Chapter 1: A Modern View of the Universe
Our Cosmic Address
Solar system: consists of the Sun, the planets and their moons, and countless smaller objects that
include rocky asteroids and icy comets
●Located a little over halfway from the galactic center to the edge of the galactic disk
Milky Way Galaxy: a huge, disk-shaped collection of stars that we belong to
●Relatively large galaxy, with over 100 billion stars that are likely orbited by planets
Galaxy: great island of stars in space, all held together by gravity and orbiting a common center
Local Group: a group of more than 70 galaxies that we are a part of
●Milky Way is one of the two largest galaxies in the Local Group
Galaxy clusters: groups of galaxies with many more large members
●Appear to be arranged in giant chains and sheets with huge voids between them
Superclusters: regions in which galaxies and galaxy clusters are most tightly packed
●Also seen as clusters of galaxy clusters
Universe: a total of all matter and energy, encompassing the superclusters and voids and
everything within them
Astronomical Distance Measurements
Astronomical unit (AU): Earth’s average distance from the Sun
●1 AU is about 6 trillion miles or 150 million kilometers
Light-year (ly): the distance that light can travel in one year
●1 ly is about 6 trillion miles or 10 trillion kilometers
●Light-year is a measure of distance, not time
Basic Astronomical Definitions
Astronomical Objects
Star: large, glowing ball of gas that generates heat and light through nuclear fusion in its core
Moon: 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
Planet: a moderately large object that orbits a star and shines primarily by reflecting light from
its star
Criteria for Being a Planet
1. Orbits a star
2. Is large enough for its gravity to make it round
3. Has cleared most objects from its orbital path
Dwarf planet: an object that meets the first two criteria, but hasn’t cleared everything in its path
Collections of Astronomical Objects
Star system: a star and any planets and other materials that orbit it
Observable Universe: the portion of the entire universe that can be seen from Earth, at least in
principle
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 the average distance between galaxies as time
progresses
The Scale of a Solar System
●Voyage scale model
○Shows sizes and distances in the solar system at a one-tenth billionth of their
actual values
○Moon about 1.5 inches away from Earth
■Trip to Mars more than 150 times as far
The Big Bang, Expansion, and the Age of the Universe
●Looking farther into space means looking further back in time
Big Bang: explosion that occurred about 14 billion years ago that created everything in space
today
●Expansion still happens but at a much slower rate
Stellar Lives and Galactic Recycling
Nuclear fusion: process in which lightweight atomic nuclei smash together and stick (or fuse) to
make heavier nuclei
●The early universe had mainly hydrogen and helium (and a trace of lithium)
●Stars converted about 2% of hydrogen and helium into heavier elements
Rotation and Orbit
●Earth rotates around its axis once a day
●Earth orbits the Sun once a year
○About 1 AU or 150 million kilometers
Ecliptic plane: flat plane of Earth’s orbital path
Axis tilt: the tilt of 23.5 degrees from a line perpendicular to the ecliptic plane, points almost
directly at a star called Polaris (North Star)
●Both move counterclockwise
Motion Within the Milky Way Galaxy
●The solar system moves relative to nearby stars in our local solar neighborhood (at
random) and the rotation of the Milky Way Galaxy
Dark matter: matter that is located outside the visible disk and is completely invisible to our
telescopes
●Significantly outweighs regular matter, and makes up most galaxies
How do galaxies move within the universe?
●Virtually every galaxy outside the Local Group is moving away from us
●The more distant the galaxy, the faster it appears to be racing away
Chapter 2: Discovering the Universe for Yourself
What does the universe look like from Earth?
●More than 2,000 stars are visible to your naked eye
○Includes Milky Way (whitish band of light)
●Patterns of stars have not changed noticeably in the past few thousand years
Constellations
Constellation: region of the sky with well-defined borders
Named by IAU in 1948
88 official constellations
Can appear very close to each other but in reality very far
Celestial Sphere
Celestial sphere: imaginary sphere on which objects in the sky appear to reside when observed
on Earth
North celestial pole: point directly over earth’s north pole
South celestial pole: point directly over earth’s south pole
Celestial equator: a projection of earth’s equator into space, which makes a complete circle
around the celestial sphere
Ecliptic: path the sun follows as it appears to circle the celestial sphere once each year; crosses
the celestial equator at 23.5 degrees because that is the tilt of the Earth’s axis
The Local Sky
Local sky: sky as seen from wherever you happen to be standing
Horizon: the boundary that divides what we can see from what we cannot see
Zenith: point directly overhead, which has an altitude of 90 degrees (straight above)
Meridian: half-circle extending from your horizon (altitude 0 degrees) due south, through your
zenith, to your horizon due north
Direction: one of the coordinates needed to pinpoint an object in the local sky (north, south, east,
west)
Altitude: angular distance between horizon and object in the sky
Angular Sizes and Distances
Angular size: the measure of the angle formed by extending imaginary lines outward from our
eyes to span an object or space between two objects
●Also known as the angular distance
Arcminute: 1/60 of a degree
Arcsecond: 1/60 of an arcminute, or 1/3600 of a degree
Why do stars rise and set?
Circumpolar star: a star that always remains above the horizon for a particular latitude (near the
north celestial pole)
●Stars near the south celestial pole never rise above the horizon at all
●All other stars have daily circles, rise in the east, and set in the west
Why do the constellations we see depend on latitude and time of year?
Latitude: angular north-south distance between Earth’s equator and a location on Earth’s surface
●0 degrees at the equator, 90 degrees at the pole
Longitude: angular east-west distance between the prime meridian and a location on Earth’s
surface
●0 degrees at the prime meridian (meridian of longitude that passes through Greenwich,
England, longitude is 0 degrees)
●The altitude of the celestial pole in your sky is equal to your latitude
Variation with Time of Year
●Night sky changes because of Earth’s changing position in orbit around the sun
Zodiac: constellations on the celestial sphere through which the ecliptic passes
What causes the seasons?
●The tilt of Earth’s axis causes sunlight to fall differently on Earth at different times of the
year
Solstices and Equinoxes
June solstice: the point on the celestial sphere where the ecliptic is farthest north of the celestial
equator and the moment in time when the sun appears at that point each year
December solstice: the point on the celestial sphere where the ecliptic is farthest south of the
celestial equator and the moment in time when the Sun appears at that point each year
March equinox: the point in Pisces on the celestial sphere where the ecliptic crosses the celestial
equator and the moment in time when the Sun appears at that point each year
September equinox: the point in Virgo on the celestial sphere where the ecliptic crosses the
celestial equator and the moment in time when the sun appears at that point each year
●Southern hemisphere seasons are milder because oceans moderate the climate
●Sun is never directly overhead
How does the orientation of Earth’s axis change with time?
Precession: gradual wobble of the axis of a rotating object around a vertical line
●Doesn’t change the amount of tilt, therefore doesn’t affect the pattern of seasons
●Caused by gravity
Why do we see phases of the Moon?
Understanding Phases
●Sunlight comes at Earth and Moon at the same time
●Moon’s phases are caused by the fact that we see different portions of its day and night
sides at different times as it orbits around Earth
●Phases from new to full are “waxing”, while those going from full to new are “waning”
●Both sides of the moon get sunlight
The Moon’s Synchronous Rotation
Synchronous rotation: rotation of an object that always shows the same face as an object that it is
orbiting because its rotation period and orbital periods are equal
●A consequence of Earth’s gravity
What causes eclipses?
Lunar eclipse: when the Moon passes through Earth’s shadow, only happens at full moon
●Total, partial or penumbral
Solar eclipse: when the moon’s shadow falls on Earth, only on the new moon
●Total, partial or annular
Shadow Regions
Full shadow: sunlight is fully blocked
Partial shadow: light from only part of the sun is blocked
Lunar Eclipses
Totality: portion of a total lunar eclipse during which the moon is fully within earth’s umbral
shadow or total solar eclipse during which the Sun’s disk is fully blocked by the Moon
Conditions for Eclipses
Nodes: two points in the Moon’s orbit where it crosses the ecliptic plane
●The moon must be near these to undergo an eclipse
Eclipse seasons: periods during which lunar and solar eclipses can occur because the nodes of
the Moon’s orbit are aligned with Earth and the Sun
Predicting Eclipses
●Hard to predict eclipses
Saros cycle: period over which the basic pattern of eclipses repeats, which is about 18 years 11 ⅓
days
Why was planetary motion so hard to explain?
Apparent retrograde motion: apparent motion of a planet during weeks or months when it moves
westward relative to the stars in our sky
Why did the ancient Greeks reject the real explanation for planetary motion?
Stellar parallax: an apparent shift in the position of a nearby star (relative to distant objects) that
occurs as we view the star from different positions in Earth’s orbit of the Sun each year
Chapter 3: The Science of Astronomy
In what ways do all humans use scientific thinking?
●Scientific thinking comes naturally to us
●Science is a way of learning about nature through careful observation and trial-and-error
experiments
How is modern science rooted in ancient astronomy?
Practical Benefits of Astronomy
●Had practical benefits for timekeeping, keeping track of seasonal changes, and navigation
○Moon-impacting seasons and rainfall
Anatomy and Measures of Time
●The length of the day is the time it takes the sun to make one full circuit of the sky
●The length of the month is based on the moon cycle
●The length of a year is based on the cycle of the seasons
●Seven days of the week were named after 7 “planets”
○Earth is not included because it cannot be seen since we are on it
Determining the Time of Day
●Sundials and the sun’s path through the sky
●Position and phase of moon, constellations, and star clocks
○Eventually changed to water clouds
●Egyptians divided the day into 12 equal parts for day and night
○AM is ante meridiem and PM is post meridiem
Marking the Seasons
●Many cultures aligned buildings and streets with cardinal directions
●Other structures marked things like solstices and equinoxes
Solar and Lunar Calendars
●Most places use solar calendars
●Some prefer the lunar calendar
○Muslims
Why does modern science trace its roots to the Greeks?
Three Philosophical Innovations
●Developed a tradition of trying to understand nature without relying on supernatural
explanations and of working communally to debate and challenge each other’s ideas
●Used mathematics to give precision to their ideas
●Saw the power of reasoning from observations
Model of Nature
Model: conceptual representation created to explain and predict observed phenomena
How did the Greeks explain planetary motion?
Geocentric model: a model that placed a spherical Earth at the center of the universe
Ptolemy’s Synthesis
Ptolemaic model: Ptolemy’s geocentric model differs from those of Eudoxus and Aristotle
●Planets moved around Earth on a small circle that turned up into a larger circle (like a
loop)
How did Copernicus, Tycho, and Kepler challenge the Earth-centered model?
Copernican revolution: a shift in the field of astronomy from a geocentric understanding of the
universe, centered around Earth, to a heliocentric understanding, centered around the Sun
Copernicus
●Believed in a sun-centered solar system, but thought planets orbited like in the Ptolemaic
model
●Ultimately rejected due to the complexity
Tycho
●Famous for naked-eye observations
●Believed that Sun orbits Earth while all other planets orbit Sun
Kepler
Ellipse: a special type of oval shape that Earth orbits the Sun in
Foci: center of the ellipse
Eccentricity: how much an ellipse is stretched out compared to a perfect circle
What are Kepler’s three laws of planetary motion?
●All three create Kepler’s laws of planetary motion
Kepler’s first law: the orbit of each planet about the Sun is an elliptical orbit with the Sun at one
focus
Perihelion: closest point to the Sun
Aphelion: farthest point from the Sun
Semimajor axis: average distance of a planet’s perihelion and aphelion
Kepler’s second law: a planet moves faster in the part of its orbit nearer the Sun and slower when
farther from the Sun, sweeping out equal areas in equal times
Kepler’s third law: more distant planets orbit the Sun at slower average speeds obeying the
precise mathematical relationship: p2=a3
●P is the orbital period and a is the average distance from the Sun
How did Galileo solidify the Copernican revolution?
●3 objections
1. Aristotle said Earth could not be moving because things would be left behind as Earth
moved along
2. The idea of noncircular orbits contradicted Aristotle’s idea that the heavens must be
perfect and unchanging
3. No one had detected stellar parallax that should be occurring if Earth orbits the Sun
Galileo’s Evidence
●A moving object remains in motion unless a force acts to stop it
●Observed sunspots and mountains on Moon with a telescope
●Saw that Milky Way had countless stars
Sealing the Case
●Four moons orbit Jupiter
●Venus goes through phases that only make sense if it orbits Sun, not Earth
●Kepler’s laws predicted a transit of mercury better than any other model
How can we distinguish science from nonscience?
Approaches to Science
Hypothesis: a supposition or proposed explanation made based on limited evidence as a starting
point for further investigation
Scientific method: a method of procedure that has characterized natural science since the 17th
century, consisting of systematic observation, measurement, and experiment, and the
formulation, testing, and modification of hypotheses
Hallmarks of Science
1. The modern science seeks explanations for observed phenomena that rely solely on
natural causes
2. Science progresses through the creation and testing of models of nature that explain the
observations as simply as possible
3. A 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
●The idea that scientists should prefer the simpler two models that agree equally well with
observations
Verifiable Observations
●Scientific data must be able to be replicated by anyone to be valid
●Eyewitness accounts by themselves are insufficient
Science and Pseudoscience
Pseudoscience: false science
Objectivity in Science
Paradigm: general patterns of thought
●Science ultimately provides a means of bringing people to an agreement
What is a scientific theory?
Theory: a powerful yet simple model that makes predictions that survive repeated and varied
testing
Chapter 4: Making Sense of the Universe: Understanding Motion, Energy, and Gravity
How do we describe motion?
Speed, Velocity, and Acceleration
Speed: how far an object will go in a certain amount of time
Velocity: speed and direction
Acceleration: change of velocity in speed or direction or both
The Acceleration of Gravity
Acceleration of Gravity
●10 m/s²
Momentum and Force
Momentum: product of an object’s mass and velocity
●momentum= m*v
Force: an influence tending to change the motion of a body or produce motion or stress in a
stationary body
Net force: combined effect of all the individual forces put together
Moving in Circles
Angular momentum: the quantity of rotation of a body, which is the product of its moment of
inertia and its angular velocity
Torque: a force that can change an object’s angular momentum
How is mass different from weight?
Mass: the amount of matter in an object
Weight: force that a scale measures when an object is placed on it
●Mass and forces combined
Free-fall and Weightlessness
Free-fall: falling without any resistance to slow an object down
Weightlessness: the state of apparently not being acted on by gravity
Weightlessness in Space
●A constant state of free-fall when in space
What are Newton’s three laws of motion?
Newton’s First Law
Newton’s first law of motion: an object moves at constant velocity if there is no net force acting
upon it
Newton’s Second Law
Newton’s second law of motion: force equals mass times acceleration
●F=m*a
Newton’s Third Law
Newton’s third law of motion: for any force, there is an equal and opposite reaction force
Why do objects move at constant velocity if no force acts on them?
Conservation of momentum: as long as there are no external forces, the total momentum of
interacting objects can’t change
What keeps a planet rotating and orbiting the Sun?
Conservation of angular momentum: as long as there is no external torque, the total angular
momentum of a set of interacting objects can’t change
Orbital Angular Momentum
●Angular momentum=m*v*r
●Two key facts:
1. Earth needs no fuel or push of any kind to keep orbiting the Sun
2. Earth’s orbital speed must be faster when it is nearer the Sun and slower when it is farther
from the Sun
Rotational Angular Momentum
●When the radius increases, the velocity of rotation decreases (and vice versa)
Where do objects get their energy?
Conservation of energy: energy cannot appear out of anywhere or disappear into nothingness
Basic Types of Energy
Kinetic energy: energy of motion
●½ mv²
Radiative energy: energy carried by light
Potential energy: stored energy
●Calories most common unit of energy
Joule: standard unit of energy
Thermal Energy- The Kinetic Energy of Many Particles
Thermal energy: collective kinetic energy of the many individual particles moving randomly
within a substance
Temperature: average kinetic energy of the particles
Kelvin: the SI base unit of thermodynamic temperature (equivalent in size to the degree Celsius),
first introduced as the unit used in the Kelvin scale
Potential Energy in Astronomy
Gravitational potential energy: energy an object possesses because of its position in a
gravitational field
●Mgh is the equation used to find it
Mass-energy: mass and energy are regarded as interconvertible manifestations of the same
phenomenon, according to the laws of relativity
●E=mc²
What determines the strength of gravity?
Universal Law of Gravitation
1. Every mass attracts every other mass through the force called gravity
2. The strength of the gravitational force attracting any 2 objects is directly proportional to
the product of their masses
3. The strength of gravity between two objects decreases with the square of the distance
between their centers
Inverse square law: a law stating that the intensity of an effect such as illumination or
gravitational force changes in inverse proportion to the square of the distance from the source
●Fg=Gm1m2/d²
Gravitational constant
●G= 6.67*10^-11 m3
How does Newton’s law of gravity extend Kepler’s laws?
Planets Are Not the Only Objects with Elliptical Orbits
●All objects orbit in an ellipse shape
Ellipses Are Not the Only Possible Orbital Paths
Bound orbits: orbits in which an object goes around another object over and over again
●Ellipses are the only possible shape
Unbound orbits: paths that bring an object closer to another object just once
●Parabola or hyperbola
Objects Orbit Their Common Center of Mass
Center of mass: the point at which the two objects would balance if they were somehow
connected
Orbital Characteristics Tell Us the Masses of Distant Objects
Newton’s version of Kepler’s third law: p²= 4π²/G(m1+m2) *a³
How do gravity and energy allow us to understand orbits?
Orbital Energy
Orbital energy: the sum of its kinetic and gravitational potential energies
Gravitational Encounters
Gravitational encounter: an encounter in which two (or more) objects pass near enough so that
each can feel the effects of the other’s gravity and they can therefore exchange energy
Escape Velocity
Escape velocity: the speed necessary for an object to completely escape the gravity of a large
body such as a moon, planet, or star
How does gravity cause tides?
The Moon’s Tidal Force
Tidal force: a force that occurs when the gravity pulling on one side of an object is larger than
that on the other side, causing the object to stretch
Tidal Friction
Tidal friction: friction within an object that is caused by a tidal force
The Moon’s Synchronous Rotation
Synchronous rotation: the rotation of an object that always shows the same face as an object that
it is orbiting because its rotation period and orbital period are equal
Chapter 5: Light and Matter: Reading Messages from the Cosmos
Spectroscopy: the process of obtaining a spectrum and reading the information it contains
Intensity: the amount of light at each wavelength
How do we experience light?
●Carries radiative energy
Energy and Power
Power: rate of energy flow
Watts: units that measure power
●1 watt is equivalent to 1 joule
Light and Color
Spectrum: a band of colors, as seen in a rainbow, produced by the separation of the components
of light by their different degrees of refraction according to wavelength
●White light occurs when all of the colors are equally present
●Blacklight means no light/color is present
●The primary colors of vision are red, green, and blue
Diffraction grating: a piece of plastic or glass etched with many closely spaced lines
How do light and matter interact?
●Transparent means that something transmits light
●Opaque means that something absorbs light
Four Basic Ways
●Emission
●Absorption
●Transmission
●Reflection/scattering
What is light?
Particles and Waves in Everyday Life
●A particle is a thing that can travel alone
●Wave is a pattern revealed by its interaction with particles
Wavelength: distance from one peak to another
●Wavelength= frequency times speed
Frequency: number of peaks passing by any point each second (measured in cycles per second
also known as Hertz (Hz))
Speed: how fast energy travels from one place to another
Light as an Electromagnetic Wave
Field: the strength of the force that a particle would experience at any point in space
Electromagnetic wave: one of the waves that are propagated by simultaneous periodic variations
of electric and magnetic field intensity and that include radio waves, infrared, visible light,
ultraviolet, X-rays, and gamma rays
Speed of light: 300,000 kilometers per second
●An increase in wavelength means a decrease in frequency
○Also works vice versa
Photons: “Particles” of Light
Photons: individual pieces that have properties of both particles and waves
What is the electromagnetic spectrum?
Electromagnetic spectrum: complete spectrum of light
Electromagnetic radiation: light itself
Visible light: light our eyes can see, 400-700 nanometers
Infrared: a type of radiant energy that's invisible to human eyes but that we can feel as heat
Radio waves: a type of electromagnetic radiation best known for their use in communication
technologies, such as television, mobile phones, and radios; these devices receive radio waves
and convert them to mechanical vibrations in the speaker to create sound waves
Microwaves: a type of electromagnetic radiation; have a range of applications, including
communications, radar, and, perhaps best known by most people, cooking
Ultraviolet: a type of electromagnetic radiation that makes black-light posters glow, and is
responsible for summer tans — and sunburns
X-rays: types of electromagnetic radiation probably most well-known for their ability to see
through a person's skin and reveal images of the bones beneath it
Gamma rays: a form of electromagnetic radiation, as are radio waves, infrared radiation,
ultraviolet radiation, X-rays, and microwaves; can be used to treat cancer, and gamma-ray bursts
are studied by astronomers
What is the structure for the matter?
Atom: the smallest component of an element having the chemical properties of the element,
consisting of a nucleus containing combinations of neutrons and protons and one or more
electrons bound to the nucleus by electrical attraction; the number of protons determines the
identity of the element
Element: each of more than one hundred substances that cannot be chemically interconverted or
broken down into simpler substances and are primary constituents of matter. Each element is
distinguished by its atomic number, i.e. the number of protons in the nuclei of its atoms
Atomic Structure
Proton: a stable subatomic particle occurring in all atomic nuclei, with a positive electric charge
equal in magnitude to that of an electron, but of the opposite sign
Neutron: a subatomic particle of about the same mass as a proton but without an electric charge,
present in all atomic nuclei except those of ordinary hydrogen
Electron: a stable subatomic particle with a charge of negative electricity, found in all atoms and
acting as the primary carrier of electricity in solids
Nucleus: the positively charged central core of an atom, consisting of protons and neutrons and
containing nearly all its mass
Electrical charge: how strongly an object will interact with electromagnetic fields
Atomic Terminology
Atomic number: number of protons in the nucleus
Atomic mass number: the combined number of protons and neutrons in the nucleus
Isotopes: versions of an element with different numbers of neutrons
Molecules
Molecule: a group of atoms bonded together, representing the smallest fundamental unit of a
chemical compound that can take part in a chemical reaction
What are the phases of matter?
Phase: physical state of matter
Solid: firm and stable in shape; not liquid or fluid
Liquid: a substance that flows freely but is of constant volume, having a consistency like that of
water or oil
Gas: a substance or matter in a state in which it will expand freely to fill the whole of a container,
having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid)
Chemical bond: interactions between electrons that hold the atoms in a molecule together
Phase Changes in Water
Vaporization: the process by which molecules break free
●Sublimation is from a solid
●Evaporation is from a liquid
Molecular Dissociation and Ionization
Molecular dissociation: the process by which molecules split into pieces
Ions: charged atoms
●Can be positively or negatively charged
Ionization: the process of stripping electrons from atoms
Plasma: hot gas in which atoms have become ionized
Phases and Pressure
Pressure: force per unit area pushing on an object’s surface
How is energy stored in atoms?
Energy Level in Atoms
Energy levels: possible energies of an atom
Energy Level Transitions
Energy level transition: when electrons rise and lower their energy levels
What are the basic types of spectra?
Three Basic Types
Continuous spectrum: a broad range of wavelengths without any interruption
Emission lines: specific wavelengths that depend on composition and temperature
Emission line spectrum: the range of wavelengths emitted by an atom or compound stimulated
by either heat or electric current
Absorption lines: a spectral line that corresponds to the absorption of electromagnetic radiation
at a specific wavelength
Absorption line spectrum: the characteristic pattern of dark lines or bands that occurs when
electromagnetic radiation is passed through an absorbing medium into a spectroscope
How does light tell us what things are made of?
Emission Line Spectra
●An increase in temperature means a higher chance of moving energy levels
○Also works vice versa
●Collisions of atoms always occurring
Chemical Fingerprints
Molecular bands: tightly bunched lines in an object’s spectrum that are produced by molecules
How does light tell us the temperature of planets and stars?
Thermal Radiation: Every Body Does It
●Large and dense objects tend to absorb a broad range of wavelengths, meaning light can’t
pass through and light emitted can’t escape
●The spectrum of an object only depends on the temperature
Thermal radiation: spectrum of radiation produced by an opaque object that depends only on the
object’s temperature
Thermal radiation spectrum: ranges in wavelength from the longest infrared rays through the
visible-light spectrum to the shortest ultraviolet rays
Two Laws of Thermal Radiation
1. Each square meter of a hotter object’s surface emits more light at all wavelengths
2. Hotter objects emit photons with a higher average energy
How does light tell us the speed of a distant object?
Doppler effect: an effect that shifts the wavelengths of spectral features in objects that are
moving toward or away from the observer
The Doppler Effect
Blueshift: doppler shift in which spectral features are shifted to shorter wavelengths, observed
when an object is moving toward the observer
Redshift: doppler shift in which spectral features are shifted to longer wavelengths, observed
when an object is moving away from the observer
●Used when talking about nonvisible light
Rest wavelength: wavelength of a spectral feature in the absence of any Doppler shift or
gravitational redshift
Components of Motion
●Doppler shifts give us information about the direction of motion, but not the speed of an
object moving on a line
Rotation Rates
●An increase in rotation means a broader range in wavelength the spectral lines become
Chapter 6: Telescopes: Portals of Discovery
How do eyes and cameras work?
Image Formation
Focus: the point at which rays of light that were initially parallel (such as those from a distant
star) converge
Focal plane: the place where the image appears in focus
Recording Images
Detector: part of a camera that makes a permanent record of the image
Exposure time: the amount of time during which light collects on the detector
Pixels: grids of picture elements that electronic chips are divided into
What are the two most important properties of a telescope?
Light-collecting area: how much total light it can collect at one time
●Measured by diameter
Angular resolution: the smallest angle over which we can tell that two objects are distinct
●Measured in arcminutes
Diffraction limit: angular resolution a telescope could achieve if it were limited only by the
interference of light waves
●Depends on the diameter of the primary mirror and the wavelength of light being
observed
What are the two basic designs of telescopes?
Refracting telescope: a telescope that uses a converging lens to collect the light
Reflecting telescope: a telescope in which a mirror is used to collect and focus light
●Most common today
What do astronomers do with telescopes?
●Imaging
Spectroscopy
Spectrograph: an instrument for dispersing radiation (such as electromagnetic radiation or sound
waves) into a spectrum and recording or mapping the spectrum
Spectral resolution: the ability of a sensor to define fine wavelength intervals
Time Monitoring
Light curves: graphs that show how an object’s intensity varies with time
How does Earth’s atmosphere affect ground-based observations?
Light Pollution
Light pollution: scattering of human-made light
Atmospheric Blurring
●Turbulence is when the atmosphere bends light in continually shifting patterns
Adaptive optics: a technology used to improve the performance of optical systems by reducing
the effect of incoming wavefront distortions by deforming a mirror to compensate for the
distortion
How do we observe invisible light?
●Radio telescopes
●Infrared telescopes
●Ultraviolet telescopes
X-ray Telescopes
Grazing incidence mirrors: mirrors that deflect x-rays by angling themselves a certain way
●Gamma-ray telescopes
Looking Beyond Light
●Neutrino
●Cosmic rays
●Gravitational waves
How can multiple telescopes work together?
Interferometry: linking 2 or more telescopes together to improve the angular resolution
Chapter 7: Our Planetary System
Comparative planetology: a branch of space science and planetary science in which different
natural processes and systems are studied by their effects and phenomena on and between
multiple bodies
What can we learn by comparing the planets to one another?
●Reveals similarities and differences among planets
●Insights into physical processes that allow us to better manage our Earth
●Apply lessons from our solar system to the study of other planetary systems
What features of our solar system provide clues to how it formed?
1. Patterns of motion among large bodies
2. Two major types of planets
3. Asteroids and comets
4. An exception to the rule
Feature 1: Patterns of Motion Among Large Bodies
●All planetary orbits are nearly circular and lie nearly in the same plane
●All planets orbit the Sun in the same direction: counterclockwise as viewed from high
above Earth’s the North Pole
●Most planets rotate in the same direction in which they orbit, with fairly small axis tilts
○Sun also rotates in this direction
●Most of the solar system’s large moons exhibit similar properties in their orbits around
their planets, such as orbiting in their planet’s equatorial plane in the same direction as
the planet rotates
Feature 2: Two Types of Planets
Terrestrial Planets
●Mercury, Venus, Earth, and Mars
●Relatively small and dense
●Rocky surface and an abundance of metals in their cores
●Few, if any, moons and no rings
Jovian Planets
●Jupiter, Saturn, Uranus, and Neptune
●Much larger and lower in average density
●Rings and many moons
●Lack solid surfaces and are made of hydrogen compounds (compounds containing
hydrogens, such as water, ammonia, and methane)
●“Gas giants”
Feature 3: Asteroids and Comets
Asteroids: rocky bodies that orbit the Sun much like planets, but are much smaller
Asteroid belt: the region in space between the orbits of Mars and Jupiter, containing the greatest
population of asteroids in our solar system: comprising about half the mass of the asteroid belt
are its four largest asteroids, namely, Ceres, Pallas, Vesta, and Hygieia
Comet: small objects made of ice mixed with rock that orbit the Sun
Kuiper Belt: a region of the solar system beyond the orbit of Neptune, believed to contain many
comets, asteroids, and other small bodies made largely of ice
Oort cloud: a spherical shell of cometary bodies believed to surround the sun far beyond the
orbits of the outermost planets and from which some are dislodged when perturbed to fall toward
the sun
How do robotic spacecraft work?
Four Major Categories
●Flyby: a spacecraft goes past a world just once and then continues on its way
●Orbiter: orbits the world for long-term study
●Lander/probe: lands on a planet’s surface/probes a planet’s atmosphere by flying through
it
●Sample return mission: makes a round trip to return a sample of the world it has studied
to Earth
Chapter 8: Formation of the Solar System
How did we arrive at a theory of solar system formation?
Criteria for a Successful Solar System Formation Theory
1. It must explain the patterns of motion
2. It must explain why planets fall into two major categories
a. Small, rocky terrestrial planets near the Sun
b. Hydrogen-rich jovian planets further out
3. It must explain the existence of huge numbers of asteroids and comets and why these
objects reside primarily in the regions we call the asteroid belt, the Kuiper Belt, and the
Oort Cloud
4. It must explain the general patterns while at the same time making allowances for
exceptions to the general rule
From Hypothesis to Theory
Nebular theory: a scientific theory that describes how our solar system formed from a cloud of
interstellar gas and dust
Where did the solar system come from?
Solar nebula: a piece of interstellar cloud from which our solar system formed
●Galactic recycling
●98% hydrogen and helium, 2% other elements
What caused the orderly patterns of motion?
Heating, Spinning, and Flattening
●Heating: conversion of energy from potential to kinetic to thermal
●Spinning: smaller radius leads to faster rotation
●Flattening: collisions that lead to merging
Why are there two major types of planets?
Condensation: Sowing the Seeds of Planets
Condensation: formation of solid/liquid particles from a cloud of gas
Four Major Categories
●Hydrogen and helium gas (98%)
●Hydrogen compounds (1.4%)
○Ice: material that is solid only at low temperatures
■Water, ammonia, and methane
●Rock (0.4%)
●Metal (0.2%)
Frost line: boundary in the solar nebula beyond which ices could condense; only metals and
rocks could condense within the frost line
●Between the present-day orbits of Mars and Jupiter
●Terrestrial and jovian planet boundary
Building the Terrestrial Planets
Accretion: the process by which small objects gather together to make larger objects
Planetesimals: building blocks of planets, formed by accretion in the solar nebula
Meteorites: a rock that falls to Earth from space
Clearing the Nebula
Solar wind: stream of charged particles ejected from the Sun
Where did asteroids and comets come from?
Impact craters: bowl-shaped depressions left by the impact of an object that strikes a planetary
surface
Heavy bombardment: the period in the first few hundred million years after the solar system
formed during which the tail end of planetary accretion created most of the craters found on
ancient planetary surfaces
How do we explain “exceptions to the rules?”
Giant Impacts
Giant impact: collision between a forming planet and a very large planetesimal
How do we measure the age of a rock?
Radiometric dating: the process of determining the age of a rock by comparing the present
amount of a radioactive substance to the amount of its decay product
Radioactive: substance whose nucleus tends to fall apart spontaneously
Half-life: the time it takes for half of the nuclei in a given quantity of a radioactive substance to
decay
Chapter 9: Planetary Geology: Earth and the Other Terrestrial Worlds
Planetary geology: the extension of the study of Earth’s surface and interior to apply to other
solid bodies in the solar system, such as terrestrial planets and jovian planet moons
What are terrestrial planets like on the inside?
Seismic waves: vibrations that travel both through the interior and along the surface after an
earthquake
Layering by Density
●Core: highest density material, primarily of nickel and iron, resides in a central core
(inner and outer)
●Mantle: rocky material of moderate density, that forms a thick mantle that surrounds the
core
●Crust: lowest-density rock, the world’s outer skin
Differentiation: the process by which gravity separates materials according to their density, with
high-density materials sinking and low-density materials rising
Layering by Rock Strength
Lithosphere: relatively rigid outer layer of a planet
●Crust and upper mantle
What causes geologic activity?
Geological activity: processes that change a planet’s surface long after the formation
●Driven by internal heat and pressure
How Interiors Get Hot
●The heat of accretion (energy from afar)
●The heat from differentiation (sinking/rising of material)
●The heat from radioactive decay
How Interiors Cool Off
Convection: energy transport process in which warm material expands and rises while cooler
material contracts and falls
Conduction: the process by which thermal energy is transferred by direct contact from warm
material to the cooler material
●Radiation
Convection cell: an individual small region of convecting material
Planetary Size Controls Geologic Activity
●Smaller planets cool more quickly and vice versa
Why do some planetary interiors create magnetic fields?
Magnetic field: region surrounding a magnet in which it can affect other magnets/charged
particles
3 Requirements
1. An interior region of electrically conducting fluid
2. Convection in that layer of fluid
3. At least moderately rapid rotation
●Only Earth meets all 3 requirements
What processes shape planetary surfaces?
Impact catering: bowl-shaped depression left by the impact of an object that strikes a planetary
surface
Volcanism: the eruption of molten rock, or lava, from a planet’s interior onto its surface
Tectonics: disruption of a planet’s surface by internal stresses
Erosion: wearing down/building up of geological features by wind, water, ice, and other
phenomena of planetary weather
Volcanism
Outgassing: the process of releasing gasses from a planetary interior, usually through volcanic
pattern
Tectonics
Plates: pieces of lithosphere that float upon the denser mantle below
Plate tectonics: the geological process in which plates are moved around by stress in a planet’s
mantle
Erosion
Sedimentary rock: rock that formed from sediments created and deposited by erosional processes
How do impact craters reveal a surface’s geological age?
The Discovery of Continental Motion
Seafloor spreading: the creation of new seafloor crust at mid-ocean ridges
Seafloor Crust and Continental Crust
Seafloor crust: a thick, dense crust of basalt created by seafloor spreading
Continental crust: a thicker lower-density crust that makes up Earth’s continents
The Conveyor Belt of Plate Tectonics
Subduction: process in which one plate slides under another
Rifts, Faults, and Earthquakes
Fault: a place where lithospheric plates slip sideways relative to one another
Hot Spots
Hot spot: a place within a plate of the lithosphere where a localized plume of hot mantle material
rises
Chapter 10: Planetary Atmospheres: Earth and Other Terrestrial Worlds
Weather: an ever-varying combination of winds, clouds, temperatures, and pressure in a planet’s
troposphere
Climate: long-term average of weather conditions
What is an atmosphere?
Atmospheric Pressure
Atmospheric pressure: surface pressure resulting from the overlying weight of an atmosphere
How Atmospheres Affect Planets
Greenhouse effect: the process by which greenhouse gasses in an atmosphere make a planet’s
surface temperature warmer than it would be in the absence of an atmosphere
How does the greenhouse effect warm a planet?
The Greenhouse Effect
Greenhouse gasses: gasses, such as carbon dioxide, water vapor, and methane, that are
particularly good absorbers of infrared light but are transparent to visible light
Greenhouse Warming of the Terrestrial Worlds
Global average temperature: An estimate of Earth's mean surface air temperature averaged over
the entire planet
Why do atmospheric properties vary with altitude?
Atmospheric structure: the layering of a planetary atmosphere due to variations in temperature
with altitude
Troposphere: lowest atmospheric layer, in which convection and weather occur
Stratosphere: the intermediate-altitude layer of Earth’s atmosphere that is warmed by the
absorption of ultraviolet light from the Sun
Thermosphere: high, hot, X-ray absorbing layer of an atmosphere, just below the exosphere
Exosphere: the hot, outer layer of an atmosphere, where the atmosphere “fades away” to space
Magnetospheres and the Solar Wind
Magnetosphere: region surrounding a planet in which charged particles are trapped by the
planet’s magnetic field
Charged particle belts: zones in which ions and electrons accumulate and encircle a planet
What creates wind and weather?
Global Wind Patterns
Global wind patterns: wind patterns that remain fixed on a global scale, determined by the
combination of surface heating and the planet’s rotation
Atmospheric Heating and Circulation Cells
Circulation cells: large-scale cells in a planet’s atmosphere that transport heat between the
equator and the poles
Rotation and the Coriolis Effect
Coriolis effect: effect due to rotation that causes air/objects on a rotating surface/planet to deviate
from straight-line trajectories
Clouds and Precipitation
Precipitation: condensed atmospheric gasses that fall to the surface in the form of rain, snow, or
hail
What factors can cause long-term climate change?
●Solar brightening
●Changes in axis tilt
●Changes in reflectivity
●Changes in greenhouse gas abundance
How does a planet gain or lose atmospheric gasses?
Sources of Atmospheric Gas
Outgassing: the process of releasing gasses from a planetary interior, usually through volcanic
eruptions
Vaporization: the process by which atoms or molecules escape into the gas phase from the liquid
or solid phase
●Surface ejection
Losses of Atmospheric Gas
●Condensation
●Chemical reactions
●Solar wind stripping
●Thermal escape
How has Mars’s climate differed in the past?
Two Types of Long-Term Climate Change
1. Changes that recur over time due to a changing axis tilt
2. Even longer-term change
Why did Mars change?
Loss of Atmospheric Gas
Carbonate rock: carbon-rich rock that forms underwater from chemical reactions between
sediments and carbon dioxide
How did Venus get so hot?
The Runaway Greenhouse Effect
Feedback processes: processes in which a small change in some property leads to changes in
other properties that either amplify or diminish the original small change
Runaway greenhouse effect: positive feedback cycle in which heating caused by the greenhouse
effect causes more greenhouse gasses to enter the atmosphere, which further enhances the
greenhouse effect
Why does Earth’s climate stay relatively stable?
The Carbon Dioxide Cycle
Carbon dioxide cycle: the process that cycles carbon dioxide between Earth’s atmosphere and
surface rocks
Ice Ages and Other Long-Term Climate Change
Ice age: periods of global cooling during which the polar ice caps, glaciers, and snow cover
extend closer to the equator
Snowball Earth: 600-700 million years ago when Earth experienced a period in which it became
cold enough for glaciers to exist worldwide, even in equatorial regions
How is human activity changing our planet?
Global warming: expected increase in Earth’s global average temperature caused by human input
of carbon dioxide and other greenhouse gasses into the atmosphere
Chapter 11: Jovian Planet Systems
What is the weather like on jovian planets?
Jupiter’s Wind and Storms
Great Red Spot: large, high-pressure storm on Jupiter
Why are Jupiter’s Galilean moons so geologically active?
Io: The Volcano World
Tidal heating: source of heating created by tidal friction
Orbital response: a situation in which one object’s orbital period is a simple ratio of another
object’s period
Chapter 12: Asteroids, Comets, and Dwarf Planets: Their Nature, Orbits, and Impacts
What are meteors and meteorites?
Meteor: a flash of light caused when a particle from space burns up in our atmosphere
Meteorite: a rock from space that lands on Earth
What do meteorites tell us about asteroids and the early solar system?
Types of Meteorites
Primitive meteorites: meteorites that formed at the same time as the solar system itself, about 4.6
billion years ago
Processed meteorites: meteorites that once were part of a larger object that “possessed” the
original material of the solar nebula into another form
Why do comets grow tails?
The Flashy Lives of Comets
Nucleus: solid portion of a comet- the only portion that exists when the comet is far from the Sun
Plasma tail: composed of ionized gas blown away from the Sun by the solar wind
Dust tail: composed of small solid particles pushed away from the Sun by the radiation pressure
of sunlight
Comet Tails and Meteor Showers
Meteor showers: a period during which many more meteors than usual can be seen
Chapter 13: Other Planetary Systems: The New Science of Distant Worlds
Extrasolar planet: a planet orbiting a star other than the Sun
Why is it so challenging to learn about extrasolar planets?
Indirect Approaches to Finding and Studying Extrasolar Planets
1. Observing the motion of a star to detect the subtle gravitational effects of orbiting
changes
2. Observing changes in a star’s brightness that occur when one of its planets passes in front
of the star as viewed from Earth
How can a star’s motion reveal the presence of planets?
The Astrometric Method
Astrometric method: detection of extrasolar planets through the side-to-side motion of a star
caused by gravitational tugs from the planet
The Doppler Method
Doppler method: detection of extrasolar planets through the motion of a star toward and away
from the observer caused by gravitational tugs from the planets
Hot Jupiter: class of planet that is Jupiter-like in size but orbits very close to its star, causing it to
have a very high surface temperature
How can changes in a star’s brightness reveal the presence of planets?
Transit: event in which a planet passes in front of a star (or the Sun) as seen from Earth
The Transit Method
Eclipse: event in which one astronomical object casts a shadow on another or crosses our line of
sight to the other object
Chapter 14: Our Star
Nuclear fission: process in which a larger nucleus splits into two or smaller particles
Nuclear fusion: process in which two (or more) smaller nuclei slam together and make one larger
nucleus
Solar activity: short-lived phenomena on the Sun
Why sunshine?
Gravitational Contraction
Gravitational contraction: process in which gravity causes an object to contract, thereby
converting gravitational potential energy into thermal energy
The Stable Sun
Gravitational equilibrium: state of balance in which the force of gravity pulling inward is
precisely counteracted by pressure pushing outward
Energy balance: balance between the rate at which fusion releases energy in the star’s core and
the rate at which the star’s surface radiates this energy into space
What is the Sun’s structure?
Basic Properties of the Sun
Sunspots: blotches on the surface of the Sun that appear darker than surrounding regions
Power: rate of energy usage
Luminosity: total power output of an object
The Sun’s Atmosphere
Solar wind: stream of charged particles ejected from the Sun
Corona: the tenuous uppermost layer of the Sun’s atmosphere
Chromosphere: the layer of the Sun’s atmosphere below the corona
Photosphere: the visible surface of the Sun, the average temperature is about 6000 K
The Sun’s Interior
Convection zone: region in which energy is transported outward by convection
Radiation zone: region of the interior in which energy is transported primarily by radiative
diffusion
How does nuclear fusion occur in the Sun?
Strong force: one of the four fundamental forces, that holds atomic nuclei together
The Proton-Proton Chain
Proton-proton chain: chain of reactions by which low-mass stars fuse hydrogen into helium
Neutrino: type of fundamental principle that has extremely low mass and responds only to the
weak force
How does the energy from fusion get out of the Sun?
Radiative diffusion: the process by which photons gradually migrate from a hot region to a
cooler region
What causes solar activity?
Sunspots and Magnetic Fields
Magnetic field lines: lines that represent how the needles on a series of compasses would point if
they were laid out in a magnetic field
Solar prominences: vaulted loops of hot gas that rise above the Sun’s surface and follow
magnetic field lines
Solar Storms
Solar flares: huge and sudden releases of energy on the solar surface
The Solar Wind
Coronal holes: regions of the corona that barely show up in x-ray images
Effects of Solar Activity on Earth
Coronal mass ejections: bursts of charged particles from the Sun’s corona that travel outward
into space
How does solar activity vary with time?
The Sunspot Cycle
Sunspot cycle: about 11 years over which the number of sunspots on the Sun rises and falls
