Transcript The Scale of the Cosmos

The Scale of the Cosmos
Astronomy: The Solar System and Beyond
5th edition
Michael Seeds
Chapter 1
The Scale of the Cosmos
The longest journey
begins with a single step.
- CONFUCIUS
The Scale of the Cosmos
• You are about to embark on a
voyage out to the end of the
universe.
The Scale of the Cosmos
• Marco Polo journeyed east and
Columbus west.
– But you will travel outward away from your home on
Earth, out past the moon, sun, and other planets, past
the stars you see in the evening sky, and past billions
more that can be seen only with the aid of the largest
telescopes.
– You will journey through great whirlpools of stars to
the most distant galaxies visible from Earth - and then
you will continue on, carried only by experience and
imagination as you look for the structure of the
universe itself.
The Scale of the Cosmos
• Imagination is your key to discovery
– It will be your scientific time machine that
transports you into the past and into the future.
• Go back to watch the birth of the
universe, the formation of the first stars,
and the origin of the sun and Earth.
– Then, rush into the future to see what will
happen when the sun dies and Earth withers.
The Scale of the Cosmos
• Although you will discover a beginning to
the universe, you will not find an edge in
space.
– No matter how far you voyage, you will not run into a
wall or limit beyond which you cannot go.
– Rather you will discover evidence that our universe
may be infinite, that it may extend in all directions
without limit.
• Such vastness may dwarf our earthly
dimensions but not our human curiosity
and imagination.
The Scale of the Cosmos
• Astronomy is more than the study of stars
and planets.
– It is the study of the universe in which we humans
exist.
• You and I live on a small planet circling a
small sun drifting through the universe.
However, astronomy can take you beyond
these boundaries and help you not only
see where you are in the universe, but
understand what you are.
– You have a right to know these things.
– Perhaps, you have a duty to know them.
The Scale of the Cosmos
• Do not be humble.
– Although astronomical sizes and distance
may dwarf you, remember that you are an
intelligent creature, and you are capable of
understanding your universe.
– It is, after all, yours.
The Scale of the Cosmos
• Astronomy will introduce you to sizes,
distances, and times far beyond your
usual experience on Earth.
– Your task in this chapter is to grasp the meaning of
these unfamiliar sizes, distances, and times.
– Believe it or not, the solution lies in a single word;
scale.
– In this chapter, you will compare objects of different
sizes to grasp the scale of the universe.
The Scale of the Cosmos
• Let’s begin with something familiar.
– The figure below shows a region about 52 feet
across occupied by a human being, a sidewalk,
and a few trees—all objects whose size you can
understand.
The Scale of the Cosmos
• Each successive picture in this chapter will
show you a region of the universe that is
100 times wider than the preceding
picture.
– That is, each step will widen your field of view
by a factor of 100.
The Scale of the Cosmos
• In the figure, your field of view widens by a
factor of 100, and you can see an area 1
mile in diameter.
– The arrow points to the scene shown in the preceding
photograph.
– People, trees, and sidewalks
have vanished, but now you
can see a college campus
and the surrounding
streets and houses.
The Scale of the Cosmos
– The dimensions of houses and streets are
familiar.
– This is the world you know, and you can
relate such objects to the scale of your
body.
The Scale of the Cosmos
• You started your adventure using feet
and miles, but you should use the
metric system of units.
– Not only is it used by all scientists around the world,
but it makes calculations much easier.
– The photo in the previous figure is 1 mile in diameter.
– A mile equals 1.609 kilometers, so you can see in the
photo that a kilometer is a bit over two-thirds of a
mile—a short walk across a neighborhood.
The Scale of the Cosmos
• The view in this figure spans 160
kilometers.
– In this infrared photo, the green foliage shows up as
various shades of red.
– The college campus is now invisible, and the patches
of gray are small cities, with
the suburbs of Philadelphia
visible at the lower right.
The Scale of the Cosmos
• At this scale, you see the natural features
of Earth’s surface.
– The Allegheny Mountains of southern
Pennsylvania cross the image in the upper left,
and the Susquehanna River flows southeast into
Chesapeake Bay.
– What look like white bumps
are a few puffs of clouds.
The Scale of the Cosmos
• These features are a reminder that you live
on the surface of a changing planet.
– Forces in Earth’s crust pushed the mountain ranges
up into parallel folds, like a rug wrinkled on a polished
floor.
– The clouds tell you that Earth’s atmosphere is rich in
water, which falls as rain and erodes the mountains,
washing material down the rivers and into the sea.
– Mountains and valleys are only temporary features on
Earth; they are constantly changing.
The Scale of the Cosmos
• As you explore the universe, you
will come to see that it—like
Earth’s surface—is always
evolving.
The Scale of the Cosmos
• Take another look at the red color
of the figure.
– This is an infrared photograph in which healthy green
leaves and crops show up as red.
– Human eyes are sensitive
to only a narrow range of
colors.
– As you explore the universe,
you will learn to use a wide
range of “colors,” from X rays
to radio waves, to reveal
sights invisible to unaided
human eyes.
The Scale of the Cosmos
• At the next step in your journey, you will
see our entire planet, which is 12,756 km
in diameter.
– The photo shows most
of the daylight side
of the planet.
– The blurriness at the
extreme right is the
sunset line.
The Scale of the Cosmos
• Earth rotates on its axis once a day,
exposing half of its surface to daylight at
any particular moment.
– The rotation of Earth
carries you eastward
and, as you cross the
sunset line into darkness,
you say the sun has set.
– It is the rotation of the
planet that causes the
cycle of day and night.
The Scale of the Cosmos
• Earth’s interior is made mostly of iron and
nickel, and its crust is mostly silicate rocks.
– Only a thin layer of water makes up the oceans,
and the atmosphere is only a few hundred
kilometers deep.
– On the scale of the photograph you just saw, the
depth of the atmosphere on which life depends is
less than the thickness of a piece of thread.
The Scale of the Cosmos
• Enlarge your field of view by a factor of 100,
and you will see a region 1,600,000 km
wide.
– Earth is the small blue dot in the center and the moon,
whose diameter is only
one-fourth that of Earth,
is an even smaller dot along
its orbit 380,000 km
from Earth.
– These numbers are so large
that it is inconvenient to
write them out.
The Scale of the Cosmos
• Astronomy is the science of big numbers,
and you will use numbers much larger
than these to discuss the universe.
• Rather than writing out these numbers as
in the previous slide, it is convenient to
write them in scientific notation.
– This is nothing more than a simple way to write
numbers without writing lots of zeros.
– In scientific notation, you would write 380,000 as 3.8
x 105.
– The universe is too big to discuss without using
scientific notation.
The Scale of the Cosmos
• When you once again enlarge your field of
view by a factor of 100, Earth, the moon, and
the moon’s orbit all lie in the small red box at
lower left.
– Now, however, you can
see the sun and two
other planets that are part
of our solar system.
– Our solar system consists
of the sun, its family of
planets, and some smaller
bodies such as moons
and comets.
The Scale of the Cosmos
• Like Earth, Venus and Mercury are planets,
small, non-luminous bodies that shine by
reflected light.
– Venus is about the size of
Earth, and Mercury is
a bit larger than
Earth’s moon.
– On this diagram, they
are both too small to
be seen as anything
but tiny dots.
The Scale of the Cosmos
• The sun is a star, a self-luminous ball
of hot gas, that generates its own
energy.
– The sun is 109 times
larger in diameter than
Earth, but it too is nothing
more than a dot in the
diagram.
The Scale of the Cosmos
• This diagram has a diameter of 1.6 x 108
km.
• One way astronomers deal with large
numbers is to define new units.
– The average distance from Earth to the sun is a unit of
distance called the
astronomical unit (AU),
a distance of 1.5 x 1011 m.
The Scale of the Cosmos
• Using this unit, you can say that the
average distance from Venus to the sun is
about 0.7 AU.
– The average distance from Mercury to the sun
is about 0.39 AU.
The Scale of the Cosmos
• The orbits of the planets are not perfect
circles, and this is particularly apparent for
Mercury.
– Its orbit carries it as close to the sun as 0.307 AU and
as far away as 0.467 AU.
– You can see this variation
in the distance from
Mercury to the sun.
– Earth’s orbit is more
circular, and its distance
from the sun varies by
only a few percent.
The Scale of the Cosmos
• Your first field of view was only 52 feet
(about 16 m) in width.
• After only six steps of enlarging by a factor
of 100, you can now see the entire solar
system.
– Your field of view is 1 trillion
(1012) times wider than in
your first view.
The Scale of the Cosmos
– The details of the earlier figure are now lost in the
red square at the center of this diagram.
– You see only the brighter, more widely separated
objects as you enlarge your view.
The Scale of the Cosmos
• The sun, Mercury, Venus, and Earth lie so
close together that you cannot separate
them at this scale.
• Mars, the next outward planet, lies only
1.5 AU from the sun.
• In contrast, Jupiter, Saturn, Uranus,
Neptune, and Pluto are so far from the sun
that they are easy to place in the diagram.
– These are cold worlds far from the sun’s warmth.
The Scale of the Cosmos
• Light from the sun reaches Earth in only 8
minutes, but it takes over 4 hours to reach
Neptune.
• Notice that Pluto’s orbit is so elliptical that
Pluto can come closer
to the sun than
Neptune does, as
it did between
1979 and 1999.
The Scale of the Cosmos
• When you again enlarge your field of view
by a factor of 100, the solar system
vanishes.
– The sun is only a point of light, and all the planets
and their orbits are now crowded into the small red
square at the center.
– The planets are too small,
and reflect too little light,
to be visible so near the
brilliance of the sun.
The Scale of the Cosmos
• Nor are any stars visible except
for the sun
– The sun is a fairly typical star, and it seems to be
located in a fairly average neighborhood in the
universe.
– Although there are many
billions of stars like the sun,
none is close enough to be
visible in the diagram, which
shows an area only
11,000 AU in diameter.
The Scale of the Cosmos
• The stars are typically separated by
distances about 10 times larger than the
diameter of the diagram.
– Except for the sun at
the center, this diagram
is empty.
– However, you will see
stars in the next field
of view.
The Scale of the Cosmos
• It is difficult to grasp the isolation
of the stars.
– If the sun were represented by a golf ball in
New York City, the nearest star would be
another golf ball in Chicago.
– Except for the widely scattered stars and a
few atoms of gas drifting between the stars,
the universe is nearly empty.
The Scale of the Cosmos
• In the diagram, your field of view has
expanded to a diameter a bit over
1 million AU.
– The sun is at the center,
and you can see a few
of the nearest stars.
– These stars are so distant
that it is not reasonable to
give their distances in
astronomical units.
The Scale of the Cosmos
• To express distances so large,
astronomers define a new unit of distance,
the light-year.
– One light-year (ly) is the distance that light travels in
one year, roughly 1013 km or
63,000 AU.
– The diameter of your field of view is 17 ly.
– The nearest star to the sun, Alpha Centauri, is 4.2 ly
from Earth.
– In other words, light from Alpha Centauri takes 4.2
years to reach Earth.
The Scale of the Cosmos
• Here’s more about Alpha Centauri.
– It is in the southern sky, so it is invisible from all
but the southernmost parts of the United States
where it occasionally peeks above the southern
horizon.
– If you ever have the chance, you should locate
the sun’s nearest companion in space.
– Later, you will discover that Alpha Centauri is
actually three stars orbiting around each other.
The Scale of the Cosmos
• Although stars are roughly the same
size as the sun, they are so far away
that you cannot see them as anything
but points of light, even with the largest
telescopes on Earth.
– Using indirect methods,
astronomers have
found nearly 200
planets orbiting
other stars.
The Scale of the Cosmos
• If you expand your field of view by a factor of
100, you see our galaxy.
– A galaxy is a great cloud of stars, gas, and dust bound
together by the combined gravity of all the matter.
Galaxies range from
1500 to over 300,000 ly in
diameter and can contain
over 100 billion stars.
– In the night sky, you see
our galaxy as a great,
cloudy wheel of stars
ringing the sky as the
Milky Way. Our galaxy
is called the Milky Way
Galaxy.
The Scale of the Cosmos
• In the figure, the sizes of the dots represent
not the sizes of the stars, but their
brightness.
– This is the custom in astronomical diagrams, and it is
also how star images are
recorded on photographs.
– Bright stars make larger
spots on a photo than faint
stars.
– The size of a star image in a
photo informs you not how
big the star is, but only how
bright it looks.
The Scale of the Cosmos
• In the diagram, you expand your field of
view by another factor of 100, and the sun
and its neighboring stars vanish into the
background of thousands of other stars.
– The field of view is
now 1,700 ly in
diameter.
The Scale of the Cosmos
– Of course, no one has ever journeyed thousands
of light-years to photograph the solar
neighborhood, so this is a representative photo of
the sky.
– The sun is a relatively
faint star that would not
be easily located in a
photo at this scale.
The Scale of the Cosmos
• What you do not see in the photograph
is critically important.
– You do not see the thin gas that fills the spaces
between the stars.
– Although those clouds of gas are thinner than the best
vacuum on Earth, it
is those clouds that give
birth to new stars.
– Our sun formed from such
a cloud about 5 billion
years ago.
– You will see evidence of
star formation in your
next field of view.
The Scale of the Cosmos
• Of course, no one can journey far enough
into space to look back and photograph our
home galaxy, so the photo shows a galaxy
similar to our own.
– Our sun would be invisible
in such a photo.
– However, if you could see
it, you would find it in the
disk of the galaxy
about two-thirds of the
way out from the center.
The Scale of the Cosmos
• Our galaxy, like many others, has graceful
spiral arms winding outward through the
disk.
– You will discover that stars are born in great
clouds of gas and dust
as they pass through
the spiral arms.
The Scale of the Cosmos
• Ours is a fairly large galaxy, roughly
75,000 ly in diameter.
– Only a century ago astronomers thought it was
the entire universe—an island universe of stars in
an otherwise empty vastness.
– Now, they know that our galaxy is not unique.
Indeed ours is only one of many billions of
galaxies scattered throughout the universe.
The Scale of the Cosmos
• As you expand your field of view by another
factor of 100, our galaxy appears as a tiny
luminous speck surrounded by other specks.
– The diagram includes a
region 17 million ly in
diameter, and each of the
dots represents a galaxy.
– Notice that our galaxy is
part of a cluster of a few
dozen galaxies.
The Scale of the Cosmos
• Galaxies are commonly grouped together
in such clusters.
– Some of these galaxies have beautiful spiral patterns
like our own galaxy, but others do not.
– Some are strangely distorted.
– One of the mysteries of modern astronomy is what
produces these differences among the galaxies.
– Astronomers have found some important clues and
have developed a fascinating theory.
The Scale of the Cosmos
• If you again expand your field of view, you
see that the clusters of galaxies are
connected in a vast network.
– Clusters are grouped into
superclusters—clusters of
clusters—and the
superclusters are linked to
form long filaments and
walls. They outline voids
that seem nearly empty of
galaxies.
– These appear to be the
largest structures in the
universe.
The Scale of the Cosmos
• Were you to expand your field of view
another time, you would probably see
a uniform fog of filaments and voids.
– When you puzzle over the origin of these
structures, you are at the frontier of
human knowledge.
The Scale of the Cosmos
• The first hurdle in studying
astronomy is keeping a proper
sense of scale.
• Remember that each of the billions
of galaxies contains billions of
stars.
The Scale of the Cosmos
• Later, you will see clear evidence that
many of those stars have families of
planets like our solar system. On some of
those planets, liquid-water oceans and
protective atmospheres may have
sheltered the spark of life.
– It is possible that at least a few other planets are
inhabited by intelligent creatures who share our
curiosity, and our wonder, at the scale of the cosmos.