What is the Difference Between the Redness of an Object and its Redshift?

-- | January 6, 2014

Question: What is the difference between Doppler shift of light and Natural light spectra (blue for young and red for old) with respect to stars and galaxies? How does astronomers differentiate between Doppler shift and Natural light spectra of deep space objects?

For Example: Consider a deep space object which is actually young (blue color) and also receding away from earth but due to its enormous distance from earth it is red-shifted (appears as red-colored in Hubble observations) due to Doppler effect. In the above condition, how does an astronomer determine the true color of the cosmic body? — Vinod

Answer: The color of an object is determined by the peak in its continuum emission, which is itself determined by how hot the object is. For example, a star that appears red is colder than a star that appears blue. Redshift, on the other hand, is a property determined by the shift in wavelength of spectral lines emitted by the gas in an object. Now, a redshifted object will also have a redshifted peak to its continuum emission, so it also will appear redder. If we can measure the redshift of this object we can determine its true color by correcting for the reddening caused by high recession velocity.

Jeff Mangum

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