PSCI 1414 GENERAL ASTRONOMY LECTURE 1: INTRODUCTION ALEXANDER C. SPAHN

WHAT IS ASTRONOMY? The term ‘astronomy’ is derived from the Greek root astron for star and nomos for arrangement or law. Astronomy is the study of the origin and evolution of the Universe, the physics and chemistry of celestial objects, and the calculation of their positions and motions. More importantly, astronomy is about us. As we study astronomy, we study ourselves and our place within the Universe. We seek answers to big questions: What are we? Where did we come from? Are we alone?

BRANCHES OF ASTRONOMY Astrometry – the precise measurements of the positions of stars and planets Planetary science – the study of asteroids, comets, and planets Stellar astronomy – the study of the origin, evolution, and death of stars Galactic astronomy – the study of the structure and components of galaxies Astrophysics – the study of the Universe and the physics of its constituents (luminosity, temp., composition, etc.) Cosmology – the study of the origin and evolution of the Universe as a whole Astrobiology – the study of the possibility of life elsewhere in the Universe

THE SCIENTIFIC METHOD Scientists explore the World around them based fundamentally on observations, logic, and skepticism. The scientific method requires that our ideas about the world around us be consistent with what we actually observe.

THE SCALE OF THE UNIVERSE AND OUR PLACE WITHIN We live on the Earth (7.4 billion people) The Earth is a part of our Solar System (8 planets, sorry Pluto) Our solar system belongs to the Milky Way Galaxy (>100 billion star systems) The Milky Way belongs to a galaxy cluster known as the Local Group ( > 70 galaxies) The Local Group is tightly packed with other galaxy clusters in the Local Supercluster Together, all of these structures make up the Universe ( 10,000,000,000,000,000,000,000 stars) Interactive Scale of the Universe

THE SCALE OF THE UNIVERSE AND OUR PLACE WITHIN

THE BRIEF HISTORY OF THE UNIVERSE The universe began with the Big Bang some 14 billion years ago and has been expanding ever since Within several hundred thousand years, neutral hydrogen had formed Within several billion years, matter had clumped into galaxies and galaxy clusters Gravity drove the collapse of clouds of gas and dust to form stars and planets Stars created heavier elements within their cores and enriched the Universe when they exploded These heavier elements had come together to allow for life (and us) to form

THE BRIEF HISTORY OF THE UNIVERSE

BASIC ASTRONOMICAL DEFINITIONS Astronomical Objects Star – A large, glowing ball of gas that generates heat and light through nuclear fusion in its core Planet – An object that orbits a star, is large enough for its own gravity to make it round, and has cleared most other objects from its orbital path Moon (or satellite) – An object that orbits a planet Asteroid – A relatively small and rocky object that orbits a star Comet – A relatively small and ice-rich object that orbits a star

BASIC ASTRONOMICAL DEFINITIONS Collections of Astronomical Objects Star system – A star and all the material that orbits it, including planets, dwarf planets like Pluto, and other small solar system bodies Galaxy – A gravitationally bound system of stars, gas, and dust Galaxy cluster (or group) – A collection of galaxies bound together by gravity Supercluster – A gigantic region of space in which many clusters of galaxies are tightly packed Universe – The total sum of all matter and energy Observable Universe – The portion of the Universe that can be seen from Earth

BASIC ASTRONOMICAL DEFINITIONS Terms Relating to Motion Rotation – The spinning of an object around its axis Orbit (revolution) – The orbital motion of one object around another due to gravity Expansion (of the Universe) – The increase in average distance between galaxies over time

BASIC ASTRONOMICAL DEFINITIONS Astronomical Distances Astronomical unit (AU) – The average distance between the Earth and the Sun 1 AU = 1.496x10 8 km = million miles Light-year (ly) – The distance that light travels in one year 1 ly = 9.46x10 12 km = 63,240 AU Parsec (pc) – The distance at which 1AU subtends and angle of 1 arcsec 1 pc = 3.09x10 13 km = 3.26 ly

POWERS OF TEN Specialized units make it easier to comprehend immense cosmic distances in which keeping track of all the zeros becomes a hassle Rather than writing out all of the zeros that accompany large numbers, we may write them using powers-of-ten notation in which the number of zeros (or how many times ten is multiplied) is given as an exponent

POWERS OF TEN

Converting a number to power-of-ten notation Numbers greater than 1: Move the decimal place to the left until only one digit remains to its left. The remaining number is then multiplied by 10 raised to an exponent equal to the number of spaces moved. Earth’s diameter = m = 6.37x10 6 m Numbers less than 1: Move the decimal to the right until it passes a non-zero number. Then multiply by 10 with a negative exponent equal to the number of spaces moved. Diameter of red blood cell = m = 7.5x10 -6 m

ARITHMETIC WITH POWERS-OF-TEN NOTATION

UNIT CONVERSIONS Scientists use SI units, which measures length in meters (m), time in seconds (s), and mass in kilograms (kg) A useful set of conversion factors for the English system are: 1 in = 2.54 cm 1 ft = m 1 mi = km Each equality can be written as a fraction equal to 1. A quantity doesn’t change if multiplied by 1

UNIT CONVERSIONS Steps to convert units 1.Start with the quantity you wish to convert 2.Multiply it by an appropriate conversion factor 3.Cancel out the original unit 4.Calculate the answer in the desired units

UNIT CONVERSIONS

WHICH OF THE FOLLOWING WOULD CAUSE A SCIENTIFIC THEORY TO BE REJECTED? A.It does not agree with previous theories. B.It does not correctly predict experimental observations. C.Most scientists consider the theory to be wrong. D.The technology has not yet been developed to make the observations that would test the theory. E.It is not supported by religious beliefs. Q1.1

WHICH OF THE FOLLOWING WOULD CAUSE A SCIENTIFIC THEORY TO BE REJECTED? A.It does not agree with previous theories. B.It does not correctly predict experimental observations. C.Most scientists consider the theory to be wrong. D.The technology has not yet been developed to make the observations that would test the theory. E.It is not supported by religious beliefs. A1.1

NEWTONIAN MECHANICS, WHICH DESCRIBES THE MOTION OF OBJECTS WHEN FORCES ARE EXERTED ON THEM, WAS ACCEPTED AS A SCIENTIFIC THEORY THAT DESCRIBES NATURE APPROXIMATELY 400 YEARS AGO. IT WAS ACCEPTED BECAUSE IT A.was proposed by someone we now know to be a genius. B.was derived from fundamental mathematical principles. C.was handed down to us by the Greeks. D.predicted how objects move under the action of a force. E.was supported by the religious authorities. Q1.2

NEWTONIAN MECHANICS, WHICH DESCRIBES THE MOTION OF OBJECTS WHEN FORCES ARE EXERTED ON THEM, WAS ACCEPTED AS A SCIENTIFIC THEORY THAT DESCRIBES NATURE APPROXIMATELY 400 YEARS AGO. IT WAS ACCEPTED BECAUSE IT A.was proposed by someone we now know to be a genius. B.was derived from fundamental mathematical principles. C.was handed down to us by the Greeks. D.predicted how objects move under the action of a force. E.was supported by the religious authorities. A1.2

WHAT LENGTHS ARE MOST CONVENIENTLY MEASURED IN ASTRONOMICAL UNITS (AU)? A.Distances on the Earth B.Distances within the solar system C.Distances between stars in our Galaxy D.Distances between galaxies E.Only the largest distances in the universe Q1.5

WHAT LENGTHS ARE MOST CONVENIENTLY MEASURED IN ASTRONOMICAL UNITS (AU)? A.Distances on the Earth B.Distances within the solar system C.Distances between stars in our Galaxy D.Distances between galaxies E.Only the largest distances in the universe A1.5

WHAT LENGTHS ARE MOST CONVENIENTLY MEASURED IN UNITS OF NANOMETERS (10 −9 M)? A.Distances between the planets B.Distances between cities on Earth C.Dimensions of the human body D.Wavelengths of visible light E.Diameter of an atom Q1.6

WHAT LENGTHS ARE MOST CONVENIENTLY MEASURED IN UNITS OF NANOMETERS (10 −9 M)? A.Distances between the planets B.Distances between cities on Earth C.Dimensions of the human body D.Wavelengths of visible light E.Diameter of an atom A1.6

WHAT LENGTHS ARE MOST CONVENIENTLY MEASURED IN UNITS OF CENTIMETERS (10 −2 M)? A.Distances between the planets B.Distances between cities on Earth C.Dimensions of the human body D.Wavelengths of visible light E.Diameter of an atom Q1.7

WHAT LENGTHS ARE MOST CONVENIENTLY MEASURED IN UNITS OF CENTIMETERS (10 −2 M)? A.Distances between the planets B.Distances between cities on Earth C.Dimensions of the human body D.Wavelengths of visible light E.Diameter of an atom A1.7

WHICH OF THE FOLLOWING STATEMENTS IS TRUE? A.A solar system may contain billions of stars. B.Stars are spread out more or less uniformly throughout the Universe. C.The Milky Way contains a variety of solar systems, stars, and galaxies. D.A galaxy contains billions of stars. E.A solar system contains billions of galaxies. Q1.12

WHICH OF THE FOLLOWING STATEMENTS IS TRUE? A.A solar system may contain billions of stars. B.Stars are spread out more or less uniformly throughout the Universe. C.The Milky Way contains a variety of solar systems, stars, and galaxies. D.A galaxy contains billions of stars. E.A solar system contains billions of galaxies. Q1.12

HOMEWORK 1: DUE FRIDAY 1-15 Questions: 1, 2, 16, 25, 28, 32 Must show all work for mathematical questions Write neatly and box answers to mathematical questions You may often find that questions will require you to read some of the book I will post the homework questions to my webpage