ASTR 1010 1st Edition Lecture 2 Outline of Last Lecture I. Our Modern View of the UniverseA. DefinitionsB. The Scale of the UniverseOutline of Current Lecture I. The History of the UniverseII. Spaceship EarthCurrent LectureI. The History of the UniverseThe average distance between galaxies is increasing, meaning the universe is expanding. In order for the universe to be expanding, it is assumed that the galaxies were once close enough for the expansion process to begin. That beginning of the expansion process is the Big Bang, which occurred about 14 billion years ago and is often cited as the beginning of life on Earth. Individual galaxies do not expand apart because the force of gravity wins out over the expansion and holds it together.A star is born when gravity pushes enough material into a cloud that forms a hot,hard center that generates energy from nuclear fusion. The star lives as long as the energy from the fusion continues and dies when it runs out. A star throws its contents back into space, but a massive star becomes a supernova before dying out. What remains is then recycled into galaxies.II. Spaceship EarthThe Earth spins, or rotates, on its axis once a day and makes a full orbit, or revolution, around the sun once a year. The Earth’s axis is the imaginary line connecting the North and South Pole. The Earth rotates from west to east, making the sun and moon appear to rise in the east and set in the west.An astrological unit (AU) is Earth’s average orbital distance. It is equivalent to 150 million kilometers (93 million miles). Earth races around the sun at an average of 100,000 km/h. Earth’s orbital path defines a plane called the ecliptic plane. Earth is tiltedon its axis at a 23.5˚ angle perpendicular to the ecliptic plane. The axis tilt points also exactly at Polaris, or the “North Star.”Our solar system completes one orbit around the Milky Way once every 230 million years, moving at a speed of 800,000
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