Study notes for the final

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Astronomy & Astrophysics
Marija Stankovic

Notes for the final Lectures 1-12, ch. 1, 3-6, 14-20, 22 Chapter 1 1.1 - Our Modern View of the Universe What is our place in the universe? Asolar system consists of a sun and all the objects which orbit it (including rocky asteroids and icy comets) Agalaxy is a great island of stars in space containing from a few hundred million to a trillion or more stars The Milky Way Galaxy is a relative large galaxy containing more than 100 billion stars Some galaxies are fairly isolated but others are found in groups (Local Group(s)) Groups of galaxies with more than a few dozen members are often called galaxy clusters The regions in which galaxies and galaxy clusters are most tightly packed are called superclus- ters, which are essentially clusters of galaxy clusters What is our cosmic address? Earth, the Solar System, the Milky Way Galaxy, Local Group, Local Supercluster, the Uni- verse How did we come to be? The universe is expanding so that the average distances between galaxies are increasing with time This implies that galaxies must have been closer in time at one point, and if we go farther back enough we must reach the point at which the expansion began - The Big Bang Estimated that the Big Bang occurred 14 (13.7) billion years ago Stellar Lives and Galactic Recycling Gravity drives the collapse of clouds of gas and dust to form stars and planets Astar is born when gravity compresses the material in a cloud to the point where the centre be- comes dense enough and hot enough to generate energy by nuclear fusion Astar livesas long as it can shine with energy from fusion, and dieswhen it exhausts its us- able field Massive stars die and create supernovae (s. supernova), and blow their matter into space from which new clouds of gas and dust are created, bringing to life new stars How can we know what the universe was like in the past? The speed of light is 300,000 kilometers per second One light year is the distance that light can travel in 1 year - about 10 trillion kilometers Alight year is a unit of distance, not time The farther away we look in distance, the further we look in time Can we see the entire universe? If we looked at a galaxy that is 7 billion light years away, wed see the universe at half its current age. If we looked at a galaxy that is 12 billion light years away wed see the universe as it was when it was 2 billion years old. If we tried to look beyond 14 billion light years wed be looking to a time more than 14 billion years ago - before the universe existed. Our observable universe is 13.7 billion light years 1.2 The Scale of the Universe How big is the universe? The Milky Way is one of roughly 100 billion galaxies in the observable universe The number of grains of sand on all Earths beaches is comparable to the number of stars in the observable universe Notes for the final Lectures 1-12, ch. 1, 3-6, 14-20, 22 1.3 Spaceship Earth How is Earth moving in our solar system? The basic motions of Earth are its daily rotation (spin) and its yearly orbit (or revolution) around the Sun Earth rotates once each day around its axis Earth completes one orbit each year Earths average orbital distance is called an astronomical unit, or AU, equivalent to about 150 million kilometers Earths orbital path defines a flat plane that we call the eliptic plane When we look outside the Local Group we discover: 1. Virtually 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 This happens because the universe is expanding Lecture 1 Planets orbit a star. They can be rocky, icy or gassy in composition and they dont produce much energy In 2006 theAstronomical Union accepted a new definition of the term planet: 1. Planets orbit star directly (not other planets) 2. Planets must be massive enough for their own gravity to compress them into spherical shape 3. Planets must have cleared their orbits. That is, there cant be other stuff orbiting the par- ent star in orbits very similar to the planet Pluto cant be a planet because its orbit is full of other objects (mainly comets) - Sorry Pluto! Amoon is a satellite, an object which orbits a planet An asteroid is a relatively small and rocky object that orbits a star Acomet is a relatively small and icy object that orbits a star Anebula is an interstellar cloud of gas (bright) and dust (dark) Asolar (star) system is a star and all the materials that orbit it, including its planets and moons Agalaxy is a great island of stars in space, all held together by gravity orbiting a common cen- tre Our nearest (large) neighbor galaxy, the Andromeda Galaxy, is 2.5 million light years from us The Universe is the totality of all space, time, energy and matter If we reduced the size of our solar system by a factor of 10 billion, with the Sun now the size of a large grapefruit, how big would the Earth be? B.Atip of a ballpoint pen The solar system is huge and the vast majority of it is empty (except for tiny quantities of gas and dust) The average distance from our Sun to the Earth is 1 AU (~1.5x10^11m) 1 AU is 1Astronomical Unit Alight year measures the distance light travels in one year 1AU = 8 light minutes (i.e., it takes 8 minutes for light leaving the Sun to reach us on Earth) Notes for the final Lectures 1-12, ch. 1, 3-6, 14-20, 22 Light travels at a finite speed Suppose an alien living on a planet 4000 light years away were to look at Earth right now through a very powerful telescope. What might it see? B.Ancient Egyptians building pyramids Chapter 3 3.4 The Nature of Science How can we distinguish science from non-science? The Idealized Scientific Method Ahypothesis is like and educated guess Hypotheses allow you to make simple predictions which can be tested Based on your test you can either confirm your hypothesis, alter it, or discard Hallmarks of Science Everything we know to be science shares the following three characteristics: 1. Modern science seeks explanations for observed phenomena that rely solely on natural causes 2. Science progresses through the creation and testing of models of nature that explain the observations as simply as possible 3.Ascientific model must make testable predictions about natural phenomena that will force us to revise or abandon the model if the predictions do not agree with observations Occams Razor Occams Razor - the simpler explanation is often the truest; to choose the simplest explana- tion which makes the least assumptions Verifiable Observations The important difference between personal testimony about a scientific test and an obser- vation of, for example, a UFO is that the former can be verified by anyone, the latter cannot There is no way to verify someones claim of having seen a UFO Science and Pseudoscience The important distinction between science and pseudoscience is that the latter makes claims on the natural world that seem to be based on observation evidence but do not treat evi- dence in a truly scientific way Pseudoscience means false science Consider if a statement fulfills the three hallmarks listed above, if not, it is pseudoscience What is a scientific theory? When a powerful yet simple model makes predictions that survive repeated and varied testing, scientists elevate its status and call it a theory Ascientific theory can never be proved true beyond all doubt, because future observations may disagree with its predictions Anything that qualifies as a scientific theory must have a large body of compelling evidence in support We can discard or replace a scientific theory only if we have an alternative way of explaining the evidence that supports it 3.5 Astrology How is astrology different from astronomy? Notes for the final Lectures 1-12, ch. 1, 3-6, 14-20, 22 The basic tenet of astrology is that the apparent positions of the Sun, Moon, and planets among the stars in our sky influence human events Astrologers and astronomers were usually one in the same in the ancient world Astrologers held esteemed positions as political advisers in the ancient world Wealthy political leaderssupport of astrology made possible much of the development of an- cient astronomy Does astrology have any scientific validity? The validity of astrology can be difficult to assess as theres no general agreement among as- trologers on such basic things as what can be predicted In science, observations and experiments are the ultimate judge of an idea; no matter how out- landish an idea might appear it cannot be dismissed if it successfully meets observational or experi- mental tests Lecture 2 There are 100 billion stars in the Milky Way Galaxy The universe appears to be bigger than we imagine it to be The brightest star in the night sky is Sirius, located 8.6 ly from Earth. If it exploded tonight, when would we see that explosion? D. In 2020 The Milky Way is one of about 100 billion galaxies The Milky Way has as many stars as grains of sand on all Earths beaches The Universe is the totality of all space, time, energy and matter. Therefore, the Universe has no outside The Universe is about 13.7 billion years old How big is the universe? - How about, how much of the Universe can be seen? If you stand in Toronto, can you see Moscow? Would building a really tall tower help you see Moscow? There is a limit imposed by n
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