Planets, Stars, and Galaxies, Third Edition

Gordon Ritter, Sophia Chen

Description

The attempt to understand the universe's seemingly endless mysteries involves a process of discovery that can evoke fascination, frustration, and continual wonder and amazement. Planets, Stars, and Galaxies, Third Edition takes readers on an incredible full-color journey through the vast universe. This engaging title tackles the questions students studying science are eager to know: How large is the universe? Is it infinite or finite? What is its structure? Is it possible to visit other worlds, and if so, what similarities might they bear to our own world?

Sujets

• Sciences et techniques
• Planets
• Physics
• Astronomy
• Galaxy
• Research
• Stars
• Galaxies
• Discovery
• Science
• Physique
• Physical attractiveness

Informations

• Publié par: Infobase Publishing
• Date de parution: 01 septembre 2021
• Nombre de lectures: 0
• EAN13: 9781646937363
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Planets, Stars, and Galaxies, Third Edition
Copyright © 2021 by Infobase
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For more information, contact:
Chelsea House An imprint of Infobase 132 West 31st Street New York NY 10001
ISBN 978-1-64693-736-3
You can find Chelsea House on the World Wide Web at http://www.infobase.com
Contents Chapters The Universe Mathematics of Motion Newton, Kepler, and Gravity Night Sky Observation Relativity Large-Scale Structure of the Universe Support Materials Glossary Bibliography About the Authors Index
Chapters
The Universe
Our solar system is situated inside the Milky Way, one galaxy of an estimated hundreds of billions of galaxies inside our universe. From this perspective, it may seem small, but with a diameter longer than nine billion kilometers, 1 our humble solar system contains a multitude of astronomical objects, whose properties astronomers study to better understand our universe.
Our solar system contains the Sun; the eight official planets (in order of increasing distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), at least six "dwarf planets" (Pluto is now classified as a dwarf planet), more than 150 satellites of the planets, and a large number of comets, asteroids, and floating debris. Some of the known planetary satellites (or "moons") are themselves comparable in size to small planets. A large asteroid belt lies between the orbits of Mars and Jupiter, containing an estimated nearly 1.1–1.9 million asteroids. 2 Even the planets show a lot of variety. From their sizes in the figure below, you may notice that Earth is very tiny compared to Uranus and Neptune, which are themselves tiny compared to Jupiter and Saturn, which are tiny when compared to the Sun. The four large planets make up more than 99% of the mass known to orbit the Sun. 3

Note that the distances to the Sun are not to scale.
Source: Infobase Learning.
Beyond Neptune lies the Kuiper Belt, a large shell of objects that begins at about 30 astronomical units, or AU, and extends outward to nearly 1,000 AU beyond the sun. (An astronomical unit equals about 150 million kilometers and is approximately the distance from Earth to the Sun.) In 1951, Gerard Kuiper suggested that comet-like debris left over from the formation of the solar system should be just beyond Neptune. Kuiper argued convincingly that it would be most unusual not to find a continuum of particles beyond the part of the solar system occupied by the large planets. He reasoned that since the large objects of the solar system were formed by the condensation of smaller particles, it is very unlikely for such a process of formation to form a sharp "edge" at its boundary. Since his prediction, astronomers have discovered some very remote and surprising Kuiper Belt objects using the Hubble Space Telescope, including, for instance, an "ice planet" half the size of Pluto. To date more than 2,000 objects in the Kuiper Belt have been identified. 4
All of the planets orbit the Sun, following elliptical paths (the precise definition of an ellipse will come later), just as Newtonian physics would predict. Each planet has its own orbital period, which is the time it takes to make one revolution around the Sun. For the Earth, this period is approximately 365 days, or one Earth-year. Each planet's elliptical orbit lies in a certain plane in three-dimensional space, and for our solar system, the planes for different planets are, in a first approximation, aligned. This means that you can think of the solar system as approximately a disk, where the planets orbit in "the plane of the solar system."
We know so much about the solar system today because of advances in both fundamental theoretical physics and in the science of observation, with one type of advance building upon previous advances. The Hubble Space Telescope gives us precise knowledge of distant parts of the universe that no one could have figured out through purely theoretical investigations, but without many important theoretical advances (such as Newtonian mechanics, general relativity, and electromagnetism), no one could have designed and built such a telescope. In the next section we will give an incomplete history of some of the most important scientific breakthroughs that enabled our current, fairly detailed understanding of the solar system to evolve into its present form.
Physics History, Legend, and Folklore
This section is dedicated to a brief, informal history of historical attempts to understand the motions of planets, stars, and galaxies, leading toward Isaac Newton's invention of calculus and of the associated theory of physics now known as Newtonian mechanics. Newton's mechanics included the first modern concept of "force" and the rules by which forces influence the motion of bodies. Newton achieved the first successful mathematical explanation of planetary motion. We will give a brief, modernized account of Newton's ideas in chapter two. While we now know his description of planetary physics is only approximately correct, the approximation is quite good even by modern standards. By the twentieth century, Einstein's general theory of relativity corrected Newton's mechanics, and it gives a description of the mechanics of our solar system that is accurate to many decimal places and has passed all experimental tests thus far.
In Alexandria, Egypt around A.D. 150, Claudius Ptolemy wrote a seminal work on astronomy now called the Almagest . The Almagest consists of 13 books containing compilations of measurements of our solar system, with accompanying mathematical theories to explain them. It also explains Ptolemy's explanation for the motion of the universe. 5 These "theories" ultimately proved of little use, because they do not explain fundamental physical processes, and thus, they do not lead to a general theory of mechanics. Instead, they are guesses about possible mathematical descriptions for the planets' motions, based on study of observational data.
One fundamental flaw in Ptolemy's "theory" is that he based it on the Earth-centered (or "geocentric") concept of Aristotle. This view of the universe presents the Earth's position as fixed, while other objects (such as the Sun, Moon, stars, and planets) rotate around it at the center. Modern physicists would agree that the statement "the Earth is fixed" has no intrinsic meaning, because it does not provide the answer to the question: "Fixed with respect to what?" Based on such premises, Ptolemy predicted the positions of the Sun, Moon, and planets using epicycles, the curves traced out by a point on a circle that rolls along another circle (see figure below). Many of these predictions do not match the data well, because the correct equations of motion are not those of an epicycle.

Ptolemy used epicycles, the curves traced out by a point on a circle that rolls along another circle, to predict the positions of the Sun, Moon, and planets.
Source: Infobase Learning.
Modern reasoning would point out that even on Earth on a dark night, a brighter torch viewed at a greater distance may appear to be the same brightness as a smaller, nearer torch. Thus, we could never determine the distance to an object in the night sky by simply observing its brightness. The determination of distance in astronomy can be very difficult, for precisely this reason, and it generally must involve at least two measurements made from different points on the Earth's orbit.
Though he lived long before Ptolemy, Aristarchus (310-230 B.C. ) had a remarkably modern picture of astronomy. Although some of Aristarchus's most important writings were lost, it is generally believed that he proposed a heliocentric (or sun-centered) model 1,700 years before Copernicus, one of the central figures in astronomy. 6 The ancient Greek mathematician, Archimedes wrote in The Sand Reckoner : "… the "universe" is the name given by most astronomers to the sphere the centre of which is the centre of the Earth, while its radius is equal to the straight line between the centre of the Sun and the centre of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same centre as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface." 7
Perhaps due to the clarity of exposition and apparent mathematical rigor of Ptolemy's Almagest , people would accept its flawed theories (including the geocentric model) for at least 1,500 years after its publication.
The sixteenth century astronomer, Nicolaus Copernicus is often credited as a founder of modern astronomy. Born in Poland, Copernicus studied mathematics and optics at Krakow University. After returning to Poland from several years' study of church law in Italy, Copernicus was appointed as a priest in the cathedral of Frauenburg (now known as Frombork, in northern Poland), where he devoted the rest of his life to scholarship. 8

Nicolaus Copernicus (1473–1543) was the astronomer who formulated the first heliocentric (sun-centered) theory of the solar system, in which the Earth spins once every 24 hours about an internal axis, while at the same time making a complete trip around t