]] with respect to the Sun, based on observed redshift of the star's light.<ref>{{cite journal | last=Huggins | first=W. | title=Further observations on the spectra of some of the stars and nebulae, with an attempt to determine therefrom whether these bodies are moving towards or from the Earth, also observations on the spectra of the Sun and of Comet&nbsp;II | journal=[[Philosophical Transactions of the Royal Society of London]] | date=1868 | volume=158 | pages=529–564 | doi=10.1098/rstl.1868.0022| bibcode=1868RSPT..158..529H}}</ref>

[[File:Planet reflex 200.gif|thumb|Diagram showing how an exoplanet's orbit changes the position and velocity of a star as they orbit a common center of mass]]

In many [[binary star]]s, the [[orbit]]al motion usually causes radial velocity variations of several kilometres per second (km/s). As the spectra of these stars vary due to the Doppler effect, they are called [[spectroscopic binaries]]. Radial velocity can be used to estimate the ratio of the [[mass]]es of the stars, and some [[orbital element]]s, such as [[eccentricity (orbit)|eccentricity]] and [[semimajor axis]]. The same method has also been used to detect [[planet]]s around stars, in the way that the movement's measurement determines the planet's orbital period, while the resulting radial-velocity [[amplitude]] allows the calculation of the lower bound on a planet's mass using the [[binary mass function]]. Radial velocity methods alone may only reveal a lower bound, since a large planet orbiting at a very high angle to the [[Sightline|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 in fact be two-planet systems of circular or near-circular resonant orbit.<ref name="Anglada-Escude">{{cite journal | first1=Guillem |last1 = Anglada-Escude |first2=Mercedes |last2=Lopez-Morales|first3= John E.|last3=Chambers | title = How eccentric orbital solutions can hide planetary systems in 2:1 resonant orbits | journal = The Astrophysical Journal Letters | arxiv = 0809.1275 | doi = 10.1088/0004-637X/709/1/168 | volume=709 | issue=1 | pages=168–78 | bibcode = 2010ApJ...709..168A|year = 2010 |s2cid = 2756148 }}</ref><ref name="KursterAA2015">{{cite journal|first1=Martin |last1=Kürster| first2=Trifon |last2=Trifonov |first3=Sabine |last3=Reffert| first4=Nadiia M. | last4=Kostogryz |first5=Florian |last5=Roder | journal=Astron. Astrophys. |year=2015|pages=A103 |doi=10.1051/0004-6361/201525872 | volume=577 |title=Disentangling 2:1 resonant radial velocity oribts from eccentric ones and a case study for HD 27894 | arxiv=1503.07769 | bibcode=2015A&A...577A.103K|s2cid=73533931}}</ref>