The term red shift refers to a certain type of change observed in the spectrum of most stars and galaxies. The wavelengths
of light in the spectra of many types of nearby stars are well known. When spectra are observed of the same types of stars lying at a great distance, however, the wavelengths usually appear longer. If the
wavelength of light measured by an observer is longer than that measured at the source, the light is said to be red shiftedÑthat is, shifted toward the red, or long-wavelength, end of the spectrum. Similarly, a blue shift is a displacement toward the blue, or short-wavelength end of the spectrum. Red and blue shifts are most easily detected when the spectrum of a distant object contains identifiable features, such as absorption or emission lines of a particular element, and the positions of these lines are compared with those on a standard spectrum of the element obtained in a laboratory (see absorption, light; spectroscope). Two types of red shift are known. The first results from the line-of-sight relative motion between the source and the observer, explained by the Doppler effect, which helps provide the equation to determine the velocity of the source relative to the observer. All stars in our galaxy have slight Doppler red or blue shifts. The value of the shift is given by the relationship z = ÆÞ/ÞV, where z is the shift, ÆÞ is the change in wavelength, and ÞV is the rest wavelength (the wavelength seen if observer and source were at rest relative to each other.) From the velocity v at which the object is moving away from the observer (or toward the observer, in the case of where c is the velocity of light (roughly 300,000 km/sec, or 186,000 mi/sec). All but the nearest
galaxies (see extragalactic systems) have substantial red shifts, indicating that nearly all galaxies are receding from our own. Thus the red shift is of central importance in modern cosmology, because the expansion of the universe indicated by galactic red shifts confirms cosmological models based on the theory of general relativity. In 1929, Edwin Hubble and Milton Humas discovered that the recessional velocity v and the distance d of galaxies are related by the equation v = HVd, where HV is a constant of proportionality called Hubble's constant. If galactic red shifts are interpreted as effects of recessional velocityÑand most astronomers make such an interpretationÑthen the red shift of a galaxy can be used to determine its distance. The second type of red shift results from the presence of a gravitational field. According to the theory of general relativity, the spectrum of light from a source located at a distance R from a mass M will suffer a gravitational red shift z when detected by a distant observer such that z = GM/Rc6, where G is the gravitational constant (see gravitation) and c is the velocity of light. Because the objects called quasars exhibit large red shifts, most astronomers think that they lie mainly toward the edge of the known universe. Some maintain, however, that they actually lie relatively nearby. The subject remains under debate. Another controversy arises from galactic red-shift studies, by U. S. astronomer William Tiftt and others, that indicate that galaxies seem to be "quantized"Ñthat is, to exist only at certain red-shift values. This finding, if true, would challenge currently accepted cosmological theories. Most astronomers tend to dismiss the concept, but further studies seem to have lent it some support.