When studying the light from a star that is moving away from us, we can still determine its emitted wavelength through a phenomenon known as the redshift. Redshift occurs because the motion of an object relative to an observer affects the wavelength of light emitted by that object.
The redshift is quantified by the cosmological redshift parameter, denoted as "z." It is calculated based on the observed wavelength of light from a distant object compared to the expected or known wavelength of that light when it was emitted.
To understand how we determine the emitted wavelength of a star, let's consider the process:
Reference spectrum: Astronomers have measured the spectra of various chemical elements under laboratory conditions on Earth, which act as reference spectra. These spectra provide us with the expected or known wavelengths of the emitted light.
Measuring observed wavelength: When the light from a distant star reaches Earth, astronomers measure the observed wavelength of specific spectral lines in that star's spectrum. Spectral lines correspond to the characteristic wavelengths at which specific elements or molecules emit or absorb light.
Comparing observed and reference wavelengths: By comparing the observed wavelength of a spectral line from the star with the known wavelength of that line in the reference spectrum, astronomers can determine the redshift, represented by the parameter "z."
The redshift (z) is calculated using the formula:
z = (λ_observed - λ_emitted) / λ_emitted
where λ_observed is the observed wavelength of the spectral line, and λ_emitted is the known or expected wavelength of that line.
Based on the redshift value, astronomers can infer the amount of the emitted wavelength that has been stretched or shifted due to the motion of the star. This shift provides information about the star's velocity and distance from Earth. The larger the redshift value (z), the greater the star's velocity and the farther it is from us.
Therefore, we do not need to be at a constant distance from the star to calculate the redshift and determine the emitted wavelength. Instead, by analyzing the observed spectrum and comparing it to known reference spectra, we can deduce the motion and distance of the star using the redshift value.