July 30th, Lecture 9.docx

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Astronomy & Astrophysics
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Marija Stankovic

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July 30 , Lecture 9, AST201 Midterm. Chapter 17 not in midterm. Info about stars and evolution, focus on the information on the PDF, read the textbook. Stars do not live forever. They are “born” and they “die”. 3 different groups, supergiants, giants, white dwarfs. Supergiants are slowly dying. Stars have life spans. Mass can determine the lifetime of a star. The smaller will live longer, the rate it diffuses hydrogen is lower. Blue and red star, red one is cooler, less energy. Red star determines temperature, diffusion rate will be slower, not as much material and energy. Blue stars die off much faster. What colour of star would you expect to mainly see where new stars are being made? Blue star. Temperature, mass, age, luminosity (blue stars are brighter) Stars are born in cool and relatively dense and clouds of gases and dusts. Obscure the light that comes from these stars. Expect to see more blue stars just because they are brighter. High luminosity they would dominate the light. Galaxies. Easily identify bright blue stars. Dominated by blue colours. Not only blue stars are more massive but shorter lifetime. Less amount of time to form high mass star than a low mass star. You make them faster, initially they will dominate the region. Much easier to see. The regions where dark parts are, tell us where the dust is. Most of the light at the spiral is dominated by blue light. Some pockets of red. We see red parts dotted by blue lights. Red nebulae  hydrogen gas. Also, dust and blue stars. Lots of blue stars  lots of young stars Lots of dust among the gas and stars The interstellar medium is filled with microscopic dust particles and gas. Cold enough to form molecules. Clouds will have molecules hydrogen, carbon monoxide. Mainly silicone and icy cover. The same galaxy seen in visible light showing hot gas and stars, and in far-infrared light showing cold dust. Infra-red longer wavelength, go over it. Dust absorbs visible light, Where a lot of dust, there are a lot of red nebulae and blue stars. The orion nebula: a nearby star-forming region. In invisible light, we see mainly gas. In infrared light, we see that a very young star cluster is embedded in the gas. In such nebulae, we tend to find lots of dark patches, which are places where the gas and dust are concentrated enough that they block out background light. Often these dark, dusty structures are studded with very young stars. They are the places where stars are formed. Usually these nebulae are rich in molecules, often quite complex ones. Molecular clouds. Barnard 68 or B68, dark patch. In visible light, there’s a dark nothing. In infrared light, we can see right through it. We can measure how much light it blocks and figure out its structure. Dust blocks short wavelengths of visible light but lets red light and infrared light through. Same as the sky. Blue light has short wavelength, cannot go through, scattered. Long wavelength able to move through. Temperature in dust clouds: doesn’t contain blue high energy light will make it cooler. Gravity vs Pressure = hydrostatic equilibrium Molecular clouds are cold (10-30K). Cold cloud  molecules move slowly  no outward pressure. Lower temperature. Higher temperature. Cold cloud  molecules move slowly  no outward pressure. Gravity wins! Cloud begins to contract. If got a little cooler, pressure would fall, and contract, gravity wins. As it contracted, conservation of angular momentum would cause it to spin faster, flatten. Angular momentum as “tendency to spin”. Cloud contracts, spin faster, doesn’t go from one big to one small, because of these laws, it contracts into a disk. L = m x v x r L is angular momentum M is mass V is rotation speed R is the radius How fast th
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