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A star's spectrum contains information about its temperature, chemical composition, and intrinsic luminosity.

An absorption spectrum is produced when a continuum passes through "cooler" gas. Photons of the appropriate energies are absorbed by the atoms in the gas. Although the photons may be re-emitted, they are effectively removed from the beam of light, resulting in a dark or absorption feature. The atmospheres of stars act as a cooler blanket around the hotter interior of a star so that typical stellar spectra are absorption spectra.

Spectroscopy is one of an astronomer’s favourite tools to help understand the Universe. Planets, stars and galaxies are just too far away to be analysed in a laboratory. Fortunately, very important information about these distant bodies is written in the light we detect with a telescope.











Earth’s Atmosphere Effects on Measurement

The are many known atmospheric absorption features (or telluric absorption lines) in the wavelength range accessible to amateur equipment. Features of moderate strength occur at wavelengths beyond about 5900 Angstroms, (although the features below 6270 Angstroms are sometimes weak enough to be ignored.)

The strongest lines occur in the A-band (7600-7630), in the B-band (6860-6890), and also between 7170 and 7350 Angstroms.

Telluric lines will vary in wavelength relative to the object spectrum due to the motion of the earth. These lines will also vary in strength with airmass. However these are not normally noticeable without sensitive equipment.

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