Chapter 1: How Science Works
Chapter 2
Early Astronomy
- The Night Sky
- Motions in the Sky
- Navigation
- Constellations and Seasons
- Cause of the Seasons
- The Magnitude System
- Angular Size and Linear Size
- Phases of the Moon
- Eclipses
- Auroras
- Dividing Time
- Solar and Lunar Calendars
- History of Astronomy
- Stonehenge
- Ancient Observatories
- Counting and Measurement
- Astrology
- Greek Astronomy
- Aristotle and Geocentric Cosmology
- Aristarchus and Heliocentric Cosmology
- The Dark Ages
- Arab Astronomy
- Indian Astronomy
- Chinese Astronomy
- Mayan Astronomy
- Questions
Chapter 3
The Copernican Revolution
- Ptolemy and the Geocentric Model
- The Renaissance
- Copernicus and the Heliocentric Model
- Tycho Brahe
- Johannes Kepler
- Elliptical Orbits
- Kepler's Laws
- Galileo Galilei
- The Trial of Galileo
- Isaac Newton
- Newton's Law of Gravity
- The Plurality of Worlds
- The Birth of Modern Science
- Layout of the Solar System
- Scale of the Solar System
- The Idea of Space Exploration
- Orbits
- History of Space Exploration
- Moon Landings
- International Space Station
- Manned versus Robotic Missions
- Commercial Space Flight
- Future of Space Exploration
- Living in Space
- Moon, Mars, and Beyond
- Societies in Space
- Questions
Chapter 4
Matter and Energy in the Universe
- Matter and Energy
- Rutherford and Atomic Structure
- Early Greek Physics
- Dalton and Atoms
- The Periodic Table
- Structure of the Atom
- Energy
- Heat and Temperature
- Potential and Kinetic Energy
- Conservation of Energy
- Velocity of Gas Particles
- States of Matter
- Thermodynamics
- Entropy
- Laws of Thermodynamics
- Heat Transfer
- Thermal Radiation
- Wien's Law
- Radiation from Planets and Stars
- Internal Heat in Planets and Stars
- Periodic Processes
- Random Processes
- Questions
Chapter 5
The Earth-Moon System
- Earth and Moon
- Early Estimates of Earth's Age
- How the Earth Cooled
- Ages Using Radioactivity
- Radioactive Half-Life
- Ages of the Earth and Moon
- Geological Activity
- Internal Structure of the Earth and Moon
- Basic Rock Types
- Layers of the Earth and Moon
- Origin of Water on Earth
- The Evolving Earth
- Plate Tectonics
- Volcanoes
- Geological Processes
- Impact Craters
- The Geological Timescale
- Mass Extinctions
- Evolution and the Cosmic Environment
- Earth's Atmosphere and Oceans
- Weather Circulation
- Environmental Change on Earth
- The Earth-Moon System
- Geological History of the Moon
- Tidal Forces
- Effects of Tidal Forces
- Historical Studies of the Moon
- Lunar Surface
- Ice on the Moon
- Origin of the Moon
- Humans on the Moon
- Questions
Chapter 6
The Terrestrial Planets
- Studying Other Planets
- The Planets
- The Terrestrial Planets
- Mercury
- Mercury's Orbit
- Mercury's Surface
- Venus
- Volcanism on Venus
- Venus and the Greenhouse Effect
- Tectonics on Venus
- Exploring Venus
- Mars in Myth and Legend
- Early Studies of Mars
- Mars Close-Up
- Modern Views of Mars
- Missions to Mars
- Geology of Mars
- Water on Mars
- Polar Caps of Mars
- Climate Change on Mars
- Terraforming Mars
- Life on Mars
- The Moons of Mars
- Martian Meteorites
- Comparative Planetology
- Incidence of Craters
- Counting Craters
- Counting Statistics
- Internal Heat and Geological Activity
- Magnetic Fields of the Terrestrial Planets
- Mountains and Rifts
- Radar Studies of Planetary Surfaces
- Laser Ranging and Altimetry
- Gravity and Atmospheres
- Normal Atmospheric Composition
- The Significance of Oxygen
- Questions
Chapter 7
The Giant Planets and Their Moons
- The Gas Giant Planets
- Atmospheres of the Gas Giant Planets
- Clouds and Weather on Gas Giant Planets
- Internal Structure of the Gas Giant Planets
- Thermal Radiation from Gas Giant Planets
- Life on Gas Giant Planets?
- Why Giant Planets are Giant
- Gas Laws
- Ring Systems of the Giant Planets
- Structure Within Ring Systems
- The Origin of Ring Particles
- The Roche Limit
- Resonance and Harmonics
- Tidal Forces in the Solar System
- Moons of Gas Giant Planets
- Geology of Large Moons
- The Voyager Missions
- Jupiter
- Jupiter's Galilean Moons
- Jupiter's Ganymede
- Jupiter's Europa
- Jupiter's Callisto
- Jupiter's Io
- Volcanoes on Io
- Saturn
- Cassini Mission to Saturn
- Saturn's Titan
- Saturn's Enceladus
- Discovery of Uranus and Neptune
- Uranus
- Uranus' Miranda
- Neptune
- Neptune's Triton
- Pluto
- The Discovery of Pluto
- Pluto as a Dwarf Planet
- Dwarf Planets
- Questions
Chapter 8
Interplanetary Bodies
- Interplanetary Bodies
- Comets
- Early Observations of Comets
- Structure of the Comet Nucleus
- Comet Chemistry
- Oort Cloud and Kuiper Belt
- Kuiper Belt
- Comet Orbits
- Life Story of Comets
- The Largest Kuiper Belt Objects
- Meteors and Meteor Showers
- Gravitational Perturbations
- Asteroids
- Surveys for Earth Crossing Asteroids
- Asteroid Shapes
- Composition of Asteroids
- Introduction to Meteorites
- Origin of Meteorites
- Types of Meteorites
- The Tunguska Event
- The Threat from Space
- Probability and Impacts
- Impact on Jupiter
- Interplanetary Opportunity
- Questions
Chapter 9
Planet Formation and Exoplanets
- Formation of the Solar System
- Early History of the Solar System
- Conservation of Angular Momentum
- Angular Momentum in a Collapsing Cloud
- Helmholtz Contraction
- Safronov and Planet Formation
- Collapse of the Solar Nebula
- Why the Solar System Collapsed
- From Planetesimals to Planets
- Accretion and Solar System Bodies
- Differentiation
- Planetary Magnetic Fields
- The Origin of Satellites
- Solar System Debris and Formation
- Gradual Evolution and a Few Catastrophies
- Chaos and Determinism
- Extrasolar Planets
- Discoveries of Exoplanets
- Doppler Detection of Exoplanets
- Transit Detection of Exoplanets
- The Kepler Mission
- Direct Detection of Exoplanets
- Properties of Exoplanets
- Implications of Exoplanet Surveys
- Future Detection of Exoplanets
- Questions
Chapter 10
Detecting Radiation from Space
- Observing the Universe
- Radiation and the Universe
- The Nature of Light
- The Electromagnetic Spectrum
- Properties of Waves
- Waves and Particles
- How Radiation Travels
- Properties of Electromagnetic Radiation
- The Doppler Effect
- Invisible Radiation
- Thermal Spectra
- The Quantum Theory
- The Uncertainty Principle
- Spectral Lines
- Emission Lines and Bands
- Absorption and Emission Spectra
- Kirchoff's Laws
- Astronomical Detection of Radiation
- The Telescope
- Optical Telescopes
- Optical Detectors
- Adaptive Optics
- Image Processing
- Digital Information
- Radio Telescopes
- Telescopes in Space
- Hubble Space Telescope
- Interferometry
- Collecting Area and Resolution
- Frontier Observatories
- Questions
Chapter 11
Our Sun: The Nearest Star
- The Sun
- The Nearest Star
- Properties of the Sun
- Kelvin and the Sun's Age
- The Sun's Composition
- Energy From Atomic Nuclei
- Mass-Energy Conversion
- Examples of Mass-Energy Conversion
- Energy From Nuclear Fission
- Energy From Nuclear Fusion
- Nuclear Reactions in the Sun
- The Sun's Interior
- Energy Flow in the Sun
- Collisions and Opacity
- Solar Neutrinos
- Solar Oscillations
- The Sun's Atmosphere
- Solar Chromosphere and Corona
- Sunspots
- The Solar Cycle
- The Solar Wind
- Effects of the Sun on the Earth
- Cosmic Energy Sources
- Questions
Chapter 12
Properties of Stars
- Stars
- Star Names
- Star Properties
- The Distance to Stars
- Apparent Brightness
- Absolute Brightness
- Measuring Star Distances
- Stellar Parallax
- Spectra of Stars
- Spectral Classification
- Temperature and Spectral Class
- Stellar Composition
- Stellar Motion
- Stellar Luminosity
- The Size of Stars
- Stefan-Boltzmann Law
- Stellar Mass
- Hydrostatic Equilibrium
- Stellar Classification
- The Hertzsprung-Russell Diagram
- Volume and Brightness Selected Samples
- Stars of Different Sizes
- Understanding the Main Sequence
- Stellar Structure
- Stellar Evolution
- Questions
Chapter 13
Star Birth and Death
- Star Birth and Death
- Understanding Star Birth and Death
- Cosmic Abundance of Elements
- Star Formation
- Molecular Clouds
- Young Stars
- T Tauri Stars
- Mass Limits for Stars
- Brown Dwarfs
- Young Star Clusters
- Cauldron of the Elements
- Main Sequence Stars
- Nuclear Reactions in Main Sequence Stars
- Main Sequence Lifetimes
- Evolved Stars
- Cycles of Star Life and Death
- The Creation of Heavy Elements
- Red Giants
- Horizontal Branch and Asymptotic Giant Branch Stars
- Variable Stars
- Magnetic Stars
- Stellar Mass Loss
- White Dwarfs
- Supernovae
- Seeing the Death of a Star
- Supernova 1987A
- Neutron Stars and Pulsars
- Special Theory of Relativity
- General Theory of Relativity
- Black Holes
- Properties of Black Holes
- Questions
Chapter 14
The Milky Way
- The Distribution of Stars in Space
- Stellar Companions
- Binary Star Systems
- Binary and Multiple Stars
- Mass Transfer in Binaries
- Binaries and Stellar Mass
- Nova and Supernova
- Exotic Binary Systems
- Gamma Ray Bursts
- How Multiple Stars Form
- Environments of Stars
- The Interstellar Medium
- Effects of Interstellar Material on Starlight
- Structure of the Interstellar Medium
- Dust Extinction and Reddening
- Groups of Stars
- Open Star Clusters
- Globular Star Clusters
- Distances to Groups of Stars
- Ages of Groups of Stars
- Layout of the Milky Way
- William Herschel
- Isotropy and Anisotropy
- Mapping the Milky Way
- Questions
Chapter 15
Galaxies
- The Milky Way Galaxy
- Mapping the Galaxy Disk
- Spiral Structure in Galaxies
- Mass of the Milky Way
- Dark Matter in the Milky Way
- Galaxy Mass
- The Galactic Center
- Black Hole in the Galactic Center
- Stellar Populations
- Formation of the Milky Way
- Galaxies
- The Shapley-Curtis Debate
- Edwin Hubble
- Distances to Galaxies
- Classifying Galaxies
- Spiral Galaxies
- Elliptical Galaxies
- Lenticular Galaxies
- Dwarf and Irregular Galaxies
- Overview of Galaxy Structures
- The Local Group
- Light Travel Time
- Galaxy Size and Luminosity
- Mass to Light Ratios
- Dark Matter in Galaxies
- Gravity of Many Bodies
- Galaxy Evolution
- Galaxy Interactions
- Galaxy Formation
- Questions
Chapter 16
The Expanding Universe
- Galaxy Redshifts
- The Expanding Universe
- Cosmological Redshifts
- The Hubble Relation
- Relating Redshift and Distance
- Galaxy Distance Indicators
- Size and Age of the Universe
- The Hubble Constant
- Large Scale Structure
- Galaxy Clustering
- Clusters of Galaxies
- Overview of Large Scale Structure
- Dark Matter on the Largest Scales
- The Most Distant Galaxies
- Black Holes in Nearby Galaxies
- Active Galaxies
- Radio Galaxies
- The Discovery of Quasars
- Quasars
- Types of Gravitational Lensing
- Properties of Quasars
- The Quasar Power Source
- Quasars as Probes of the Universe
- Star Formation History of the Universe
- Expansion History of the Universe
- Questions
Chapter 17
Cosmology
- Cosmology
- Early Cosmologies
- Relativity and Cosmology
- The Big Bang Model
- The Cosmological Principle
- Universal Expansion
- Cosmic Nucleosynthesis
- Cosmic Microwave Background Radiation
- Discovery of the Microwave Background Radiation
- Measuring Space Curvature
- Cosmic Evolution
- Evolution of Structure
- Mean Cosmic Density
- Critical Density
- Dark Matter and Dark Energy
- Age of the Universe
- Precision Cosmology
- The Future of the Contents of the Universe
- Fate of the Universe
- Alternatives to the Big Bang Model
- Space-Time
- Particles and Radiation
- The Very Early Universe
- Mass and Energy in the Early Universe
- Matter and Antimatter
- The Forces of Nature
- Fine-Tuning in Cosmology
- The Anthropic Principle in Cosmology
- String Theory and Cosmology
- The Multiverse
- The Limits of Knowledge
- Questions
Chapter 18
Life On Earth
- Nature of Life
- Chemistry of Life
- Molecules of Life
- The Origin of Life on Earth
- Origin of Complex Molecules
- Miller-Urey Experiment
- Pre-RNA World
- RNA World
- From Molecules to Cells
- Metabolism
- Anaerobes
- Extremophiles
- Thermophiles
- Psychrophiles
- Xerophiles
- Halophiles
- Barophiles
- Acidophiles
- Alkaliphiles
- Radiation Resistant Biology
- Importance of Water for Life
- Hydrothermal Systems
- Silicon Versus Carbon
- DNA and Heredity
- Life as Digital Information
- Synthetic Biology
- Life in a Computer
- Natural Selection
- Tree Of Life
- Evolution and Intelligence
- Culture and Technology
- The Gaia Hypothesis
- Life and the Cosmic Environment
Chapter 19
Life in the Universe
- Life in the Universe
- Astrobiology
- Life Beyond Earth
- Sites for Life
- Complex Molecules in Space
- Life in the Solar System
- Lowell and Canals on Mars
- Implications of Life on Mars
- Extreme Environments in the Solar System
- Rare Earth Hypothesis
- Are We Alone?
- Unidentified Flying Objects or UFOs
- The Search for Extraterrestrial Intelligence
- The Drake Equation
- The History of SETI
- Recent SETI Projects
- Recognizing a Message
- The Best Way to Communicate
- The Fermi Question
- The Anthropic Principle
- Where Are They?
Units and the Metric System
The subject of measurement has a rich and fascinating history. Ever since humans have traded goods, they have needed a standard system of weights and measures. Many of our familiar units are part of the English system, which dates back to the Middle Ages. For example, length units were based on the human body: an inch is the length of the end joint of a thumb, a foot is of course the length of a foot, a yard is the distance from the tip of the nose to the end of an outstretched arm, and a fathom is the distance between the fingertips of two arms held straight out. However, these units are variable, since people are different sizes! As early as 1215, King John of England recognized the need for standardization in the Magna Carta, "There shall be standard measures of wine, corn, and ale throughout the kingdom."
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Even after standardization, the English system of units is a mess. It consists of a welter of different units, many of which are divided by different amounts. There are 8 pints in a gallon, 12 inches in a foot, and 16 ounces in a pound. Many units are accidents of history, but the use of these multiples came about because very few people have ever been numerate. It is easy to divide up objects when they are contained in groups of 8 (= 2 × 2 × 2) or 12 (= 2 × 2 × 3) or 16 (= 2 × 2 × 2 × 2).
The metric system was established by the French Academy of Sciences in 1791. It grew out of the French Revolution and a desire to rationalize many aspects of human affairs. The goal was to create a measurement system based on invariable quantities in nature, rather than on parts of the human body. Thus, the meter was defined to be 1 ten-millionth of the distance from the Earth's equator to the North Pole, the gram was defined to be the mass of a cubic centimeter of water at 4º C, and the second was defined to be 1/86,400 of a solar day (1/60 × 1/60 × 1/24). The metric system makes a lot of sense. There is only one basic unit for each quantity and all subdivisions and multiples are powers of ten. Thomas Jefferson was impressed by the metric system and pushed for its adoption in the United States. Finally, the United States and sixteen other countries signed the Treaty of the Meter in 1879. But as you look around, you will see that we are not metric in everyday life. In this regard, the United States is almost a lone holdout — not even the English use the English system anymore!
Scientists have embraced the metric system completely. Here are the conversion factors between some familiar quantities in the English system and their counterparts in the metric system:
• 1 m = 39.37 in and the reverse 1 in = 0.0254 m = 25.4 mm
• 1 m = 1.094 yd and the reverse 1 yd = 0.914 m = 914 mm
• 1 km = 0.621 mi and the reverse 1 mi = 1.609 km = 8/5 km
• 1 g = 0.0353 oz and the reverse 1 oz = 28.3 g = 0.0283 kg
• 1 kg = 2.205 lb and the reverse 1 lb = 0.454 kg = 454 g
• 1 liter = 1.06 qt and the reverse 1 qt = 0.94 liters = 940 cm3
• 1 liter = 0.264 gal and the reverse 1 gal = 3.79 liters = 3790 cm3
• 1 W = 0.00134 hp and the reverse 1 horsepower = 745.7 W
• 1 J = 0.00024 cal and the reverse 1 calorie = 4186 J
These are just a few of the most common units from the English system. There are many more. You can be glad you do not have to remember the distinctions between cubits and furlongs and pecks and pottles and bushels and jacks and gills! Here are other important quantities that can all be derived from the fundamental units of mass, length, and time:
• Frequency: measured in cycles/s or Hertz (Hz)
• Force: measured in kg m/s2 or Newton (N)
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• Energy: measured in kg m2/s2 or Joules (J)
• Power: measured in J/s or Watts (W)
• Temperature: measured in K or Kelvin (K)
In science, temperature is given on the Kelvin scale, named after the Scottish physicist William Thomson, Lord Kelvin. The Kelvin scale is measured with reference to absolute zero — the temperature at which an object contains no heat and all atomic motions are frozen. Some scientists also use the Celsius scale, named after the Swedish astronomer Anders Celsius. The Celsius scale is also called the centigrade scale, since it is based on the division of the range from the freezing to the boiling points of water into 100 equal degrees. A third temperature scale was named after German physicist Gabriel Fahrenheit, who made the first successful mercury thermometer in 1720. The Fahrenheit scale is not used by scientists, and it is only used in the United States.
The information below gives some important temperature markers and tells you how to convert from one temperature scale to another.
• Absolute zero: 0 K = -273 °C = -459 °F
• Water freezes: 273 K = 0 °C = 32 °F
• Water boils: 373 K = 100°C = 212 °F
• Surface of the Sun: 5700 K = 5427 °C = 9797 °F
• Any level: °C + 273 = 5/9(°F-32) = (9/5)°C+32
Despite the variety of measurements we can make in the natural world, there are three fundamental properties that cannot be described in any simpler terms. These fundamental properties are mass, length, and time. Every quantity you see in basic astronomy can be expressed as some combination of these three. For example, velocity is distance (or length) divided by time, and density is mass divided by volume (or length cubed).
The metric system has been refined over time. The modern version of the metric system was agreed on in 1960; it is called the International System of Units (SI). We now have fundamental and extremely precise definitions of the units of mass, length, and time. The standard unit for mass is the kilogram (kg). The "true" kilogram is a metal cylinder kept at the International Bureau of Weights and Measures in Paris. The standard unit of length is the meter (m). The meter is defined to be the distance that light travels in 1/299,792,458 seconds; this definition is precise because we have measured the speed of light very accurately. The standard unit of time is the second (s). One second is defined as the length of time required for 9,192,631,770 vibrations of the cesium-133 atom. These exact definitions are not important — scales and rulers and clocks work well enough in everyday life. But it is important to know that metric units have precise definitions.