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Astronomy book notes - CH 01.docx

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Concordia University
PHYS 384

Textbook notes Chapter One: Our Place in the Universe 1.1 Our Modern View of the Universe What is our place in the universe?  Section Summary: o Earth is a planet orbiting the Sun o Our Sun is one of more than 100 billion stars in the Milky Way Galaxy o Our galaxy is one of about 40 galaxies in the Local Group o The Local Group is one small part of the Local Supercluster, which is one small part of the universe.  Text notes: o Ancestors -> heavens revolved around stationary Earth  Earth centered = geocentric o Earth orbits sun (large star) o Cosmic Address = our place in the universe  Earth (planet) in Solar System (sun + all orbiting objects/planets)  Sun + stars = Milky Way Galaxy (galaxy = island of stars)  Billions of galaxy in space, some isolated, many in groups  Local Group = galaxies in a group  Galaxy cluster = more than a few dozen galaxies  Arranged in chains; huge voids between them  Supercluster = cluster of galaxy clusters; tightly packed o Our Local Group is on outskirts of Local Supercluster o Universe = sum of everything (all matter, energy, superclusters, voids, etc.) How did we come to be?  Section Summary: o The universe began in the Big Bang and has been expanding ever since, except in localized regions where gravity has caused matter to collapse into galaxies and stars. o The Big Bang essentially produced only two chemical elements: hydrogen and helium. o The rest have been produced by stars, which is why we are “star stuff.”  Text notes: o Universe is expanding = avg. distance btn galaxies is increasing with time  Galaxies in past were closer together, ultimately as one o Big Bang = beginning of the Universe  Rate of expansion implies14 billion years ago o Universe continues to expand on smaller scales o Galaxies & clusters lie where gravity beat expansion  Universe expands, but galaxies/clusters DON’T  Matter clumped into galaxies/clusters as expansion happened o Gravity causes gas/dust to collapse & form stars/planets  Stars are NOT living, but have life cycles  Born -> gas/dust compressed by gravity  Dense/hot enough -> nuclear fusion -> atomic nuclei fuse together  Lives -> long as generates enough energy from fusion  Dies -> uses up all its fuel  Content goes back into space o Supernovae -> massive star explosion  Matter becomes new gas/dust; cycle restarts  Yields “generations” of stars/planets  Our solar system = many generations of recycling  Universe beginning -> hydrogen & helium  Earth -> carbon, nitrogen, oxygen, iron o Must have come from past star cycles  Solar System = 4 billion years old How can we know what the universe was like in the past?  Section Summary: o Light takes time to travel through space, so the farther away we look in distance, the further back we look in time. o When we look billions of light-years away, we see pieces of the universe as they were billions of years ago.  Text notes: o Determine history of Universe -> study light from distant stars/galaxies o Speed of light = 300,000 km/s  Circle Earth 8 times in 1 sec.  1 sec. to reach Earth from Moon  8 mins to reach Earth from Sun o Light Years = distance light travels in 1 year (10 trillion km) -> DISTANCE, not TIME o Farther you look in distance = farther you look into past  8 light years away = 8 years for light to reach us (viewing it 8 years in the past) Can we see the entire universe?  Section Summary: o No. The age of the universe limits the extent of our observable universe. o Because the universe is about 14 billion years old, our observable universe extends to a distance of about 14 billion light-years. o If we tried to look beyond that distance, we’d be trying to look to a time before the universe existed.  Text notes: o (Universe = 14 billion years old) + (distance in space = distance in time) = limit on observable universe o If try to look more than 14 billion light years -> looking beyond 14 billion years ago (Universe DIDN’T exist yet = nothing to see)  Doesn’t limit size of ENTIRE universe, just what we can see o Observable Universe = 14 billion light years; portion of universe we can see 1.2 The Scale of the Universe How big is Earth compared to our solar system?  Section Summary: o On a scale of 1 to 10 billion, the Sun is about the size of a grapefruit. o Planets are much smaller, with Earth the size of a ball point and Jupiter the size of a marble on this scale. o The distances between planets are huge compared to their sizes, with Earth orbiting 15 meters from the Sun on this scale.  Text notes: o Voyage Scale Model (Wash. DC) -> “walk” the Solar System  Sun = large grapefruit, Jupiter = marble, Earth = pin tip  Sun is larger than any planets & outweighs all combined by 1,000x  Has everything in straight line; if spread out for orbits, = 300 football fields  Moon -> 4 cm from Earth (Mars is 200x farther) How far away are the stars?  Section Summary: o On the 1-to-10-billion scale, it is possible to walk from the Sun to Pluto in just a few minutes. o On the same scale, the nearest stars besides the Sun are thousands of kilometers away.  Text notes: o Voyage Scale Model (Wash. DC)  600 meter distance from Sun to Pluto  Would need to walk to California to reach next star (on that scale)  Nearest star system = 4.4 light years away (4,400 km on scale) o Blackness of sky allows naked eye to see the star light  Telescopes make it brighter (still can’t see star surface features) o Voyager 2 -> spacecraft; flown to Jupiter, Saturn, Uranus, and Neptune  50,000 km/hr (take 100,000 yrs to reach closest star system @ speed) How big is the Milky Way Galaxy?  Section Summary: o Using a scale on which the Milky Way galaxy is the size of a football field, the distance to the nearest star would be only about 4 millimeters. o There are so many stars in our galaxy that it would take thousands of years just to count them.  Text notes: o Reduced scale: 1 light year = 1 mm  100,000 LR diameter of Milky Way = 100 meters (length of football field)  Galaxy centered mid-field  Solar System = microscopic dot on 20 yard line  Solar System to nearest star system = 4.4 mm o More than 100 billion stars = 100 billion seconds to count them all (3,000 years!) How big is the universe?  Section Summary: o The observable universe contains roughly 100 billion galaxies, and the total number of stars is comparable to the number of grains of dry sand on all the beaches on Earth.  Text notes: o Milky Way is 1/100 billion galaxies of observable universe o 100 billion stars/galaxy (estimate), 100 bil. Galaxies x 100 bil. Stars =
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