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lecture-08 - These lecture notes were prepared for Rutgers...

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These lecture notes were prepared for Rutgers Physics 341/342: Principles of Astrophysics by Prof. Chuck Keeton, and modified by Profs. Saurabh Jha and Eric Gawiser. All rights reserved. c 2011 Lecture 8: Binary Stars I. Background: Stellar Spectra (This discussion draws from § 5.4 and 8.1 of Carroll & Ostlie.) Sometimes it is possible to see stars in a binary system move around on the sky, i.e., we can see the orbits as shown in the left-hand panel above. More often, though, we can only measure the velocities along the line of sight, using the Doppler effect. Because this is so important for studies of binary stars — and extrasolar planets — it is worthwhile to take a moment to discuss how we do this. Figure 1: (Left) Schematic diagram of hydrogen energy levels. Balmer emission lines are produced when an electron jumps from a high energy level down to n = 2, while Balmer absorption lines are produced when an electron jumps from n = 2 up to a higher level. (The first two Balmer lines, H α and H β , are shown here.) (Right) Energy level diagram for hydrogen. (From Carroll & Ostlie Figs. 5.6 and 5.7.) Stars are big balls of gas that glow because they are hot. The heat is supplied by nuclear fusion deep in the core, but the light comes from near the surface. Not right at the surface (if it is even possible to discuss the “surface” of a ball of gas), but a little ways in. This means that the light actually passes through some gas on its way out. Some of the light may 1
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be absorbed by the gas it passes through. Since atoms and molecules have discrete energy levels, only certain wavelengths of light can be absorbed. The energy levels of hydrogen are illustrated above. The spectral lines serve as a fingerprint of each atom and molecule. It turns out that different stars have different sets of spectral lines. The reasons will be discussed in detail in Physics 342. For now, the main point is that stellar spectra can be grouped into seven classes, which are shown below. In each class, the absorption lines are understood in terms of the atoms and molecules in the star’s atmosphere.
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