Astronomy
Spectroscopic Techniques for Measuring Stellar Radial Velocities
Quick fact
The radial velocity method has discovered thousands of exoplanets, including the first exoplanet orbiting a sun-like star, 51 Pegasi b in 1995.
Why this is interesting
A star that looks perfectly still in the night sky may actually be drifting towards or away from us. How can we possibly detect such motion from light-years away?
Read the full explanation
Understanding Spectroscopic Techniques for Measuring Stellar Radial Velocities
When a star moves toward us, the light waves it emits get compressed, shifting them to shorter, bluer wavelengths—a blueshift. When it moves away, the waves stretch out, shifting to longer, redder wavelengths—a redshift. This is the same Doppler effect we hear when a siren's pitch changes as it passes by. To measure a star's radial velocity, astronomers spread its light into a rainbow-like spectrum using a spectrograph. Superimposed on this continuum are dark absorption lines—'fingerprints' of the elements in the star's atmosphere. By precisely measuring how these lines have shifted from their known laboratory wavelengths, we determine how fast the star is moving toward or away from us. It's like listening for a change in a familiar tune to tell if the musician is walking closer or farther away.
A deeper explanation
The key mechanism is the Doppler shift of spectral lines. Since each element absorbs light at specific wavelengths, those patterns serve as fixed reference points. A spectrograph disperses starlight, and a high-resolution detector records the exact positions of these lines. To achieve accuracy, astronomers calibrate with a reference source, often a gas cell like iodine or Thorium-Argon, ensuring known wavelengths. Any displacement in the star's lines from their rest wavelengths indicates radial motion. The velocity is calculated using the equation Δλ/λ = v/c , where Δλ is the shift, λ is the rest wavelength, v is the radial velocity, and c is the speed of light. By recording how these velocities change over time, we can detect a periodic wobble caused by an orbiting companion—this is the basis of the radial velocity method for discovering exoplanets. Thus, this technique not only measures stellar motion but also reveals the presence of invisible planets and contributes to our understanding of the universe's structure.