Radial Velocity

Another way to measure properties of stars is to calculate something called the radial velocity of the star. This is just the velocity of the star as it is moving either towards us or away from us. Remember from the Doppler Effect, that as the star is moving towards us, the star will appear bluer, i.e. the spectrum moves towards the blue end of the visible light spectrum. As the star moves away from us, the spectrum moves towards the red end of the visible light spectrum. How can we use this to find the radial velocity?


As discussed in the post about spectroscopy, each element has a unique spectral pattern based on the energy levels of electrons orbiting the nucleus. By comparing a star's spectrum to the standard spectra of the elements, one can match up lines found in the stellar spectrum. By measuring the wavelength of those lines in the spectrum, and comparing them to the standard spectrum of the element, just using the Doppler equation can give you the radial velocity of the star.


This obviously only works if the star is moving away from or towards you. If the star is moving across you line of sight, astrometry is used instead and what you are measuring is called proper motion.