![]() ![]() When dealing with distant galaxies, it is always positive.ġ. But as I said, this isn't always correct - there are times when "z" isn't equal to v/c, so it is sometimes easier to just leave it as "z".Īnd if the value of "z" is positive it is a "redshift" indicating motion away, but it can also be negative, which would be a "blueshift", indicating motion towards us. ![]() If we look at the previous formula (Doppler shift), "z" is just the value of v/c, the ratio of the velocity to the speed of light. There are reasons for that since you may not be able to calculate the velocity using the regular Doppler formula, but may have to use the Relativistic formula (next formula down). Redshift - this is sort of like the previous case, except the last steps aren't carried through - the velocity value isn't calculated. Now you can plug the values into the formula. Since this is a positive value, the object is moving away from us. Solution: First you need to determine the value for, which is the change in wavelength How fast is this galaxy moving, and is it moving towards or away from you? An absorption feature of calcium usually has a wavelength of 3934 Å, but it is observed in a galaxy to have a wavelength of 4002 Å. Typically for galaxies we tend to use km/s, so the speed of light in this case would be 3.0 x 10 5 km/sġ. The units you use for velocity and for "c" are sort of arbitrary. If it is positive, the motion is away from you, if it is negative the motion is towards you. It should be noted that is the difference or change in the value of the spectral feature from its normal wavelength ( ), and the change could be positive or negative.
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