# How to calculate the redshift of a line in a bunch of line spectra?

If in a spectra, there are many lines, mixed with gravitational redshift and non-redshift lines. For example, in this paper, they identify lines both redshifted and no-redshifted. Then how can i distinguish those redshifted from those do not?

• Red-shift is not selective for frequency. Either the whole spectrum of a source is red-shifted, or nothing at all. The only type of lines which are NOT red-shifted are the absorption lines from Earth's atmosphere - and those can actually work nicely as calibration lines (though professionally you might want to use a iodine cell for more precision). For a rotating source you additionally have line-broadening. You might want to refine your question; what exactly gives you a problem in calculating red-shift when you have a line at wavelength $\lambda_z$ instead of $\lambda_r$. – planetmaker Jul 27 '20 at 13:25
• For a spectra, the lines can be rediated from different part of a source, so the lines can be both redshifted or no-redshifted. So i want to know the method to distinguish them. – Chen Jul 27 '20 at 14:17

For instance, the O VII Ly$$\alpha$$ line has a rest wavelength of 19 Å, but is seen at ~25.6 Å, i.e. a factor 1.35 higher, so its redshift is $$z = 0.35$$.