PHY122 Ch 12 Notes The Milky Way Galaxy

PHY122 Ch 12 Notes The Milky Way Galaxy - Astronomy Chapter...

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Astronomy Chapter 12: The Milky Way Galaxy - The hazy band of the Milky Way is our wheel-shaped galaxy seen from within, but its size and shape are not obvious. - Structure of galaxy hard to determine because – 1. We are inside 2. Distance measurements are difficult 3. Our view towards the center is obscured by gas and dust - First attempt to unveil the structure of the galaxy by William Hershell (1785), based on optical observations. - The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center, but they could not see very far into space because of gas and dust. - In the early 20 th century, Henrietta Leavitt found many variable stars called Cepheids and noticed that that the brighter the star is, the longer its periods were. She concluded that there is a relationship between period and luminosity. Many Cepheid variables are known today; they have periods from 1 to 60 days and lie in a region of the H-R diagram known as the instability strip . From their position in the diagram, you can tell that Cepheids are giant and supergiant stars A related kind of variable star, an RR Lyrae variable star , has a period of about ½ a day and lies at the bottom of the instability strip Measuring Distances: The Cepheid Method (period – luminosity relation) - More luminous (brighter) Cepheid variable stars, have longer pulsation periods and pulsate slower - Observing the period gives a gauge of star’s luminosity, and so its distance! - The Cepheid Method allows us to measure the distances to star clusters throughout the Milky Way. - The absolute magnitude M v is the apparent magnitude a star would have if it were at 10 parsec away M v – M v = -5 + 5log 10 (d[pc]) D[pc] = 10 (mv –Mv +5)/5 - Harlow Shapley realized that he could study the size and extent of the star system by studying globular clusters and measuring their distances. -
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This note was uploaded on 12/11/2011 for the course PHY 121 taught by Professor Weinstein during the Fall '08 term at SUNY Buffalo.

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PHY122 Ch 12 Notes The Milky Way Galaxy - Astronomy Chapter...

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