Chapter 1 AST.doc

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

1 Chapter 1 - A Modern View of the Universe 1.1: The Scale of the Universe - earth = planet that orbits an average star (sun) in a vast universe What is our place in the universe? - pic. on page 1 displays a galaxy filled with billions of stars; this photo can tell a lot about our place on the universe... Our Cosmic Address - earth = planet in our solar system, which consists of the Sun (a star), the planets and their moons, and countless small objects that include rocky asteroids and icy comets - our solar system belongs to the huge, disc-shaped collection of stars called the Milky Way Galaxy - a galaxy is a great island of stars in space, containing b/t a few hundred million and a trillion or more stars - Milky Way is a large galaxy containing more than 100 billion stars - our solar system is located a little over halfway from the galactic center to the edge of the galactic disk - billions of galaxies scattered in space, some are isolated but many are found in groups - Milky Way is one of the 2 largest among about 40 galaxies in the Local Group - groups of galaxies with more than a few dozen members are often called galaxy clusters - galaxies and galaxy clusters appear to be arranged in giant chains and sheets with huge voids b/t them - regions in which galaxy and galaxy clusters are most tightly packed are called superclusters, which are essentially clusters of galaxy clusters - all these structures together make up our universe - the universe is the sum total of all matter and energy, encompassing the superclusters and voids and everything within them Astronomical Distance Measurements - one astronomical unit (AU) is earth's average distance from the sun, which is about 150 million km (93 million miles) - distances within solar system usually described in AU - light year (ly) is the distance that light can travel in 1 year, which is about 10 trillion km (6 trillion miles) - distances of stars and galaxies are described in light years - note: light year is a unit of distance, not time - light travels 300 000 km/second (one light-second) Looking Back in Time - light takes 1 second to reach earth from moon, 8 min to reach earth from sun - stars are so far away that their light takes years to reach us, which is why distance is measured in light-years - Sirius = brightest star in might sky, located 8 light years away 2 - idea that light takes time to travel through space leads to this fact: farther away we look in distance, the further back we look in time; ex. Orion Nebula is 1500 light-years away, which means we see it as it looked 1500 years ago - most snapshots of space are a picture of space and time (ex. Andromedla Galaxy is 100 000 light years in diameter, thus light from the far side must left galaxy 100 000 years before the light we see on the near side) - impossible to separate space and time when studying universe Figure 1.1 = pg. 3 , Fig. 1.4 = p. 5, Fig. 1.5 = p.7, Fig.10 = p.1
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