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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Astronomy study notes for midterm Chapter 1 key conceptsy We used to believe we lived in a geocentric Earth centered universe y Solar systems consists of stars with planets orbiting them y A galaxy is a create island of stars in space with few hundred million to a trillion or more stars The milky way has more than 100 billion stars y Other galaxies are scattered throughout space some galaxiesare isolated but others are in groups y Our local group has about 40 galaxies y Galaxy cluster groups of galaxies with more than a few dozen members y Superclusters regions in which galaxies and galaxy clusters are tightly packed y Universe sum total of all matter and energyImportant concept y The farther we look in distance the further we look back in time o Why Because the light that we see from distance starsgalaxies took time to reach us And that time is how much were looking back in timeie Sirius is 8 light years away o Speed of light 300 000 Kms o Sun to earth time for light8 minutes o One light yearthe distance light can travel in 1 year about 10 trillion KM o Pictures of distant galaxies is a picture looking back in time o The Observable Universe portion of the universe that we can potentially observe our observable universe is about 137 billion light years o However we can see even more distant objects because the universe is expanding as light travels through it The true size of the universe is not simply the time light took to reach usy The moon is the most distant place humans have visited y Voyager 2 is the farthest man made vehicle in space y Rotationspin orbitrevolutiony Astronomical unitearths average orbital distance1 AU This is also the distance between the sun to the Earth y Earths orbital path is called ecliptic Which means that its not a perfect circle around the sun but rather is closer at one point than another y Earths axis is tilted 235 degrees from a line perpendicular to the ecliptic plane The tilt is orientated so that it points directly to the star Polaris or the north star y Milky way is also rotating y Galaxys have more mass than what is seen However it is also comprised of dark matter no light comes from itImportant concept y Edwin Hubble concluded that1 Every galaxy outside the local group is moving away from us 2 The more distant the galaxy the faster it appears to be racing away y Explanation the universe is expanding y Raisin cake model imagine youre on a raisin in a raisin cake As its in the oven the raisins start to spread apart the ones in the distance appear to be moving faster
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