An increase in the wavelength of spectral lines from a galaxy or star, known as a redshift, can be used to infer its motion through a phenomenon called the Doppler effect. The Doppler effect describes the change in frequency or wavelength of a wave (such as light or sound) as a result of relative motion between the source of the wave and the observer.
When an object emitting light or other electromagnetic radiation moves away from an observer, the wavelengths of the emitted radiation appear longer or "stretched out" due to the motion-induced increase in the observed wavelength. This is known as a redshift. Conversely, when an object moves towards an observer, the wavelengths appear shorter or "compressed," resulting in a blueshift.
By analyzing the redshift (or blueshift) of the spectral lines in the light emitted by a galaxy or star, scientists can infer the object's motion relative to the observer. Here's how it works:
Known reference spectrum: Scientists first obtain a high-quality spectrum of a reference object or laboratory sample with known spectral lines. This serves as a baseline against which the observed spectrum can be compared.
Comparison with reference: They then compare the observed spectrum of the galaxy or star with the reference spectrum. If the spectral lines in the observed spectrum are shifted towards longer wavelengths (redshifted) compared to the reference spectrum, it indicates that the object is moving away from the observer.
Determining the redshift: The amount of redshift can be quantified by measuring the change in wavelength of specific spectral lines. The redshift is typically expressed as a dimensionless quantity called "z," which represents the fractional change in wavelength. The formula for redshift is given by z = Δλ/λ, where Δλ is the change in wavelength and λ is the rest wavelength of the spectral line.
Inferring motion: Based on the redshift value, scientists can determine the object's velocity and direction of motion. Higher redshift values correspond to greater recessional velocities, indicating that the object is moving away at a faster rate. The Hubble's law, which describes the relationship between the redshift of distant galaxies and their recession velocities, is often used to estimate the object's motion in cosmological contexts.
By examining the redshift of spectral lines, astronomers have been able to gather valuable information about the motion, velocity, and expansion of celestial objects, contributing to our understanding of the universe's large-scale structure and dynamics.