Radial velocity is the velocity of an object that is either moving away from or towards the line of sight of an observer. If an object is approaching, it has a negative radial velocity, correspondingly if the object is moving away, it has a positive radial velocity.
For stars and other luminous astronomical objects, the radial velocity is accurately measured by observing the spectrum of the object and comparing the measured wavelengths of its spectral lines to wavelengths from laboratory measurements. The amount of radial velocity is calculated by the displacement of the spectral lines from their normal position. If the object is moving towards us, those spectral lines will be blueshifted, if the object is moving away, the spectral lines instead will be redshifted. This is known as the Doppler effect. The greater the blueshift or redshift, the larger the radial velocity of the object.
Object detection through radical velocity
The radical velocity method, also known as Doppler spectroscopy, is one of the many techniques used for searching for extrasolar planets. This method works because planets cause their host stars to move in a small counter-orbit as the planets orbit the star. As such, the radial velocity of the star increases and decreases by a slight amount as it moves towards or away from an astronomically distant observer. If the planet's mass is adequately large, its will create a great enough "back and forth" motion in the star through its counter-orbit to be detected as small, regular periodic blue and red shifts in the spectral lines of that star. This method though only allows for the calculation of the lower mass limit of any orbiting companion to the star that is detected.
This method is also used to determine the minimum mass of otherwise invisible binary stars companions. The Doppler effect of each star of the binary star will occur as the two stars orbit each other. As each star moves towards and then away from the distant observer in their mutual orbit around a common center of mass, their spectral lines correspondingly blueshifts and redshifts. Binary companions found in such ways are called spectroscopic binaries.
- The Alchemy of the Heavens, K. Croswell, Anchor Books (1995)