SPECTRAL ANALYSIS: LINES AND MK SPECTRAL CLASSIFICATION

We have just barely scratched the surface of how astronomers utilize stellar spectra to reveal the properties of stars. For the next few lectures we will investigate how stars produce their spectra and how information is extracted from stellar spectra. In the left panel above, a series of stellar spectra are shown from hot stars to cool stars. We can see immediately that the color of the stars shift from bluish to reddish and that different series of dark absorption lines appear. We have already pointed out why the color changes;

the wavelength at which the star appears the brightest depends on its temperature. The hotter is the star, the shorter the wavelength at which its emission peaks. This is often times quantified using Wien's Law, the property of a blackbody radiator star that explicitly states how the wavelength of blackbody emssion depends inversely on the temperature of the blackbody.

For the next lecture we discuss how we also determine the surface temperature of stars using the line spectrum of the star. We pay particular attention to why the absorption line patterns differ from star to star. We will cover:

• Spectral Formation--Kirchhoff made three statements about spectral formation in stars that have become known as Kirchhoff's Three Laws of Spectral Formation. Kirchhoff's laws allow one to predict the type of spectrum an astrohysical object is likely to produce, a hot dense gas produces a continuous spectrum, a hot dense gas with a cooler gas sitting on top of it produces an absorption line spectrum and a hot gas sitting by itself produces an emission line spectrum.

• Spectral Analysis-- Based on the early experiments of Bunsen and Kirchhoff, and the theroretical work of Bohr, it was learned that the wavelengths (energies) of the spectral lines for particular elements were the same every time they were measured and the patterns of the lines were unique to individual elements. Atomic lines serve as fingerprints for atoms. We understand why this is true because of the pioneering work of Niels Bohr and many other workers in the field of Quantum Mechanics which elucidated the structure of atoms.

• Spectral Classification--Stellar spectra were organized in a Spectral Classification scheme developed in the late 1800s and early 1900s by Annie Jump Cannon and co-workers at Harvard. The scheme was based primarily on the appearance of the absorption lines produced by hydrogen with the stars beoming ranked according to their surface temperatures. Annoyingly, the scheme ranked stars in order of decreasing surface temperature ranging from 30,000-50,000 K to 2,500-3,000 K. In the scheme, the ordering of the Spectral Classes is O, B, A, F, G, K, M. The seemingly random ordering reflected the fact that temperature determined the differences in the appearance of the stellar spectra and was not because stars were made of different materials. With the realization of the pre-eminent role of temperature, it became clear that most stars are eseentially made of the same things, hydrogen, helium, and small amounts of everything else! In this schene, the Sun has classification G2. G stars have temperatures ranging from 6,000 K to 5,000 K, the 2 means that the Sun is around 0.2 of the way from G to K (the next spectral class), T around 5,800 K.

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