Redshift and Blueshift
- Introduction to Astronomy
- The Celestial Sphere - Right Ascension and Declination
- What is Angular Size?
- What is the Milky Way?
- The Magnitude Scale
- Sidereal Time, Civil Time and Solar Time
- Parallax, Distance and Parsecs
- Apparent Magnitude, Absolute Magnitude and Distance
- Variable Stars
- Spectroscopy and Spectrometry
- Redshift and Blueshift
- Spectral Classification of Stars
- Hertzsprung-Russell Diagram
- Kepler's Laws of Planetary Motion
- The Lagrange Points
- What is an Exoplanet?
- Glossary of Astronomy & Photographic Terms
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.
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).
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.
Equation 27 - Redshift
Plugging in our values for the observed wavelengths gives:
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.
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.