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Chapter 3

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McGill University
CHEM 222
Karine Auclair

3.1.6 Stellar Evolution - A star spends most of its life on the main sequence – it produces energy through hydrogen to helium fusion • It slowly becomes hotter and brighter - It moves off the main sequence as it begins helium fusion (typically becomes larger, cooler, and brighter) - Large stars move through stages quickly • Stars larger than 8 solar masses finish their lives as supernovae - Stars the size of the sun move into a red giant phase before gently collapsing into white dwarves • If they’re a bit larger they can collapse into neutron stars or black holes. - Small stars (smaller than 5 solar masses) collapse under their own gravity • They become smaller and temporarily hotter as gravitational energy converts into heat – white dwarves • They can also pull nearby material in with their gravitational force, producing a Type Ia supernova - Very large stars (between 5-20 solar masses) go through a yellow-supergiant, Cepheid-variable phase late in their lives • Variation in brightness is related to absolute magnitude. 3.1.7 A Distance Scale Based on the Intrinsic Brightness of Stars - From H-R Diagram, we can know how intrinsicly bright a star is. - By comparing the apparent and absolute magnitudes, we can determine the distance to the star. 3.1.8 A Distance Scale Based on the Intrinsic Brightness of Galaxies - For the closest galaxies, we can determine their size if we know their distance. - Larger galaxies = brighter galaxies - We measure the distance of galaxies by using the apparent brightness of the brightest resolvable stars in the galaxy • We can now measure distances to galaxies out to 10 Mpc - One of the brightest kinds of stars is the Cepheid variable type • Extremely large, yellow stars thought to be in the stage of life following hydrogen fusion. • Polaris (our North Star) is a Cepheid. - If we can recognize the kind of star, and we know how far away they are, we can figure out how intrinsically bright they are - At great distances, we run into another kind of galaxy, the Quasar. • They produce enormous amounts of light and energy • They are probably enormous young galaxies in formation that have nearby galaxies or black holes consuming enormous amounts of mass from them. - Quasars are very far away and very old. - The extreme brightness of quasars allow us to extend our distance scale out to the boundaries of our universe (3000 Mpc). 3.1.9 Hubble: “Red-shift” and the “Age” of the Universe - In the 1920s, astronomers noticed a spectral shift in galaxies that were moving away. • The spectrum tended to shift towards the red end as they appeared smaller in the field of vision. - Edwin Hubble used the luminosity-period relationship of Cepheid variable stars as a means of measuring distance to many galaxies. • The farther away the galaxy, the more red light it
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