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astronomy midterm 2

astronomy midterm 2 - 12:23:00 PM Review Topics/Questions 1...

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17/03/2008 13:23:00 Review Topics/Questions 1. Explain the basis of the stellar classification scheme that involves the “spectral type” (O,B,A,F,G,K,M). Astronomers classify stars according to surface temperature by assigning a spectral type determined from the spectral lines present in a star’s spectrum. The hottest stars with the bluest colors are called spectrial type O, followed in the orderof declining suface temperature by spectral types B,A,F,G,K,M o “Oh, be a fine guy kiss me!” we measure a star’s surface temperature from its color or spectrum, and we classify spectra according to the sequence of spectral types, OBAFGKM, which runs from hottest to coolest. Cool, red starts of spectral shape M are much more common than hot, blue stars of Spectral How do the colors of these stars vary? Some stars fail to achieve a proper balance between the amount of fusion energy welling up from their cores and the amount of radiative enegery emanating from their surfaces. The surfaces of these stars therefore pulsate in and out, periodically rising and falling in luminosity. o Stars come in different colors because they emit thermal radiation. Recall that an ideal thermal radiation spectrum depends only on the surface temerpature of the object that emits it. What is really being measured by the spectral type? A spectral type is determined from the spectral lines present in a star’s spectrum. The hotest stars with the bluest colors are called spectral type O, and M being the coolest. o Cool, red stars are much more common than hot, blue stars. 2. What is the difference between “apparent” and absolute” magnitude when describing the brightness of a star? Apparent magnitudes is how bright stars “appear” in the sky. Apparent magnitudes are directly related to apparent brightness, except the scale runs backwards. A larger apparent magnitude means a dimmer apparent brightness. o Example: A star of a magnitude 4 is dimmer in the sky than a star magnitude of 1.
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A stars absolute magnitude is the apparent magnitude it would have IF it were at a distance of 10 parsecs (32.6 light years) from Earth. o Example: The Sun’s absolute magnitude is about 4.8 meaning that the Sun would have an apparent magnitude of 4.8 if it were 10 parsecs away from us, bright enough to be visible but not conspicuous on a dark night. 3. What is stellar parallax? The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in the Earth’s orbit of the Sun each year Why do measurements of parallax provide a good way to measure distance? Be sure you can explain how parallax works (remember the finger and eyes example from class).
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