Chapter 1

Science and the Universe: A Brief Tour

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Summary

It is also humanity’s attempt to organize what we learn into a clear history of the universe, from the instant of its birth in the Big Bang to the present moment. Throughout this book, we emphasize that science is a progress report —one that changes constantly as new techniques and instruments allow us to probe the universe more deeply In considering the history of the universe, we will see again and again that the cosmos evolves ; it changes in profound ways over long periods of time. For example, the universe made the carbon, the calcium, and the oxygen necessary to construct something as interesting and complicated as you.

Key terms

universe

one of the most important (and satisfying) parts of modern astronomy

Chapter 2

Observing the Sky: The Birth of Astronomy

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Summary

The Sun’s annual path on the celestial sphere is the ecliptic—a line that runs through the center of the zodiac, which is the 18-degree-wide strip of the sky within which we always find the Moon and planets. The direct evidence of our senses supports a geocentric perspective, with the celestial sphere pivoting on the celestial poles and rotating about a stationary Earth. Ancient Greeks such as Aristotle recognized that Earth and the Moon are spheres, and understood the phases of the Moon, but because of their inability to detect stellar parallax, they rejected the idea that Earth moves. Ptolemy of Alexandria summarized classic astronomy in his Almagest ; he explained planetary motions, including retrograde motion, with remarkably good accuracy using a model centered on Earth.

Key terms

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

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

Moon

spheres, and understood the phases of the Moon, but because of their inability to detect stellar parallax, they rejected the idea that Earth moves

zenith

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

parallax

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

retrograde motion

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

zodiac

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

Chapter 3

Orbits and Gravity

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Summary

Angular momentum is a measure of the motion of a spinning or revolving object and depends on its mass, velocity, and distance from the point around which it revolves. Kepler’s laws describe the behavior of planets in their orbits as follows: (1) planetary orbits are ellipses with the Sun at one focus; (2) in equal intervals, a planet’s orbit sweeps out equal areas; and (3) the… Momentum is a measure of the motion of an object and depends on both its mass and its velocity. Gravity, the attractive force between all masses, is what keeps the planets in orbit.

Key terms

orbit

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

gravity

the mutual attraction of material bodies or particles

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

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

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

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

velocity

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

mass

a measure of the amount of material within an object

Chapter 4

Earth, Moon, and Sky

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Summary

An analogous celestial coordinate system is called right ascension (RA) and declination, with 0° of declination starting at the vernal equinox. The terrestrial system of latitude and longitude makes use of the great circles called meridians. The Foucault pendulum is a way to demonstrate that Earth is turning At the summer solstice, the Sun is higher in the sky and its rays strike Earth more directly.

Key terms

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

Foucault pendulum

a way to demonstrate that Earth is turning

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…

analogous celestial coordinate system

called right ascension (RA) and declination, with 0° of declination starting at the vernal equinox

meridian

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

declination

the angular distance north or south of the celestial equator

Sun

higher in the sky and its rays strike Earth more directly

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

Chapter 5

Radiation and Spectra

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Summary

The electromagnetic spectrum consists of gamma rays, X-rays, ultraviolet radiation, visible light, infrared, and radio radiation. Light is one form of this electromagnetic radiation. Electromagnetic radiation sometimes behaves like waves, but at other times, it behaves as if it were a particle—a little packet of energy, called a photon. The apparent brightness of a source of electromagnetic energy decreases with increasing distance from that source in proportion to the square of the distance—a relationship known as the inverse square law

Key terms

ion

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

electromagnetic radiation

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

gamma rays

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

infrared

electromagnetic radiation of wavelength 10 3 -10 6 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

X-rays

electromagnetic radiation with wavelengths between 0.01 nanometer and 10 nanometers; intermediate between those of ultraviolet radiation and gamma rays

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

Chapter 6

Astronomical Instruments

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Summary

The light-gathering power of a telescope is determined by the diameter of its aperture, or opening—that is, by the area of its largest or primary lens or mirror. The primary optical element in a telescope is either a convex lens (in a refracting telescope) or a concave mirror (in a reflector) that brings the light to a focus. A telescope collects the faint light from astronomical sources and brings it to a focus, where an instrument can sort the light according to wavelength. The resolution of a visible-light or infrared telescope is degraded by turbulence in Earth’s atmosphere.

Key terms

light-gathering power of a telescope

determined by the diameter of its aperture, or opening—that is, by the area of its largest or primary lens or mirror

seeing

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

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

detector

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

focus

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

telescope

instrument for collecting visible-light or other electromagnetic radiation

aperture

diameter of the primary lens or mirror of a telescope

Chapter 7

Other Worlds: An Introduction to the Solar System

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Summary

The planets can be divided into two groups: the inner terrestrial planets and the outer giant planets. The giant planets are composed mostly of liquids and gases. Smaller members of the solar system include asteroids (including the dwarf planet Ceres), which are rocky and metallic objects found mostly between Mars and Jupiter; comets, which are made mostly of frozen gases and… When a meteor survives its passage through our atmosphere and falls to Earth, we call it a meteorite

Key terms

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

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

terrestrial planet

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

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…

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

meteorite

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

giant planets

composed mostly of liquids and gases

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…

Chapter 8

Earth as a Planet

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Summary

Our planet’s geology is dominated by plate tectonics, in which crustal plates move slowly in response to mantle convection. The outer layer, or crust, consists primarily of oceanic basalt and continental granite. Its interior composition and structure are probed using seismic waves. Such studies reveal that Earth has a metal core and a silicate mantle.

Key terms

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

seismic wave

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

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…

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…

primitive rock

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

plate tectonics

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

granite

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

mantle

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

Chapter 9

Cratered Worlds

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Summary

The Moon’s heavily cratered highlands are made of rocks more than 4 billion years old. Generally, the surface is dominated by impacts, including continuing small impacts that produce its fine-grained soil Most of what we know about the Moon derives from the Apollo program, including 400 kilograms of lunar samples still being intensively studied. The Moon has one-eightieth the mass of Earth and is severely depleted in both metals and volatile materials.

Key terms

highlands

the lighter, heavily cratered regions of the Moon, which are generally several kilometers higher than the maria

surface

dominated by impacts, including continuing small impacts that produce its fine-grained soil

Moon’s heavily cratered highlands

made of rocks more than 4 billion years old

mare

(plural: maria) Latin for “sea;” the name applied to the dark, relatively smooth features that cover 17% of the Moon’s surface

Chapter 10

Earthlike Planets: Venus and Mars

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Summary

Venus, the nearest planet, is a great disappointment through the telescope because of its impenetrable cloud cover. Mars is more tantalizing, with dark markings and polar caps. Early in the twentieth century, it was widely believed that the “canals” of Mars indicated intelligent life there. Mars has only 11% the mass of Earth, but Venus is nearly our twin in size and mass.

Key terms

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

Chapter 11

The Giant Planets

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Summary

The outer solar system contains the four giant planets: Jupiter, Saturn, Uranus, and Neptune. The gas giants Jupiter and Saturn have overall compositions similar to that of the Sun. These planets have been explored by the Pioneer, Voyager, Galileo, and Cassini spacecraft. Voyager 2, perhaps the most successful of all space-science missions, explored Jupiter (1979), Saturn (1981), Uranus (1986), and Neptune (1989)—a grand tour of the giant planets—and these flybys have been the only…

Key terms

synchrotron radiation

the radiation emitted by charged particles being accelerated in magnetic fields and moving at speeds near that of light

photochemistry

chemical changes caused by electromagnetic radiation

Chapter 12

Rings, Moons, and Pluto

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Summary

The four jovian planets are accompanied by impressive systems of moons and rings. Of the four ring systems, Saturn’s is the largest and is composed primarily of water ice; in contrast, Uranus and Neptune have narrow rings of dark material, and Jupiter has a tenuous ring of dust Over 410 moons have been discovered in the outer solar system. Jupiter’s largest moons are Ganymede and Callisto, both low-density objects that are composed of more than half water ice.

Key terms

largest and

composed primarily of water ice; in contrast, Uranus and Neptune have narrow rings of dark material, and Jupiter has a tenuous ring of dust

four jovian planets

accompanied by impressive systems of moons and rings

resonance

an orbital condition in which one object is subject to periodic gravitational perturbations by another, most commonly arising when two objects orbiting a third have periods of…

tidal heating

the heating of a planet or moon’s interior by variable tidal forces caused by changing gravitational pull from a nearby planet or moon

Chapter 13

Comets and Asteroids: Debris of the Solar System

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Summary

The rocky ones are generally called asteroids. Ceres is the largest asteroid; about 15 are larger than 250 kilometers and about 100,000 are larger than 1 kilometer. Most are in the asteroid belt between Mars and Jupiter. The presence of asteroid families in the belt indicates that many asteroids are the remnants of ancient collisions and fragmentation.

Key terms

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…

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…

largest asteroid; about 15

larger than 250 kilometers and about 100,000 are larger than 1 kilometer

inner belt

S-type (stony) asteroids, with a few M-type (metallic) ones

rocky ones

generally called asteroids

Kuiper belt

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

Chapter 14

Cosmic Samples and the Origin of the Solar System

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Summary

Meteorites are called finds or falls according to how they are discovered; the most productive source today is the Antarctic ice cap. The most primitive are the carbonaceous meteorites, such as Murchison and Allende. Streams of dust particles traveling through space together produce meteor showers, in which we see meteors diverging from a spot in the sky called the radiant of the shower. Many meteor showers recur each year and are associated with particular comets that have left dust behind as they come close to the Sun and their ices evaporate (or have broken up into smaller pieces)

Key terms

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

most primitive

the carbonaceous meteorites, such as Murchison and Allende

stony-iron meteorite

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

accretion

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

stony meteorite

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

Chapter 15

The Sun: A Garden-Variety Star

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Summary

In order of increasing distance from the center of the Sun, they are the photosphere, with a temperature that ranges from 4500 K to about 6800 K; the chromosphere, with a typical temperature of 10 4 K; the transition… Sunspots are dark regions where the temperature is up to 2000 K cooler than the surrounding photosphere. Solar wind particles stream out into the solar system through coronal holes. The Sun’s surface is mottled with upwelling convection currents seen as hot, bright granules.

Key terms

coronal hole

a region in the Sun’s outer atmosphere that appears darker because there is less hot gas there

aurora

light radiated by atoms and ions in the ionosphere excited by charged particles from the Sun, mostly seen in the magnetic polar regions

photosphere

the region of the solar (or stellar) atmosphere from which continuous radiation escapes into space

sunspot

large, dark features seen on the surface of the Sun caused by increased magnetic activity

chromosphere

the part of the solar atmosphere that lies immediately above the photospheric layers

Sun’s surface

mottled with upwelling convection currents seen as hot, bright granules

temperature

up to 2000 K cooler than the surrounding photosphere

corona

(of the Sun) the outer (hot) atmosphere of the Sun

Chapter 16

The Sun: A Nuclear Powerhouse

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Summary

Solar energy is produced by interactions of particles—that is, protons, neutrons, electrons, positrons, and neutrinos. Specifically, the source of the Sun’s energy is the fusion of hydrogen to form helium. Since Earth and the solar system are roughly 4.5 billion years old, this means that the Sun has been producing vast amounts for energy for a very, very long time. The series of reactions required to convert hydrogen to helium is called the proton-proton chain.

Key terms

neutrino

fundamental particle that has no charge and a mass that is tiny relative to an electron; it rarely interacts with ordinary matter and comes in three different types

solar system

roughly 4.5 billion years old, this means that the Sun has been producing vast amounts for energy for a very, very long time

helium atom

about 0.71% less massive than the four hydrogen atoms that combine to form it, and that lost mass is converted to energy (with the amount of energy given by the formula E

proton-proton chain

series of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium

positron

particle with the same mass as an electron, but positively charged

fusion

building up of heavier atomic nuclei from lighter ones

source of the Sun’s energy

the fusion of hydrogen to form helium

conduction

process by which heat is directly transmitted through a substance when there is a difference of temperature between adjoining regions caused by atomic or molecular collisions

Chapter 17

Analyzing Starlight

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Summary

The apparent brightnesses of stars are often expressed in terms of magnitudes, which is an old system based on how human vision interprets relative light intensity A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars The apparent brightness of a star depends on both its luminosity and its distance from Earth. Thus, the determination of apparent brightness and measurement of the distance to a star provide enough information to calculate its luminosity.

Key terms

apparent brightness

a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky, as contrasted with its luminosity

apparent brightnesses of stars

often expressed in terms of magnitudes, which is an old system based on how human vision interprets relative light intensity

color index of a star

the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars

color index

difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B-V) magnitudes

magnitude

an older system of measuring the amount of light we receive from a star or other luminous object; the larger the magnitude, the less radiation we receive from the object

luminosity

the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object

brown dwarf

an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining…

spectral class

(or spectral type) the classification of stars according to their temperatures using the characteristics of their spectra; the types are O, B, A, F, G, K, and M with L, T, and Y…

Chapter 18

The Stars: A Celestial Census

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Summary

Most of the nearest stars are intrinsically so faint that they can be seen only with the aid of a telescope. Most of the brown dwarfs in the local neighborhood have not yet been discovered The masses of stars can be determined by analysis of the orbit of binary stars—two stars that orbit a common center of mass. In visual binaries, the two stars can be seen separately in a telescope, whereas in a spectroscopic binary, only the spectrum reveals the presence of two stars.

Key terms

brown dwarf

an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining…

spectroscopic binary

a binary star in which the components are not resolved but whose binary nature is indicated by periodic variations in radial velocity, indicating orbital motion

nearest stars

intrinsically so faint that they can be seen only with the aid of a telescope

binary stars

two stars that revolve about each other

eclipsing binary

a binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that the light of one star is periodically diminished by the other…

white dwarf

a low-mass star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final state of life

visual binary

a binary star in which the two components are telescopically resolved

main sequence

a sequence of stars on the Hertzsprung-Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right

Chapter 19

Celestial Distances

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Summary

Half the shift in a nearby star’s position relative to very distant background stars, as viewed from opposite sides of Earth’s orbit, is called the parallax of that star and is a measure of its distance. The units used to measure stellar distance are the light-year, the distance light travels in 1 year, and the parsec (pc), the distance of a star with a parallax of 1 arcsecond (1 parsec = 3.26 light-years). Distances within the solar system are now determined by timing how long it takes radar signals to travel from Earth to the surface of a planet or other body and then return The closest star, a red dwarf, is over 1 parsec away.

Key terms

parsec

a unit of distance in astronomy, equal to 3.26 light-years; at a distance of 1 parsec, a star has a parallax of 1 arcsecond

solar system

now determined by timing how long it takes radar signals to travel from Earth to the surface of a planet or other body and then return

parallax

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

cepheid

a star that belongs to a class of yellow supergiant pulsating stars; these stars vary periodically in brightness, and the relationship between their periods and luminosities is…

light curve

a graph that displays the time variation of the light from a variable or eclipsing binary star or, more generally, from any other object whose radiation output changes with time

luminosity class

a classification of a star according to its luminosity within a given spectral class; our Sun, a G2V star, has luminosity class V, for example

RR Lyrae

one of a class of giant pulsating stars with periods shorter than 1 day, useful for finding distances

Chapter 20

Between the Stars: Gas and Dust in Space

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Summary

About 1% of the interstellar matter is in the form of solid interstellar dust grains Glowing clouds (nebulae) of ionized hydrogen are called H II regions and have temperatures of about 10,000 K. About 15% of the visible matter in the Galaxy is in the form of gas and dust, serving as the raw material for new stars. About 99% of this interstellar matter is in the form of gas—individual atoms or molecules.

Key terms

interstellar dust

tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are…

nebula

a cloud of interstellar gas or dust; the term is most often used for clouds that are seen to glow with visible light or infrared

visible matter in the Galaxy

in the form of gas and dust, serving as the raw material for new stars

electron

captured by an ion and cascades down to lower-energy levels

H II region

the region of ionized hydrogen in interstellar space

interstellar matter

in the form of solid interstellar dust grains

cosmic rays

atomic nuclei (mostly protons) and electrons that are observed to strike Earth’s atmosphere with exceedingly high energies

Local Bubble

(or Local Hot Bubble) a region of low-density, million degree gas in which the Sun and solar system are currently located

Chapter 21

The Birth of Stars and the Discovery of Planets outside the Solar System

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Summary

The accumulation of material halts when a protostar develops a strong stellar wind, leading to jets of material being observed coming from the star. Most stars form in giant molecular clouds with masses as large as 3 × 10 6 solar masses. The most well-studied molecular cloud is Orion, where star formation is currently taking place. Protostars generate energy (and internal heat) through gravitational contraction that typically continues for millions of years, until the star reaches the main sequence

Key terms

giant molecular clouds

large, cold interstellar clouds with diameters of dozens of light-years and typical masses of 10 5 solar masses; found in the spiral arms of galaxies, these clouds are where…

most well-studied molecular cloud

Orion, where star formation is currently taking place

stellar wind

the outflow of gas, sometimes at speeds as high as hundreds of kilometers per second, from a star

protostar

a very young star still in the process of formation, before nuclear fusion begins

mini-Neptune

a planet that is intermediate between the largest terrestrial planet in our solar system (Earth) and the smallest jovian planet (Neptune); generally, mini-Neptunes have sizes…

Herbig-Haro (HH) object

luminous knots of gas in an area of star formation that are set to glow by jets of material from a protostar

super-Earth

a planet larger than Earth, generally between 1.4 and 2.8 times the size of our planet

transit

when one astronomical object moves in front of another

Chapter 22

Stars from Adolescence to Old Age

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Summary

When stars first begin to fuse hydrogen to helium, they lie on the zero-age main sequence. The core of a red giant is contracting, but the outer layers are expanding as a result of hydrogen fusion in a shell outside the core. The amount of time a star spends in the main-sequence stage depends on its mass. More massive stars complete each stage of evolution more quickly than lower-mass stars.

Key terms

zero-age main sequence

a line denoting the main sequence on the H-R diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy…

core of a red giant

contracting, but the outer layers are expanding as a result of hydrogen fusion in a shell outside the core

planetary nebula

a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the…

triple-alpha process

a nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus

globular cluster

one of about 150 large, spherical star clusters (each with hundreds of thousands of stars) that form a spherical halo around the center of our Galaxy

open cluster

a comparatively loose cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of our Galaxy; sometimes referred to as a…

association

a loose group of young stars whose spectral types, motions, and positions in the sky indicate a common origin

helium flash

a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star

Chapter 23

The Death of Stars

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Summary

If the mass of a star’s iron core exceeds the Chandrasekhar limit (but is less than 3 M Sun ), the core collapses until its density exceeds that of an atomic nucleus, forming a neutron star with a typical diameter of… Stars with masses of 8 M Sun or less can lose enough mass to become white dwarfs, which have masses less than the Chandrasekhar limit (about 1.4 M Sun ). In a massive star, hydrogen fusion in the core is followed by several other fusion reactions involving heavier elements. Just before it exhausts all sources of energy, a massive star has an iron core surrounded by shells of silicon, sulfur, oxygen, neon, carbon, helium, and hydrogen.

Key terms

Chandrasekhar limit (but

less than 3 M Sun ), the core collapses until its density exceeds that of an atomic nucleus, forming a neutron star with a typical diameter of…

neutron star

a compact object of extremely high density composed almost entirely of neutrons

core

followed by several other fusion reactions involving heavier elements

Chandrasekhar limit

the upper limit to the mass of a white dwarf

millisecond pulsar

a pulsar that rotates so quickly that it can give off hundreds of pulses per second (and its period is therefore measured in milliseconds)

pulsar

a variable radio source of small physical size that emits very rapid radio pulses in very regular periods that range from fractions of a second to several seconds; now understood…

degenerate gas

a gas that resists further compression because no two electrons can be in the same place at the same time doing the same thing (Pauli exclusion principle)

nova

the cataclysmic explosion produced in a binary system, temporarily increasing its luminosity by hundreds to thousands of times

Chapter 24

Black Holes and Curved Spacetime

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Summary

Einstein proposed the equivalence principle as the foundation of the general theory of relativity. By considering the consequences of the equivalence principle, Einstein concluded that we live in a curved spacetime. Light must change its path near a massive object not because light is bent by gravity, but because spacetime is The distribution of matter determines the curvature of spacetime; other objects (and even light) entering a region of spacetime must follow its curvature.

Key terms

black hole

a region in spacetime where gravity is so strong that nothing—not even light—can escape

spacetime

system of one time and three space coordinates, with respect to which the time and place of an event can be specified

equivalence principle

concept that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment

massive object not because light

bent by gravity, but because spacetime is

general theory of relativity

Einstein’s theory relating gravity and the structure (geometry) of space and time

position where Mercury

at perihelion would change by 43 arcsec per century even if there…

accretion disk

the disk of gas and dust found orbiting newborn stars, as well as compact stellar remnants such as white dwarfs, neutron stars, and black holes when they are in binary systems…

event horizon

a boundary in spacetime such that events inside the boundary can have no effect on the world outside it—that is, the boundary of the region around a black hole where the…

Chapter 25

The Milky Way Galaxy

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Summary

The Milky Way Galaxy consists of a thin disk containing dust, gas, and young and old stars; a spherical halo containing populations of very old stars, including RR Lyrae variable stars and globular star clusters; a… The gaseous distribution in the Galaxy’s disk has two main spiral arms that emerge from the ends of the central bar, along with several fainter arms and short spurs; the Sun is located in one of those spurs. The total mass of the Galaxy is about 2 × 10 12 M Sun. The Sun is located roughly halfway out of the Milky Way, about 26,000 light-years from the center

Key terms

Milky Way Galaxy

the band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy

spiral arm

a spiral-shaped region, characterized by relatively dense interstellar material and young stars, that is observed in the disks of spiral galaxies

halo

the outermost extent of our Galaxy (or another galaxy), containing a sparse distribution of stars and globular clusters in a more or less spherical distribution

Sun

located roughly halfway out of the Milky Way, about 26,000 light-years from the center

dark matter

nonluminous mass, whose presence can be inferred only because of its gravitational influence on luminous matter; the composition of the dark matter is not known

supermassive black hole

the object in the center of most large galaxies that is so massive and compact that light cannot escape from it; the Milky Way’s supermassive black hole contains 4.6 millions of…

dark matter halo

the mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy; although we…

Chapter 26

Galaxies

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Summary

The majority of bright galaxies are either spirals or ellipticals. Faint star clusters, clouds of glowing gas, and galaxies all appeared as faint patches of light (or nebulae) in the telescopes available at the beginning of the twentieth century. It was only when Hubble measured the distance to the Andromeda galaxy using cepheid variables with the giant 2.5-meter reflector on Mount Wilson in 1924 that the existence of other galaxies similar to the Milky Way in… Spiral galaxies contain both old and young stars, as well as interstellar matter, and have typical masses in the range of 10 9 to 10 12 M Sun .

Key terms

majority of bright galaxies

either spirals or ellipticals

mass-to-light ratio

the ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication…

redshift

when lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum)

Hubble’s law

a rule that the radial velocities of remote galaxies are proportional to their distances from us

elliptical galaxy

a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material

spiral galaxy

a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, winding out from its central bulge

type Ia supernova

a supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about 4.5 × 10 9 L Sun ; can be used to determine distances to galaxies on a…

Hubble constant

a constant of proportionality in the law relating the velocities of remote galaxies to their distances

Chapter 27

Active Galaxies, Quasars, and Supermassive Black Holes

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Summary

The redshifts of the underlying galaxies match the redshifts of the quasars embedded in their centers, thereby proving that quasars obey the Hubble law and are at the great distances implied by their redshifts. The first quasars discovered looked like stars but had strong radio emission. The quasar spectra obtained so far show redshifts ranging from 15% to more than 96% the speed of light. Observations with the Hubble Space Telescope show that quasars lie at the centers of galaxies and that both spirals and ellipticals can harbor quasars.

Key terms

quasar

an object of very high redshift that looks like a star but is extragalactic and highly luminous; also called a quasi-stellar object, or QSO

active galaxies

galaxies that house active galactic nuclei

Hubble law and

at the great distances implied by their redshifts

active galactic nuclei (AGN)

galaxies that are almost as luminous as quasars and share many of their properties, although to a less spectacular degree; abnormal amounts of energy are produced in their centers

Chapter 28

The Evolution and Distribution of Galaxies

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Summary

The universe now is 13.8 billion years old. The color of a galaxy is an indicator of the age of the stars that populate it. When we look at distant galaxies, we are looking back in time. We have now seen galaxies as they were when the universe was about 300 million years old—only about two percent as old as it is now.

Key terms

color of a galaxy

an indicator of the age of the stars that populate it

cosmological principle

the assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous

hot dark matter

massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation; hot dark matter is faster-moving material than cold dark matter

supercluster

a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies

dark energy

an energy that is causing the expansion of the universe to accelerate; the source of this energy is not yet understood

homogeneous

having a consistent and even distribution of matter that is the same everywhere

cold dark matter

slow-moving massive particles, not yet identified, that don’t absorb, emit, or reflect light or other electromagnetic radiation

Chapter 29

The Big Bang

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Summary

Cosmology is the study of the organization and evolution of the universe. Measurements of distant supernovae show that when the universe was about half its current age, dark energy began to dominate the rate of expansion and caused it to speed up. After allowing for these effects, astronomers estimate that all of the matter within the observable universe was concentrated in an extremely small volume 13.8 billion years ago, a time we call the Big Bang The universe is expanding, and this is one of the key observational starting points for modern cosmological theories.

Key terms

Big Bang

the theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy)

dark energy

the energy that is causing the expansion of the universe to accelerate; its existence is inferred from observations of distant supernovae

universe

expanding, and this is one of the key observational starting points for modern cosmological theories

cosmology

the study of the organization and evolution of the universe

flat universe

a model of the universe that has a critical density and in which the geometry of the universe is flat, like a sheet of paper

inflationary universe

a theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion when the universe was about 10 -35 second old; after this period of rapid…

critical density

in cosmology, the density that is just sufficient to bring the expansion of the universe to a stop after infinite time

open universe

a model in which the density of the universe is not high enough to bring the expansion of the universe to a halt

Chapter 30

Life in the Universe

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Summary

Life on Earth is based on the presence of a key unit known as an organic molecule, a molecule that contains carbon, especially complex hydrocarbons. Our solar system formed about 5 billion years ago from a cloud of gas and dust enriched by several generations of heavier element production in stars. The study of life in the universe, including its origin on Earth, is called astrobiology. Life is made up of chemical combinations of these elements made by stars.

Key terms

astrobiology

the multidisciplinary study of life in the universe: its origin, evolution, distribution, and fate; similar terms are exobiology and bioastronomy

organic molecule

a combination of carbon and other atoms—primarily hydrogen, oxygen, nitrogen, phosphorus, and sulfur—some of which serve as the basis for our biochemistry

gene

the basic functional unit that carries the genetic (hereditary) material contained in a cell

stromatolites

solid, layered rock formations that are thought to be the fossils of oxygen-producing photosynthetic bacteria in rocks that are 3.5 billion years old

amino acids

organic compounds that are the molecular building blocks of proteins

biomarker

evidence of the presence of life, especially a global indication of life on a planet that could be detected remotely (such as an unusual atmospheric composition)

Drake equation

a formula for estimating the number of intelligent, technological civilizations in our Galaxy, first suggested by Frank Drake

extremophile

an organism (usually a microbe) that tolerates or even thrives under conditions that most of the life around us would consider hostile, such as very high or low temperature or…