General Astronomy/The Spectrum

Violet scatters the most, and red the least.

Definition
Spectrum is a word that has taken on a broad meaning in English, first used by scientists such as Isaac Newton in the 1600s to specify the range of colors obtained by passing sunlight through a glass prism, or produce through the natural mechanism of a rainbow. Today it is applied in almost any situation for a broad range of values. Specifically in physics and astronomy it still denotes the range of colors of visible light, but also includes invisible forms of electromagnetic energy ranging from very long wavelength radio waves to the ultra short wavelength gamma radiation.

Producing a Spectrum
Color is analogous to wavelength when we speak of visible light. There are relatively long wavelengths of red light and relatively short wavelengths of blue and violet light. These wavelengths are also indicative of temperature for a heated body; red is cooler while blue is hotter. White light, such as that from the Sun, is not composed of a single color or wavelength, but of a mixture of many colors or wavelengths, which the eye interprets as white.

While all wavelengths of light travel at the same speed through a vacuum, the speed of different wavelengths varies as light passes through a transparent medium such as glass, water or even air. As light passes from one medium (such as air) into another medium (such as glass), its speed changes according to the index of refraction of the two media. In the case of this example, light slows down as it passes into the glass. Blue or violet light is slowed slightly more than red light as it passes from a medium of lower refractive index to one of higher refractive index. This, along with the particular shape of a glass prism, acts to bend or disperse light, spreading the colors out. Since blue light is bent more than red light, the original mixture of light is spread out into its constituent colors to form a spectrum, somewhat like an artificial rainbow.

“The brightness of a star is also dependent on its temperature, and the temperature will have an effect on the spectrum the star emits. If two stars with identical spectra are observed, and the distance of one of the stars through parallax measurement is known, their brightness can be compared. The variance in brightness is attributable to the difference in distance. Using the inverse square law, the distance of the star whose distance was previously unknown can then be determined. Stars can give off radiation not only in the visible spectrum but also as radio waves, x-rays, and gamma rays. All of these different parts of the electromagnetic spectrum can be used in conjunction with the techniques already discussed to make astronomical measurements.”

Types
Spectra types are
 * continuous, which are broad bands created by a incandescent solid (such as the red hot element of an electric stove), liquid (such as molten lava) or a high-pressure gas (such as the surface of a star).
 * emission, characterized by narrow bright lines, created by an excited, low-pressure gas. Examples of emission spectra sources are a comet's coma and tail, and the Rosette nebula.
 * absorption, characterized by narrow dark lines, created by a continuous spectrum that is passed through a low-pressure gas. This is seen in the spectra of the Sun and stars, and is caused by light absorption in the cooler, lower pressure gas atmosphere of the star.

By studying spectra, astronomers can discover many things about stars, most specifically the chemical elements found in the star. “The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.”

Most stars exhibit absorption line spectra, but a few rare stars show emission lines. Wolf-Rayet stars have emission spectra caused by UV (ultraviolet) radiation from a hot star passed through low-pressure gas. Certain nebulosity or gas clouds also exhibit emission lines. Spectral lines also are detectable in non-visible light such as ultraviolet and microwaves.