Blackbody: A perfect absorber and emitter of radiation. The simplest example of a blackbody is a sealed box with a small hole cut into it. We see the hole as black because the light which enters it bounces around inside the box and does not escape; the light is perfectly absorbed. If the hole is made too large, light entering the hole and reflecting from an object within the box (or from a wall of the box) can leave the box, and the hole is no longer black. If the box with the small hole is made of metal, when it is heated it will radiate over a range of wavelengths. The wavelength of peak radiation intensity is inversely proportional to the temperature, easily seen graphically. More on blackbody.

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Cepheid Variables: Stars which brighten and dim in the visible wavelenths with precise regularity. A cepheid's maximum brightness (luminosity) is closely related to the time for it to go from maximum brightness to its dimmest, back to maximum brightness (the period). The most famous cepheid is Polaris, the North Pole star, although its variability is barely noticeable to the naked eye. Delta Cephei, the first cepheid discovered, is visible to the naked eye. Measuring a cepheid's period allows a determination of its brightness, which enables a calculation of its distance.

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Doppler Effect: A change in the color (wavelenght) of light because of the relative motion of the light source either toward (blue shift, shorter wavelength) or away (red shift, longer wavelength) from the observer. Speeds of stars and galaxies are determined by using the Doppler effect.

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Galaxy: A large grouping (about 100 billion) of stars. Galaxies are of two basic types: elliptical, and spiral. Our Sun is part of the Milky Way, a spiral galaxy.

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Red-shift: A special case of the Doppler effect where the light source has a relative motion away from the observer, causing the light's wavelength to shift toward the red part of the spectrum.

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Supernova: an exploding star.

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Theory: a hypothesis supported by all available scientific evidence, but which nonetheless could be proved wrong or shown to have limited validity in a future experiment.

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