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Unformatted text preview: Astronomy 100Lg Nick Warner, Fall 2009 NOTES ON STARS 1. FROM OBSERVATION TO THEORY Fundamentally important: 1) Understand how distances to stars are measured. 2) The parsec and the light year as important units of distance (for once, it might be in order to learn their values). 3) The basic notion of proper motion (and the typical amount of motion), as well as that of stellar size. 4) The connection between apparent brightness, distance and luminosity: the inverse square law. The idea of apparent magnitude. The definition of absolute magnitude. Color of stars and the relation to temperature. Learn the principal spectral types (the famous mnemonic). All this allows drawing the Hertzsprung-Russel (HR) diagram. Allow yourself to be amazed that observed stars do not form a random scatter diagram, but a very characteristic structure. Learn the principal features of the HR diagram (main sequence, red giants, white dwarfs). How the HR diagram encodes the radius of a star. Know the basic idea of spectral type, and how it reflects the temperature of the star’s photosphere. Understand the reasons why absorption line strength depends upon temperature. How does spectral type and the star’s apparent color give information about the amount of interstellar absorption of light (“garbage/crud”)? Note how spectroscopy helps to resolve ambiguities about where a star of given color is in the HR diagram (Luminosity Class). This is crucial in order to use our knowledge of stars to determine stellar distances. Finally, the mass of stars: Learn how it is determined. Note the mass-luminosity relation of main-sequence stars. Such strikingly simple relations do cry out for a simple explanation. Lifetimes of stars and how they depend upon mass. Understand that, for a theorist, the mass determines essentially every bulk property of a star, and almost very aspect of a its evolution. 2. AN OVERVIEW OF THE THEORY OF STELLAR EVOLUTION • Imagine an interstellar cloud, so big that it creates a gravitational force that prevents its hydrogen and helium (and some other...) atoms from flying away • If gravity is big enough (and if it isn’t, just imagine a yet bigger cloud, there must be a critical mass that can do it), the cloud begins to contract. • Simple laws of gas physics (Kelvin-Helmholtz) heat the cloud up. • At the surface, the cloud radiates away energy, like any black body. To compensate for the energy lost, the cloud must fall more into itself (it loses gravitational energy, turning it into heat). • But by contracting more, it becomes hotter and hotter. The Stefan-Boltzmann law says that more and more energy will be radiated away (recall: for instance, 16 times more 1 radiation energy at the double temperature.). This is a runaway process: the contraction becomes faster and faster....
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This note was uploaded on 11/27/2009 for the course ASTR 50800 at USC.