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    Britannica Illustrated Science Library: Universe 2008

    Printed in China

    www.britannica.comUniverseContents PICTURE ON PAGE 1

    Image of a planetary nebula.

    Planetary nebulae are among

    the most photogenic objects

    in astronomy.

    What Is the

    Universe?

    Page 6

    What Is in the

    Universe?

    Page 18

    The Solar

    System

    Page 38

    The Earth

    and the Moon

    Page 66

    Observing

    the Universe

    Page 80T

    here was a time when people believed

    that the stars were bonfires lit by

    other tribes in the sky, that the

    universe was a flat plate resting on the shell

    of a giant turtle, and that the Earth,according to the Greek astronomer Ptolemy,was at the center of the universe. From the

    most remote of times, people have been

    curious about what lies hidden beyond the

    celestial sphere. This curiosity has led them

    to build telescopes that show with clarity

    otherwise blurry and distant objects. In this

    book you will find the history of the cosmos

    illustrated with spectacular images that

    show in detail how the cosmos was formed,the nature of the many points of light that

    adorn the night sky, and what lies ahead.

    You will also discover how the suns that

    inhabit space live and die, what dark matter

    and black holes are, and what our place is in

    this vastness. Certainly, the opportunity to

    compare the destiny of other worlds similar

    to ours will help us understand that for the

    time being there is no better place than the

    Earth to live. At least for now.

    I

    n the Milky Way—according to

    mathematical and physical

    calculations—there are more than 100

    billion stars, and such a multitude leads to

    the question: Is it possible that our Sun is

    the only star that possesses an inhabited

    planet? Astronomers are more convinced

    than ever of the possibility of life in other

    worlds. We just need to find them. Reading

    this book will let you become better

    acquainted with our neighbors in the solar

    system—the other planets—and the most

    important characteristics that distinguish

    them. All this information that explores the

    mysteries of space is accompanied by

    recent images captured by the newest

    telescopes. They reveal many details about

    the planets and their satellites, such as the

    volcanoes and craters found on the surface

    of some of them. You will also learn more

    about the asteroids and comets that orbit

    the Sun and about Pluto, a dwarf planet,which is to be visited by a space probe for

    the first time. Less than a decade ago,astronomers began observing frozen worlds,much smaller than a planet, in a region of

    the solar system called the Kuiper belt. We

    invite you to explore all of this. The images

    and illustrations that accompany the text

    will prove very helpful in studying and

    understanding the structure of all the visible

    and invisible objects (such as dark matter)

    that form part of the universe. There are

    stellar maps showing the constellations, the

    groups of stars that since ancient times have

    served as a guide for navigation and for the

    development of calendars. There is also a

    review through history: from Ptolemy, who

    thought the planets orbited around the

    Earth, and Copernicus, who put the Sun in

    the center, and Galileo, the first to aim a

    telescope skyward, up to the most recent

    astronomical theories, such as those of

    Stephen Hawking, the genius of space and

    time who continues to amaze with his

    discoveries about the greatest mysteries of

    the cosmos. You will find these and many

    more topics no matter where you look in this

    fantastic book that puts the universe and its

    secrets in your hands.

    The Secrets of

    the Universe

    CONE NEBULA

    This nebula got its name

    from its cone shape, as

    shown in the image.What Is the Universe?

    T

    he universe is everything that

    exists, from the smallest

    particles to the largest ones,together with all matter and

    energy. The universe includes

    visible and invisible things, such as dark

    matter, the great, secret component of

    the cosmos. The search for dark matter

    is currently one of the most important

    tasks of cosmology. Dark matter may

    literally determine the density of all of

    space, as well as decide the destiny of

    the universe. Did you know that, second

    by second, the universe grows and

    grows? The question that astronomers

    are asking—the question that concerns

    them the most—is how much longer the

    universe can continue to expand like a

    balloon before turning into something

    cold and dark.

    X-RAY OF THE COSMOS 8-9

    THE INSTANT OF CREATION 10-13

    EVERYTHING COMES TO AN END 14-15

    THE FORCES OF THE UNIVERSE 16-17

    DARK MATTER

    Evidence exists that dark matter, though invisible

    to telescopes, betrays itself by the gravitational

    pull it exerts over other heavenly bodies.UNIVERSE 9 8 WHAT IS THE UNIVERSE?

    T

    he universe, marvelous in its majesty, is an ensemble of a hundred

    billion galaxies. Each of these galaxies (which tend to be found in

    large groups) has billions of stars. These galactic concentrations

    surround empty spaces, called cosmic voids. The immensity of the

    cosmos can be better grasped by realizing that the size of our fragile

    planet Earth, or even that of the Milky Way, is insignificant

    compared to the size of the remainder of the cosmos.

    Originating nearly 14 billion years ago

    in an immense explosion, the universe

    today is too large to be able to conceive. The

    innumerable stars and galaxies that populate it

    promise to continue expanding for a long time.

    Though it might sound strange today, for many

    years, astronomers thought that the Milky Way,where the Earth is located, constituted the entire

    universe. Only recently—in the 20th century—was outer

    space recognized as not only much vaster than previously

    thought but also as being in a state of ongoing expansion.

    The Universe

    NEAR STARS Found closer

    than 20 light-years from the

    Sun, they make up our solar

    neighborhood.

    2.

    NEIGHBORS Within a space

    of one million light-years,we find the Milky Way and

    its closest galaxies.

    3.

    NEAREST GALAXIES. At a scale

    of one hundred million light-years,the galactic clusters nearest to

    the Milky Way can be seen.

    5.

    FILAMENTS. From five billion

    light-years away, the immensity of

    the cosmos is evident in its

    galactic filaments, each one home

    to millions and millions of galaxies.

    7.

    SUPERCLUSTERS. Within a

    distance of a billion light-years,groups of millions of galaxies,called superclusters, can be seen.

    6.

    LOCAL GROUP. Ten

    million light-years away

    is Andromeda, the

    closest to the Earth.

    4.

    EARTH Originated, together

    with the solar system, when

    the universe was already 9.1

    billion years old. It is the only

    known planet that is home to life.

    1.

    EARTH

    Neptune

    G51-15

    Ross

    128

    Lalande

    21185

    Wolf

    359

    Luyten’s

    Star

    Procyon

    Uranus

    Saturn

    Jupiter

    Pluto

    SUN

    Alpha

    Centauri

    Sirius

    270°

    90°

    180°

    0.5

    0°

    180°

    0°

    12.5

    Epsilon

    Eridani

    L372-58

    L726-8

    L725-32

    Epsilon

    Indi

    Lacaille

    9352 Ceti

    7.5

    2.5

    Struve

    2398

    Ross

    248

    Ross

    154

    Groombridge

    34

    61 Cygni

    Bernard’s

    Star

    L789-6

    L789-6

    0°

    Sextans

    Dwarf

    Ursa

    Minor Dwarf

    Leo A

    Leo I

    Leo II

    Andromeda I

    Sextans B

    Sextans A

    Antila

    Dwarf

    NGC

    3109

    Draco

    Dwarf

    Sagittarius

    Dwarf

    Tucana

    Dwarf

    Phoenix

    Dwarf

    Cetus

    Dwarf Sagittarius

    Irregular

    Dwarf

    Aquarius

    Dwarf

    LGS 3

    Pegasus

    Dwarf

    IC

    1613

    WLM

    Canis

    Major

    Small

    Magellanic

    Cloud

    Large

    Magellanic

    Cloud

    Carina

    Dwarf

    MILKY WAY

    MILKY WAY

    NGC

    6822

    Triangle

    Andromeda

    M32

    M110

    NGC

    185

    NGC

    147

    IC 10

    0.12

    0.25

    0.37

    1.2

    2.5

    3.7

    0°

    180°

    0°

    180°

    NGC

    7582

    NGC

    6744

    Capricornus

    Supercluster

    Pavo-Indus

    Supercluster

    Sculptor

    Supercluster

    Sculptor

    Void

    Pisces-Cetus

    Superclusters

    Pisces-Perseus

    Supercluster

    Coma

    Supercluster

    Centaurus

    Supercluster

    Hercules

    Supercluster

    Shapley

    Supercluster

    Bo?tes

    Void

    Leo

    Supercluster

    Ursa Major

    Supercluster

    Bo?tes

    Supercluster

    Corona Borealis

    Supercluster

    Hydra

    Horologium

    Superclusters Columba

    Supercluster

    NGC

    1023

    NGC

    2997

    NGC

    5128

    NGC

    5033

    NGC

    4697

    12.5

    25

    37.5

    50

    1,000

    750

    250

    Dorado

    Sculptor

    Maffei

    M81

    M101

    Leo I

    Canis

    Ursa Major

    Group

    Virgo

    Group

    Leo III

    Group

    Virgo III

    Group

    Fornax

    Cluster

    Eridanus

    Cluster

    LOCAL

    GROUP

    VIRGO

    Sextans

    Supercluster

    X-Ray of the Cosmos

    100 billion The total number of galaxies that exist,indicating that the universe is both larger

    and older than was previously thoughtUNIVERSE 11 10 WHAT IS THE UNIVERSE?

    Region 1 Region 3

    Region 2

    Region 4

    Region 5

    Galaxy 1 Galaxy 2

    Galaxy 5

    Galaxy 4

    Galaxy 3

    The Instant of Creation

    I

    t is impossible to know precisely how, out of nothing, the universe began to exist. According to the big

    bang theory—the theory most widely accepted in the scientific community—in the beginning, there

    appeared an infinitely small and dense burning ball that gave rise to space, matter, and energy. This

    happened 13.7 billion years ago. The great, unanswered question is what caused a small dot of light—filled

    with concentrated energy from which matter and antimatter were created—to arise from nothingness. In

    very little time, the young universe began to expand and cool. Several billion years later, it acquired the

    form we know today.

    10-43

    sec 10-12

    sec 3min

    Scientists theorize that, from

    nothing, something infinitely

    small, dense, and hot appeared.

    All that exists today was

    compressed into a ball smaller than

    the nucleus of an atom.

    TIME

    TEMPERATURE

    Cosmic Inflation Theory

    Although big bang theorists understood the universe as originating

    in an extremely small, hot, and condensed ball, they could not

    understand the reason for its staggering growth. In 1981, physicist Alan

    Guth proposed a solution to the problem with his inflationary theory. In an

    extremely short period of time (less than a thousandth of a second), the

    universe grew more than a trillion trillion trillion times. Near the end of this

    period of expansion, the temperature approached absolute zero.

    HOW IT DID

    NOT GROW Had the universe not

    undergone inflation,it would be a

    collection of different

    regions, each with its

    own particular types

    of galaxies and each

    clearly

    distinguishable from

    the others.

    HOW IT GREW Cosmic inflation was

    an expansion of the

    entire universe. The

    Earth's galactic

    neighborhood appears

    fairly uniform.

    Everywhere you look,the types of galaxies

    and the background

    temperature are

    essentially the same.

    FROM PARTICLES TO MATTER

    The quarks, among the oldest particles,interact with each other by forces

    transmitted through gluons. Later protons

    and neutrons will join to form nuclei.

    Photon

    Massless elemental

    luminous particle

    Gluon

    Responsible for

    the interactions

    between quarks

    Quark

    Light, elemental

    particle

    Graviton

    It is believed to

    transmit gravitation.

    0

    Gravity

    SUPERFORCE

    Strong nuclear

    Weak nuclear

    Electromagnetism

    EXPANSION

    10-38

    sec

    1032

    ° F (and C) 1029

    ° F (and C) 1015

    ° F (and C) 2x109

    ° F (1x109

    ° C) -

    1

    At the closest moment to

    zero time, which physics has

    been able to reach, the

    temperature is extremely

    high. Before the universe's inflation,a superforce governed everything.

    2

    The universe is unstable. Only

    10-38 seconds after the big

    bang, the universe increases in

    size more than a trillion trillion

    trillion times. The expansion of the

    universe and the division of its forces begin.

    3

    The universe experiences a

    gigantic cooldown. Gravity

    has already become

    distinguishable, and the

    electromagnetic force and the strong

    and weak nuclear interactions appear.

    4

    5sec

    9x109

    ° F (5x109

    ° C)

    The electrons and their

    antiparticles,positrons, annihilate

    each other until the

    positrons disappear. The

    remaining electrons form atoms.

    6

    10-4

    sec

    1012

    ° F (and C)

    Protons and neutrons

    appear, formed by three

    quarks apiece. Because

    all light is trapped within

    the web of particles, the universe

    is still dark.

    5

    The nuclei of the

    lightest elements,hydrogen and

    helium, form.

    Protons and neutrons unite to

    form the nuclei of atoms.

    7

    WMAP (WILKINSON MICROWAVE ANISOTROPY PROBE)

    NASA's WMAP project maps the background radiation of the universe. In the

    image, hotter (red-yellow) regions and colder (blue-green) regions can be

    observed. WMAP makes it possible to determine the amount of dark matter.

    THE SEPARATION OF FORCES

    Before the universe expanded, during a period of

    radiation, only one unified force governed all

    physical interactions. The first distinguishable

    force was gravity, followed by electromagnetism

    and nuclear interactions. Upon the division of the

    universe's forces, matter was created. Energetic Radiation

    The burning ball that gave rise to the universe remained a

    source of permanent radiation. Subatomic particles and

    antiparticles annihilated each other. The ball's high density

    spontaneously produced matter and destroyed it. Had this state

    of affairs continued, the universe would never have undergone the

    growth that scientists believe followed cosmic inflation.

    1 sec

    1

    A gluon interacts

    with a quark.

    2 Quarks join by means

    of gluons to form

    protons and neutrons.

    3 Protons and

    neutrons unite to

    create nuclei.

    Electron

    Negatively charged

    elemental particle

    ELEMENTARY PARTICLES

    In its beginnings, the universe was a soup of particles that interacted with each other

    because of high levels of radiation. Later, as the universe expanded, quarks formed the

    nuclei of the elements and then joined with electrons to form atoms.

    The neutrinos separate from the initial particle soup through the disintegration

    of neutrons. Though having extremely little mass, the neutrinos might

    nevertheless form the greatest part of the universe's dark matter.

    Proton

    Neutron

    Quark

    GluonUNIVERSE 13 12 WHAT IS THE UNIVERSE?

    380,000 500 million TIME

    (in years)

    TEMPERATURE 4,900° F (2,700° C) -405° F (-243° C)

    380,000 years after the big

    bang, atoms form. Electrons

    orbit the nuclei, attracted by

    the protons. The universe

    becomes transparent. Photons travel

    through space.

    8 Galaxies acquire their definitive

    shape: islands of millions and

    millions of stars and masses of

    gases and dust. The stars explode

    as supernovas and disperse heavier

    elements, such as carbon.

    9

    FIRST ATOMS

    Hydrogen and helium were the first elements to

    be formed at the atomic level. They are the main

    components of stars and planets. They are by far

    the most abundant elements in the universe.

    The vast span of time related to the history of

    the universe can be readily understood if it is

    scaled to correspond to a single year—a year

    that spans the beginning of the universe, the

    appearance of humans on the Earth, and the

    voyage of Columbus to America. On January 1

    of this imaginary year—at midnight—the big

    bang takes place. Homo sapiens appears at

    11:56 P.M. on December 31, and Columbus sets

    sail on the last second of the last day of the

    year. One second on this timescale is equivalent

    to 500 true years.

    1

    Hydrogen

    An electron is attracted by

    and orbits the nucleus, which

    has a proton and a neutron.

    Proton

    Neutron

    Electron

    NUCLEUS 2

    NUCLEUS 1

    2 Helium

    Since the nucleus

    has two protons,two electrons are

    attracted to it.

    3 Carbon

    With time, heavier and more complex elements

    were formed. Carbon, the key to human life, has six

    protons in its nucleus and six electrons orbiting it.

    Quasar

    Star

    cluster

    Nebula

    Elliptical

    galaxy

    Irregular

    galaxy

    Star

    Spiral

    galaxy

    Barred

    spiral

    galaxy

    Galaxy

    cluster

    COLUMBUS'S

    ARRIVAL

    takes place on

    the last second

    of December 31.

    THE SOLAR

    SYSTEM

    is created on

    August 24 of

    this timescale.

    BIG BANG

    occurs on the

    first second of

    the first day of

    the year.

    JANUARY DECEMBER

    13.7 billion

    -454° F (-270° C)

    The universe continues to expand. Countless galaxies

    are surrounded by dark matter, which represents 22

    percent of the mass and energy in the universe. The

    ordinary matter, of which stars and planets are

    made, represents just 4 percent of the total. The predominant

    form of energy is also of an unknown type. Called dark energy, it

    constitutes 74 percent of the total mass and energy.

    11

    DARK MATTER

    The visible objects in the

    cosmos represent only a

    small fraction of the total

    matter within the universe.

    Most of it is invisible even to

    the most powerful

    telescopes. Galaxies and their

    stars move as they do

    because of the gravitational

    forces exerted by this

    material, which astronomers

    call dark matter.

    THE UNIVERSE TODAY

    TIMESCALE

    The Transparent Universe

    With the creation of atoms and overall cooling, the once opaque and

    dense universe became transparent. Electrons were attracted by the

    protons of hydrogen and helium nuclei, and together they formed atoms.

    Photons (massless particles of light) could now pass freely through the

    universe. With the cooling, radiation remained abundant but was no longer the

    sole governing factor of the universe. Matter, through gravitational force, could

    now direct its own destiny. The gaseous lumps that were present in this

    process grew larger and larger. After 100 million years, they formed even

    larger objects. Their shapes not yet defined, they constituted protogalaxies.

    Gravitation gave shape to the first galaxies some 500 million years after the

    big bang, and the first stars began to shine in the densest regions of these

    galaxies. One mystery that could not be solved was why galaxies were

    distributed and shaped the way they were. The solution that astronomers have

    been able to find through indirect evidence is that there exists material called

    dark matter whose presence would have played a role in galaxy formation.

    9.1 billion THE EARTH IS CREATED

    Like the rest of the planets, the Earth is made of

    material that remained after the formation of the solar

    system. The Earth is the only planet known to have life.

    EVOLUTION OF MATTER

    What can be observed in the universe today is a great

    quantity of matter grouped into galaxies. But that was not

    the original form of the universe. What the big bang initially

    produced was a cloud of uniformly dispersed gas. Just three

    million years later, the gas began to organize itself into

    filaments. Today the universe can be seen as a network of

    galactic filaments with enormous voids between them.

    1

    Gaseous cloud

    The first gases

    and dust resulting

    from the Big Bang

    form a cloud.

    2 First filaments

    Because of the

    gravitational pull of dark

    matter, the gases joined

    in the form of filaments.

    3 Filament networks

    The universe has

    large-scale filaments

    that contain millions

    and millions of galaxies.

    9 billion

    -432° F (-258° C)

    Nine billion years after the big

    bang, the solar system

    emerged. A mass of gas and

    dust collapsed until it gave rise

    to the Sun. Later the planetary system was

    formed from the leftover material.

    10UNIVERSE 15 14 WHAT IS THE UNIVERSE?

    Everything Comes to an End

    T

    he big bang theory helped solve the enigma of the early moments of the universe. What has yet to

    be resolved is the mystery surrounding the future that awaits. To unravel this mystery, the total

    mass of the universe must be known, but that figure has not yet been reliably determined. The

    most recent observations have removed some of this uncertainty. It seems that the mass of the universe

    is far too little to stop its expansion. If this is this case, the universe's present growth is merely the last

    step before its total death in complete darkness.

    Black hole

    Universe 1

    Universe 1

    Universe 4

    Universe 3

    Black

    hole

    Universe 2

    Object in three

    dimensions

    Object that changes

    with time

    Universe 3

    New universe

    Inflection point

    DISCOVERIES

    The key discovery that led to the big

    bang theory was made in the early

    1920s by Edwin Hubble, who

    discovered that galaxies were moving

    away from each other. In the 1940s,George Gamow developed the idea

    that the universe began with a

    primordial explosion. A consequence

    of such an event would be the

    existence of background radiation,which Arno Penzias and Robert

    Wilson accidentally detected in the

    mid-1960s.

    There is a critical amount of mass

    for which the universe would

    expand at a declining rate without

    ever totally stopping. The result of this

    eternal expansion would be the existence of

    an ever-increasing number of galaxies and

    stars. If the universe were flat, we could

    talk about a cosmos born from an explosion,but it would be a universe continuing

    outward forever. It is difficult to think

    about a universe with these characteristics.

    Flat Universe

    BIG BANG

    BIG

    CRUNCH

    HOW IT IS MADE UP

    Dark energy is hypothesized to be

    the predominant energy in the

    universe. It is believed to speed up

    the expansion of the universe.

    BLACK HOLES

    Some theorists believe

    that, by entering a

    black hole, travel

    through space to

    other universes might

    be possible because of

    antigravitational

    effects.

    1

    The universe

    expands violently.

    2 The universe's

    growth slows.

    3 The universe collapses

    upon itself, forming a

    dense, hot spot.

    1

    The universe

    continuously

    expands and

    evolves.

    TIME

    2 The universe's

    expansion is

    unceasing but

    ever slower.

    2 Expansion is

    continuous and

    pronounced.

    3 Gravity is not

    sufficient to bring a

    complete stop to the

    universe's expansion.

    4 The universe

    expands indefinitely.

    1

    Self-generated

    Universes

    A less widely accepted theory about

    the nature of the universe suggests

    that universes generate themselves.

    If this is the case, universes would be

    created continuously like the branches of a

    tree, and they might be linked by

    supermassive black holes.

    According to this theory, universes

    continuously sprout other universes. But

    in this case, one universe would be

    created from the death or disappearance of

    another. Each dead universe in a final collapse, or

    Big Crunch, would give rise to a supermassive

    black hole, from which another universe would

    be born. This process could repeat itself

    indefinitely, making the number of universes

    impossible to determine.

    Baby Universes

    5

    Closed Universe

    If the universe had more than

    critical mass, it would expand

    until reaching a point where

    gravity stopped the expansion. Then,the universe would contract in the Big

    Crunch, a total collapse culminating in

    an infinitely small, dense, and hot spot

    similar to the one from which the

    universe was formed. Gravity's pull on

    the universe's excess matter would stop

    the expansion and reverse the process.

    2

    Open Universe

    The most accepted theory about

    the future of the cosmos says

    that the universe possesses a

    mass smaller than the critical value. The

    latest measurements seem to indicate that

    the present time is just a phase before the

    death of the universe, in which it goes

    completely dark.

    4 1

    After the original

    expansion, the

    universe grows.

    3 reaches a point where

    everything grows dark

    and life is extinguished.

    3

    74% dark energy

    22% dark matter

    4% visible matter

    GALACTIC EXPANSION

    By noting a redshift toward the red end of the

    spectrum, Hubble was able to demonstrate that

    galaxies were moving away from each other.

    1920s

    GAMOW'S SUSPICION

    Gamow first hypothesized the big bang,holding that the early universe was a

    “cauldron” of particles.

    1940s

    BACKGROUND RADIATION

    Penzias and Wilson detected radio signals

    that came from across the entire sky—the

    uniform signal of background radiation.

    1965

    THE HAWKING UNIVERSE

    The universe was composed originally of four

    spatial dimensions without the dimension of

    time. Since there is no change without time,one of these dimensions, according to Hawking,transformed spontaneously on a small scale

    into a temporal dimension, and the universe

    began to expand.UNIVERSE 17 16 WHAT IS THE UNIVERSE?

    The Forces of the Universe

    T

    he four fundamental forces of nature are those that are not derived from basic forces. Physicists

    believe that, at one time, all physical forces functioned as a single force and that during the

    expansion of the universe, they became distinct from each other. Each force now governs different

    processes, and each interaction affects different types of particles. Gravity, electromagnetism, strong

    nuclear interactions, and weak nuclear interactions are essential to our understanding of the behavior of

    the many objects that exist in the universe. In recent years, many scientists have tried with little success

    to show how all forces are manifestations of a single type of exchange.

    The universe, if it were empty, could be

    pictured in this way.

    The universe is deformed by the mass

    of the objects it contains.

    MOLECULAR MAGNETISM

    In atoms and molecules, the electromagnetic force is

    dominant. It is the force that causes the attraction

    between protons and electrons in an atom and the

    attraction or repulsion between ionized atoms.

    NEWTON'S EQUATION

    BENDING LIGHT

    Light also bends because of the curvature of space-time.

    When seen from a telescope, the real position of an object

    is distorted. What is perceived through the telescope is a

    false location, generated by the curvature of the light. It

    is not possible to see the actual position of the object.

    The biggest contribution to our comprehension of the universe's internal

    workings was made by Albert Einstein in 1915. Building on Newton's

    theory of universal gravitation, Einstein thought of space as linked to time. To

    Newton, gravity was merely the force that attracted two objects, but Einstein

    hypothesized that it was a consequence of what he called the curvature of space-

    time. According to his general theory of relativity, the universe curves in the

    presence of objects with mass. Gravity, according to this theory, is a distortion of

    space that determines whether one object rolls toward another. Einstein's general

    theory of relativity required scientists to consider the universe in terms of a non-

    Euclidian geometry, since it is not compatible with the idea of a flat universe.

    In Einsteinian space, two parallel lines can meet.

    General Theory of Relativity

    UNIVERSAL GRAVITATION

    The gravitation proposed by Newton is

    the mutual attraction between bodies

    having mass. The equation developed by

    Newton to calculate this force states

    that the attraction experienced by two

    bodies is directly proportional to the

    product of their masses and inversely

    proportional to the square of the

    distance between them. Newton

    represented the constant of

    proportionality resulting from this

    interaction as G. The shortcoming of

    Newton's law, an accepted paradigm

    until Einstein's theory of general

    relativity, lies in its failure to make time

    an essential component in the

    interaction between objects. According

    to Newton, the gravitational attraction

    between two objects with mass did not

    depend on the properties of space but

    was an intrinsic property of the objects

    themselves. Nevertheless, Newton's law

    of universal gravitation was a

    foundation for Einstein's theory.

    The strong nuclear force holds the protons and neutrons

    of atomic nuclei together. Both protons and neutrons are

    subject to this force. Gluons are particles that carry the

    strong nuclear force, and they bind quarks together to form

    protons and neutrons. Atomic nuclei are held together by

    residual forces in the interaction between quarks and gluons.

    Strong Nuclear Force

    3

    The weak nuclear force is not as strong as the other

    forces. The weak nuclear interaction influences the beta

    decay of a neutron, which releases a proton and a

    neutrino that later transforms into an electron. This force takes

    part in the natural radioactive phenomena associated with certain

    types of atoms.

    Weak Nuclear Force

    4

    Electromagnetism is the force that affects

    electrically charged bodies. It is involved in the

    chemical and physical transformations of the

    atoms and molecules of the various elements. The

    electromagnetic force can be one of attraction or repulsion,with two types of charges or poles.

    Electromagnetism

    2

    Gravity was the first force to

    become distinguishable from the

    original superforce. Today

    scientists understand gravity in Einstein's

    terms as an effect of the curvature of

    space-time. If the universe were thought of

    as a cube, the presence of any object with

    mass in space would deform the cube.

    Gravity can act at great distances (just as

    electromagnetism can) and always exerts a

    force of attraction. Despite the many

    attempts to find antigravity (which could

    counteract the effects of black holes), it

    has yet to be found.

    Gravity

    1

    E=mc2

    In Einstein's equation, energy and mass are

    interchangeable. If an object increases its

    mass, its energy increases, and vice versa.

    F=Gxm1xm2

    d2

    SUN

    EARTH

    What

    we see

    LUMINOUS TRAJECTORY

    Real

    position

    Two bodies with mass attract each other. Whichever

    body has the greatest mass will exert a greater force

    on the other. The greater the distance between the

    objects, the smaller the force they exert on each other.

    d

    F m1 m2

    Quark

    Force

    Positive

    pole

    Positive

    pole

    Negative

    pole

    Negative

    pole

    Nucleus

    Electron

    Helium

    Hydrogen

    Neutron WIMP

    Nucleus

    Proton

    HYDROGEN ATOM

    HELIUM ISOTOPE

    Gluon

    Force

    Proton

    Electron

    Electron

    1

    Quarks and gluons

    The strong nuclear interaction

    takes place when the gluon

    interacts with quarks.

    Attraction

    Two atoms are drawn together,and the electrons rotate

    around the new molecule.

    1

    Hydrogen

    A hydrogen atom interacts

    with a weak, light particle

    (WIMP). A neutron's

    bottom quark transforms

    into a top quark.

    2 Union

    Quarks join and form

    nuclear protons and

    neutrons.

    2 Helium

    The neutron transforms

    into a proton. An electron

    is released, and the helium

    isotope that is formed has

    no nuclear neutrons.THE FINAL DARKNESS 30-31

    ANATOMY OF GALAXIES 32-33

    ACTIVE GALAXIES 34-35

    STELLAR METROPOLIS 36-37

    What Is in the Universe?

    T

    he universe is populated on a

    grand scale by strands of

    superclusters surrounding

    vacant areas. Sometimes the

    galaxies collide with each

    other, triggering the formation of stars.

    In the vast cosmos, there are also

    quasars, pulsars, and black holes.

    Thanks to current technology, we can

    enjoy the displays of light and shadow

    that make up, for example, the Eta

    Carinae Nebula (shown), which is

    composed of jets of hot, fluorescent

    gases. Although not all the objects in the

    universe are known, it can be said

    without a doubt that most of the atoms

    that make up our bodies have been born

    in the interior of stars.

    LUMINOUS 20-21

    STELLAR EVOLUTION 22-23

    RED, DANGER, AND DEATH 24-25

    GAS SHELLS 26-27

    SUPERNOVAE 28-29

    ETA CARINAE NEBULA

    With a diameter of more than 200 light-years, it is

    one of biggest and brightest nebulae of our galaxy.

    This young, supermassive star is expected to become

    a supernova in the near future.100,000

    SUN

    Main

    sequence

    Supergiants

    Red giants

    White dwarfs

    10,000

    1,000

    100

    10

    1

    0.1

    0.01

    0.001

    0.0001

    INTRINSIC

    LUMINOSITY (SUN = 1)

    SPECTRAL CLASSES

    OB A F G KM

    UNIVERSE 21 20 WHAT IS IN THE UNIVERSE?

    Luminous

    F

    or a long time stars were a mystery to humans, and it was only as recently

    as the 19th century that astronomers began to understand the true nature

    of stars. Today we know that they are gigantic spheres of incandescent

    gas—mostly hydrogen, with a smaller proportion of helium. As a star radiates

    light, astronomers can precisely measure its brightness, color, and temperature.

    Because of their enormous distance from the Earth, stars beyond the Sun only

    appear as points of light, and even the most powerful telescopes do not reveal

    any surface features.

    COLORS The hottest stars are

    bluish-white (spectral classes

    O, B, and A). The coolest stars

    are orange, yellow, and red

    (spectral classes G, K, and M).

    Calcium Hydrogen Hydrogen Hydrogen Sodium

    Wavelength longest on the red side

    When a star moves toward or away from an observer, its

    wavelengths of light shift, a phenomenon called the Doppler effect.

    If the star is approaching the Earth, the dark lines in its spectrum

    experience a blueshift. If it moves away from the Earth, the lines

    experience a redshift.

    DOPPLER EFFECT

    SCORPIUS REGION

    0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 27 28 29 30

    PARSECS

    PRINCIPAL STARS WITHIN 100 LY FROM THE SUN

    SUN

    (G2)

    The H-R diagram plots the intrinsic

    luminosity of stars against their

    spectral class, which corresponds to their

    temperature or the wavelengths of light

    they emit. The most massive stars are

    those with greatest intrinsic luminosity.

    They include blue stars, red giants, and

    red supergiants. Stars spend 90 percent

    of their lives in what is known as the

    main sequence.

    Hertzsprung-Russell (H-R) Diagram

    In measuring the great distances

    between stars, both light-years (ly)

    and parsecs (pc) are used. A light-year is

    the distance that light travels in a year—

    5.9 trillion miles (10 trillion km). A light-

    year is a unit of distance, not time. A parsec

    is equivalent to the distance between the

    star and the Earth if the parallax angle is of

    one second arc. A pc is equal to 3.26 light-

    years, or 19 trillion miles (31 trillion km).

    Light-years and Parsecs

    When the Earth orbits the Sun, the closest stars

    appear to move in front of a background of more

    distant stars. The angle described by the movement of a

    star in a six-month period of the Earth's rotation is called

    its parallax. The parallax of the most distant stars are too

    small to measure. The closer a star is to the Earth, the

    greater its parallax.

    Measuring Distance

    The electromagnetic waves that make up light have

    different wavelengths. When light from a hot

    object, such as a star, is split into its different

    wavelengths, a band of colors, or spectrum, is obtained.

    Patterns of dark lines typically appear in the spectrum of

    a star. These patterns can be studied to determine the

    elements that make up the star.

    Spectral Analysis

    Dark lines deviate toward the blue end of the spectrum.

    BLUESHIFT of a star moving toward the Earth.

    OPEN CLUSTER

    The Pleiades are a formation of

    some 400 stars that will eventually

    move apart.

    GLOBULAR CLUSTER

    More than a million stars are

    grouped together into a spherical

    cluster called Omega Centauri.

    ALPHA

    CENTAURI

    (G2, K1, M5)

    SIRIUS

    (A0 and

    dwarf star)

    PROCYON

    (F5 and

    dwarf star)

    ALTAIR

    (A7)

    VEGA

    (A0)

    POLLUX

    (K0 giant)

    ARCTURUS

    (K2 giant)

    CAPELLA

    (G6 and G2

    giants)

    CASTOR

    (A2, A1, and M1)

    ALDEBARAN

    (K5 giant)

    ALIOTH

    (A0 giant)

    MENKALINAN

    (A2 and A2)

    ALGOL

    (B8 and K0)

    REGULUS

    (B7 and K1)

    25

    0 1 2345 67 89 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

    LIGHT-YEARS

    GACRUX

    (M4 giant)

    TYPE O

    52,000-72,000° F

    (29,000-40,000° C)

    TYPE B

    17,500-52,000° F

    (9,700-29,000° C)

    TYPE A

    13,000-17,500° F

    (7,200-9,700° C)

    TYPE F

    10,500-13,000° F

    (5,800-7,200° C)

    TYPE G

    8,500-10,500° F

    (4,700-5,800° C)

    TYPE K

    6,000-8,500° F

    (3,300-4,700° C)

    TYPE M

    4,000-6,000° F

    (2,100-3,300° C)

    Star Earth

    Wavelength is compressed by

    the movement of the star.

    Because the parallax

    of star A is small, we

    see that it is distant

    from the Earth.

    Position of

    the Earth in

    January

    Position of

    the Earth in

    July

    PARALLAX

    SUN

    A

    The parallax of star B

    is greater than that of

    star A, so we see that

    B is closer to the

    Earth.

    BA CLOUD OF GAS AND DUST collapses

    because of gravitational forces. In doing

    so it heats up and divides into smaller

    clouds. Each one of these clouds will

    form a protostar.

    PROTOSTAR

    A protostar has a

    dense, gaseous

    core surrounded by

    a cloud of dust.

    1.

    RED SUPERGIANT

    The star swells and heats up.

    Through nuclear reactions, a

    heavy core of iron is formed.

    3.

    NEUTRON STAR

    If the star's initial mass is between

    eight and 20 solar masses, it ends up

    as a neutron star.

    BLACK HOLE If the star's initial

    mass is 20 solar masses or more, its

    nucleus is denser and it turns into a

    black hole, whose gravitational force

    is extremely strong.

    5.

    5.

    PROTOSTAR

    A protostar is formed

    by the separation of gas

    and dust. Gravitational

    effects cause its core to

    rotate.

    1.

    PLANETARY NEBULA When

    the star's fuel is depleted, its

    core condenses, and its outer

    layers detach, expelling

    gases in an expanding shell

    of gases.

    WHITE DWARF

    The star remains

    surrounded by

    gases and is dim.

    5.

    22 WHAT IS IN THE UNIVERSE? UNIVERSE 23

    S

    tars are born in nebulae, which are giant clouds of gas (mainly hydrogen)

    and dust that float in space. Stars can have a life span of millions,or even billions, of years. The biggest stars have the

    shortest lives, because they consume their nuclear

    fuel (hydrogen) at a very accelerated rate. Other

    stars, like the Sun, burn fuel at a slower rate and

    may live some 10 billion years. Many times, a

    star's size indicates its age. Smaller stars are the

    youngest, and bigger stars are approaching

    their end, either through cooling or by

    exploding as a supernova.

    Stellar Evolution

    Nebula

    Small star

    Less than 8 solar masses

    STAR

    A star is finally born. It

    fuses hydrogen to form

    helium and lies along

    the main sequence.

    2.

    The evolution of a star depends on its mass. The

    smallest ones, like the Sun, have relatively long and

    modest lives. Such a star begins to burn helium when its

    hydrogen is depleted. In this way, its external layers

    begin to swell until the star turns into a red giant. It

    ends its life as white dwarfs, eventually fading away

    completely, ejecting remaining outer layers, and forming

    a planetary nebula. A massive star, because of its higher

    density, can form elements heavier than helium from its

    nuclear reactions. In the final stage of its life, its core

    collapses and the star explodes. All that remains is a

    hyperdense remnant, a neutron star. The most massive

    stars end by forming black holes.

    Life Cycle of a Star

    end their lives as white dwarfs. Other (larger)

    stars explode as supernovae, illuminating

    galaxies for weeks, although their brightness is

    often obscured by the gases and dust.

    STAR The star shines and

    slowly consumes its

    hydrogen. It begins to fuse

    helium as its size increases.

    2.

    RED GIANT The star continues to

    expand, but its mass remains

    constant and its core heats up.

    When the star's helium is depleted,it fuses carbon and oxygen.

    3.

    Massive star

    More than 8 solar masses

    95% of stars

    BLACK DWARF

    If a white dwarf

    fades out

    completely, it

    becomes a black

    dwarf.

    6.

    4.

    4.

    SUPERNOVA When the star can no longer

    fuse any more elements, its core collapses,causing a strong emission of energy.Red

    giant

    3 4

    5

    6

    7

    2

    3

    4

    6

    7

    LIFE CYCLE OF A

    STAR

    5

    When a star exhausts its hydrogen, it begins to die. The

    helium that now makes up the star's core begins to

    undergo nuclear reactions, and the star remains

    bright. When the star's helium is depleted, fusion of

    carbon and oxygen begins, which causes the star's

    core to contract. The star continues to live, though its

    surface layers begin to expand and cool as the star turns

    into a red giant. Stars similar to the Sun (solar-type stars)

    follow this process. After billions of years, they end up as

    white dwarfs. When they are fully extinguished, they will

    be black dwarfs, invisible in space.

    UNIVERSE 25 24 WHAT IS IN THE UNIVERSE?

    Red, Danger, and Death

    DIAMETER

    All stars go through a red-giant

    stage. Depending on a star's

    mass, it may collapse or it may simply

    die enveloped in gaseous layers. The

    core of a red giant is 10 times smaller

    than it was originally since it shrinks

    from a lack of hydrogen. A supergiant

    star (one with an initial mass greater

    than eight solar masses) lives a much

    shorter life. Because of the high density

    attained by its core, it eventually

    collapses in on itself and explodes.

    Red Giant

    HERTZSPRUNG-RUSSELL

    When a white dwarf leaves the

    red-giant stage, it occupies the

    lower-left corner of the H-R

    diagram. Its temperature may be

    double that of a typical red giant.

    A massive white dwarf can

    collapse in on itself and end its

    life as a neutron star.

    HYDROGEN

    Hydrogen continues undergoing

    nuclear fusion in the exterior of

    the core even when the inner

    core has run out of hydrogen.

    HELIUM Helium is produced by the fusion

    of hydrogen during the main

    sequence.

    CARBON AND OXYGEN

    Carbon and oxygen are produced

    by the fusion of helium within the

    core of the red giant.

    1

    2

    3

    SUN

    Convection

    Cells

    Convection cells carry heat toward

    the surface of a star. The ascending

    currents of gas eventually reach

    the surface of the star, carrying

    with them a few elements that

    formed in the star's core.

    Hot Spots

    Hot spots appear when large

    jets of incandescent gas

    reach the star's surface.

    They can be detected on the

    surface of red giants.

    REGION OF THE CORE

    TEMPERATURE

    As the helium undergoes fusion,the temperature of the core

    reaches millions of degrees

    Fahrenheit (millions of degrees

    Celsius).

    4

    Venus's orbit

    Mercury's orbit

    Earth's orbit

    Mars's orbit

    Jupiter's orbit

    Saturn's orbit

    Red supergiant. Placed

    at the center of the

    solar system, it would

    swallow up Mars and

    Jupiter.

    Red giant. Placed at

    the center of the solar

    system, it could reach

    only the nearer planets,such as Mercury,Venus, and the Earth.

    1% The scale of the

    diameter of the Sun

    to the diameter of

    a typical red giant.

    After going through the red-giant stage, a solar-type star loses its

    outer layers, giving rise to a planetary nebula. In its center remains a

    white dwarf—a relatively small, very hot (360,000° F [200,000° C]), dense

    star. After cooling for millions of years, it shuts down completely and

    becomes a black dwarf.

    White Dwarf

    On leaving the main sequence,the star enlarges to 200 times

    the size of the Sun. When the

    star begins to burn helium, its

    size decreases to between 10 and

    100 times the size of the Sun.

    The star then remains stable until

    it becomes a white dwarf.

    SPECTACULAR DIMENSIONS

    HERTZSPRUNG-RUSSELL

    When the star exhausts its

    hydrogen, it leaves the main

    sequence and burns helium

    as a red giant (or a

    supergiant). The smallest

    stars take billions of years to

    leave the main sequences.

    The color of a red giant is

    caused by its relatively cool

    surface temperature of

    3,600° F (2,000° C).

    Sun

    NEBULaNGC 6751

    After the nuclear reaction in the star's core ceases, the star

    ejects its outer layers, which then form a planetary nebula.

    WHITE DWARF

    Mars Venus

    Sun

    Earth Mercury

    Mars Venus

    Sun

    Earth Mercury

    Mars Venus

    Sun

    Earth

    Like any typical star, the Sun burns hydrogen

    during its main sequence. After taking

    approximately five billion years to exhaust its

    supply of hydrogen, it will begin its

    transformation into a red giant, doubling in

    brightness and expanding until it swallows

    Mercury. At its maximum size, it may even

    envelop the Earth. Once it has stabilized, it will

    continue as a red giant for two billion years and

    then become a white dwarf.

    THE FUTURE OF THE SUN

    Dust Grains

    Dust grains condense in the star's outer

    atmosphere and later disperse in the form of

    stellar winds. The dust acquires a dark

    appearance and is swept into interstellar

    space, where new generations of stars will

    form. The outer layer of the star may

    extend across several light-years of

    interstellar space.

    RED GIANT

    The radius of the

    Sun reaches the

    Earth's orbit.

    Earth

    1

    2Gas Shells

    HOURGLASS

    HELIX

    SPIROGRAPH

    When a small star dies, all that remains is an expanding

    gas shell known as a planetary nebula, which has

    nothing to do with the planets. In general,planetary nebulae are symmetrical or spherical

    objects. Although it has not been possible to

    determine why they exist in such diversity, the reason

    may be related to the effects of the magnetic field of the

    dying central star. Viewed through a telescope, several

    nebulae can be seen to contain a central dwarf star, a mere

    remnant of its precursor star.

    UNIVERSE 27

    The two rings of colored

    gas form the silhouette of

    this hourglass-shaped

    nebula. The red in the

    photograph corresponds to

    nitrogen, and the green

    corresponds to hydrogen.

    This nebula is 8,000 light-

    years from the Earth.

    MYCN 18

    NGC 7293

    BUTTERFLY

    The density of a white dwarf is a million

    times greater than the density of water.

    In other words, each cubic meter of a

    white dwarf star weighs a million tons.

    The mass of a star is indirectly

    proportional to its diameter. A white

    dwarf with a diameter 100 times smaller

    than the Sun has a mass 70 times greater.

    M2-9

    The Butterfly Nebula contains

    a star in addition to a white

    dwarf. Each orbits the other

    inside a gas disk that is 10

    times larger than Pluto's

    orbit. The Butterfly Nebula

    is located 2,100 light-years

    from Earth.

    IC 418

    The Spirograph Nebula has a

    hot, luminous core that

    excites nearby atoms,causing them to glow. The

    Spirograph Nebula is about

    0.1 light-year wide and is

    located 2,000 light-years

    from Earth.

    White

    Dwarf

    The remains of the red

    giant, in which the

    fusion of carbon and

    oxygen has ceased, lie

    at the center of the

    nebula. The star slowly

    cools and fades.

    Hydrogen

    The continuously expanding

    masses of gas surrounding the

    star contain mostly hydrogen,with helium and lesser amounts

    of oxygen, nitrogen, and other

    elements.

    Concentric

    circles

    of gas, resembling the inside of an onion, form a

    multilayered structure around the white dwarf.

    Each layer has a mass greater than the combined

    mass of all the planets in the solar system.

    TWICE THE

    TEMPERATURE OF

    THE SUN

    is reached at the surface of a

    white dwarf, causing it to

    appear white even though its

    luminosity is a thousand times

    less than that of the Sun.

    is the weight of a single

    tablespoon of a white

    dwarf. A white dwarf is

    very massive in spite of

    the fact that its

    diameter of 9,300 miles

    (or 15,000 km) is

    comparable to the

    Earth's.

    The astrophysicist

    Subrahmanyan

    Chandrasekhar, winner of the

    Nobel Prize for Physics in

    1983, calculated the maximum

    mass a star could have so that

    it would not eventually collapse

    on itself. If a star's mass exceeds

    this limit, the star will eventually

    explode in a supernova.

    CHANDRASEKHAR

    LIMIT

    1.44 SOLAR MASSES

    is the limit Chandrasekhar

    obtained. In excess of this value,a dwarf star cannot support its

    own gravity and collapses.

    NGC 6542 CAT'S EYE

    26 WHAT IS IN THE UNIVERSE?

    SMALLER DIAMETER

    More massive white dwarf

    LARGER DIAMETER

    Less massive white dwarf

    DENSITY OF A WHITE DWARF

    The Helix is a

    planetary nebula that

    was created at the

    end of the life of a

    solar-type star. It is

    650 light-years from

    the Earth and is

    located in the

    constellation Aquarius.

    Planetary

    nebula

    1

    2

    3 4

    5

    6

    7

    2

    3

    4

    6

    7

    LIFE CYCLE OF

    A STAR

    5

    3 tons28 WHAT IS IN THE UNIVERSE?

    Supernovae

    Asupernova is an extraordinary explosion of a giant star at

    the end of its life, accompanied by a sudden increase

    in brightness and the release of a great amount

    of energy. In 10 seconds, a supernova releases 100

    times more energy than the Sun will release in its

    entire life. After the explosion of the star that gives

    rise to a supernova, the gaseous remnant expands and

    shines for millions of years. It is estimated that, in our Milky

    Way galaxy, two supernovae occur per century.

    Supernova

    GAS AND DUST

    Gas and dust that have

    accumulated in the two visible lobes

    absorb the blue light and ultraviolet

    rays emitted from its center.

    FUSION

    The nuclear

    reactions in a

    dying star occur

    at a faster rate

    than they do in a

    red giant.

    GASEOUS FILAMENTS

    Gaseous filaments are ejected

    by the supernova at 620 miles

    (1,000 km) per second.

    THE END

    Either a neutron star

    or a black hole may

    form depending on the

    initial mass of the star

    that has died.

    Stellar Remnant

    When the star explodes as a supernova, it

    leaves as a legacy in space the heavy

    elements (such as carbon, oxygen, and iron) that

    were in the star's nucleus before its collapse. The

    Crab Nebula (M1) was created by a supernova

    seen in 1054 by Chinese astronomers. The Crab

    Nebula is located 6.5 light-years from Earth and

    has a diameter of six light-years. The star that

    gave rise to the Crab Nebula may have had an

    initial mass close to 10 solar masses. In 1969, a

    pulsar radiating X-rays and rotating 33 times per

    second was discovered at the center of the

    nebula, making the Crab Nebula a very powerful

    source of radiation.

    The explosion that marks the

    end of a supergiant's life occurs

    because the star's extremely heavy

    core has become incapable of

    supporting its own gravity any longer.

    In the absence of fusion in its interior,the star falls in upon itself, expelling

    its remaining gases, which will expand

    and shine for hundreds—or even

    thousands—of years. The explosion of

    the star injects new material into

    interstellar space and contributes

    heavy atoms that can give rise to new

    generations of stars.

    The Twilight of a Star

    Supergiant

    The diameter of the star may

    increase to more than 1,000

    times that of the Sun. Through

    nuclear fusion, the star can

    produce elements even heavier

    than carbon and oxygen.

    When a star's iron core

    increases in density to 1.44

    solar masses, the star can

    no longer support its own

    weight and it collapses

    upon itself. The resulting

    explosion causes the

    formation of elements that

    are heavier than iron, such

    as gold and uranium.

    Other Elements

    Core

    A star's core can be seen to

    be separated into distinct

    layers that correspond to

    the different elements

    created during nuclear

    fusion. The last

    element created

    before the star's

    collapse is iron.

    DENSE

    CORE

    CRAB NEBULA

    Explosion

    The star's life ends in an immense

    explosion. During the weeks

    following the explosion, great

    quantities of energy are radiated

    that are sometimes greater than the

    energy emitted by the star's parent

    galaxy. A supernova may illuminate

    its galaxy for weeks.

    ETA CARINAE SUPERMASSIVE

    The mass of Eta Carinae

    is 100 times greater

    than that of the Sun.

    Astronomers believe

    that Eta Carinae is

    about to explode,but no one knows

    when.

    The image at left shows a sector of

    the Large Magellanic Cloud, an

    irregular galaxy located 170,000 light-

    years from the Earth, depicted before

    the explosion of supernova 1987A. The

    image at right shows the supernova.

    BEFORE AND AFTER

    FEBRUARY

    23, 1987

    After the supernova

    explosion, increased

    brightness is

    observed in the

    region near the star.

    FEBRUARY 22,1987

    This star is in its last

    moments of life. Because it

    is very massive, it will end

    its life in an explosion. The

    galaxy exhibits only its usual

    luminosity.

    LIFE CYCLE OF

    A STAR 1

    2

    3 4

    5

    6

    7

    2

    3

    4

    6

    7

    5UNIVERSE 31 30 WHAT IS IN THE UNIVERSE?

    The Final Darkness

    T

    he last stage in the evolution of a star's core is its

    transformation into a very dense, compact stellar body.

    Its particulars depend upon the amount of mass

    involved in its collapse. The largest stars become black

    holes, their density so great that their gravitational

    forces capture even light. The only way to detect these

    dead stars is by searching for the effects of their

    gravitation.

    Discovery of Black Holes

    The only way of detecting the presence of

    a black hole in space is by its effect on

    neighboring stars. Since the gravitational force

    exerted by a black hole is so powerful, the gases

    of nearby stars are absorbed at great speed,spiraling toward the black hole and forming a

    structure called an accretion disk. The friction

    of the gases heats them until they shine

    brightly. The hottest parts of the accretion disk

    may reach 100,000,000° C and are a source of

    X-rays. The black hole, by exerting such

    powerful gravitational force, attracts everything

    that passes close to it, letting nothing escape.

    Since even light is not exempt from this

    phenomenon, black holes are opaque and

    invisible to even the most advanced telescopes.

    Some astronomers believe that

    supermassive black holes might have

    a mass of millions, or even

    billions, of solar masses.

    When a star's initial mass is between

    10 and 20 solar masses, its final mass

    will be larger than the mass of the Sun.

    Despite losing great quantities of matter

    during nuclear reactions, the star finishes

    with a very dense core. Because of its intense

    magnetic and gravitational fields, a neutron

    star can end up as a pulsar. A pulsar is a

    rapidly spinning neutron star that gives off a

    beam of radio waves or other radiation. As

    the beam sweeps around the object, the

    radiation is observed in very regular pulses.

    tons is what one tablespoon of a

    neutron star would weigh. Its small

    diameter causes the star to have a

    compact, dense core accompanied by

    intense gravitational effects.

    1 billion

    RED GIANT

    A red giant leaves

    the main sequence.

    Its diameter is 100

    times greater than

    the Sun's.

    1

    SUPERGIANT

    A supergiant grows

    and rapidly fuses

    heavier chemical

    elements, forming

    carbon, oxygen, and

    finally iron.

    2

    EXPLOSION

    The star's iron core

    collapses. Protons

    and electrons

    annihilate each other

    and form neutrons.

    3

    DENSE CORE

    The core's exact

    composition is

    presently unknown.

    Most of its

    interacting particles

    are neutrons.

    4

    Pulsars

    The first pulsar (a neutron star radiating

    radio waves) was discovered in 1967.

    Pulsars rotate approximately 30 times per second

    and have very intense magnetic fields. Pulsars

    emit radio waves from their two magnetic poles

    when they rotate. If a pulsar absorbs gas from a

    neighboring star, a hot spot that radiates X-rays

    is produced on the pulsar's surface.

    Devouring gas from

    a supergiant

    Located within a binary system, the pulsar can

    follow the same process as a black hole. The pulsar's

    gravitational force causes it to absorb the gas of

    smaller, neighboring stars, heating up the pulsar's

    surface and causing it to emit X-rays.

    CURVED SPACE

    THE SUN forms a shallow

    gravitational well.

    1

    A WHITE

    DWARF

    generates a

    deeper

    gravitational

    well, drawing

    in objects at a

    higher speed.

    2

    Accretion Disk

    An accretion disk is a gaseous accumulation

    of matter that the black hole draws from

    nearby stars. In the regions of the disk

    very close to the black hole, X-rays are

    emitted. The gas that accumulates

    rotates at very high speeds. When

    the gases from other stars ......