When optical spectral lines produced by hot gas in distant galaxies is examined, a systematic shift in frequency with respect to laboratory measurements is found. The more distant a galaxy, the lower the frequency (the longer the wavelength) of the light received on earth. This is known as the redshift, with light waves being stretched to the redder part of the spectrum for the most distant objects. Those distances are independently measured through a variety of techniques involving variable stars and the observed properties of supernova explosions. Early in the 20th century Edwin Hubble defined this relationship and one of the principle goals of the Hubble Space Telescope was to pin down this redshift law, as it is called, more accurately. By knowing how redshift and distance are related, the redshift measured for any newly discovered object can be quickly converted into an accurate distance.
KeywordsBlack Hole Radio Emission Radio Source Spiral Galaxy Radio Galaxy
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