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<text><span class="style10">he Universe and Cosmology (4 of 6)</span><span class="style7"></span><span class="style10">Red shift</span><span class="style7">In 1868, the English amateur astronomer Sir William Huggins (1824-1910) noticed that lines in the spectra of certain stars were displaced towards the red end of the spectrum. Huggins realized that this was due to the </span><span class="style26">Doppler effect</span><span class="style7">, which had been discovered in 1842. Just as the noise from a moving vehicle will appear to change pitch as it passes, the color of light from a star will change in wavelength as the star moves towards us, or away from us. Stars moving away from the Earth have their light moved towards the red end of the spectrum (</span><span class="style26">red shift</span><span class="style7">), while those moving towards us exhibit a shift towards the blue end.</span><span class="style10">Hubble's law</span><span class="style7">In 1929, Edwin Hubble (1889-1953) - who also worked on the classification of galaxies - analyzed the red shifts of a number of galaxies. He found that the speed at which a galaxy is moving away from us is proportional to its distance - i.e. the more distant a galaxy, the faster it is receding. This principle was formulated as Hubble's law, which can be written in the form: speed = H x distance, where H is the </span><span class="style26">Hubble constant.</span><span class="style7">Various values for the Hubble constant have been proposed, but the generally accepted value is 56 km (35 mi) per second per megaparsec (a megaparsec is 3.26 million light years; see box). Thus a galaxy that is receding from the Earth at 56 km/sec will be 326 000 light years distant.</span></text>
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<text><span class="style10">he expanding universe</span><span class="style7"> . The speed (indicated by the length of arrow) at which a galaxy is moving away from the observer becomes greater the further the galaxy is from the observer. Wherever the observer is in the universe, all other galaxies are seen to be receding.</span></text>
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<text>ΓÇó TIMEΓÇó STARS AND GALAXIESΓÇó THE HISTORY OF ASTRONOMYΓÇó NEWTON AND FORCEΓÇó QUANTUM THEORY AND RELATIVITYΓÇó WAVE THEORYΓÇó OPTICSΓÇó ATOMS AND SUBATOMIC PARTICLES</text>
