Red Shift

Hydrogen redshift example
Calculation

The light from distant stars and more distant galaxies is not featureless, but has distinct spectral features characteristic of the atoms in the gases around the stars. When these spectra are examined, they are found to be shifted toward the red end of the spectrum. This shift is apparently a Doppler shift and indicates that essentially all of the galaxies are moving away from us. Using the results from the nearer ones, it becomes evident that the more distant galaxies are moving away from us faster. This is the kind of result one would expect for an expanding universe.

The building up of methods for measuring distance to stars and galaxies led Hubble to the fact that the red shift (recession speed) is proportional to distance. If this proportionality (called Hubble's Law) holds true, it can be used as a distance measuring tool itself.

The measured red shifts are usually stated in terms of a z parameter. The largest measured z values are associated with the quasars.

Examples of observed redshifts
Redshift calculation example
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Measured Red Shifts

Measured Doppler red shifts give the recession velocity of stars or galaxies, presuming that the Hubble law is valid. It is common practice to express this velocity as a fraction of the speed of light and to use a parameter z defined by:
For z =

v/c =
Calculating v/c from this expression gives:

The largest red shifts measured are those of the quasars. Measurements of >100 quasars have given a range of 0.16 to 3.53, corresponding to recession speeds of 0.15c to 0.91c.

Examples of observed redshifts
Redshift calculation example
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Red Shift of Galaxy 8C1435+635

Reported in November 1994 in Monthly Notices of the Royal Astronomical Society is a galaxy with a measured red shift of z=4.25 , a new record. This value for the z parameter corresponds to a recession speed of .93c. The galaxy 8C1435+635 was observed in a systematic search for faint, radio-emitting galaxies carried out by a team at Leiden Observatory led by George Miley. After discovery, the optical spectra was observed by the William Hershel Telescope in La Palma, Canary Islands. Two emission lines of ionized carbon and hydrogen were measured to obtain the red shift. This red shift corresponds to a distance of 12 to 15 million light years, depending on what value of Hubble constant is used.

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