Download PSC100 Summary Chapters 10 to Chapter 20

Summary and Important Ideas from Discovery by Seeds, 10th Ed
for Astronomy 100 Classes
Summer 1009
Chapters 10 through 20
Chapter 10 - The Deaths of Stars
The life cycle of most stars begins with the onset of the nuclear fusion of
hydrogen into helium. As long as this process continues, the star is said to be in its
MAIN SEQUENCE LIFE TIME. This time that a star remains on the main
sequence is determined by the mass of the star in a rather peculiar way. The more
massive the star, the hotter the core and the faster the hydrogen fuel is used up.
Our Sun will be on the main sequence for about 10 billion years, but a star 15
times as massive will be in its hydrogen-burning stage for only about 1/100 of that
time. As long as hydrogen fuel is available in the core of a star, enough radiation
pressure is continually produced to balance the force of gravity, and the star
experiences a long period of very stable equilibrium.
When the hydrogen gas in the star's core is depleted, the radiation pressure
suddenly drops, and gravity collapses the core of the star, producing higher
temperatures and densities in the core that push the outer layers of the star outward.
These outer layers cool so that the star not only appears much larger but also takes
on a reddish coloration. This period of a star's life is called the RED GIANT
phase. Meanwhile the core is really much hotter due to the energy produced by
gravitational collapse of the central region of the star. This higher temperature
makes it possible for a new nuclear reaction to begin which converts the helium
which was produced in the previous fusion reaction into carbon, oxygen, silicon,
and other heavier elements up to the atom iron in the periodic table. The "burning"
of helium and other light elements increases the radiation pressure and again brings
the star into equilibrium for a period of time about equal to 1/3 of its main
sequence lifetime.
Finally all of the lighter elements have been converted into heavier atoms
such as iron, and there is no more nuclear fuel left. This time the fuel crisis is fatal,
and the force of gravity collapses the star into its final form. There are three
possible ends for a star after its nuclear fuel has been completely exhausted. The
star can become either a WHITE DWARF, a NEUTRON STAR or a BLACK
HOLE. Which of these end products a star becomes is determined entirely by the
mass of the star. If the star has a mass less than 1.4 times the mass of our Sun, it
will become a simple dead core called a black dwarf, because it is very small and
no longer gives off any visible light. The difference between a WHITE DWARF
and a BLACK DWARF is in the light that is given off from the star's surface. A
white dwarf is the very hot remnant of the collapse of a small red-giant star (mass
less than 1.4 solar masses). The core is still so hot that it give off all of the colors
in the visible spectrum of light and so appears to be a "white" color. As this core
cools, it gives off less and less radiation, and its color slowly changes from white
hot to reddish. Finally it is so cool that NO visible light is given off anymore. At
this point the core is known as a "black" dwarf because it no long gives off any
detectable light. Black dwarfs remain forever as the cold, dead, "cinder-like"
remains of stars like our Sun.
Note that a few of the lowest mass stars on the main sequence do not have
enough mass to produce an ignition of hydrogen burning in their cores. These
stars, known as BROWN DWARF main sequence stars never get hot enough to
ignite hydrogen fusion in their cores and thus never get further than a
gravitationally heated small star that radiates mostly in the infra-red and
eventually simply cools off and remain small, cold gas balls forever.
Chapter 11 - Neutron Stars and Black Holes
If the star is between 1.4 and about 5 solar masses, it will become a neutron
star, so named because its core is made up entirely of neutrons. In the process of
becoming a neutron star, a tremendous shock wave is produced by the final
gravitational collapse of the core, and a spectacular event called a SUPERNOVA
occurs. In these spectacular supernova explosions, it is believed that all of the
existing elements heavier than iron in the periodic table are formed. If the star is
more massive than about 5 solar masses, there is no known way to stop the final
collapse of the star's core, and it becomes a strong supermassive entity with so
strong a gravitational pull that nothing, not even light waves, can escape. The
supermassive entity is called a BLACK HOLE. The text describes the processes
leading to these final stellar states in more detail, so be sure that you study this
material carefully.
It is also possible for some massive stars to "thrown off" some of their mass
before the final collapse. The process involve here are the SOLAR WINDS,
OUTER LAYER LOSSES, in the red giant phase, and PLANETARY NEBULAE.
The latter of these produce beautiful glowing rings around the hot remaining cores
of dwarf stars. Seen through our giant telescopes, these planetary nebulae are
some of the most beautiful objects in the sky.
This chapter also shows in detail how the life cycle of a star can be traced on
an H-R or TEMPERATURE-LUMINOSITY DIAGRAM. Using and H=R
diagram, we can explain and predict many things about a star such as its main
sequence lifetime, its luminosity as a red giant, its passage through certain
instability regions where its luminosity will vary in a regular way; producing
cepheid and RR Lyrae variable stars, and even the final state that the star will
attain after its death. Careful study of the placement of all of the stars in a stellar
cluster on an H-R diagram can even be used to determine the age of that star
cluster. Again these topics are described in detail in the text, so look them over
well.
This chapter continues the discussion of the end results of stellar evolution.
After a supernova explosion, medium mass stars are left with a massive core that
can no longer withstand the collapsing force of gravity in their normal atomic
form. When gravity is too strong, electrons can no longer stay in orbit around the
nuclei of individual atoms. as these atoms collapse, the electrons and protons
combine in the nucleus to form more and more neutrons until, in a very short
period of time, the only thing left is a gigantic nucleus composed of nothing but
neutrons. This final en-product of a star's evolution is simply called a NEUTRON
STAR. Some neutron stars are also called PULSARS because as they rotate
rapidly, they tend to give off lighthouse-like beams of radio waves that flash past
the Earth at regular intervals, producing "pulses" that give rise to the name
"pulsar." There are over 200 known pulsars, the best known of which is the one at
the center of the CRAB NEBULA.
BLACK HOLES are the final states of stars that are even more massive than
neutron stars. The force of gravity becomes so strong that even neutrons can no
longer support themselves against so great a force. So even they collapse into a
state that we cannot adequately describe but which produces such a tremendous
gravitational pull that NOTHING, not even light ore other types of electromagnetic
radiation, can escape. Since no signals of any kind can ever be observed from a
black hole itself, its name is self-explanatory and also very descriptive.
Just because we can never see a black hole itself does not mean that we can
never detect one. as ions and other uncharged particles swirl into a black hole,
they form an ACCRETION DISC that is still outside of the EVENT HORIZON (or
SWARTSCHILD RADIUS) for the black hole and so can still emit detectable
radiation. Thus it is possible that an accretion disc may someday be seen. We
could also observe the extreme gravity of such an object as it affects other nearby
massive objects. Binary stars are there fore good places to look for black holes.
There are also such things as MINI BLACK HOLES and SUPERMASSIVE
BLACK HOLES that you should study in the text material.
CLOSE BINARY STARS are discussed, and you should be familiar with
such terms as the ROCHE LOBE, CONTACT BINARIES, and SEMIDETACHED BINARIES. These latter types are the basis for the explanation of
NOVAE. You should also look over the discussion of the other "strange"
astronomical objects covered in this chapter. It is possible that the observation of
some of these strange objects may prove the existence of GRAVITY WAVES and
thus furnish another important proof of Einstein's General Theory of Relativity.
Chapter 12 - The Milky Way Galaxy
When we look out into space from the Earth, even with our best telescopes,
the stars appear to be quite alike. There are, of course, different patterns to be seen
in various areas of the sky, but aside from our Sun and the planet of our solar
system, all of the other points of light seem much the same. We have already
learned in lesson 3 that the light from these distant points can be carefully analyzed
to give us much more information than just the brightness and spatial location of
the stars.
A few of the light sources that can be seen in the night sky have a "fuzzy
structure" that was the subject of great mystery and interest for many years. As our
telescope become large and more powerful, these objects were studied in great
detail. They were even formulated into their own unique list called the Messier
Catalog of the Nebula. The term "nebulae" is used here because it is the Latin
word for cloud, which is what these light sources resembled. Some of the nebula
actually turned out to be clouds of gas and dust, but many others eventually proved
to be giant collections of stars held together by their mutual gravitational attraction.
Study of the stars in the Andromeda Nebula, now known to be another
galaxy, showed Edwin Hubble that these stars were much too far away to be part
of our own local collection of stars. This idea, combined with Doppler Shift
observations by Vesto Slipher, showed beyond any doubt that the universe is made
up of many large collections of stars that are held together by mutual gravitation.
These star groups, known as galaxies, are located at tremendous distance from our
own galaxy. The Doppler Effect is one of the most important tools that we have to
use in our study of astronomy. When Doppler Effect readings on various celestial
objects are correlated, we find the most surprising and interesting piece of
information ever discovered about the cosmos -- The Principle of Universal
Expansion, which states that every part of the universe is moving away from every
other part at incredibly high rates of speed. This whole expansion process is now
believed to have begun long ago with a strange "singularity" known as the "Big
Bang," the very beginning of the universe as we know it today.
In this lesson we will deal with a description of the discover of the overall
galactic structure of the universe and continue with a formulation of the basic
theory of the origin of the universe. We will also study the methods by which the
age of the universe can be estimated and see that it is even possible to formulate a
history of the early universe that adds detail and credibility to the expansion
process and to the "Big Bang." We will also see why the various areas of the
universe have evolved into what we observe today.l You should look closely at the
roles that photons, matter-antimatter, and the microwave background have played
in our understanding of the universe. You should also examine the various future
outcomes that are possible for our universe as cosmological expansion continues.
Remember that the further our telescopes allow us to look into space, the further
into the past we can make observations. this is not only interesting but may hold
the eventual key to determine the details of the origin of the universe and possible
even the long-term fate of the universe as well.
Galaxies, like al other objects in the universe, are affected by mutual
gravitation. group of galaxies that are linked strongly together by gravity are
usually quite large and are called Galactic Clusters. (See Chapter 5 for more
details on the general characteristics of galactic clusters.) Our Milky Way Galaxy
is not part of such a large cluster. As a matter of fact, it is associated with only
about two dozen other galaxies in what has come to be known simply as THE
LOCAL GROUP. The primary constituents of the Local group are two averagesized spiral galaxies, the Milky Way and Andromeda, along with one other small
spiral, a few irregulars, and several small ellipticals. Two of the irregulars are of
special importance because they are our nearest neighbor galaxies and are actually
so close to us that a few astronomers consider them to be satellites of the milky
Way. These two galaxies are called the Large and the Small Magellanic Clouds
because of their nebular appearance when they were seen with the naked eye by
Magellan and his crew as they sailed the southern seas. The Magellanic clouds
cannot be seen in the night sky from North america because they lie far south of
the celestial equator.
Another interesting thing is that although we have observed large elliptical
galaxies in abundance in other parts of the universe, we have none in our Local
Group. Since this type of galaxy is so common, it would be helpful to have a good
representative sample located close enough to us os that we could study it easily.
This, unfortunately, is not the case, as we do not know as much about large
elliptical galaxies as we would like to.
Note that the measurement of distance is again of major importance when
discussing galaxies. The existence of other galaxies was actually confirmed by the
study of CEPHEID VARIABLE STARS, whose rates of variation in observed
brightness have been shown to indicate their ABSOLUTE MAGNITUDE. These
variable stars can then be used as "standard candles" to measure their individual
distances using the inverse square law. )This was explained completely in Chapter
5.) The distances to cepheid variable stars in the Andromeda galaxy were
measured by Edwin Hubble using his method. Hubble's calculations showed that
the variable stars in Andromeda were so far away that they could not be part of our
own galaxy. This was done in so conclusive a way that the astronomical
community was finally convinced of the existence of galaxies other than our own
Milky Way.
Because objects close to us are easier to study than those far away, our Sun
was the first star to be studied in detail. It was felt for some time that our Sun
represented a true model for all stars. It was later found that another distinct class
of stars exist, because they show a different spectral line pattern than our Sun. The
first stars studied, those like our Sun, were given the name POPULATION I
STARS, while the second type were named POPULATION II STARS. It turns out
that the Population I stars are young stars made up of the debris of II stars. It turns
out that the Population I stars are your stars made of the debris of earlier
generations of stars, while the Population II stars are older and were formed from
the original gas and dust clouds left after the "Big Bang." The Population II stars,
in their natural aging and death cycles, have provided the heavier elements present
in the younger Population I stars. The primary difference between the two types,
besides their ages, is their content of "heavy metals." In astronomy, heavy metals
mean any of the elements above helium in the periodic table. The early universe
was composed primarily of hydrogen and helium gas with only very small amounts
of the heavier elements. Population II stars have this same chemical composition
with only about 0.1% of the heavy metals. Population I stars like our Sun,
however, were formed after the older stars had generated more of these heavier
elements in their cores and subsequently distributed them throughout space as their
like cycles ended. The younger stars are made up of the resultant gas and dust
clouds and have about 10 times more of these heavy metals than the older stars. It
is interesting to note that Population I stars are found in regions where gas and dust
still exist from which new stars can still form; that i s, in the discs of spiral galaxies
and in irregular galaxies. Population II stars on the other hand, are found where
little or no gas and dust still exist; that is, in the halos or spiral galaxies and in
elliptical galaxies.
Gravity not only causes galaxies to form and remain intact over long periods
of time, but it also causes stars to form into groups within galaxies called "star
clusters." There are three types of star groups that can form: globular clusters,
open clusters, and stellar associations. These are, in genera, made up of different
types of stars and are found in different portions of galaxies. Upon close
examination this grouping is consistent with the age of the two star types and the
length of time that the different regions of galaxies have been in existence. We
thus find globular clusters in elliptical galaxies and in the haloes of spirals.s
Globular clusters are made up exclusively of Population II stars. Open clusters and
stellar associations are made up primarily of Population I stars and are found in
spiral discs and in irregular galaxies where large amounts of gas and dust are still
available from which new stars can be formed.
Our Sun is part of a stellar association. STELLAR ASSOCIATIONS are
small groups of stars that were formed at about the same time as were the stars in
clusters, but lack the strong gravitational interaction necessary to stay together for
long periods of time. After only a few million years, most stellar associations split
apart, and it is no longer possible to tell which stars were originally in these loosely
attached groups. We can not even determine which of the nearby stars in our own
galaxy were once "sisters" of our Un at its birth. The contraction of stellar
association is believed to have been triggered by a nearby supernova explosion
over 6 billion years ago.
Many stars are not part of any galactic subgroups at all. They move about
the galactic center in elliptical orbits at varying speeds without any strong
interaction with any other individual stars. Some interesting examples of such
unattached stars are the HIGH-VELOCITY STARS which appear to move much
more rapidly than the other stars around us. This process is only an illusion. All
stars move through space. Most of the nearby stars move with our Sun in orbits
around the center of our galaxy. These stars that are part of the spiral disc move
with us and so have small relative velocities with respect to our own motion. The
so-called high-velocity stars are part of the halo and as such move in random
orbits around the galactic center. These stars from the halo sometimes move past
us at nearly right angles to our own direction of travel. Just as cars crossing the
road in front of us seem to be moving much faster than those traveling along in the
same direction with us, these stars from the halo SEEM to be moving faster even
though they are really traveling at about the same actual speed. This apparent
higher speed gives them their name, high-velocity stars.
Our galaxy, and many other regions of space, contain large clouds of gas and
dust that are called NEBULAE, because they appear cloudy of fuzzy when viewed
through even our best telescopes. Nebular regions are important to observational
astronomers. These are the areas where new stars are still forming, and nebular
gas clouds are also often found to be regions where old stars are dying. Thus these
areas are of special interest in the study of the evolution of the universe. Nebulae
can also be a problem to observation astronomers because they tend to block our
view of certain portions of the universe. One such region is the core of our own
galaxy, which is obscured by large clouds of gas and dust. The development of
radio telescopes has helped to solve this problem somewhat because radio waves
penetrate these clouds of gas and dust much hotter than optical signals. The
electron spin flips of hydrogen gas produce a very distinctive type of
electromagnetic radiation in the radio portion of the spectrum that can be used to
map the positions of hydrogen gas clouds. We will discuss the importance of radio
astronomy in more detail in the next chapter.
Dust clouds can be detected and mapped because as light coming from
distant sources passes through dust, the shorter wavelengths of light, blues and
greens, are scattered more than the yellow and reds. This means that more of the
longer wavelength light gets through, producing what is known as
INTERSTELLAR REDDENING. By studying the reddened color change of the
light from a distant star, astronomers can calculate the amount of dust that the light
has traversed in its journey from that star to our telescopes.
The central core of our own galaxy is still quite a mystery to astronomers.
This chapter tells us a little of what is known about this important region of our
galaxy and also tells us why we are having difficulty studying this area. You
should pay particular attention to such topics as the ZONE OF AVOIDANCE,
high-energy emission processes that are occurring within our galactic center, and
the possibility of the presence of a large BLACK HOLE at the very hub of the
Milky Way. There is also a discussion of the reason why the spiral-arm structure
of our galaxy, and other similar galaxies, persists over long periods of time. You
should look over the DENSITY-WAVE theory carefully. I like to visualize the
density-wave theory as a fire sweeping across a field of dry grass. The fire itself is
analogous to the highly visible region in which bright new stars are being formed.
The region in which the fire is burning is much more visible than the regions of
new fuel or burned out fuel that lie on either side of the fire itself. This is similar
to the regions of a spiral disc that are most visible in which hot, bright, new stars
are born. It is these areas of enhanced visibility that given the spiral galaxies their
characteristic appearance, even though the density of stars in the spiral arms is very
nearly the same as that for the halo and the other less visible portions of the disc
itself.
Chapter 13: Galaxies
Once the existence of other galaxies was determined, it was only natural for
astronomers to study them in detail. Galaxies are classified by the appearance that
they have as viewed from Earth. These categories indicate their overall spiral,
elliptical, or irregular shapes. Other properties of galaxies such as brightness,
mass, and spectral composition have also been studied, along with the Doppler
Shift measurements indicating the general motions of these gigantic star groups.
From this data. a good deal of information has been obtained about galaxies which
shows them all to be surprisingly similar to our own Milky Way Galaxy, yet
different in fascinating details.
Astronomers now feel that they understand the formation processes by
which galaxies originate out of clouds of gas and dust. Clumping of matter is the
natural result of gravitation on gas and dust clouds, and the observed rotational
characteristics of galaxies are clearly the result of the conservation of angular
momentum as these great clouds shrink and contract. Individual stars within
galaxies are simply a more localized gravitational collapse of smaller, more
localized gas and dust clouds. There is still some question as to just what
differences there are in the formation processes for spiral and elliptical galaxies. In
the next lesson, we will study the observable results that these different processes
produce, but it is still not clear just why these differences occurred.
This lesson again involves us in one of the most interesting but difficult
measurements that astronomers have to make: determining the distances to various
parts of the universe. The further an object is from us, the harder it is to make
accurate estimates of this exact distance. As you will see, standard candles play a
large part in distance determination, so you should study this concept carefully.
Using a standard candle involves first establishing a close similarity between two
luminous objects, one of which we have already established as being a certain
distance away from us. This similarity can be determined by the object's color,
spectral line classification or other specific criteria, but once this similarity has
been confirmed, we immediately assume that the absolute magnitude of the two
objects are the same. Since the two objects (say two stars) have the same absolute
light output, any difference in observed brightness must be due to the difference in
distance that the two objects are from us. By using the inverse square law of
intensity, we can then estimate the actual distance to the second object. Notice that
this process may involve considerable uncertainty, so that even though the distance
to objects is of critical importance to astronomers, it is one of the most difficult
things to measure accurately. When all else fails, astronomer fall back on Doppler
shift measurements and use the linear relationship known as Hubbles' Law to
estimate the distance. The faster an object is moving away from us, as measure by
it Doppler shift, the further it is away from us. These kinds of estimates are
responsible for one of the biggest puzzles in astronomy today -- that of quasars.
The Doppler shifts of quasars suggest that they are very far away, but they are so
bright that they must produce more light and other types of electromagnetic
radiation than we can account for by any energy-producing processes that we know
of. Thus the quasar is not only on the distance frontier, but also on the true frontier
of research for astronomers today.
Another interesting fact that has been discovered only recently is that certain
galaxies give off radio signals much strong than would be expected from our
standard theories of energy production. Radio galaxies and Seyfert galaxies are
therefore well documents but not well understood. What we see of them is
tantalizing, but as yet we do not know what keys they hold to our understanding of
the universe as a whole. Do not be surprised if you read of new and exciting
discoveries about quasars and radio galaxies as our facilities for observing distant
parts of the universe improve over the next few years, especially when the space
telescope becomes operational.
Chapter 14: Galaxies with Active Nuclei
Although most galaxies that we have studied are quite placid, a few have
been discovered to be emitting tremendous amounts of energy into space. This
has been attributed to the fact that these galaxies have energy producing central
regions which have come to be known as active galactic nuclei (AGN). These
peculiar galaxies were first discovered in the 1950's when astronomers first
detected strong radio signals from certain regions of space. Many of these were
soon found to be associated with active galaxies that have come to be known as
RADIO GALAXIES.
Seyfert galaxies were among the first to be discovered. Seen through an
optical telescope these galaxies look like normal spiral galaxies that possess highly
luminous nuclei with peculiar spectra. Although only about 2 percent of all
known spiral galaxies appear to be Seyferts, these galaxies are still quite important
and have been further classified into groups, Type 1 Seyferts that show strong Xray and ultraviolet emissions, and Type 2 Seyferts that produce fewer X-rays and
narrower emission lines.
A second strange galaxy group were discovered by radio telescopes and are
now referred to as double-lobed radio sources. This is because the high powered
radio signals emitting by these galaxies do not come from the galaxies themselves,
but from regions on either side of their central regions, sometime extending
outward for vast distances into space. The basic explanation for these radio
sources is the double-exhaust model and seem to be associated with the interaction
of two galaxies. Deeper investigation has shown that these are associated with
active galactic nuclei which turn out to be the manifestation of central black holes
into which matter is falling. The resulting accretion disks are then the actual
source of the high levels of radio and other types of emissions that are pouring out
of these gravitational vortexes.
Remember that synchrotron radiation (Review Chapter 10) is associated
with strong magnetic fields and high velocity charged particles. The accretion disk
of a supermassive black hole is perfect explanation of how this type of radiation
can be produced. The collapsed star that originally formed the black hole has
retained its initial magnetic field which has been condensed into a very small area
in which the magnetic field is extremely strong. As gravity pulls nearby matter in
toward the event horizon of the black hole, the matter heats up and is ionized so
that there is a large influx of charged particles that are further accelerated to high
speeds by the intense pull of gravity. When the field lines are warped and twisted
they can produce the jets of high temperature gas and ions that ultimately produce
the intense radio lobes of bright nuclear beams that signal the presence of active
galactic nuclei.
Black holes that are being fed by inflowing matter are then the explanation
of these extreme energy sources found at the centers of active galaxies. A unified
model has been proposed to complete the details of this process and is also able to
explain the variations in intensity seen in entire galaxies. These galaxies have
come to be known as blazars, short for BL Lac objects. Another interesting subject
in this chapter involved the origin of supermassive black holes and the realization
that these intense gravitational singularities can even erupt creating even larger
emissions of energy into space. Make sure that you look over this material
carefully.
Finally one last great class of objects has been discovered. These are
sources of light that look like normal stars through an optical telescope, but that
have very strange emission spectra. These objects, now called quasars (short for
quasi-stellar objects), show extremely large red shifts in their spectra indicationing
that they are moving away from us at fantastic speeds often nearly the speed of
light itself. For this to be true, Hubble’s Law tells us that they must be at very,
very great distances and are now believed to be the most distant objects that we can
detect in the entire universe. Again supermassive black holes with accretion disks
whose high energy jets are pointed directly toward Earth are believed to be the
model that best explains the observed properties of quasars.
Study of this chapter leads us to believe the most galaxies contain black
holes at their centers, but that only a few of them have nearby sources of gas and
dust that can continually feed into these black holes to produce accretion disks and
high energy jets. These are the ones that show active galactic nuclei and that have
exotic properties such as quasars and Seyfert galaxies. The rest of the galaxies in
the universe, like our own Milky Way galaxy are quiet, that is they surround
sleeping supermassive black holes that provide a strong gravitational core for the
millions of stars that make up these galaxies but do not have inward falling matter
to fuel the giant energy producing regions in active galaxies.
Chapter 15: Cosmology in the 21st Century
Cosmology is essentially the study of the BIG BANG and how the universe
has evolved since its occurrence around 14 billion years ago. The actual reason for
the big bang is still a mystery, but astronomers and physicists have been able to
discover details about the after effects of this traumultuous event from only a few
millionths of a second after it occurred, right up to the present day. It is believed
that understanding this evolution of the universe is not only a fascinating subject to
study, but that knowledge gained from the study of cosmology can greatly advance
the future well-being of mankind.
Olber’s paradox leads us to believe that the universe which consists of
everything that exists could be infinite, whereas the observable universe that we
can actually see is limited by the travel time of light from the most distant regions
of space. The most distant stars are so far away the light has not had time to reach
us from their location in the universe and if we could look out far enough we
would be observing a time when stars had not yet begun to shine. Current studies
indicate that the universe is infinite and therefore can have no edges. If there are
no edges then the question of where the center of the universe is located turns out
to be futile since with no definable edges, there can be no center.
From the presence of red shifts in the spectra of nearly all objects observed
and because of the discovery of the 3 degree background radiation also known as
the cosmic microwave background radiation in space, it is generally accepted that
the universe is undergoing cosmic expansion. This has not always been at a
constant rate. There is evidence that there was a time of rapid expansion known as
inflation that occurred shortly after the big bang. Since the universe contains large
quantities of mass, it is logical to think that self gravitation should be pulling the
universe back together and slowing the expansion rate more and more over time.
This, however, does not seem to be the case. Recent observations point to an
increase in the expansion rate of the universe which leads to the necessity of a new
type of dark energy to power this increased expansion rate. Dark energy is one of
the hottest topics of current study in astronomy and is leading to many new and
exciting theories about the structure and composition of matter and space.
There have been several distinct periods in the history of the universe. The
initial universe was so hot that existing photons could produce matter in a process
called pair production but the matter was almost immediately destroyed in pair
annihilation. During this time Einstein’s mass/energy relationship prevented any
permanent solid matter from forming but particles of both matter and antimatter
were numerous but short lived. Eventually the expanding universe cooled so that
protons and neutrons could exist but electrons and positrons were still being
created and destroyed by high energy photons. It was not until about 4 minutes
after the big bang that permanent electrons finally could form in the cooling
environment. Selectively protons and electrons survived, but most of the
antiprotons and positrons did not, so the universe as we know it is today made up
of normal matter and not antimatter.
Eventually as the cooling continued protons and neutrons could combine to
form deuterium and helium but about 75% of the nuclear particles remained as free
protons. Additional cooling ended total ionization and actual atoms of hydrogen
and helium began to form in a process called recombination. Finally clouds of gas
and dust began to form from these atoms although the universe was quite
transparent through this period and had a natural glow because of the continued
presence of photons in the visible portion of the spectrum. Eventually these
photons cooled in to the microwave region of the spectrum, hence the 3 degree
microwave background that continues to this day. At this point there was no light
and the universe existed in a state of perpetual darkness, referred to as the dark age,
for hundreds of millions of years.
The universe remained dark until gravity managed to pull the gas and dust
clouds together to form the first stars. As the clouds collapsed the gas and dust
heated up until first infrared, then light, and finally ultraviolet light began to shine.
These ultraviolet photons had enough energy to start ionizing the gas atoms again
and a new era of reionization began. Soon after, nuclear fusion of hydrogen began
in the cores of these now dense clouds and the first true stars were finally born.
This account is a very brief summary covering the early life of the universe,
so make sure that you carefully read the explanations in the textbook so that you
will understand more of the actual details of the processes mentioned above. Also
pay attention to the reintroduction of the terms isotropy, homogeneous, and the
cosmological principle that we touched on in chapter one.
Overall models of the universe lead to different possible fates for our
expanding cosmos. Carefully look over how critical density relates to the ideas of
an open, flat, or closed universe and how there concepts lead to a description of
space as curved. New studies of hot and cold dark matter and dark energy are
leading astronomers into new and interesting areas of research that promise to
answer many age old questions but as always are introducing strange and exciting
additions puzzles for our consideration. How do the basic forces of nature relate to
each other under extremely high energy conditions and can these four forces,
gravitational force, electro-magnetic force, weak force, and strong force, all be
united into one single theory? The electro-weak theory and the grand unification
theory have already shown that the possibility exists and that all of these forces
may have been unified at the very beginning of the universe soon after the big
bang.
It is possible that the question of the overall fate of the universe will be
answered by studying the large-scale structure of the universe and its relationship
to the curvature of space. Superclusters, voids, walls and other formations of
galaxies throughout space and how the size of these large scale structures vary are
new and fascinating areas of study in astronomy that is leading to many exciting
ideas and theories and may eventually lead to a solution to the problem of the
ultimate fate of the entire universe.
Chapter 16 - The Origin of the Solar System
Compared to the gigantic size of the universe we have been discussing in our
recent study of cosmology our solar system is very small indeed, but it is probably
the only part os the universe that mankind will ever actually visit. A round-trip to
the nearest neighbor star to our Sun would take over 10 years even if we could
devise a way to travel at or near the speed of light which would take extremely
powerful engines and vast amounts of energy. This means that though many may
still dream of interstellar travel, the truth is that what we have in our solar system is
about all we can ever hope to study up close and use as a resource to build the
future of our race. That does not mean that the detailed study of our solar system
is not a fascinating and important thing to study so this chapter is devoted to just
that.
The origin of our solar system should come as no surprise since we have
already studied the birth of stars in some detail. Remembering the only about half
of the stars we see are part of a binary star system, we can surmise that the rest
probably have some sort of planetary system surrounding them to account for the
large amount of angular momentum present in the initial swirling clouds of gas and
dust from which they were formed. Since these extended clouds speed up their
rotation rate as they are drawn together, they would fly apart if all of this initial
angular momentum were confined to a single fast rotating star.
The particular solar system surrounding our Sun is of course of great interest
for two reasons, first our home planet Earth is a part of it, and secondly this solar
system is the only one close enough to study in high detail. Make sure that you
understand the solar nebula hypothesis as it relates to our own planetary system.
Then you will know how our small collection of planets came into existence.
Our solar system consists primarily of the Sun itself which accounts for
about 99% of its entire mass, two types of planets (Terrestrial and Jovian), an
asteroid belt, and the Kuiper belt. There are also comets and meteoroids that
capture our attention from time to time. All of these are revolving around the Sun
and most are rotating on their own internal axes. Make sure you understand the
difference between these two terms. They may sound quite similar but they are
actually quite different in meaning. Also note that the age of our solar system can
be quite accurately determined by radioactive dating processes and you should
study the various radioactive materials which allow this to be done and to
understand the nature of the term half-life and how it varies from one material to
another enabling us to determine both short term and long term ages of various
parts of our environment.
The actual formation of the planets involved the processes of condensation,
accretion, gravitational collapse, differentiation, and outgassing. These processes
occurred as the planets grew out of the gas and dust found orbiting the young Sun
through the formation of planetesimals, protoplanets, and finally into the actual
planets that we find orbiting the Sun today. Note that these are two models of
planet building that depend on the stability of the temperature during the solar
nebula period. If the temperature remained nearly constant, the process of
differentiation must have been much more active in separating the iron core and
dense inner mantle from the lower-density outer regions. If the temperature of the
solar nebula changed during planet formation, the early planetesimals that formed
the core could have already been mostly metals. In either case, heat either from
internal radioactive decay or from rapid gravitational collapse must have produced
some differentiation, resulting in the low-density crust that forms the outer layers
today.
The planets swept up most of the loose materials around them often
resulting in heavy bombardment of the planet’s surfaces resulting in craters on
their surfaces in the cases of the terrestrial planets. The regions between the planets
were gradually cleared of gas and dust by radiation pressure and the solar wind
from the Sun leaving a near vacuum in the space between most of the planets. The
exceptions were the areas now occupied by the asteroid belt and the Keiper belt
which still contain large amounts of loose debris that we will study in more detail
later.
It is believed that similar processes took place around many other stars as the
universe aged. We have been able to detect some of the largest extrasolar planets
that orbit other nearby stars by observing the perturbations that the orbit of these
large bodies make in the motion of each individual star through space but planets
the size of Earth are have too small an effect to be observed. We can, however,
also detect large clumps in the gas and dust rings surrounding other stars that are
probably an indication that planets are in the process of forming in these regions.
Chapter 17 - Comparative Planetology of the Terrestrial Planets
The solar system is divided into two quite different types of planets. Those
closest to the Sun known as the terrestrial planets and those that formed outside the
asteroid belt further from the Sun called the Jovian planets. In this chapter
comparative planetology will be used to show how the members of each group are
similar and how they differ. This will help you grasp the underlying characteristics
rather than forcing you to remember many individual facts about each planet.
The terrestrial planets all have a very dense central core, and thick mantle of
dense rock, and a low-density rocky crust. The core of Earth has a very hot inner
region that is under so much pressure that it behaves as a solid. It is composed of
iron and nickle and its temperature is nearly as high as the surface of the Sun. The
outer layer of the core is still metallic and slightly cooler but has a lower pressure
and this region exists in a liquid state. The outer core is where the Earth’s
magnetic field is generated by convection currents within the molten liquid. This
combined with the facts that it is rotating as the Earth turns and the metal is highly
conductive to electric currents, causes a strong magnetic field to be produced. We
know as much as we do about the interior of our Earth because of systemic studies
and analysis of the P-waves and S-waves that travel through it when earthquakes
and volcanic eruptions occur. The study of these waves is known as seismology.
The other terrestrial planets have similar overall structures including varying
strength magnetic fields, but the details of the inner core regions are less clearly
understood. Because of the similar internal structures, all of the terrestrial planets
have high densities, between 3.94 gm/cm3 and 5.597 gm/cm3 with Earth having the
highest density and Mars the lowest.
There is one other major object in our solar system that is quite similar to the
four terrestrial planets. That object is the Moon that orbits Earth. It is only about
1/4 of the size of Earth and has a density even a little lower than that of Mars, but it
has a solid surface, a deep crust, and a small iron core. It appears that the Moon
was created when a large planetesimal (about the size of Mars) smashed into Earth
and resulted in the formation of two binary objets orbiting around each other, the
Earth itself and the Moon. Note that there are three other hypothesis that have
been proposed for the formation of our Moon, but current evidence does not fully
support any of them. This leaves us with the large-impact hypothesis as the best
available theory of lunar formation. Mars is the only other terrestrial planet to
have moons. There are two and these are quite small, irregular in shape and have
rocky structures. Current theory suggests that they are objects that were probably
captured from the asteroid belt.
One other important feature of the terrestrial planets is their atmosphere.
Mercury is quite small and so close to the Sun that the combination of its low
gravity and the high solar heat have driven away nearly all of its atmosphere. The
Moon also has no atmosphere, but the other three terrestrial planets do. Mars has a
very thin atmosphere composed mainly of carbon dioxide with very small
percentages of nitrogen and argon. Venus also has a carbon dioxide rich
atmosphere, again with small amounts of nitrogen and argon plus sulfur dioxide,
sulfuric acid, hydrochloride acid, and hydrofluoric acid. Venus’s atmosphere is
over 90 times denser than Earth’s atmosphere, which makes it only about 10 times
less dense than water. The temperatures at the surface of Venus are over 700 K.
The high pressure and temperature found in the Venetian atmosphere makes it
almost impossible for man to ever land on the surface of Venus and it certainly is
not a good prospect for human colonization.
Earth’s atmosphere is quire unique. It is the only atmosphere in our solar
system to contain oxygen (about 20%) with the remainder being mostly nitrogen
with small amounts of carbon dioxide and a few other gases. It is fortunate that
carbon dioxide is easily dissolved in water and a large portion of the rocky crust of
Earth is covered with water (oceans). This keeps the carbon dioxide content low
and prevents the greenhouse effect from warming the surface of Earth to
unreasonable levels. Today many processes used by man to produce energy
release large amounts of carbon dioxide into the air. This is leading to a serious
problem known as global warming. This results from to the higher heat retention
of the greenhouse gas carbon dioxide when it accumulates in the atmosphere in
increased amounts . Man must learn to reduce the amount of atmospheric waste
produced by our everyday activities or we may eventually make the Earth an
uninhabitable planet.
The surfaces of the terrestrial objects has been shaped by several processes,
the most important of which are cratering and volcanism. Cratering occurs when
large chunks of space debris falls to the surface and gouge out sections of the
existing crust. Heat within the cores can also melt the rocky material just below
the crust and spew this molten rock (lava) up onto the surface as volcanic
eruptions. These processes produce mountains and valleys on the crust. In
addition wind and rain can erode away surface features causing the high mountains
and low valleys to equalize and flatten the overall surface contours. These
processes vary between each of the terrestrial plants, so make sure you look over
how these processes and others have, and are still, effecting the surfaces of our
Earth and other terrestrial worlds like it.
Study the structure, formation, geologic history, and atmospheres of each of
the terrestrial planets and of our Moon carefully. Since Earth is our home planet
and the other terrestrial planets are the only celestial objects close enough for us to
visit with relative ease, these worlds are of great importance and interest to us and
should served as the basis for our understanding of the explorations and
developments of man and his future in this region of space.
Chapter 18 - Comparative Planetology of the Outer Planets
The outer planets, those whose orbits are outside the asteroid bolt, are quite
different from the terrestrial planets. We must be careful in the use of the term
“outer planets”, it is a little ambiguous. If we use the orbit of Earth as our
reference, Mars is also an outer planet. If we use the asteroid belt, it is not. For
this reason it is better to refer to these four planets inside the asteroid belt as
terrestrial planets and those outside this region as the Jovian planets. This will
completely avoid such difficulties.
Although all of the Jovian planets can be seen from Earth, even our best
telescopes can tell us little about them. Fortunately a series of space probes have
been able to send back quite detailed information about them and their moons,
beginning with the Pioneer and Voyager probes in 1970 and continuing through
Cassini in 2004. What they found was in same cases quite a surprise.
First, there is no solid surface for the Jovian planets. The outer layers are
simply gas and for that reason these planets were all referred to as the gas giants
for many years. Now we know that the farthest ones, Uranus and Neptune, should
probably be more accurately described as ice giants. All of the Jovian planets are
rich in hydrogen and helium, gases not found in great abundance on the terrestrial
planets. This is because the Sun is so far away from these Jovian planets that the
heat and solar wind cannot strip off these light gases and the gravitational pull of
their gigantic masses also helps to retain these lighter gases.
Jupiter and Saturn both have outer layers composed of liquid hydrogen, a
second inner layer of liquid metallic hydrogen and finally a heavy-element core of
rock and metal. In addition Saturn has a relatively thin layer of gaseous hydrogen
above the liquid hydrogen layer. Uranus and Neptune have thick outer layers of
hydrogen and helium gas, and ice / rock outer core and a heavy-element inner core.
Little detail is known about the interior of these giant planets, but studies of their
densities, which are much lower than that of the terrestrial planets, combined with
theories about the temperature and pressures that must be present given their
general structures can give us strong clues to help us deduce the general structural
features listed above. All of the Jovian planets exhibit magnetic fields, but these
are probably not generated in quite the same way as the magnetic fields of the
terrestrial planets.
Secondly, all of the Jovian planets have ring systems surrounding them as
well as a wealth of objects large enough to be considered moons in orbit around
them. Saturn’s rings are quite dominant and can easily be seen from Earth even
using very small telescopes. The rings of the other Jovial planets can only be
studied in detail using space probes even though some indication of their presence
can be observed when these planets passed in front of more distant stars. The light
passing the planets was effected by the material in the rings so they can be detected
but not studied in detail.
Saturn’s rings are made up predominantly of ice particles confined in orbit
by the larger moons (Shepard satellites) that orbit the planet. This interaction
keeps the icy material orbiting the planet in a thin ring structure. Jupiter’s rings
appear much different. Instead of shining brightly as the rings of Saturn, Jupiter’s
rings are very dark and somewhat reddish in color. This indicates that the rings are
rocky rather than icy and in addition these rings are made up of microscopic
particles more like dust that rock chunks. The rings of Uranus and Neptune are
also confined by Shepard satellites (moons) and appear dark. The particles here
are probably methane ice coated with carbon dust. Neptune’s rings are brighter
and probably have a larger dust content than those around Uranus.
Thirdly, there are many moons orbiting the Jovian planets. Some are very
large like Io, Europa, Ganymede, and Callisto that orbit Jupiter, other are so small
that they barely qualify as moons at all. These moons greatly effect the respective
ring systems found around the Jovian planets. They actually keep the ring material
confined to a thin disc around the planets and in addition break the rings up into
bands of varying density. This produces regions where almost no ring material is
present at all and others where the ring material is quit prevalent. It further appears
that the material in the rings is not permanent and must be replaced with new dust
caused by impacts on the surfaces of the larger moons or by gravitational tidal
forces that cause the distraction of some of the smallest moons that orbit close to
the planet itself.
Pay particular attention to the large Galilean moons that orbit the giant
planets. The four largest moons of Jupiter are quite varied and show us a great
deal about the way object form and behave in strong gravitational fields. All four
of these moons are about the size of our own Moon and thus might even be
suitable for manned or unmanned space probe landing sites. Io orbits so close to
Jupiter that tidal forces produce tremendous amounts of heat in its interior resulting
in a large amount of volcanic activity. Over 100 volcanic vents are evident on Io’s
surface and active volcanoes belch forth gas and ash rich in sulfur that collects on
the surface obscuring any previous impact craters. Io is mostly rock but its high
density suggests that it may have a metallic core making it very similar to our own
Moon.
Europa is farther from Jupiter and has a thin icy crust over its rocky interior.
Again little impact evidence is visible because of the icy coating but low
mountains suggest that the ice is often shifted, again because of gravitational tidal
forces. Ganymede is the largest moon in the entire solar system and is actually a
little large than the planet Mercury. It’s density is fairly low suggesting that it is
not solid rock but a mixture of ices and rock. Ganymede’s surface shows signs of
cracking where older cratered regions of dark ice are alternated with younger,
brighter regions. There is also a weak magnetic field that suggests a metallic core.
The furthest Galilean moon of Jupiter is Callisto. It is larger by half than the
Earth’s Moon and consists of a mixture of about half rock and half ice that has
been differentiated to produce a heavy core and lower-density crust. This crust is
highly cratered. Of considerable interest to scientists is the indication that below
Callisto’s crust lies a mineral-rich ocean of water in its liquid state. This might
prove to be a location where extra-terrestrial life could have originated and may
still exist. We will explore this possibility further in Chapter 20.
After all of this discussion of gas/ice giants its is somewhat surprising that
the ninth world found to be orbiting even farther from our Sun is quite different.
Pluto has had a varied history of sometimes being referred to as a planet and
sometimes not. In 2006 this status was finally decided by the AIU and Pluto was
dropped from the list of planets and officially classified as a dwarf planet. By the
new definition, dwarf planets must orbit the Sun (not be a moon of some other
planet) and have enough mass to pull themselves into a reasonably spherical shape
by their self-gravitation. Pluto is not the only dwarf planet in our solas system.
One dwarf planet exists in the asteroid bolt. It is called Ceres. Several others are
located in the Kuiper Belt, far outside the orbit of Neptune. The largest of these
that have been discovered so far are Eris, Sedna, and Orcus.
Another new term is presented here, Plutino. A Plutino is a massive object
located outside the orbit of Neptune in the Kuiper belt that, like Pluto, have a
special relationship in their orbital period to that of Neptune itself. If they orbit the
Sun twice in the same time period that it takes Neptune to orbit the Sun three
times, they are said to be caught in a 3:2 resonance. Since Pluto is also in a 3:2
resonance with Neptune, all of these smaller planet like objects have been assigned
to the name Plutinos.
Pluto itself is an icy world that is so far from the Sun that most even gases
are frozen to a solid ice state. Even nitrogen becomes solid at these temperatures.
Pluto has a very thin atmosphere over a solid base consisting of about 1/3 ice and
2/3 rock. Pluto has three moons the largest of which, Charon, is about 1/12 the
mass but nearly one-half of the diameter of the planet itself. Charon travels in a
highly eccentric orbit around this dwarf planet. Pluto also has very eccentric orbit
as it travels around the Sun and sometimes is actually somewhat closer to the Sun
than Neptune, although the orbits of the two do not actually intersect because the
orbit of Pluto is canted at a large angle to that of the orbits of both the terrestrial
and the Jovian planets. Pluto’s other two moons, Nix and Hydra, are quite small.
Pluto, along with the other Plutinos formed in the outer solar nebula but
were captured into resonances of orbit when the planets Uranus and Neptune were
gradually shifted outward during an early restructuring of the solar system. This
locked the Kuiper belt objects in their orbits around the Sun. Although the planets
seem to have readjusted their orbits early in the history of our solar system, what
we observe today seem quits stable so we expect our solar system to remail much
as we now observe it.
Capter 19 - Meteorites, Asteroids, and Comets
Our solar system is much more complicated than just eight planets and
several moons in orbit around each other and around the Sun. There are three
areas of interest that do not involve planets or moons that we must look at before
our study of the solar system is complete. There are also several types of mobile
objects that reside in remote areas of our solar system and that sometimes leave
these distant areas to travel closer to the Earth and the Sun. The three specific
areas in question are the asteroid belt, the Kuiper belt, and the Oort Cloud.
The asteroid belt lies between the orbits of Mars and Jupiter, about 2.8 AU
from the Sun. The asteroids themselves are rocky objects, many of which have
low enough average densities to be considered loose piles of rocks rather than solid
rock lumps. They are usually single objects but sometimes are found in binary
pairs where one orbits around the other. All asteroids follow generally
counterclockwise orbits around the Sun and a few of these rocky objects have
orbits that overlap the orbits of the nearby planets. It is a little troubling to know
that a few thousand of these asteroids that are larger than 1 km in diameter actually
cross Earth’s orbit. These are referred to as NEOs, near-Earth objects. If any of
them were to actually intercept Earth, it could prove devastating. We must hope
that none ever intersect the orbit of Earth and crash into us. For this reason several
astronomical groups are trying to track and to catalogue these asteroids in an
attempt to provide some warning if a collision with Earth is ever likely.
There are three classes of asteroid. S-type asteroids were formed when
fragmentation of differentiated planetesimals knocked large chunks of matter from
their crust and mantle. M-type asteroids originated from the metallic core regions
of these same planetesimals. C-type asteroids seem to have formed early in the
history of the solar system, by accretion in the outer region of the asteroid belt
where cool conditions allowed the development of carbonaceous material into
large enough bodies to be considered asteroids. Some of the larger asteroids have
accumulated enough mass so that their inner cores could become heated by
gravitational collapse . Some of these asteroids even show indications of geologic
activity such as volcanism. Asteroids are further classified by their color and by
their albedo, the amount of light that they reflect back into space.
The Kuiper belt houses quite different material and begins just outside the
orbit of Neptune. It’s residents are more a combination of ice and rock, lighter in
density than asteroids. This is due primarily to the large distance from the Sun to
this region of the solar system. The Kuiper belt lies outside of the orbit of Neptune
but still remains predominantly in the disk of the solar system. It’s icy/rocky
objects also revolve counterclockwise around the Sun. Although the most easily
observed Kuiper belt objects are quite large, Pluto is one of the largest and is now
classified as a dwarf planet, there are probably 100 million or so smaller Kuiper
belt objects that occasionally can be perturbed in their orbits and fall in toward the
Sun becoming comets.
The Oort cloud is located much farther from the Sun than the outer-most
planets and is populated by objects quite similar to Kuiper belt objects. Oort cloud
bodies were probably formed in the outer portions of the solar nebula but were
more or less randomly ejected outward in orbits many AUs (10,000 to 100,000
AUs) outside the rest of the solar system by interaction with the newly formed
massive Jovian planets early in the history of the solar system. For this reason the
general shape of the Oort cloud is spherical and the orbits of objects within it are
randomly orientated in direction as they circle the Sun. Objects in the Oort cloud
ares not confined to a disk-like region as are the bodies in the asteroid belt and in
the Kuiper belt, so the Oort cloud forms a sort of uniform halo around the outer
most portions of our solar system.
There are two types of small moving objects besides the planets and moons
that orbit our Sun. One type is the meteoroid, the other is the comet. As we have
already seen in Chapter 16, meteoroids are rocky/metallic objects that may have
originated as fragments of the collisions of planetesimals in the asteroid belt. They
can also be the remnants of the debris left by comets that have been broken apart
by the heat of the Sun when these fiery bodies swing in close to the center of our
solar system. If meteoroids intersect the orbit of Earth they are called meteors.
These often enter our atmosphere and appear as bright flashes of light in the night
sky. Meteors are therefore often referred to as shooting stars. If a fragment of
such a meteor survives its passage through Earth’s atmosphere and actually
impacts the surface of Earth it becomes a meteorite.
Large numbers of meteors impact Earth’s atmosphere every day, but most of
them are too small and fragile to survive the trip through the air and travel all the
way to Earth’s surface. At certain times of the year meteor showers occur when
Earth passes through the old orbit of a comet in which many debris remnants
remain. These comet fragments are observed as meteors in the night sky and 40 to
50 such events can occur per minutes during heavy showers. These displays of
“nature’s fireworks” are quite spectacular and are well worth staying up late or
getting up early to observe.
Meteors fall toward Earth quite often but are mostly burned up as they fall
through the atmosphere. As we stated above, those that do survive the trip through
the atmosphere and actually strike the Earth’s surface are called meteorites.
Meteorites are often too small or too similar to Earth rocks to be easily found.
Those that are retrieved by collectors have proved to be very important to
astronomers and other scientists because they are real samples of extra-terrestrial
material that can be analyzed and studied. High quality meteorite samples are
much prized by museums and private collectors alike.
Meteorites are classified into three groups; iron meteorites that are
composed of iron and nickle crystals, stony meteorites that are silicates and are
often quite similar to Earth rocks and therefore more difficult to locate and identify
when they fall, and stony-iron meteorites which are a combination of both of these
other types. Stony meteorites are also called chondrites (resembling matter from
the Sun, but without the volatile gases found there). Chondrites often contain
chondrules, small rounded pieces of glassy rock fragments. When volatile
compounds are still present in the rock, the term carbonaceous chondrites is used
because these samples usually also contain significant quantities of the element
carbon. Stony meteorites that do not contain chondrules are referred to as
achondrites.
Returning to our discussion of comets, these celestial objects are also
sometimes seen in the night sky as glowing balls with long luminous tails. Each
comet has, to varying degrees, two kinds of tails; gaseous tails (Type I) that are
pushed away from the Sun by the solar wind leaving a straight tail on the comet,
and dust tails (Type II) that are also pushed outward by the pressure of sunlight but
have a decidedly more curved shape.
The nucleus of a comet is sometimes referred to as a “dirty snowball”
because it is made up of ice and embedded rock fragments and gases, held together
in an icy state. When a comet’s orbit takes it in close to the Sun, this ice begins to
melt and the released dust and gases released by the melting ice are forced out into
space forming the comet’s tails as described above. Note that the tails of a comet
always point away from the Sun no matter what direction the nucleus of the comet
is actually traveling.
In addition to the tails and nucleus of a comet there is also a spherical coma
surrounding the nucleus that is composed of newly released gas and dust and
which can extend over 1 million kilometers out into space. Most of this gas is
hydrogen, but small amounts of other gases may also be present. These released
gases often glow brightly. Thus it is the coma and the tails of a comet that we see
when observing a comet from Earth, not the nucleus itself. The nucleus is quite
small and hidden within the glowing ball of the coma.
Comets are divided into two classes, short-period comets (periods of orbit
around the Sun that are less than 200 years) and long-period comets whose orbital
periods are longer than 200 years. About 100 short-period comets are known to
exist, the best known of which is probably Halley’s comet with a period of 76
years. Short-period comets tend to follow paths within the disk of the solar
system and probably originate by being pulled from the Kuiper belt by
gravitational interactions with the Jovian planets, although a few may have come
from the Oort cloud and had their long-period orbits altered by close encounters
with the massive gas-giant planets. Most long-period comets appear to have come
from the Oort cloud. These comets are pulled free when their original orbits within
the cloud are perturbed by the gravity of an occasional close-passing star. There
are 500 or so known long-period comets in orbit about our Sun.
Impacts of any large celestial object with Earth can have catastrophic effects
on the weather and environment. Regardless of whether these impacts are from
meteorites or comets these collisions with our planet can dramatically change the
environmental conditions on Earth and such impacts may have been responsible
for several major extinctions of animal and plant life on Earth since it was formed.
For this reason astronomical groups like the Lowell Observatory Near Earth Object
Search (LONEOS) scans the entire sky every month to identify any possible
collisions that might occur between such objects and our Earth. Although these
types of collisions are very rare, it is important to have as much early warning of
such an event as possible, even though it is not currently known exactly what could
be done to prevent a such a collision if one were predicted.
hapter 20 - Life on Other Worlds
With what we have learned about our universe, it is now time to take a few
minutes to review our place as human beings in the grand structure of things and
speculate about the concepts of life itself and if it can exist in other places. There
are no conclusive answers to these questions but we can still make some informed
and educated guesses.
Life is a very complex process and as such, may be quite hard to duplicate.
On the other hand the basic building blocks of life are not so complicated and have
been duplicated in the laboratory by human scientists. We have not yet been able
to bridge the final gap between these basic components of life and true life itself.
Nevertheless, it appears that the fundamental ingredients for life can be found as
what is known as a primordial soup in the oceans and other wet environments on
Earth. Therefore, as long as there is or has been liquid water present, it is not too
huge a step to assume that these basic components might also have combined to
produce life on any of a number of other planets in other solar systems or even on
some of the planets or moons in our own Sun-centered system.
Organic molecules are the key to life as we know it on Earth. Long carbonchain structures form in many different combinations with other elements to form
amino acids, enzymes, and proteins that, under the right conditions of temperature,
pressure, and electrical current stimulation seem to be able to produce life in a very
wide variety of forms. Life in the arctic, the tropical jungles, and even under
extreme conditions of temperature and pressure in deep sea trenches at the floor of
Earth’s oceans, all have developed their own unique life forms that persist and
thrive on Earth today.
If life was simply a continuous random process, there would be no
propagation of species and complex life forms such as animals and humans would
not have been able to develop and survive. The key to replicating complex life
forms lies in the chemical recipes that are stored in DNA molecules. This
complex template for cell formation allows genetic information to be handed down
from one generation to the next, insuring that the offspring produced will be
similar if not exact copies of the parents. This reproduction is not always identical
and when environmental conditions change, evolution often occurs causing slight
or even sometimes drastic mutations in the next generation. As life evolves, a
process known as natural selection tends to pick out the strongest traits that will, in
the long run, insure that the life form being reproduced will survive and life will
go on in step with environmental conditions that themselves often evolve in the
areas where life is trying to survive.
The first life appears to have originated on Earth about 3.5 billion years ago
as simple hydrocarbon-based organisms. More complex life forms such as higher
level animals, did not develop until about 600 million years ago and man himself
did not come upon the scene until much more recently. An interesting
representation of the entire age of Earth compared to a single year in time is
presented in Figure 20-8 in the textbook. Look this over carefully. It may give
you a better feeling for the relationship of the age of man to the overall time scale
for the development of our home planet.
Discovering extraterrestrial life has been the dream of many scientists for
years. Where might such life be found? Within our solar system there are several
possibilities. Mars is the most Earth-like planet and many scientists thought that
Mars might be a good place to start our search for extraterrestrial life, but
unmanned Mars explorers have so far found no conclusive proof of past or present
life on this nearby planet. None of the rest of the planets in our solar system
appear to have environments conducive to the formation of life as we know it.
Limited atmosphere, high temperature and/or pressure, lack of water, etc. makes
life on the other planets in our solar system appear quite unlikely. The satellites of
the Jovian planets, however, show much better possibilities. Moons such as
Europa and Titan seem to present the necessary conditions and are thus our best
hopes for finding new life forms. Unfortunately, it will probably be a long time
before definitive exploration of these distant moons can prove or disprove the
notion that life exists there.
Finally, what about life on other planets in other solar systems? Any planet
that supports living things would have to have a stable orbit around a main
sequence star like our Sun and also be located in an orbit that would provide the
necessary range of temperature and pressure. These and other conditions define a
life zone or ecosphere that is necessary for life as we know it on Earth to exist.
Some scientists believe that with all of the millions upon million of stable stars that
we know to exist in the universe, that these conditions must exist somewhere else
so the possibility of life in other solar systems is almost certain. Others are not so
sure. Remember we have proved that the basic building blocks of life are fairly
easy to produce, but that last step bridging the presence of the right organic
material in the right environment appears to be much more elusive. Just because
the correct conditions exist is not proof that life has actually begin.
If there is life in other solar systems, is it intelligent? Can we visit it or even
communicate with it? These questions are also quite complex and require the
development of much more sophisticated space ships if visits are planned, or the
formulation of a common language if complex communication is to be achieved.
None of this appears to be in man’s immediate future. The other major problem is
distance. Even the nearest stars are several light-years away. Remember that this
means that even if we could send and receive intelligent signals, it would take
about 10 years to send on message and get a reply because the fastest means of
communication is by radio waves (electromagnetic radiation) which can only
travel at the speed of light and thus requires years of turn-around time for any
messages exchanged.
Perhaps someday we will receive some sort of message from space that
indicates that intelligent life really does exist in other parts of the universe. Even
so, it is quite unlikely that we will be able to establish meaningful communication
between our species. We do have a model based on the Drake equation that can
help us calculate the probability of life on other worlds, but again a theory is not
the same thing as proof. Large radio telescopes like the one at Arecibo regularly
transmit signals toward stars that appear to have the proper conditions for planet
formation, and a project known as SETI (Search for Extra-terrestrial Intelligence)
probes the skies for incoming radio signals, but so far no good candidates for
intelligent messages have been found. We have taken an additional step to show
our presence to other intelligent life in the universe. When the deep space probe
Voyager was launched in the 1980s, it carried on-board an anti-coded message
(one that we hope could be easily decoded) in binary format that contained
information about our planet and the humans that inhabit it. Will other intelligent
beings ever recover this probe and learn that we exist? Even if they do we may
never know.
Afterword
Now that you have had a chance to see how vast and how diverse the
universe is, it is time to look back and reflect ton where our Earth and the human
race fits into this grand scheme. Our everyday concept of nature as applied to our
home planet takes on a whole new meaning when we consider the tremendous
interactions of energy and mass that have formed the myriad stars and galaxies in
the universe.
No matter how many other planets may be capable of supporting life, most
of the universe is still lifeless. This means that life is special and intelligence is
precious. Being able to use our intelligence to understand the universe is one of the
high points in the existence of the human race. This process of understanding
nature is the true goal of science, and astronomy is this understanding on the
largest possible scale.
Mankind faces a difficult future in this vast universe. With Earth’s limited
resources and growing population, it is going to become harder and harder to
survive as a species. Even if we can overcome our current short-term problems of
birth control and pollution, the Earth itself is only temporary when considered on
the cosmic scale. In a few billion years our Sun will evolve into a red giant and
Earth as a viable planet will cease to exist. This may be a very long time in the
future and man may be able to figure out ways to survive the ultimate demise of
our current home planet, but in the meantime we must all assume the responsibility
for becoming dependable custodians of our planet. We must make a concerted
effort to maintain our search for an understanding of our place in the universe as a
whole. Our species may not be perfect, but humanity has been given the gift of
intelligence and we must be willing to apply this gift, because according to the
author of our textbook, human intelligence is the “finest thing that the planet has
ever produced”.
Our journey through space is concluded now and we hope that you have
acquired a liking for the thrill of exploring and understanding our universe. We
can expect the study of astronomy to continue to evolve as new and better ways are
developed to gather data and analyze the information gained more completely.
This course has given you the opportunity to look rather deeply into the structure
and beauty of our extended world, from our home planet to the outer limits of
man’s investigations of cosmology and universal structure. There will always be
more to learn about our universe and about our home planet, Earth. Perhaps now
that you have a more complete understanding of your own position in the natural
world around us, you will be able to help in our continuing effort to maintain the
Earth as a viable residence for humanity and all of the other varied kinds of life
that depend on it.