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Astronomy & Astrophysics
Michael Reid

* The definition of Universe The Universe is the totality of all space, time, energy, and matter. Therefore, the Universe has no outside. * The age and size of the Universe The Univserse is about 13.7 billion years old. The size of the universe is a tough question to answer. So, a better question to ask is how much of the universe can we see. * The observable Universe But, if the universe is 13.7 billion years old, the light from the galaxy would not have reached you yet. It will take 86.3 billion years before the light reaches us. Therefore, there is limit imposed by nature to how far we can see: observable universe vs. unobservable universe. So, whenever we mentionrefer to the term universe we will usually mean the observable universe. * Light travel time and the light year Speed of light = c = 300 million ms. A light year is the distance light travels in one year: 1 ly = 9.46 x 10 m or 9.46 trillion km By definition of a light year, light takes 1 year to travel 1 light year and 100 billion years to travel 100 billion light years. Because light takes time to travel to us, looking far away means literally looking back in time. For example, the distance to the nearest large galaxy is about 2.5 million light years, we seeit the way it was 2.5 million years ago. * The contents of the Universe Planets: spherical bodies which orbit stars directly; can be rocky, gassy, liquid, or icy. Stars: enormous spheres of very hot gas which produce energy by nuclear fusion. The Sun is one. Solar System: one or more planets orbiting a star. Galaxy: a collection of billions of stars all orbiting a common center. * Our location in the Universe www.notesolution.comEarth is in the Milky Way Galaxy, Our solar system is somewhere in the middle (between the center and outer edge of the solar system). Our nearest neighbor galaxy is the Andromeda Galaxy. Both Andromeda and Milky Way are spiral galaxies. The Milky Way, Andromeda and a few dozen small galaxies make up the Local Group of galaxies, all of which orbit one another. All the galaxy clusters in the Universe are moving apart from one another. Enormous group of galaxies all orbiting one another are called galaxy superclusters. They may have 1000s or millions of member galaxies. Our supercluster is called the Virgo because many of its galaxies appear to us in the direction of the constellation Virgo. *Science Science is the process of trying to find natural explanations for natural phenomena by repeatedly following a cycle of hypothesis, prediction, and observation or experiment. For a proposition to be scientific, it must be falsifiable. A falsifiable proposition is one which could, by experiment, be proven false (if indeed it is false). * Gravity and motion Speed: rate at which object moves, Example: 10 ms Velocity: speed and direction, Example: 10 ms, due east Acceleration: Any change in velocity units of speedtime (it may involve a change of direction, not speed per se) All falling objects accelerate at the same rate (not counting friction of air resistance). On Earth g = 10 ms i.e. speed increases by 10 metres every second of falling. Galileo showed that g is the same for all falling objects, regardless of their mass. Newton discovered the laws of motion and gravity. He realized that the same physical laws that operate on Earth also operate in the heavens (one universe). * Forces and Newtons Laws Newtons three laws of motion: 1) An object moves at constant velocity unless a net force acts to change its speed or direction. Example: spacecraft spend most of their coasting (no engines running). 2) Force = mass x acceleration. Example: www.notesolution.com
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