A Physical Universe: an Introduction to Astronomy Book Redesign

Frank H. Shu

THE PHYSICAL

UNIVERSE:

AN INTRODUCTION

TO

ASTRONOMY

THE PHYSICAL UNIVERSE: AN INTRODUCTION TO ASTRONOMY Frank H. Shu

Professor of Astronomy University of California, Berkely

To my parents

BERKELY FRANK H. SHU PROFESSOR OF ASTRONOMY, UNIVERSITY OF CALIFORNIA,

THE PHYSICAL UNIVERSE AN INTRODUCTION TO ASTRONOMY

PART 1:

BASIC PRINCIPLES

PAGE.3 1.THE BIRTH OF SCIENCE THE CONSTELLATIONS AS NAVIGATIONAL AIDS................3 THE CONSTELLATIONS AS TIMEKEEPING AIDS................4 THE RISE OF ASTROLOGY...........9 THE RISE OF ASTRONOMY...........9 MODERN ASTRONOMY..10 ROUGH SCALES OF THE ASTRONOMICAL UNIVERSE...........11 CONTENTS OF THE UNIVERSE...........12

PAGE.33 2.THE GREAT LAWS OF MICROSCOPIC PHYSICS MECHANICS.........33 QUANTUM MECHANICS.........16 SPECIAL RELATIVITY........20

PAGE.62 4.THE GREAT LAWS OF MACROSCOPIC PHYSICS

PAGE.14 3.CLASSCAL MECHANICS / LIGHT / ASTRONOMICAL TELESCOPES CLASSICAL MECHANICS..........14 THE NATURE OF LIGHT..............16 ASTRONOMICAL TELESCOPES.........20

THERMODYNAMICS....33 STATISTICAL MECHANICS.........64 THERMODYNAMIC BEHAVIOR OF MATTER............66 THERMODYNAMIC BEHAVIOR OF RADIATION.........77 AN EXAMPLE........80 PHILOSOPHICAL COMMENT...........80

PART 2: THE STARS

PAGE.81 5.THE SUN AS A STAR

PAGE.125

COPYRIGHT 1982

7.END STATES OF STARS

BY UNIVERSITY

WHITE DWARF.......126 NEUTRON STARS.....129 BLACK HOLES.......134 CONCLUDING PHILOSOPHIC REMARK.........143

Reproduction or translation of any part of this work beyond that permitted

by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.

Requests for permission or further information should be addressed to the Permission Department, University Science Books, P.O. Box 605.

THE ATMOSPHERE OF THE SUN............84 THE INTERIOR OF THE SUN............86 THE CHROMOSPHERE OF AND CORONA OF THE SUN............96 THE RELATIONSHIP OF THE SUN TO OTHER STARS AND TO US...100

PAGE.102 6.NUCLEAR ENERGY / SYNTHESIS OF THE ELEMENTS MATTER AND THE FOUR FORCES............103 NUCLEAR FORCES AND NUCLEAR REACTIONS.........108 SPECULATION ABOUT THE FUTURE........123

PAGE.144 8.EVOLUTION OF THE STARS THEORETICAL H-R DIAGRAM...........145 EVOLUTION OF LOW-MASS STARS.............147 EVOLUTION OF HIGH MASS STARS........153 CONCLUDING PHILOSOPHIC REMARK.........157

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PART. THREE

GALAXIES & COSMOLOGY

Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from the Spitzer Space Telescope, scientists have discovered that the Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

The annotated artist’s concept illustrates the new view of the Milky Way. The galaxy’s two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located just between the major arms.

for ma

an,

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THE MATERIAL BETWEEN THE STARS

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B

etween the stars in the universe lies a vast amount of interstellar material in the form of both gas and dust. The interstellar gas and dust medium of our Galaxy probably has a massively mass of several billion M 0. Thus, the total mass of the material between the stars is a non-negligible fraction—only a few percent—of the cummulative mass of all the visible stars in the Galaxy. Yet the presence of interstellar matter, both gas and dust, is much less obvious than the presence of stars, since the mass contained in ordinary stars has been compacted by self-gravity into a readily observable dense state. In contrast, the interstellar gas and dust is spread very thinly over the vast distances between the stars; this diffused gas is much more rarefied than the best, so-called “vacuums” produced in terrestrial laboratories. Selfgravity plays a relatively minor role for this gas, and it should not even surprise us to learn that even astronomers were slow to realize the existence of interstellar gas and dust in the Galaxy.

The Discovery of Interstellar Dust More than 200 years ago William

or faulty data. The conclusive proof

Herschel described Tholes in the sky.-

for the presence of a general and se-

where there were an apparent deficit

lective absorption came in 1930, with

of stars (Figure 11.1). The most plau-

the epochal work of R. S. Trumpler on

sible explanation for this deficit was

the properties of star clusters.

the existence of obscuring material

We

may

reproduce Trumpler’s

along the line of sight which blocked

reasoning as follows. Let D be the

the light from the back-ground stars.

real (linear) diameter of an open

Claims were occasionally made for

cluster and r equal its distance. Since

the definitive discovery of other ob-

D should be independent of the open

servational evidence for this absorp-

cluster’s distance r, we expect the

tion, but careful follow-up analyses

angular diameter of an open cluster

invariably showed these claims to be

to be 0 = Direction.so as to keep it de-

based on either incorrect reasoning

creaseing with its increasing distance

THE DISCOVERY OF INTERSTELLAR DUST

and there is also scattering about the

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theoretical line due to intrinsic variations and systematic departure. Thus, we may expect that the square root of its angular diameter,

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d = (D/r�)

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of) an open cluster should also vary as the inverse square of its distance from us, because, on the average, the

systematic departure of the observed

intrinsic brightness (luminosity) L of a

points from the theoretical line. This

typical cluster should be independent

might be explained in several ways.

of its position relative to the Earth.

(a) Perhaps far-away clusters look

Thus, if apparent brightnessos of

intrinsically bigger than their mea-

open clusters arc plotted ‘Tains t an-

sured apparent brightnesses f would

gfIlp r diameters squared, we should

predict. If this were a real effect, then

expect theoretically to see a straight-

our position in the universe would be

line relation (Figure 11.2a). The actual

special, since open clusters which are

result is shown schematically in

close to the Earth would then he in-

Figure 11.2b. The inter-pretation of this

trinsically small, and those which are

result is the following.

far away would be intrinsically large.

First, there is scatter about the the-

Such a special location for Earth has

oretical line, because there must he

been anathema for astronomers since

intrinsic variations of the luminosities

Copernicus, and this interpretation

L and diameters D of open clusters

may therefore be rejected.

about some mean values. (That is,

(b) Could this effect be one of ob-

some open clusters are intrinsically

servational selec-tion? That is, could

more less luminous than average and

this effect arise because observers

some are intrinsically bigger or small-

find it easier to see intrinsically big

er than average.) Second, there is a

open clusters than intrinsically small

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ones if they are far away? Trumpler was well aware of such observational

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biases, and he was able to show convincingly that his data set did not con-tain errors of this kind. (c) Perhaps far-away clusters look fainter than their measured squares of

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THE MATERIAL BETWEEN THE STARS

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angular diameters 92 would predict.

The Copernican viewpoint prevents us from thinking that this could be a

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real effect. (d) The remaining possibility is, then, that far-away clusters have been dimmed by a general obscuration of starlight which increases with in-

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creasing distance, as more and more obscuration occurs along the line of sight. This interpretation is the one accepted today, and it was the one

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given by Robert Julius Trumpler.

Astronomers now know that this obscuring material is in the form of small solid specks, dust grains whose

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chemical composition may be silicates (like sand) or carbon-containing compounds (like graphite or silicon carbide). The obscuration of starlight

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is believed to arise from a. combination of true absorption and scattering, and this combination goes by the general name of star.

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Interstellar reddening occurs because blue light is scattered out of a beam of starlight directed toward us mere than red light is. A similar effect occurs in the Earth’s atmosphere.

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because of the scattering of sunlight by

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atmospheric

molecules.

The

light from the setting Sun (or rising

plain interstellar reddening, because

Sun) has to travel through a greater

these processes are not exercised in

column of air and undergoes more

the operation throughout the seper-

scattering and more reddening than

ating sequences. The obscuration of

light from the noon Sun (Figure 11.4).

starlight is also believed to arise from

On a microscopic level (Chapter 3),

a. combination of true absorption and

we understand this result to arise

scattering, and this combination goes

because air molecules interact more

by the general name of every stars.

strongly with blue light than with red

In other words, whereas the wave-

light and, therefore, scatter blue light

lengths of the absorption lines from

preferentially out of the sunbeams,

the photospheres of the stars in the

leaving mostly reddened sunlight.

binary system shifted back and forth

Exactly the same process cannot ex-

as the stars revolved in their orbits, there were also absorption lines of the

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stars whose wavelengths remained stationary and does not change.

THE DISCOVERY OF INTERSTELLAR GAS

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fig. 11.1

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The Coalsack nebula in the southern Milky Way gives the impression of a large region in the sky where there is a marked deficit of stars. In fact, the apparent “hole in the sky” is eplained by the obscuration of the light of many background stars by and intervening cloud of dark material.

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The Discovery of Interstellar Gas Historically, the conclusive evi-

interstellar lines there-fore appear

dence for the existence of interstellar

“stationary” relative to the changing

gas was presented somewhat earlier

pattern of lines presented by the

than the evidence for interstellar dust.

spectroscopic binary.

In 1904, Hartmann dis-covered that a

Hartmann’s

hypothesis

of

an

set of absorption lines of once-ionized

intervening gas cloud did not gain

calcium did not undergo the periodic

immediate acceptance. because the

Doppler shifts of the absorption lines

ab-sorbing gas might have resided

in the spectrum of a spectroscopic

near the star in question rather than

binary. In other words, whereas the

a long distance away. This issue of

wavelengths of the absorption lines

whether the absorbing gas is “cir-

from the photospheres of the stars

cumstellar- or “interstellar-remains

in the binary system shifted back and

difficult to resolve in any one ease

forth as the stars revolved in their

even today. The “interstellar” inter-

orbits, there were also absorption

pretation was demonstrated by Plas-

lines whose wavelengths remained

kett, Struve, Eddington, and Bok. who

stationary. Hartmann con-cluded that

showed that the ionization stages, or

the “stationary lines” arose from ab-

Galactic distribution, or veloci-ties

sorption produced by a cold interstel-

of the “stationary lines” were often

lar cloud of gas which lay between the

incompatible.

binary system and Earth (Figure 11.5).

Further insight into the interstellar

There may be also a fixed displace-

gas was provided by Beats, Adams,

ment of the inter-stellar absorption

Munch, and Zirin, who found that

line from the position of a similar line

many stars show multiple interstellar

produced in a terrestrial laboratory,

absorption lines, i.e., the same lines,

because the cloud may have a com-

say, of once-ionized calcium at sev-

ponent of velocity lin along the line

eral different Doppler velocities and

of sight. However, the latter velocity

inter-stellar lines, or an approximate

does not vary on the timescale which

a spherical dust grain of radius R to

characterizes the orbital motion of

have a cross-sectional area 7R’ for

the background binary system. The

blocking visual light.

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THE MATERIAL BETWEEN THE STARS

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Different Optical Manifestations of Gaseous Nebulae We now know that the dust and the

ble to or smaller than the wavelength

gas in interstellar space are intimately

of visual light, but it is valid enough for

mixed, and that both generally reside

our purposes here.) A photon mean-

in clouds or complexes of clouds.

free-path I is defined to be the length

The ratio of dust mass to gas mass in

between successive encounters with

the Galaxy is about I percent. Since

dust grains (contrast with Problem

the gas itself is only a few percent of

9.11). If the number density of dust

the mass of the stars in the Galaxy,

grains is n, show that I = 1innR2. If a

interstellar dust constitutes a very

beam of photons from a star travels

minor fraction of the total mass of

a distance i toward an observer, the

our Galaxy. Nevertheless, this dust

beam will suffer an extinction in

has a great influence on how we see

intensity by a factor of e = 2.718 .... Ex-

the Galaxy, because it obscures so

tinction observations indicate that at

much starlight in many directions.

the position of the Sun in the Galaxy. I

Problem 11.1. A rough estimate of the

equals about 3.000 light-years. Given

mass fraction of dust in the Galaxy

the estimate R = 10 5 cm, calculate the

proceeds as follows. Approximate

value of n in units of cm’. How many

a spherical dust grain of radius R to

interstellar dust grains would you

have a cross-sectional area 7R’ for

expect to find in a volume equal to a

blocking visual light. (This “shadow”

football stadium? (Assume a typical

formula breaks down if R is compara-

dimension of 100 meters in all three directions.) Solid material typically has a mass density of 2 gm/cm’.

Estimate now the mass ni of a typical inter-stellar dust grain. Let V be the

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DIFFERENT OPTICAL MANISFETATIONS OF GASEOUS NEBULAE

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TIME BOX

1.1

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A page full of surprises Fun things you’ll want to know Interstellar dust?

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THE MATERIAL BETWEEN THE STARS

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fig. 11.2

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volume in which one typically finds

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one solar mass of stars in the Galaxy.

The dark gobules lie in front of the extended background of light from the Rosette Nebula.

If V = 300 (It-yr)3, what is the mass M = nVm of dust grains in this same volume? What, then, is the mass frac-

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tion of dust compared to stars at the solar position in the Galaxy? (In actual practice, the average mass fraction is even smaller, because the dust is confined to a layer in the Galaxy whichis

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about three times thinner than the stars.) The appearance of clouds of gas and dust depends, in part, on the

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wavelength regime in which they are

Crab nebula, an amorphous region

ob-served and, in part, on how close

which emits continuum light by the

they are to neighboring stars. We first

synchrotron process may coexist with

explore the different optical appear-

the filamentary structure. The radio

ance of interstellar gas clouds. Such

emission from supernova remnants is

gas clouds are also generally called

invariably nonthermal, but the X-ray

gaseous nebulae (See Box 11.2). BOX

emission and optical-line emission

11.2 Optical Classification of Gaseous

may arise from thermal pro-cesses

Nebulae Dark nebulae: observed by

in a shock-heated gas. Dark Nebulae

the obscuration of background stars

A gas and dust cloud placed in a rich

or some other background wIlik.11

field of back-ground stars would block

is otherwise bright (such as an Ha

most or all of the starlight behind it.

region). Reflection nebulae: observed

We would easily see many stars to the

by the scattered light from embedded

sides of the dust cloud (and a few in

stars. The spectrum is the (reflected)

front of it); hence such a dark nebulae

absorption-line

the

would manifest itself as one of Her-

embedded stars. His regions: bright

schel’s “holes in the sky” (Figure 11.6).

ionized regions surrounding newborn

Especially interesting among the dark

hot and bright stars (of spectral type

nebulae are the round ones studied

0 and B). The spectrum is dominated

by Barnard and by Bok. The regular

by emission lilies. Thermal radio-con-

shapes of the “Bok globules” suggest

tinuum emission is found. Planetary

that these objects are self-gravitating,

nebulae: similar to Hu regions, but

and led Bok to propose that they are

the exciting object is a very hot

probably sites where new stars are

evolved star in the throes of death.

forming. This suggestion is almost

Planetary nebulae also tend to be

certainly correct for the dark clouds

denser and more compact than opti-

in giant complexes, as we will discuss

cal Hit regions. Supernova remnants:

later. Whether isolated Bok globules

optical emission usually strongest

are :ilso collapsing to form stars is be-

from filaments, whose spectra are

ing debated now. Reflection Nebulae

domi-nated by emission lines. In a

A gas and dust cloud which surrounds

young supernova remnant like the

a star or a group of stars can shine by

spectrum

of

DIFFERENT OPTICAL MANISFETATIONS OF GASEOUS NEBULAE

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reflected light. This effect was demon-

limit (see Chapter 3) can ionize the

strated observationally by Hubble,

hydrogen atoms. The part of an in-

and explained theoretically in terms of

terstellar cloud where hydrogen has

scattering from dust grains by Russell

been once-ionized (the maxi-mum

in 1922 (see Figure 11.7). The reflection

possible for hydrogen atoms) is called

nebula Problem 11.2. If you were to

an Hu re-gion. The roman numeral a

take a spectrum of a reflec-tion neb-

distinguishes once-ionized hydrogen,

ula, would you see absorption lines,

Ha, from neutral atomic hydrogen,

emission lines, or no spectral lines?

Hi. Bengt Stromgren showed that

How would this help to show that the

the division between Hu regions and

illumination is by reflection from the

Hi regions will he quite sharp if the

central star? Thermal Emission Neb-

surrounding gas cloud is so massive

ulae: Htt Regions Hydrogen atoms in

that all the ultraviolet photons from

an interstellar gas cloud located near

the central 0 or B star are used up

a very hot star- -say, a newborn 0 or

before the Ha region can encompass

B star--will be exposed to the copious

the entire cloud Figure 11.8). Inside

outpouring of ultraviolet ra-diation

the Hit region, the hydrogen plasma

from such a star. Ultraviolet photons

is constantly trying to recombine to

with ener-gies greater than the Lyman

form neutral hydrogen atoms, but

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the plasma is kept almost completely ionized by the continued outpouring of ultraviolet photons from the central source. These ultraviolet photons break apart any newly formed hydro-

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gen atoms. and the ions and electrons so formed keep recombining to form new atoms. The part of the Hit region where the ultraviolet output of the

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central star is able to keep a balance between recombina-tion and ionization is called the Stromgren sphere.

Problem 11.3. To calculate the size of

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the Stromgren sphere, idealize the problem by considering a pure hydrogen gas of uniform density.

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fig. 11.3

The Coalsack nebula Milky Way gives the large region in the is a marked deficit

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in the Southern impression of a sky where there of stars.

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“

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CLUSTERS OF GALAXY AND THE EXPANSION OF THE UNIVERSE

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S

o far we have discussed galaxies as if they were isolated entities, free to pursue their evolution apart from the influence of other galaxies. In practice, just as there are interacting binary stars, so are there actually interacting binary galaxies in the universe. Just as there are clusters of stars, so are there also clusters of galaxies.

Interacting Binary Galaxies Strongly interacting pairs of galax-

galaxy can then pull out material from

ies constitute a very small percentage

the near side of the larger galaxy into

of all galaxies, but the more spec-

a bridge which temporarily spans the

tacular examples produce intriguing

gulf between the two galaxies (Figure

structures which are not present in

14.2). Except for the noncircular orbit

single galaxies (Figure 14.1). Spectac-

and the flattened original distribution

ular examples of bridges, tails, and

of matter, the bridges are analogous

rings have been catalogued observa-

to the mass-transfer streams that form

tionally by Vorontsov-Velyaminov and

in semidetached binaries (Chapter 11).

by Arp. Our theoretical understanding

The close encounter of two bound

of such systems has been advanced

spiral galaxies, containing much ener-

greatly by the numerical simulations

gy in the ordered forms of spinning

carried out by Hohnberg, Alladin,

disks and orbiting centers, must tend

Toomre, Wright, and others.

eventually to produce a merger into a by

single pile of stars, containing much

Toomre and Toomre of closely grav-

The

computer

simulations

more energy in the disordered form

itating disk galaxies are especially

of random motions. On the other

interesting. Tvhe greatest commotion

hand, if the encountering galaxies

results from orbits which cause the

have nearly equal masses, one long

spinning near edge of a participating

tail from each galaxy can develop

disk to travel in the same direction as

that generally extends away from the

the passing galaxy. If the two galaxies

main bodies (Figure 14.3). This nonin-

have very different masses, the small

tuitive result arises because the tidal

MERGERS

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If we pursue the closer analogy

ters between two bound galaxies

with inter-acting binary stars, long

must tend to bring the two galaxies

tails are analogous to mass lost from

closer together, consistent with the

the outer Lagrangian points, L2 and

constraints of the conservation of

L3 (consult Figure 10.7). Long bridges

total angular momentum and total

and tails are best produced when the

energy. This expectation is based on

orbit of the two galaxies are bound,

the second law of thermodynamics,

so that the two systems are not fly-

which when applied to trillions of

ing past one another too fast when

stars still states that order tends to be

they reach closest approach. The

replaced by disorder (Chapter 4). The

interstellar gas clouds might bang

close encounter of two bound spiral

together inelastically, would tend to

galaxies, containing much energy in

conserve its total energy. In the much

the ordered forms of spinning disks

more rare case when the two bodies

and orbiting centers, must tend

of the galaxies interpenetrate, very

eventually to produce a merger into

exotic looking “ring galaxies” can be

a single pile of stars, containing much

formed. The conclusion that interstel-

more energy in the disordered form

lar extinction and reddening are both

of random motions. This merger pro-

caused by interstellar dust.

cess involves a form of “violent re-

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laxation,” in which violently changing

Mergers

gravitational fields help to produce a final (relaxed) smooth distribution of

Very close encounters between

stars. Except in having relatively great

much

amounts of interstellar gas and dust,

internal motion in them. The energy

such a pile of stars would probably

to

strongly resemble an elliptical galaxy.

galaxies

obviously

produce

these

excite motions

must

come from the orbital motion. In a

Francois

Schweizer

has

found

system consisting of a collection of

an excellent candidate for such a

gravvitating points (stars), kinetic

merger process (Figure 14.5). In

energy of motion cannot be dissipat-

some expo-sures, the system looks

ed, eventually to leave the system as

like a crumpled spider with no close

radiation. In an encounter between

neighbors. The central body of the

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two galaxies, the individual stars fly right by one another, suffering only gravitational

deflections

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produced

by the entire collection of stars. The interstellar gas clouds might bang together inelastically, would tend to

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conserve its total energy. The stars might, however, transform one kind of energy into another kind, say, orbital energy into random motions.

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Indeed, it can be argued on statistical pounds that repeated encoun-

fig. 14.1

Two numerical simulations of ring-producing encounters. An interpretation of a disk galaxy by another massive body yields rippling waves of rings.

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CLUSTERS OF GALAXIES AND THE EXPANSION OF THE UNIVERSE

fig. 14.2

Successfully deeper photographs (a—e) of NGC7525 reveals a comple set of filaments surrounding a central body that resembles a giant elliptical galaxy. maintained for more than a few times 108 years. If each such interacting pair

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of galaxies (which are probably gravitationally bound) eventually leads to a merger, then during 1010 years, we

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can expect on the order of 400 of the present 4,000 NGC galaxies to have resulted

from

such

coales-cence.

In other words, mergers of spiral

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galaxies would cause 10 percent of all galaxies to become elliptical gal-axies. The actual fraction of ellipticals is more like 20 or 30 percent, and many of these ellipticals are to be

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found in rich clusters; however, it is conceivable that mergers were either more common in the past than now or more common in clusters than in

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the general field.

As attractive as Toomre’s proposal

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system looks surprisingly like a giant

is, several strong objections to it

elliptical galaxy. The luminosity of the

have been raised. The most damaging

system is equal to that of two lumi-

concern the systematic properties

nosity class I spirals. We apparently

of giant elliptical galaxies—such as

have here the gravitational merger of

the Fish-Freeman relation and the

two giant spiral galaxies into a single

Faber-Jackson relation (Chapter 13)—

pile of stars. After the gas and dust

which would be difficult to explain

has been completely used up to form

in terms of random mergings of a

more stars, the resulting system will

variety of spiral galaxies. In particular,

probably be classified as ordinary.

one would naivety expect the merged

Alar Toomre has speculated that

product to have fewer central stars

perhaps all elliptical galaxies formed

than the constituent spirals, because

in this way. His argument is beguiling,

the excess orbital energy must he

and proceeds as follows. Of the 4,000

absorbed into the merger product

or so NGC galaxies, perhaps a dozen

for hierarchial clustering, supposing

are interacting systems exhibiting

m to have radius R and gravitational

spectacular bridges or tails. On the

attraction of M. For this reason do sin-

other hand, the numerical simulations

gle galaxies develop spiral structure

show that such geometric forms are

(Chapter 12). For this reason binary

transient phenomena that cannot be

stars spiral to fuse to a single object.

HIERARCHIAL CLUSTERING

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Astronomy compels the Hierarchial Clustering soul to look upwards and leads us from this world to another.

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As catalogued by Abell, Zwicky,

and others, rich clusters of galaxies may contain several thousand galax-

ies that extend over a radius of ten

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million light-years. On even larger

scales, say, a radius of a hundred million

light-years,

astronomers

have discovered another level to this

hierarchical clustering. It is not known whether clusters of superclusters exist or not. From detailed statistical

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Rich Clusters of Galaxies

analyses of galaxy counts by Shane

About seven times further away

and Wirtanen, Peebles find examples

than Virgo is the Coma cluster, the

of clustering at all sizes,up to 60 mil-

nearest

lion light-years, beyond the amount

thousands of galaxies. Most of the

of clustering drops dramatically. The

galaxies in the Coma cluster are

closest fairly rich cluster to ourselves

ellipticals or SOs. Rood estimated

is the Virgo cluster, located about 50

that only 15 percent of the systems in

million light-years away in the direc-

Coma are spirals or irregulars. Since

tion of the constellation of Virgo (Fig-

iron is believed to be synthesized

ure 14.7). About 200 bright galaxies

only in the deep interior of massive

reside in the Virgo cluster, of which

stars which are destined to supernova

68 percent are spirals, 19 percent are

current belief is that this gas comes

ellipticals, and the rest are irregulars

from the galaxies of the cluster. There

or unclassified. Although spiral’s are

are several theoretical ways in which

more numerous in the Virgo chister,

gas spewed from supernovae might

the four brightest galaxies in Virgo are

eventually find its way into the cluster

ellipticals: among them is, of course,

mediums. There appears to be some

the infamous M87 (Chapter 13).

general features of rich clusters: they

great

cluster

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containing

Plato

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CLUSTERS OF GALAXIES AND THE EXPANSION OF THE UNIVERSE

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are deficient in spiral galaxies, which are quite common in the field and in poorer groups. This mechanism would work for both isolated ellip-

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ticals and those in rich clusters. In

Galactic Cannibalism

spiral galaxies, there is probably too

A defining property of a cD

much gas in the disk to maintain a

galaxy is the possession of a very

very high temperature. In the more

distended envelope of stars. A good

general case, gas in the inner regions

example is NGC6166, a radio galaxy

flows toward the nucleus and gas in

that resides in the Abell cluster 2199.

the outer regions blows away in a

This cD galaxy has a visible radius of

galactic wind. The Coma cluster has

about one million light-years, roughly

another characteristic that is shared

20 times larger than an ordinary

by many rich clusters: the presence of

giant elliptical or spiral. Oemler has

one or two very luminous supergiant

performed surface photometry on the

elliptical galaxies near the center of

extended envelopes of cD galaxies.

the cluster. Such a supergiant ellipti-

He finds that they typically drop off

cal (called a cD galaxy for historical

with in-creasing distance from the

reasons which are not particu-larly

center at a slower rate than given by

illuminating)

dominate

de Vaucouleurs law (equation 13.1),

the appearance of the whole cluster.

will

often

which is only valid for ordinary and

Despite the fact that cD systems are

elliptical galaxies.

intrinsically quite rare, they are the

Tidal stripping is simple to un-

most common optical counterpart

derstand. An object of mass m and

member of the universe.

radius R, which is held together by self-gravitation and approaches within a distance r of a massive body of M, will be ripped apart by tidal forces naturally when r becomes too small.

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HIERARCHIAL CLUSTERING

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box AN EDUCATIONAL PROBLEM OF THE

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INTERNATIONAL AERONAUTICS AND SPACE EDUCATION

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JOURNEY GALAXY

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to the

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CLUSTERS OF GALAXIES AND THE EXPANSION OF THE UNIVERSE

Thus, a cD galaxy with a mass M which is 500 times bigger than its victim’s mass m will begin to rip the

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latter’s stars from it at a radius r which is 10 times the victim’s radius R. This process

presumably

explains

fig. 14.3

The central region of the Coma Cluster contains two supergiant ellipticals. These cD systems may have grown bloated by cannibalizing their smaller neighbours.

the

formation of the extensive envelope

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around a cD galaxy, although not all the shredded material need be captured gravitationally by the cD galaxy.

Some of the stars may enter into orbit about the cluster as a whole cluster of

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stars (see Figure 14.13).

Equation (14.1) has a simple physical interpretation. Imagine spreading the mass m of the small galaxy

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uni-formly over a sphere of radius R equal to its size. The average density would

be

3m/47R3. Astronomers

now know that a direct collision

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between two galaxies of comparable size is a relatively rare event: so the odd optical appearance of Cygnus A more probably results from the

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recent gobbling of a gas-rich ordinary galaxy by a supergiant elliptical.

Imagine spreading the mass Al of the big galaxy over a sphere of radius r equal to the distance of the small

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galaxy. The average density would be 3M/4rrr3. Equation (14.1) can now be interpreted as stating that the Roche limit occurs when the average density

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of the small galaxy is twice that of the large galaxy. Now, we see a potential limitation to the applicability of equation (14.1). Galaxies do have

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spread-out mass distributions, and a simple estimate gives:

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r = (2M/m)”R (14.1)

HIERARCHIAL CLUSTERING

of dynamical friction is not new. In

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gravitating systems, it originated with the work of Chandrasekhar on stellar dynamics. Chandrasekhar’s own work was inspired by Einstein’s paper on

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Brownian motion. In Chapter 9, we pointed out that encounters between stars of different masses in a star The rarefied outer portions of the

cluster tend to bring about a state

small galaxy will exceed the Roche

of equipartition of energy, where

criterion at relatively large distances r

high-mass stars have low random

from the big galaxy. and tidal stripping

speeds and low-mass stars have

will operate rather efficiently on the

high speeds. Such a thermodynamic

matter there. The dense cores of even

distribu-tion is actually brought about

small galaxies. however, will have to

by two opposing processes. Random

plummet quite far into the heart of

gravitational scatterings of stars by

a cD system before they encounter

one another cause them statistically

interior densities comparable to their

to random walk to higher and higher

own. Since such plungitz, orbits are

velocity dispersions. However, each

a priori rare, how does a cD.galaxy

star also suffers dynamical friction

manage to gobble the cores of other

as described in Figure 14.14, and this

galaxies? By the second course of the

tends to reduce their random veloci-

meal: dynamical friction. For gravitat-

ties. The balance between diffusion

ing bodies, dynamical friction arises

to higher random velocities and drag

for the reason outlined. As a heavy

to lower ones leads, in a steady state,

dense core m (approximated to be a

to a statistical distribution where

point mass) moves through a medium

stars of a given mass tend to have a

containing stars (the envelope of the

certain velocity dispersion (Box 14.1).

cD galaxy), the core deflect the stars it

The relation between the velocity

passes. These deflec-tions statistically

dispersions and masses of stars is

tend to give a slight excess of stars in

given by the principle of equipartition

back of in. (In the language of plasma

of energy, contrast to a star cluster.

physics, the presence of the heavy

Moreover, the stellar masses are

mass polarizes the ambient medium.)

minuscule in comparison with that of

This excess mass pulls on the mass

the system as a whole. Under galactic

m, and thereby tends to reduce its

conditions, neither velocity diffusionv

motion relative to the distribution

nor dynamical friction have enough

of stars. The net effect of dynamical

time to affect the behavior of the stars

friction, therefore, is to bring the

in the universe.

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galactic core to the center of the cD galaxy. A simple estimate gives: thus the galactic cores’ ambient densities comparable to its own and is chewed

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up by tidal forces, releasing its stars to join the others in the central region of the cD system. The concept

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PART. FOUR THE SOLAR SYSTEM & LIFE

On July 20, 1969, astronaut Neil Armstrong put his left foot on the rocky Moon. It was the first human footprint on the Moon.

People all over the world watched when it happened.

Right image: The first footprints on the Moon will be there for a million years. There is no wind to blow them away.

The two astronauts walked on the Moon. They picked up rocks and dirt to bring back to Earth. The astronauts had much work to do.

Then, the Eagle went back to meet astronaut Collins.

He was in the Command Module working.

Apollo 11 splashed down in the Pacific Ocean on July 24, 1969. The astronauts were safe at home.

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THE SOLAR SYSTEM

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I

n this book so far, we have taken a long journey to explore the universe, and finally have reached the most extreme outposts of observable space and the earliest instants of measurable time. It is now time for us pull back now. Let us return past the Big Bang, past the quasars, past the distant clusters of galaxies, past the nearby groups of galaxies, past the Local Group of galaxies, past the interstellar dusts and gases, into our own Milky Way system of stars and gas clouds, into our own solar system. It’s been a very long journey; it’s really good to be back.

The first bodies we encounter as

during recent spacecraft missions

we approach the solar system are fro-

which fiew past Saturn have we got-

zen balls of gas and dust that exist in

ten a really clear look at this spectac-

the billions, usually at a considerable

ular ringed giant. The next planet in,

fraction of the distance to the nearest

Jupiter, is even more massive. Its face

stars. Look there! There’s one of

is covered by an intricate system of

those frozen balls that has wandered

zones and belts, and a giant red spot

too close to the Sun; it’s taken the

further attests to the fierce winds and

familiar, but ever-fascinating, form

storms which wrack this lord of the

of a comet. That’s Pluto. Usually it’s

planets. Like any regal figure, Jupiter

the most distant planet from the Sun,

is surrounded by a retinue of satel-

but during 1979-2000, it’s eccentric

lites; fifteen known moons, great and

orbit has actually carried it inside

small, with lesser bodies quite likely

the orbit of Neptune (Figure 17.2b).

to be discovered in the future. Four of

Also inside Neptune lies Uranus, the

these moons were known to Galileo:

seventh planet of the solar system.

Io, Europa, Ganymede, and Callisto.

Pluto, Neptune, and Uranus are all so

These grand moons are large enough

far away from us that their properties

that they would be called planets in

are relatively poorly known. For ex-

their own right were they not dwarfed

ample, it was only in 1977 that a team

by the great lord.

of astronomers led by James Elliott

Inside the orbit of Jupiter lies a vast

discovered in an airborne observation

wasteland, littered by the debris of

that Uranus was surrounded by a set

failed planets: the asteroids. What is

of thin rings; and it was only in 1978

the true story of their failure? Closer

that James Christy discovered on

yet to the Sun lies the fourth planet,

several high quality photographs that Pluto possessed a satellite (named Charon, the mythical figure who ferried souls across the river Styx to the

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god of the underworld, Pluto).

The sixth planet, Saturn, is close enough and bright enough that it was known to the ancients. But only

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THE SOLAR SYSTEM

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Mars, considered the god of war by

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the ancients because of its fierce red color. Its extraordinarily scarred face bear swims to an active past. Let’s press on, past the third planet, and

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as we approach the Sun, we pass the second planet, Venus, considered by the ancients to be the goddess of love.

A thick veil of sulfuric-acid clouds hides her face from us, and modern

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explorations have revealed that her passions are too fiery for mortal man. Closest to the Sun is Mercury.

Fleetest of the planets, Mercury, the

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smallest planet in the Solar System was considered by the ancients to be the messenger of the gods. How did he acquire such a pitted face?

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At the center of the solar system lies the true monarch of the realm, the mighty Sun. So plentiful are the Sun’s riches that in one second it squanders more energy than lies in oil under the sands of all Arabia (Chapter 5).

Even great Jupiter bows under the

fig 17.1

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A galactic crash captured by the Hubble Telescope

influence of the Sun. As we swing

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around the Sun, we pass Mercury and Venus, and approach again the third planet. We see that she has a faithful

flows over its surface. X-ray pictures

companion, the Moon. A serene if

of rich clusters taken by the Hubble

pock-marked figure, the Moon shines

Telescope find that they present two

by reflected light from the Sun. Last,

morphological classes. The first type

the third planet comes into view, the

has a smooth distribution of cluster

Earth. An insignificant speck of dust

gas, very similar to what simple the-

in this immense universe, yet in our

oretical models predict. The second

eyes, no more beautiful sight exists

shows a clumpy appearance, with the

than Earth, our home, the only planet

gas concentrated in lumps around

which provides us with liquid that

individual galaxies.

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THE SOLAR SYSTEM

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Inventory of the Solar System p.417

Planets & Their Satellites Table 17.1 gives a quick inventory of

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the nine planets which circle the Sun, and shows that the planets divide into two categories: the inner or terrestrial planets, which are small and have a

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mean density of 4-5 gm/cm’; and the outer or Jovian planets, which are large (except for Pluto) and have a mean density of 1-2 gm/cm’.

Each of the Jovian planets-Jupiter,

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Saturn, Uranus, and Neptune-has more than one moon. Jupiter has at least fifteen moons; the thirteenth was discovered in 1974 in ground-based

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observations conducted by Charles Kowal; the fourteenth and fifteenth, in the 1979 fiyby of Voyager 1. Saturn has at least twenty-two moons, only nine

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of which were known before space missions to this spectacular planet Uranus has five known moons; and Neptune, two. However, more small

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moons may be discovered as future and present spacecraft inspect these giant planets at closer range.

In addition to extensive satellite

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systems, the Jovian planets may also all possess rings. The rings of Saturn were discovered by Galileo in 1610, but their true geometry was

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not understood until Huygens offered the correct solution in 1655, and in 1856 Maxwell showed theoretically that the rings must consist of many independent particles. Uranus was

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discovered to have rings by stellar occultation studies in 1977 (Figure

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fig 17.2 Lunar surface hit by meteorites of the past and present. These crater will last for a very long time as there is no wind to blow them away.

INVENTORY OF THE SOLAR SYSTEM

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p.416 17.4), and direct imaging during the Voyager missions revealed Jupiter to have rings in 1979 (Figure 175). We

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may have to await until 1989, when Voyager 2 is scheduled to encounter Neptune, to discover whether this

Celestial mechanics experiments

Minor Planets or Asteroids, Meteoroids, Meteors & Meteorites

plus ultraviolet photometry by Pioneer

In addition to the nine major

11 showed that the total mass of Sat-

planets and their satellites, there are

urn’s rings cannot exmd 3 millionths

between 10‘ and 106 minor planets or

of the mass of the planet. It used to

asteroids (perhaps more) With orbits

be thought that this mass consisted

that lie between Mars and Jupiter at

mostly of small icy specks; however,

distances of 2 2 to 3 3 astronomical

analysis of radio waves transmitted

units from the Sun. There may also

through the rings during the Voyager

be minor planets further from the

1 flight past Saturn revealed many

Sun, like Chiron (discovered by Kowal

boulders with diameters of several

in 1977), whose orbit lies mostly

meters. ln contrast, the rings of Jupi-

between Saturn and Uranus. The total

ter and Uranus are probably made of

mass of all the asteroids has been

much smaller and less reflective par-

estimated by Kresak to be less than

ticulate material compared to Saturn.

10” 3 of the mass of the Earth where

fourth Jovian planet also has faint rings like Jupiter and Uranus.

The

terrestrial

planets-Mercury,

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we human call home.

Venus, Earth, and Mars-have no rings

Although most of the asteroids

and very few moons. Mercury has no

lie in a “belt” between Mars and

moon; Venus also has none; Earth has

Jupiter, there are some-called me-

one (Luna or the Moon); and Mars has

teoroids-which have Earth-crossing

two (Phobos and Deimos, “Fear” and

orbits. Figure 17 6a shows schemat-

“Panic,” the chariot horses of the god

ically the prevalent theory about

of war). Moreover, in comparison with

the production of such meteoroids

the large moons of the Jovian planets,

from asteroids. The basic idea is that

the moons of the terrestrial planets,

occasionally asteroids collide with

except for Luna, are quite small.

one another. These collisions can

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shatter the bodies, and some of the fragments may be thrown into resonant interactions with Jupiter (see Chapter 18) that ultimately give them

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Earth-crossing orbits, like our Earth’s lunar orbit, the moon.

A meteoroid which actually intersects the Earth and enters the Earth’s

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atmosphere will heat up from the friction generated in the passage. It will then appear as a fiery, as they call it, “shooting star”, is called a meteor.

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THE SOLAR SYSTEM

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If the mass of the meteor is less than 10“ ‘° gm, it may slow down so fast that it survives the flight; if so, it is called a micrometeorite. On the

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other hand, if the mass of the meteor

Meteorites come in three basic

is greater than 103 gm (a kilogram), it

types, which depend on their chemical

may have enough material to survive

composition: “stones,” “stony irons,”

the ablation and also make it to the

and “irons”. Stones resemble rocks;

ground. If subsequently found, such

stony irons have some metal-rich

an object is called a meteorite. The

inclusions; and irons contain mostly

impact of an especially large meteor

metals like iron and nickel. Fascinat-

may create an enormous crater (Fig-

ing among the stones are a subclass

ure 17.6c). The famous pock-marked

called the carbonaceous chondrites

surface appearances of Mercury and

(Figure

the Moon which look like pores (Fig-

contain carbon-bearing compounds

ures 17.3 and 17.6d) attest to the heavy

(“organic” compounds) in rounded

bombardment that all the terrestrial

inclusions called chondrules. Ordi-

planets must have suffered early in

nary white dwarfs are, of course, dead

their history, and to this time still suf-

stars, whose primary support comes

fer from it. The lack of wind erosion on

from the degeneracy pressure of the

Mercury and the Moon have merely

free electrons. We shall discuss the

preserved a more vivid record of the

significance of these findings for life

cratering process than on the planets

in the universe in Chapter 20. Indeed,

Venus, Earth, and Mars, like the foot-

since the two small moons of Mars

print left by man on the Moon.

are probably captured asteroids, the

17.6e).

These

meteorites

Earth-Moon system is unique among the terrestrial planets, and ought to be regarded, not as a planet-satellite system, but as a double planet.

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Box 15.2

INVENTORY OF THE SOLAR SYSTEM p.415

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THE SOLAR SYSTEM

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Comets

convection or by conduction, not by

Greek name for “long-haired star.”

radiative diffusion (in the interiors).

For naked-eye observers, comets are

Finally, the internal temperatures

one of astronomy’s most spectacular

of planets are far too low to allow

displays (Color Plate 38). Figure 17.7

thermonuclear reactions (see Chapter

shows schematically some features

8, which gave the lower mass limit

commonly found in comets: a head

for luminous stars as 0.08MO). The

consisting, it is believed, of two

loss of internal heat therefore either

parts-a nucleus and a coma-plus one

must lead to a gradual cooling of the

or two tails. The nucleus of the comet

planet, or must be made up by other

is the essential part, since it is the

sources (e.g, radioactive decay, slow

ultimate source of all the mass.

gravitational contraction). Let us now

In the most widely accepted theory

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place in them also, but primarily by

The word comet derives from the

examine these issues in more detail.

of comets, Fred Whipple proposed

In any case, Jan Oort proposed that

that the nucleus is composed of

there are billions of such small bodies

chunks of dust and frozen ices of

in a “cloud” of about 105 AU in radius

compounds such as methane (CH4),

surrounding the Sun. Some of these

ammonia (NH 3), water (H 20), and

have very elongated orbits, and these

carbon dioxide (Co2). Whipple has

occasionally suffer a gravitational

likened the nucleus of a comet to a

perturbation from a nearby passing

“dirty snowball,” but given that it may

star which sends the body closer to

span a few kilometers across, a “dirty iceberg” might be a more appropriate

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name. In more recent models, the cometary nucleus is thought to have a rocky core, surrounded by a mantle of “dirty ices”. Planets are not now

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collapsing gravitationally; so they must also be in a state of mechanical balance. This balance differs, however, from that of the Sun, in that the matter in the interior of a planet

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resembles nothing like a perfect gas.

The interiors of planets contain solid or liquid matter, or a combination of both. As a consequence, mechanical

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balance in planets is largely independent of heat transfer and energy balance. Because the interiors of planets are generally still hotter than

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their surfaces, heat transfer does take

fig 17.2 Comet 67P/Churyumov-Gerasimenko shot by Rosetta’s OSIRIS narrowangle camera on 3 August from a distance of 285 km.

INVENTORY OF THE SOLAR SYSTEM

When we meet people who are astronauts or deal in astronomy, it’s always really fascinating.

be thrown out of the solar system

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altogether; see Figure 9.18 for an analogy.) Indeed, astronomers know of some 30-odd “Apollo objects,” which are asteroid-like bodies whose

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orbits cross the orbit of Earth. Some scientists believe these objects to be the exposed rocky cores of such “extinct” comets. Wetherill has estimated

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that such Apollo objects strike the Earth once every 250,000 years or so, releasing an amount of energy equal to 100,000 megaton nuclear bombs.

the Sun. When the comet approaches

No such collision has occurred during

within a critical distance of the Sun,

recorded history, but some prehistoric

the ices in a skin around the mantle

impacts may have produced some of

vaporize and form a huge ball of

the larger craters that exist on Earth,

expanding fluorescent gas. This coma

and an especially destructive collision

constitutes the part of the head that is

may have led to the extinction of the

visible on Earth. Two processes then

dinosaurs (Chapter 20). In Chapter 5,

contribute to the development of the

we learned that the Sun represents a

tail(s). Radiation pressure can push

mass of gas in which:

Legend, John

on the dust particles embedded in the

(a) the inward pull of self-gravita-

expanding coma; and as Ludwig Bier-

tion is balanced by the internal (ther-

mann was the first to realize, a solar

mal) pressure of the gas (mechanical

wind can blow past the ionized gas

equilibrium);

and drag on it. These two processes

(b) heat diffuses outward from the

sweep the gas and dust into one or

hot interior, eventually to leave the

two long tails which generally point

surface in the form of freely escaping

radially away from the Sun (Figure

photons (heat transfer); and

17.8). Notice, in particular, that the

(c) the central temperature is raised

tails do not necessarily correspond to

high enough to release enough nucle-

the direction of motion of the comet.

ar energy to balance the heat trans-

Some comets, like Hailey’s comet,

ferred outwards (energy generation

make periodic returns. These must

and thermal equilibrium). Planets are

ultimately have all their volatile com-

not now collapsing gravitationally; so

pounds outgassed from the mantle.

they must also be in a state of me-

(Other comets may be perturbed

chanical balance. This balance differs,

by a close passage to Jupiter and

however, from that of the Sun, in that

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the matter in the interior of a planet resembles nothing like a perfect gas.

The interiors of planets, though, contain solid or liquid matter.

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