Big Bang Theory

All measurements of the stars must necessarily be made from the neighborhood of the Earth, since the distances involved are enormous. The nearest star other than the Sun is more than four light-years away, and most objects seen from earth, even with the naked eye, are much farther off. (A light-year is the distance that light travels in one year: 5.88 trillion miles [9.46 trillion kilometers], or about 60,000 times the distance from the Earth to the Sun.)

There are two fairly direct ways to determine the distance to the nearest stars. The first is to measure their parallax or apparent change in position during the year. As the Earth circles the Sun, stars are seen from a shifting vantage point. The furthest objects do not appear to move because the Earth's change in position is too small to affect our view, but the nearest stars seem to move back and forth slightly during the course of a year. Parallax can be seen by holding up a finger a few inches before your eyes and closing first one eye, then the other, thus repeatedly shifting your point of view by the distance between your eyes: the finger seems to jump back and forth dramatically, while objects across the room move much less. The shift in the finger's apparent position is its parallax. By measuring the parallax of a nearby star over a six-month period (during which the Earth moves from one side of its orbit to the other), and knowing the radius of the Earth's orbit, it is a matter of straightforward trigonometry to determine the distance to that star.

Another technique to determine stellar distances is to measure the proper motion of a star. This is the apparent motion of a star with respect to other stars caused by the star's actual motion through the sky. (All stars are moving, including the Sun.) Although the motion of distant stars is too small to detect, closer stars can be seen changing position with respect to more distant stars over the years.

Such techniques are only applicable to a few of the nearest stars, however, and disclose nothing about the large-scale structure of the Universe. More sophisticated methods had to be developed for this task, requiring different astronomical observations. One such method depends on the examination of a star's (or other celestial object's) spectrum, that is, the intensity of its radiation (including, but not limited to, its visible light) at various wavelengths. If the light from a star is divided into its component wavelengths using a prism, a continuous spread of wavelengths punctuated by a number of dark lines can be seen. (The visible part of our Sun's spectrum is the rainbow.) These absorption lines are caused by elements in the star's outer atmosphere that absorb light at specific wavelengths. Each dark line in a star's spectrum corresponds to a specific element; the absorption lines in a star's spectrum thus give a catalogue of the substances in its outer layers. Furthermore, these lines can reveal how fast the star is approaching the Earth or receding from it using the Doppler effect, a fundamental property of all traveling waves (including light waves). The absorption lines in the spectra of objects moving away from the Earth are shifted to longer wavelengths, while absorption lines in the spectra of objects moving towardthe Earth are shifted to shorter wavelengths. A shift to longer wavelengths is called a redshift because red light appears near the long-wavelength end of the visible spectrum, while a shift to shorter wavelengths is called a blueshift. Measurement of the Doppler shifting of spectral lines has made it possible to map the large-scale structure of the cosmos, and it is this structure that the theory of the Big Bang and the theory of general relativity explain.