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Using Spectra to Measure Planet & Stellar Properties

Most bright astronomical objects shine because they are hot. In such a case, the continuum they emit tells us what the temperature is. Recall, Wien's Law, relates color to temperature and the Stefan-Boltzmann Law, tells us hotter objects are brighter. But also, we can use spectra to tell us about the composition of the objects emitting that light.

The earliest studies of spectra where made of the sun by Joseph Fraunhofer, in the mid-1800s. His studies revealed the sun to have many absorption lines (dark lines against the brighter continuum). The full Solar Spectrum looks something like this below. Fraunhofer labeled them with letters, because he didn't know their origin. Today we do. To the right is a listing of what elements created which lines, now referred to as the Fraunhofer Lines. The precise origin of these Fraunhofer lines remained in doubt for many years, until Gustav Kirchhoff, in 1859, announced that the same substance can either produce emission lines (when a hot gas is emitting its own light) or absorption lines (when a light from a brighter, and usually hotter, source is shone through it). See here Now scientists had the means to determine the chemical composition of stars through spectroscopy!

Lines Due To Wavelengths (Å) A - (band) O2 7594 - 7621 B - (band) O2 6867 - 6884 C H 6563 a - (band) O2 6276 - 6287 D - 1, 2 Na 5896 & 5890 E Fe 5270 b - 1, 2 Mg 5184 & 5173 c Fe 4958 F H 4861 d Fe 4668 e Fe 4384 f H 4340 G Fe & Ca 4308 g Ca 4227 h H 4102 H Ca 3968 K Ca 3934