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Spectroscopy & Stars

What kinds of useful information can be gathered by collecting electromagnetic radiation, like visible light or infrared radiation?

By collecting the radiation stars emit astronomers can determine the brightness of an object and the spectra of that object (e.g., with a visible light telescope one can determine the color spectrum). The brightness can tells us the distance of a star, while the spectrum tells us temperature, mass, chemical make-up, diameter, and distance. In fact, spectra are such an information-rich measurement that astronomers do not usually directly look through their large telescopes. Instead, they collect light in an instrument called a spectroscope, which itself is connected to a computer for data analysis.

A spectroscope is an instrument that consists of a prism or a grating spreads the incoming beam of radiation into its different wavelengths and some kind of screen to project the spectrum (see below).

Cartoon of a spectroscope: white light is spread by a prism into long and short wavelengths.
Spectra come in several types:
  1. Continuous spectra: a smooth gradient of electromagnetic radiation without any gaps – e.g., the spectrum of incandescent solids.
  2. Absorption spectra: an incomplete spectrum with missing gaps (which appear as dark lines) due to the absorption of a continuous electromagnetic radiation by a cooler medium, like a gas. Such absorbed energy can be re-emitted, but the absorbed energy is essentially removed from a telescope’s view. Since the “cooler,” outer gaseous surface of a star tends to absorb the radiation produced in the hotter, inner part, the spectra of most stars are absorption spectra.
  3. Emission spectra: a spectrum that represents all the wavelengths emitted by atoms or molecules.

Astronomers take advantage of something from physics called Wien’s Displacement Law, a mathematical relationship that basically says that the hotter a body (like a star) is, the shorter the wavelength of light will be emitted from it:

λpeak T = 2.898 x 10-3 m · K

where λpeak is the peak (i.e., maximum) wavelength that the star emits and T is the star’s surface temperature. Hence, a red star, with a maximum wavelength of 966 nanometers, has a surface temperature of “only” 3000 Kelvin while a blue star, emitting at a maximum wavelength of 290 nanometers, has a surface temperature of 10, 000 Kelvin!

In addition, the stars have been classified into “spectral classifications” (labeled by a letter) based on their surface temperature. These spectral types also organize the stars by their chemical make-up and their main sequence lifetimes, that is, the lifetime of the star based on calculations of its available fuel and the rate at which it is consuming that fuel (as interpreted by its luminosity).

Chart illustrating the relations between stars’ spectral type, surface temperature, color and lifetime. Note that Myr = Millions of years and Gyr = Billions of years.

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