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1 equals average earth-sun distance=93 million miles or 150 kilometers |
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the distance light travels through space in one year 10 to the 13th km/yr |
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nearest star other than our sun |
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proxima centauri is 4.2 light years away |
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measuring distance to stars (indirect methods) Stellar Parallax |
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shift in apparent position caused by earth orbit (determined by photographing nearby stars against the back drop of more distant objects) |
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stellar brightness is controlled by what 3 factors? |
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1. how large the star is 2. how hot the star is 3. how far away the star is |
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star brightness as it appears from earth with the unaided eye. (brightest is 1st magnitude, faintest is 6th magnitude.) each goes up by 2.5x |
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apparent= -26.7 absolute=5 |
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stars that fluctuate in brightness are known as variables |
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fluctuate due to expansion and contraction CLASSICAL CEPHEIDS |
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sudden brightening of a star called a nova |
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stellar color and temperature |
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hot stars (30000K) emit dominantly short wave radiation appear BLUE |
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stellar color and temp. cont'd |
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red stars (3000K) emit longer wave radiation because they are cooler. |
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binary stars and stellar mass |
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18th century- astronomers discovered numerous star pairs. |
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second star in the handle of the big dipper, is actually a double star. |
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revealed that many star pairs actually orbit each other. United by mutual gravition called visual binaries 50% of the stars in the universe are multiples |
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binary star and star mass |
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determined by the size of their orbits |
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accumulations of gas and dust in space |
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glow because they are close proximity to a hot blue star |
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gaseous masses composed mostly of hydrogen |
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reflected light from nearby stars thought to be composed of dense clouds or dust |
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not close enough to a bright star to be illuminated appear as a dark mass or holes in the heavens |
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dark, cool, innerstellar clouds called a protostar |
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gravitational contraction continues -when core reaches 10 million degrees K fusion begins -a balance between gravity (squeezing) and gas pressure (expansion) is reached and it becomes a main sequence star |
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hydrogen burning continues for a few billion years different stars age at different rates hot massive blue stars only last a few million years smallest red main sequence stars may last hundreds of billions of years average star spends 90% of it's life as a hydrogen burning main-sequence star, when hydrogen depleted star evolves rapidly and dies |
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inert helium core all hydrogen in core is consumed -hydrogen fusion still progressing in outer shell -star's outer envelope is expanding -as the star expands, it cools -internal pressure is increasing until the star begins to consume carbon in its core -eventually all usable nuclear fuel is consumed -different types of stars die ar different rates |
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somewhat speculative what we know is... -a star, no matter what size will eventually exhaust its fuel supply Low mass stars(<0.5*sun) -may remain sequence for 100 billion years -never become red giants because they do not achieve sufficient heat and pressure to fuse anything but hydrogen |
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