Key-Terms Quiz Answers

• A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano. (Crater)
• A bright celestial object that gives off x-rays as a major portion of its radiation. (X-ray Star)
• A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals. (Planet)
• An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky. (Ecliptic)
• An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography; can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves. (Telescope)
• The amount of light emitted by a star. (Luminosity)
• The angular distance of an object in the sky from the celestial equator. (Declination)
• The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed. (Spectroscopy)
• Electromagnetic radiation that are visible to the human eye. (Visible Light)
• The two points at which the Sun crosses the celestial equator in its yearly path in the sky; occur on or near March 21 and September 22. (Equinox)

Cosmic Samples and the Origin of the Solar System

 

accretion

the gradual accumulation of mass, as by a planet forming from colliding particles in the solar nebula

exoplanet

a planet orbiting a star other than our Sun

iron meteorite

a meteorite composed primarily of iron and nickel

meteor

a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light

meteor shower

many meteors appearing to radiate from one point in the sky; produced when Earth passes through a cometary dust stream

meteorite

a portion of a meteor that survives passage through the atmosphere and strikes the ground

stony meteorite

a meteorite composed mostly of stony material, either primitive or differentiated

stony-iron meteorite

a type of differentiated meteorite that is a blend of nickel-iron and silicate materials

Comets and Asteroids: Debris of the Solar System

 

asteroid

a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets

asteroid belt

the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun

comet

a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail

Kuiper belt

a region of space beyond Neptune that is dynamically stable (like the asteroid belt); the source region for most short-period comets

near-Earth asteroid (NEA)

an Earth-approaching asteroid, one whose orbit could bring it on a collision course with our planet

near-Earth object (NEO)

a comet or asteroid whose path intersects the orbit of Earth

nucleus (of a comet)

the solid chunk of ice and dust in the head of a comet

Oort cloud

the large spherical region around the Sun from which most “new” comets come; a reservoir of objects with aphelia at about 50,000 AU

tail

(of a comet) a tail consisting of two parts: the dust tail is made of dust loosened by the sublimation of ice in a comet that is then pushed by photons from the Sun into a curved stream; the ion tail is a stream of ionized particles evaporated from a comet and then swept away from the Sun by the solar wind

Earthlike Planets: Venus and Mars

 

runaway greenhouse effect

the process by which the greenhouse effect, rather than remaining stable or being lessened through intervention, continues to grow at an increasing rate

tectonic

geological features that result from stresses and pressures in the crust of a planet; tectonic forces can lead to earthquakes and motion of the crust

Earth as a Planet

 

bar

a force of 100,000 Newtons acting on a surface area of 1 square meter; the average pressure of Earth’s atmosphere at sea level is 1.013 bars

basalt

igneous rock produced by the cooling of lava; makes up most of Earth’s oceanic crust and is found on other planets that have experienced extensive volcanic activity

convection

movement caused within a gas or liquid by the tendency of hotter, and therefore less dense material, to rise and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat

core

the central part of the planet; consists of higher density material

crust

the outer layer of a terrestrial planet

fault

in geology, a crack or break in the crust of a planet along which slippage or movement can take place, accompanied by seismic activity

granite

a type of igneous silicate rock that makes up most of Earth’s continental crust

greenhouse effect

the blanketing (absorption) of infrared radiation near the surface of a planet—for example, by CO2 in its atmosphere

greenhouse gas

a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range; on Earth, these atmospheric gases primarily include carbon dioxide, methane, and water vapor

igneous rock

rock produced by cooling from a molten state

magnetosphere

the region around a planet in which its intrinsic magnetic field dominates the interplanetary field carried by the solar wind; hence, the region within which charged particles can be trapped by the planetary magnetic field

mantle

the largest part of Earth’s interior; lies between the crust and the core

mass extinction

the sudden disappearance in the fossil record of a large number of species of life, to be replaced by fossils of new species in subsequent layers; mass extinctions are indicators of catastrophic changes in the environment, such as might be produced by a large impact on Earth

metamorphic rock

rock produced by physical and chemical alteration (without melting) under high temperature and pressure

ozone

(O3) a heavy molecule of oxygen that contains three atoms rather than the more normal two

photosynthesis

a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product

plate tectonics

the motion of segments or plates of the outer layer of a planet over the underlying mantle

primitive rock

rock that has not experienced great heat or pressure and therefore remains representative of the original condensed materials from the solar nebula

rift zone

in geology, a place where the crust is being torn apart by internal forces generally associated with the injection of new material from the mantle and with the slow separation of tectonic plates

sedimentary rock

rock formed by the deposition and cementing of fine grains of material, such as pieces of igneous rock or the shells of living things

seismic wave

a vibration that travels through the interior of Earth or any other object; on Earth, these are generally caused by earthquakes

stratosphere

the layer of Earth’s atmosphere above the troposphere and below the ionosphere

subduction

the sideways and downward movement of the edge of a plate of Earth’s crust into the mantle beneath another plate

troposphere

the lowest level of Earth’s atmosphere, where most weather takes place

volcano

a place where material from a planet’s mantle erupts on its surface

Other Worlds: An Introduction to the Solar System

 

asteroid

a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets

comet

a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail

differentiation

gravitational separation of materials of different density into layers in the interior of a planet or moon

giant planet

any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems

half-life

time required for half of the radioactive atoms in a sample to disintegrate

meteor

a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light

meteorite

a portion of a meteor that survives passage through an atmosphere and strikes the ground

planetesimals

objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today; the comets and some asteroids may be leftover planetesimals

radioactivity

process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays

solar nebula

the cloud of gas and dust from which the solar system formed

terrestrial planet

any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list

Astronomical Instruments

 

adaptive optics

systems used with telescopes that can compensate for distortions in an image introduced by the atmosphere, thus resulting in sharper images

aperture

diameter of the primary lens or mirror of a telescope

charge-coupled device (CCD)

array of high-sensitivity electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens) to record an image or spectrum

chromatic aberration

distortion that causes an image to appear fuzzy when each wavelength coming into a transparent material focuses at a different spot

detector

device sensitive to electromagnetic radiation that makes a record of astronomical observations

eyepiece

magnifying lens used to view the image produced by the objective lens or primary mirror of a telescope

focus

(of telescope) point where the rays of light converged by a mirror or lens meet

interference

process in which waves mix together such that their crests and troughs can alternately reinforce and cancel one another

interferometer

instrument that combines electromagnetic radiation from one or more telescopes to obtain a resolution equivalent to what would be obtained with a single telescope with a diameter equal to the baseline separating the individual separate telescopes

interferometer array

combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers

prime focus

point in a telescope where the objective lens or primary mirror focuses the light

radar

technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object or to form images of it

reflecting telescope

telescope in which the principal light collector is a concave mirror

refracting telescope

telescope in which the principal light collector is a lens or system of lenses

resolution

detail in an image; specifically, the smallest angular (or linear) features that can be distinguished

seeing

unsteadiness of Earth’s atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady

telescope

instrument for collecting visible-light or other electromagnetic radiation

Radiation and Spectra

 

absorption spectrum

a series or pattern of dark lines superimposed on a continuous spectrum

blackbody

an idealized object that absorbs all electromagnetic energy that falls onto it

continuous spectrum

a spectrum of light composed of radiation of a continuous range of wavelengths or colors, rather than only certain discrete wavelengths

dispersion

separation of different wavelengths of white light through refraction of different amounts

Doppler effect

the apparent change in wavelength or frequency of the radiation from a source due to its relative motion away from or toward the observer

electromagnetic radiation

radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light

electromagnetic spectrum

the whole array or family of electromagnetic waves, from radio to gamma rays

emission spectrum

a series or pattern of bright lines superimposed on a continuous spectrum

energy flux

the amount of energy passing through a unit area (for example, 1 square meter) per second; the units of flux are watts per square meter

energy level

a particular level, or amount, of energy possessed by an atom or ion above the energy it possesses in its least energetic state; also used to refer to the states of energy an electron can have in an atom

excitation

the process of giving an atom or an ion an amount of energy greater than it has in its lowest energy (ground) state

frequency

the number of waves that cross a given point per unit time (in radiation)

gamma rays

photons (of electromagnetic radiation) of energy with wavelengths no longer than 0.01 nanometer; the most energetic form of electromagnetic radiation

ground state

the lowest energy state of an atom

infrared

electromagnetic radiation of wavelength 103–106 nanometers; longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths

inverse square law

(for light) the amount of energy (light) flowing through a given area in a given time decreases in proportion to the square of the distance from the source of energy or light

ion

an atom that has become electrically charged by the addition or loss of one or more electrons

ionization

the process by which an atom gains or loses electrons

isotope

any of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons

microwave

electromagnetic radiation of wavelengths from 1 millimeter to 1 meter; longer than infrared but shorter than radio waves

nucleus (of an atom)

the massive part of an atom, composed mostly of protons and neutrons, and about which the electrons revolve

photon

a discrete unit (or “packet”) of electromagnetic energy

radial velocity

motion toward or away from the observer; the component of relative velocity that lies in the line of sight

radio waves

all electromagnetic waves longer than microwaves, including radar waves and AM radio waves

spectrometer

an instrument for obtaining a spectrum; in astronomy, usually attached to a telescope to record the spectrum of a star, galaxy, or other astronomical object

Stefan-Boltzmann law

a formula from which the rate at which a blackbody radiates energy can be computed; the total rate of energy emission from a unit area of a blackbody is proportional to the fourth power of its absolute temperature: F = σT4

ultraviolet

electromagnetic radiation of wavelengths 10 to 400 nanometers; shorter than the shortest visible wavelengths

visible light

electromagnetic radiation with wavelengths of roughly 400–700 nanometers; visible to the human eye

wavelength

the distance from crest to crest or trough to trough in a wave

Wien’s law

formula that relates the temperature of a blackbody to the wavelength at which it emits the greatest intensity of radiation

X-rays

electromagnetic radiation with wavelengths between 0.01 nanometer and 20 nanometers; intermediate between those of ultraviolet radiation and gamma rays

Earth, Moon, and Sky

 

apparent solar time

time as measured by the position of the Sun in the sky (the time that would be indicated by a sundial)

declination

the angular distance north or south of the celestial equator

great circle

a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center

International Date Line

an arbitrary line on the surface of Earth near longitude 180° across which the date changes by one day

lunar eclipse

an eclipse of the Moon, in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon

mean solar time

time based on the rotation of Earth; mean solar time passes at a constant rate, unlike apparent solar time

meridian

a great circle on the terrestrial or celestial sphere that passes through the poles

phases of the Moon

the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle, from new moon to full moon and back to new moon

right ascension

the coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body

sidereal day

Earth’s rotation period as defined by the positions of the stars in the sky; the time between successive passages of the same star through the meridian

sidereal month

the period of the Moon’s revolution about Earth measured with respect to the stars

solar day

Earth’s rotation period as defined by the position of the Sun in the sky; the time between successive passages of the Sun through the meridian

solar eclipse

an eclipse of the Sun by the Moon, caused by the passage of the Moon in front of the Sun; solar eclipses can occur only at the time of the new moon

solar month

the time interval in which the phases repeat—say, from full to full phase

synchronous rotation

when a body (for example, the Moon) rotates at the same rate that it revolves around another body

tides

alternate rising and falling of sea level caused by the difference in the strength of the Moon’s gravitational pull on different parts of Earth

Orbits and Gravity

 

angular momentum

the measure of the motion of a rotating object in terms of its speed and how widely the object’s mass is distributed around its axis

aphelion

the point in its orbit where a planet (or other orbiting object) is farthest from the Sun

apogee

the point in its orbit where an Earth satellite is farthest from Earth

asteroid belt

the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun

astronomical unit (AU)

the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 × 108 kilometers

density

the ratio of the mass of an object to its volume

eccentricity

in an ellipse, the ratio of the distance between the foci to the major axis

ellipse

a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same

escape speed

the speed a body must achieve to break away from the gravity of another body

focus

(plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant

gravity

the mutual attraction of material bodies or particles

Kepler’s first law

each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse

Kepler’s second law

the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time

Kepler’s third law

the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit

major axis

the maximum diameter of an ellipse

mass

a measure of the amount of material within an object

momentum

the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved

Newton’s first law

every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force

Newton’s second law

the change of motion of a body is proportional to and in the direction of the force acting on it

Newton’s third law

for every action there is an equal and opposite reaction (or: the mutual actions of two bodies upon each other are always equal and act in opposite directions)

orbit

the path of an object that is in revolution about another object or point

orbital period (P)

the time it takes an object to travel once around the Sun

orbital speed

the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse

perigee

the point in its orbit where an Earth satellite is closest to Earth

perihelion

the point in its orbit where a planet (or other orbiting object) is nearest to the Sun

perturbation

a small disturbing effect on the motion or orbit of a body produced by a third body

satellite

an object that revolves around a planet

semimajor axis

half of the major axis of a conic section, such as an ellipse

velocity

the speed and direction a body is moving—for example, 44 kilometers per second toward the north galactic pole

Observing the Sky: The Birth of Astronomy

 

accelerate

to change velocity; to speed up, slow down, or change direction.

apparent magnitude

a measure of how bright a star looks in the sky; the larger the number, the dimmer the star appears to us

astrology

the pseudoscience that deals with the supposed influences on human destiny of the configurations and locations in the sky of the Sun, Moon, and planets

celestial equator

a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator

celestial poles

points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis

celestial sphere

the apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere

circumpolar zone

those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon

cosmology

the study of the organization and evolution of the universe

ecliptic

the apparent annual path of the Sun on the celestial sphere

epicycle

the circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent)

geocentric

centered on Earth

heliocentric

centered on the Sun

horizon (astronomical)

a great circle on the celestial sphere 90° from the zenith; more popularly, the circle around us where the dome of the sky meets Earth

horoscope

a chart used by astrologers that shows the positions along the zodiac and in the sky of the Sun, Moon, and planets at some given instant and as seen from a particular place on Earth—usually corresponding to the time and place of a person’s birth

parallax

the apparent displacement of a nearby star that results from the motion of Earth around the Sun

planet

today, any of the larger objects revolving about the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars

precession (of Earth)

the slow, conical motion of Earth’s axis of rotation caused principally by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge

retrograde motion

the apparent westward motion of a planet on the celestial sphere or with respect to the stars

year

the period of revolution of Earth around the Sun

zenith

the point on the celestial sphere opposite the direction of gravity; point directly above the observer

zodiac

a belt around the sky about 18° wide centered on the ecliptic

Speed at which a Wave Moves Equals the Frequency Times the Wavelength

For any wave motion, the speed at which a wave moves equals the frequency times the wavelength.
Waves with longer wavelengths have lower frequencies.
Mathematically, we can express this as

c =  f  λ
where the Greek letter for “l”—lambda, λ—is used to denote wavelength and
c is the scientific symbol for the speed of light.
Solving for the wavelength, this is expressed as:

λ = c / f

Kepler’s Third Law