stars - Astronomy 100, Fall 2008, N. Katz SUPPLEMENTAL...

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Astronomy 100, Fall 2008, N. Katz SUPPLEMENTAL 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 memorize a number. .. their relation to each other!). 3) The basic notion of proper motion (and their typical magnitude), as well as that of stellar size. 4) The connection between apparent brightness and luminosity (note the inverse square law!). 5) The definition of the absolute brightness. Further, bring color (and with it, temperature [why?]) into the game. Learn the principal spectral types (the famous mnemonic!). All this allows drawing the Hertzsprung-Russell (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). Note how spectroscopy helps to resolve ambiguities about where a star of given color is in the HR diagram. 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! 2. AN OVERVIEW OF THE THEORY OF STELLAR EVOLUTION • Imagine an interstellar cloud, so big that it creates a gravitational force that prevents its constituents (mainly hydrogen and helium atoms) from flying away. • If the gravitational attraction between the different pieces of the cloud is big enough, the cloud begins to contract (If it isn’t, just imagine an even bigger cloud: there must be some “critical mass” where this happens!), • Simple laws of gas physics cause heating up of the cloud (this is analogous to what happens with a bicycle pump!) • At the edge of the cloud, radiation leaves (=the cloud shines like any black body with a Planck curve; when it is still cold, the radiation is in radio, when it gets warmer, the radiation becomes infrared). The result is that the cloud loses energy. To compensate for this energy loss, the cloud must fall more into itself (a physicist would say that the cloud loses gravitational energy, which is converted in to heat inside and in radiation sent out.) • But by contracting more, the cloud is becoming hotter and hotter. According to Stefan’s law the increase in energy output is considerable: for instance, 16 times more radiation energy when the temperature has doubled!). This looks like (it really is!) a runaway process: the contraction becomes faster and faster. Note for aficionados: When a substance loses energy and becomes thereby hotter something very
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stars - Astronomy 100, Fall 2008, N. Katz SUPPLEMENTAL...

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