Redshift and Blueshift

By , 26th March 2009 in Astronomy

Redshift is an increase in the wavelength of light coming from an object due to its motion away from Earth. Blueshift is the opposite, a decrease in wavelength due to its motion towards Earth.
 

Redshift and Blueshift are visible effects of the Doppler effect. You have probably witnessed the Doppler effect yourself. The best example is a siren coming towards you fast. As it approaches the siren is a much higher pitch than when it passes and is moving away from you. This corresponds with an increase in frequency. This is demonstrated in the video below.

The same thing happens with light. As an object moves towards us the wavelength is altered so that it shifts towards the blue end of the spectum. As the object moves away from us the light is "stretched" towards the red. The shifts we can observe occur in the change of position in the spectral lines.

Redshift and Blueshift
Redshift and Blueshift
Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decr
Absorption lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the optical spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decr

History

The Doppler effect is named after Christian Andreas Doppler who offered the first known physical explanation for the phenomenon in 1842. The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845.

The first Doppler redshift was described in 1848 by French physicist Armand-Hippolyte-Louis Fizeau, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is sometimes called the "Doppler-Fizeau effect". In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method.

In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1901 Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors.

Looking for Redshift

The spectrum of light coming from a distant object can be measured through spectroscopy. To determine the redshift features in the spectrum (such as absorption lines, emission lines, or other variations in light intensity) are searched for and if found compared with known features in the spectrum of various elements. A very common element in space is hydrogen.

In the diagram above you can see two spectra. One from out Sun (a known spectra - we know each of the absorption lines) and one from a supercluster of distant galaxies. When we compare the two we see a correlation between the Hydrogen lines of the Sun and of the distant galaxies, the only difference is that the absorption lines in the galaxies are all moved up (towards the red). This indicates a redshift and we can tell that the galaxies are moving away from us (or we are moving away from the galaxies).

Calculation

Once we find a known spectral line we can work out it's wavelength in the spectra. We can then use this to calculate the exact redshift.

From the graphic above we can take the Hydrogen Alpha emission line at 656.2 nm. We can then calculate the wavelength from the observed spectra based on the spectrum (for this example the observed line is at 675 nm). We can then use a simple equation to calculate the redshift value.

z={lambda_obsv-lambda_emit}/lambda_emit
Equation 27 - Redshift

Plugging in our values for the observed wavelengths gives:

z={675-656.2}/656.2
z=0.02
Equation 28 - Redshift example working

z is a dimensionless quantity that is traditionally used. A positive value of z indicates redshift, a negative value represents blueshift.

Example Redshifts

Currently the objects with the highest known redshifts are galaxies. The most reliable redshifts are from spectroscopic data and the highest confirmed spectroscopic redshift of a galaxy is that of IOK-1 at a redshift z = 6.96.

The most distant observed gamma ray burst is GRB 080913, which had a redshift of 6.7.

Further Reading
Comments
  1. Mo King
    Mo King

    The red and blue-shift in relation to photons seems to be grossly misused for the determination of stellar motion.Gravity lensing is a phenomena caused by the slingshot effect around gravity bodies whereby the photons gain energy but not speed, which has the effect of shortening the wavelength to a higher energy band thus negating the use for measurement of large interstellar distances. The wavelength of all radiation can be increased or decreased in space by any and all large gravity bodies, therefore its use in distance measurement is nonsensical.Lastly the radiation over large distances will dissipate its energies effecting a red shift regardless of direction (towards or away).
    MoK

  2. Nicholas Jessup
    Nicholas Jessup

    1. What are the Negative Aspects of Red Shift

    2. Are there other pieces of evidence that show the universe is changing besides red shift.

  3. Axel
    Axel

    we can also say z=v/c, where c is the speed of light and v is the velocity of the source. From this we can calculate how fast the object is moving away or towards us.

    1. Tim Trott
      Tim Trott

      True, but that equation only works in a non-relativistic Universe. For a unrelativistic speeds it is <em>nearly</em> right.

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