Mid-Term Test #1.docx

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Astronomy & Astrophysics
Michael Reid

AST201 Mid-Term Test #1 CHAPTER 1: OUR PLACE IN THE UNIVERSE Universe Universe the totality of all space, time, energy and matter o About 13.7 billion years old o Unobservable universe is not limited by lack of technology; rather, the laws of physics prevents it this means there is a limit imposed by nature to how far we can see How big is the universe? o Big problem because (a) it can't be measured; and (b) you must measure the distance with your eyes without touching Solar system one or more planets orbiting a star (e.g. planets and their moons; asteroids; comets) Galaxy a collection of billions of stars all orbiting a common center o Milky Way Galaxy and Andromeda Galaxy are spiral galaxies Star a large glowing ball of very hot gas that generates heat and light (energy) through nuclear fusion in its core (e.g. Sun) o Born when gravity compresses the material into a cloud to the point where the center becomes dense enough and hot enough to generate energy by nuclear fusion, the process in which lightweight atomic nuclei smash together and stick (or fuse) to make heavier nuclei o Star "lives" as long as it can shine with energy from fusion; "dies" when it exhausts its usable fuel Moon (or satellite) an object that orbits a planet Local Group a group of galaxies among 40 galaxies Galaxy clusters groups of galaxies with more than a few dozen members SOLAR SYSTEM < GALAXY < GALAXY GROUP < GALAXY CLUSTER < SUPERCLUSTER < WALL or FILAMENT Light travels at a constant, finite speed c = 300 million m/s Light year the distance light travels in one year o 1 light year = 9.46 trillion km If there was a galaxy 100 billion light years away from you, and given that the universe is only 13.7 billion years old, the light would not have reached you yet When we see Andromeda (i.e. 2.5 million light years away), we see it as it WAS 2.5 million years ago A cupboard that is 0.0000000000002 light years away we see it as it was 0.0000000000002 years ago Astronomical unit (AU) the average distance from Earth to the Sun o 1 AU = 150 million km CHAPTER 3: THE SCIENCE OF ASTRONOMY Ancient astronomy was often astrological in nature o Astrology making observations in the sky and correlating them to make predictions Ancient astronomy was geocentric, or Earth-centred For thousands of years, astronomers insisted on the Pythagorean ideal of circular motion Planet = wanderers (in Greek) because they move in space Nicholas Copernicus (1473-1543) o Sun-centred model of solar system Galileo Galilei (1564-1642) o First to use telescope to look at sky saw craters on the moon which shattered the belief that everything in space was perfect o First evidence that something orbited something other than the Earth o Published a book in Italian so that the commoners could understand it too (before it was only Latin) Used characters that resembled himself and the pope o Contributed the idea that if you have a scientific theory, you should check it against reality (scientific method) Johannes Kepler (1571-1630) o Took Galileo's data and codified them into a couple laws of planetary motion Isaac Newton (1642-1727) o Invented three laws of motion; contributed earliest observations of optics Kepler's Laws of Planetary Motion 1 LAW the orbit of each planet around the Sun is an ellipse with the Sun at one focus o Closest at the point called perihelion; furthest at point called aphelion 2ND LAW as a planet moves around its orbit, it sweeps out equal areas in equal times o Planet travels faster when nearer to the Sun; slower when further away from the Sun 3RD LAW more distant planets orbit the Sun at slower average speeds; p = a3 o p = period, time taken to complete one orbit; a = orbital semimajor axis, a half of the longest axis of an ellipse o More distant planets move more slowly Hallmarks of Science Modern science seeks explanations for observed phenomena that rely solely on natural causes Science progresses through the creation and testing of models of nature that explain the observations as simply as possible As scientific 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 Occam's Razor principle holding that scientists should prefer the simpler of two models that agree equally well with observations Falsifiable something that can be tested to determine whether true of false Scientific Method MAKE OBSERVATIONS ASK A QUESTION SUGGEST A HYPOTHESIS MAKE A PREDICTION PERFORM A TEST (experiment or additional observation) o If test DOES NOT support hypothesis revise hypothesis or make new one o If test DOES support hypothesis make additional predictions and test them Theory a model of some aspect of nature that has been rigorously tested and has passed all tests to date o Often misused phrase o In science, nothing is superior to a theory CHAPTER 4: MAKING SENSE OF THE UNIVERSE Understanding Motion, Energy, and Gravity Acceleration when velocity is changing in any way, either in speed or direction or both 2 Acceleration of gravity the acceleration of a falling object; approximately = 9.8 m/s Momentum product of an object's mass and velocity; momentum = mass x velocity Force anything that can cause a change in momentum Net force overall force to which an object responds; equal to the rate of change in the object's momentum, or equivalently to the object's mass x acceleration Angular momentum momentum attributable to rotation or revolution; equals m x v x r Free-fall condition in which an object is falling without resistance; objects are weightless when in free-fall o Only external force acting on it is gravityThe Four Fundamental Forces Strong nuclear force holds atomic nuclei together Electromagnetism makes atoms cling to each other Weak nuclear force breaks atoms apart Gravity makes masses attract one another o Because gravity's range is infinite, everything in the universe is aware of your mass o Unique because it only ever adds up there is no "anti-gravity"; the more electric charges you add, the greater the mass becomes o Gravity only pulls, never pushes Newton's Three Laws of Motion 1 LAW an object at rest will remain at rest, and a moving object will remain at constant velocity unless acted upon by a net force to change its speed or direction ND 2 LAW force = mass x acceleration (Fnetma) 3RD LAW for every action force, there is always an equal and opposite reaction force Conservation Laws Conservation of momentum principle that, in the absence of net force, the total momentum of a system remains constant Conservation of angular momentum principle that, in the absence of net torque (twisting force), the total angular momentum of a system remains constant Conservation of energy principle that energy (including mass-energy) can be neither created nor destroyed, but can only change from one form to another Types of Energy Kinetic energy of motion Radiative energy carried by light Potential energy stored for later conversion into kinetic energy Thermal
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