Stellar_properties - Measuring the Properties of Stars(ch 17 Stars(and everything are undoubtedly much more complex than our descriptions of them

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Measuring the Properties of Stars (ch. 17) Stars (and everything) are undoubtedly much more complex than our descriptions of them. All representations are reductions or abstractions of something more complex. Confusing the abstraction or theory or model for the actual phenomenon is called reification . Nevertheless, we will try to reduce the nature of stars to a fairly small number of properties that can be used in an attempt to answer questions like: Why do there appear to be different “types” of stars? How are stars born, evolve, and die? First we discuss how the properties of stars are measured and how they can be interpreted ( ch. 17 ). Then we do the same for the gas between the stars (the “interstellar medium,” ch. 18 ) and try to put them together to understand how stars form (ch. 19) , especially as a function of mass . [For the next exam, however, we will probably have to omit ch. 19.]
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Finally, in the next section of the course we will consider in detail how stars of different mass evolve from birth to death. [Not on the exam : Discover 17-1, p. 441; More Precisely 17-1, p. 445; you should just have a basic idea of what the “magnitude system” means when referring to stellar brightnesses; More precisely 17-3. I won’t test you on the different types of binary stars (pp. 469-470) or specifically how masses are determined for each type, but you should be comfortable with the general idea; recall that we have been talking about this, off and on, since we discussed how to get the mass of the Sun from the Earth’s semimajor axis and period, using Kepler’s 2 nd law. ]
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Basic properties of stars 1. Distances . The most basic method is to measure a star’s parallax angle, a subject we discussed early in the course. (See Fig. 17.1 for a useful illustration.) This method gives rise to the unit of distance we will use throughout the remainder of the course, the parsec , which is the distance of a star with a parallax of one second of arc . (The nearest stars are a few parsecs distant from us, while our Galaxy is about 30,000 parsecs across.) A parsec is about 3 x 10 18 cm, or over a hundred thousand times larger than the distance from the Earth to the Sun (1AU). Distance (in parsecs) is equal to the inverse of the parallax angle (expressed in seconds of arc). For example, a star 10 pc distant has a parallax angle of 0.1 seconds of arc. Distant stars have such small parallax angles that they cannot be measured (recall our discussion of the diffraction and seeing limits for telescopes). So there is a distance limit for this method, and it is only about 100-500 pc.
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(Think: size of our Galaxy ~ 30,000 pc, nearest other galaxies millions of pc away, most distant galaxies we can see are billions of pc away.) The Hipparcos space mission revolutionized our knowledge of parallaxes (p. 452). Planned future space missions (around 2010; SIM and GAIA) aim to enormously extend the distances to which parallaxes
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This note was uploaded on 04/20/2008 for the course AST 301 taught by Professor Harvey during the Fall '07 term at University of Texas at Austin.

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Stellar_properties - Measuring the Properties of Stars(ch 17 Stars(and everything are undoubtedly much more complex than our descriptions of them

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