Download Beyond Our Solar System

Chapter 16 Lecture Outline
Beyond Our
Solar System
– The figure 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.
• Each successive picture in the 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—the
region you can see in the image—by a factor of 100.
In this 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 photo.
– People, trees, and sidewalks
have vanished, but now you
can see a college campus
and the surrounding
streets and houses.
– 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.
• 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 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 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.
– The patches of gray are
small cities, with Wilmington,
Delaware, visible at the
lower right.
• 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.
– The Susquehanna River flows southeast into Chesapeake Bay.
– What look like white bumps
are a few puffs of clouds.
• Notice the red color.
– 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.
• At the next step in your journey, you will see your entire planet—which is
12,756 km in diameter.
• Earth rotates on its axis once a day, exposing half of its
surface to daylight at
any particular moment.
– The photo shows most of the
daylight side of the planet.
– The blurriness at the
extreme right is the sunset
line.
• The rotation of Earth carries you eastward.
– 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.
• 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.
– 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.
– 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.
• 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.
• Like Earth, Venus and Mercury are planets—small, nonluminous 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 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.
• This diagram has a diameter of 1.6 x 108 km.
– 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.
• 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 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 in the figure.
– Earth’s orbit is more circular,
and its distance from the
sun varies by only a few
percent.
• Enlarge your field of view again, and you can see the entire solar system.
• The details of the preceding figure are now lost in the red square at the
center of the diagram.
– You see only the brighter,
more widely separated
objects.
• 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.
– Light from the sun reaches
Earth in only 8 minutes,
but it takes over 4 hours to
reach Neptune.
• Pluto’s orbit is so elliptical that it can come closer to
the sun than Neptune does—as Pluto did between
1979 and 1999.
• 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.
• 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 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.
• Now, 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.
• To express distances so large, 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.
•
It is a common misconception that a
light-year is a time.
– Have you heard people say, “It will take me light-years
to finish my term paper”?
– Next time, you can tell them that a light-year is a
distance, not a time.
• The diameter of your field of view in the figure 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.
• 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 photos.
– 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.
• Now, 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
1,700 ly in diameter.
• 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.
• If you expand your field of view by a factor of 100, you see our
galaxy—a disk of stars about 75,000 ly in diameter.
– A galaxy is a great cloud of stars, gas, and dust bound together by the combined
gravity of all the matter.
– Galaxies range from
1,500 to over 300,000 ly
in diameter and can
contain over 100 billion
stars.
• 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.
• 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.
– The superclusters are linked
to form long filaments and
walls outlining voids that
seem nearly empty of
galaxies.
– These appear to be the
largest structures in the
universe.
Focus Question 16.1
What was Hubble’s big contribution to
cosmology?
The Universe
• Cosmology
– Study of the Universe
• I. Kant – mid 1700’s
– Island Universes - fuzzy patches
• Edwin Hubble – 1919
– Cephid variables
• Compare absolute magitude to observed
brightness
– Determined the fuzzy patches were
indeed out of our galaxy
• Light-year
– Distance light travels in one year
– Slightly less than 10 trillion km
Properties of Stars
 Distance
 Measuring a star's distance can be very
difficult
 Stellar parallax
 Used for measuring distance to a star
 Apparent shift in a star's position due to the
orbital motion of Earth
 Measured as an angle
 Near stars have the largest parallax
 Largest parallax is less than one second of arc
 Distance
 Distances to the stars are very large
 Units of measurement
 Kilometers or astronomical units are too cumbersome to use
 Light-year is used most often
 Distance that light travels in 1 year
 One light-year is 9.5 trillion km (5.8 trillion miles)
 Other methods for measuring distance are also used
Properties of Stars
 Stellar brightness
 Controlled by three factors
 Size
 Temperature
 Distance
 Magnitude
 Measure of a star's brightness
Properties of Stars
 Stellar brightness
 Magnitude
 Two types of measurement
 Apparent magnitude
 Brightness when a star is viewed
from Earth
 Decreases with distance
 Numbers are used to designate
magnitudes—Dim stars have large
numbers and negative numbers are
also used
Properties of Stars
 Stellar brightness
 Magnitude
 Absolute magnitude
 "True" or intrinsic brightness of a star
 Brightness at a standard distance of
32.6 light-years
 Most stars' absolute magnitudes are
between -5 and +15
 Color and temperature
Properties of Stars
 Hot star
 Temperature above 30,000 K
 Emits short-wavelength light
 Appears blue
 Cool star
 Temperature less than 3000 K
 Emits longer-wavelength light
 Appears red
 Color and temperature
Properties of Stars
 Between 5000 and 6000 K
 Stars appear yellow
 Binary stars and stellar mass
 Binary stars
 Two stars orbiting one another
 Stars are held together by mutual
gravitation
 Both orbit around a common center of
mass
 Binary stars
and stellar
mass
Properties
of Stars
 Binary stars
 Visual binaries are resolved telescopically
 More than 50% of the stars in the universe
are binary stars
 Used to determine stellar mass
 Stellar mass
 Determined using binary stars—The center
of mass is closest to the most massive star
 Mass of most stars is between one-tenth
and fifty times the mass of the Sun
Binary Stars
Orbit Each
Other
Around Their
Common
Center of
Mass
The Universe
The Universe
The Universe
Focus Question 16.2
Why are reflection nebulae generally blue?
Interstellar Matter: Nursery of the Stars
Interstellar matter
• Strings and clumps of matter
• Nebulae (clouds) 2 TYPES:
–Bright nebulae
 Glows if it close to a very hot star
 Two types of bright nebulae
 Emission nebula
 Reflection nebula
Interstellar Matter: Nursery of the Stars
Bright nebulae
• Emission nebulae
‒ Active, star-forming regions
‒ UV light emitted under low pressure
‒ Appear red
Interstellar Matter: Nursery of the Stars
Bright nebulae
• Reflection nebulae
‒ Reflect light of nearby stars
‒ Appear blue
Interstellar Matter: Nursery of the Stars
Bright nebulae
• Planetary nebulae
‒ Originate from dying stars
‒ Resemble giant planets
Interstellar Matter: Nursery of the Stars
Dark nebulae
• Made of hydrogen (90%), helium, and
interstellar dust
• Too distant from stars to be illuminated
• Appear as:
‒ Opaque objects against bright backgrounds
‒ Starless regions in space
‒ Contains the material that forms stars and planets
Focus Question 16.3
How does the H-R diagram classify stars?
Classifying Stars: H-R Diagrams
Hertzsprung-Russell Diagram
• Shows the relation between stellar
brightness (absolute magnitude)
and temperature
• Diagram is made by plotting each
star’s:
–Luminosity (brightness) and
–Temperature
Classifying Stars: H-R Diagrams
• Main-sequence stars
– 90% of all stars
– Band through the center of the H-R diagram
– Sun is in the main-sequence
• Giants (or red giants)
– Large and very luminous
– Upper-right on the H-R diagram
– Very large giants are called supergiants
– Only a few percent of all stars
• White dwarfs
– Fainter than main-sequence stars
– Small (approximate the size of Earth)
– Lower-central area on the H-R diagram
– Not all are white
Classifying Stars: H-R Diagrams
Focus Question 16.4
Why are less massive stars thought to age
more slowly than more massive stars, despite
the fact they have much less “fuel”?
Stellar Evolution
• Stars exist because of gravity
• Two opposing forces in a star are:
1. Gravity: contracts
2. Thermal nuclear energy: expands
• Stages
– Birth
• In dark, cool, interstellar clouds
• Gravity contracts the cloud
• Temperature rises
• Becomes a protostar
Stellar Evolution
Protostar
• Gravitational contraction of gasses
continues
• Core reaches 10 million K
• Hydrogen nuclei fuse
–Become helium nuclei
–Process is called hydrogen fusion
• Energy is released
• Outward pressure balanced by gravity
• Star becomes a stable main-sequence star
Stellar Evolution
Main-sequence stage
• Stars age at different rates
–Massive stars
• Use fuel faster
• Exist for only a few million years
–Small stars
• Use fuel slowly
• Exist for perhaps hundreds of billions of years
• 90% of a star’s life is in the main-sequence
Stellar Evolution
Red giant stage
• Hydrogen burning migrates outward
• Star’s outer envelope expands
–Surface cools
–Surface becomes red
• Core collapses as helium converts to
carbon
• Eventually all nuclear fuel is used
• Gravity squeezes the star
Stellar Evolution
Stellar Evolution
Burnout and death
• Final stage depends on mass
– Low-mass star
• 0.5 solar mass
• Red giant collapses and becomes a white dwarf
– Medium-mass star
• Between 0.5 and 3 solar masses
• Red giant collapses, planetary nebula forms, then
becomes a white dwarf
Stellar Evolution
– Massive star
• Over 3 solar masses
• Terminates in a supernova
• Interior condenses and may produce a hot, dense
object that is either a neutron star or a black hole
Focus Question 16.5
Explain how it is possible for the smallest white
dwarfs to be the most massive.
Stellar Remnants
White dwarf
• Small (some no larger than Earth)
• Dense
–Can be more massive than the Sun
–Spoonful weighs several tons
–Atoms take up less space
• Electrons displaced inward
• Called degenerate matter
• Hot surface
• Cools to become a black dwarf
Stellar Remnants
Neutron stars
• Forms from a more massive star
–Star has more gravity
–Squeezes itself smaller
• Remnant of a supernova
• Gravitational force collapses atoms
–Electrons combine with protons to produce
neutrons
–Small size
Stellar Remnants
Neutron stars
• Pea size sample
–Weighs 100 million tons
–Same density as an atomic nucleus
• Strong magnetic field
• First one discovered in early 1970s
–Pulsar (pulsating radio source)
–Found in the Crab Nebula (remnant of an
A.D. 1054 supernova)
Stellar Remnants
Stellar Remnants
Black holes
• More dense than neutron stars
• Intense surface gravity lets no light escape
• As matter is pulled into it
–Becomes very hot
–Emits x-rays
• Likely candidate is Cygnus X-1, a strong xray source
Stellar Remnants
Focus Question 16.6
What are the 3 major classes of Galaxies?
Galaxies and Galactic Clusters
Three basic types of galaxies
1. Spiral galaxy
• Arms extending from nucleus
• About 30 percent of all galaxies
• Large diameter of 20,000 to 125,000
light years
• Contains both young and old stars
• e.g., Milky Way
 Milky Way galaxy
 Rotation
 Around the galactic nucleus
 Outermost stars move the slowest
 Sun rotates around the galactic nucleus once about
every 200 million years
 Halo surrounds the galactic disk
 Spherical
 Very tenuous gas
 Numerous globular clusters
Galaxies and Galactic Clusters
Galaxies and Galactic Clusters
Galaxies and Galactic Clusters
2. Elliptical galaxy
• Ellipsoidal shape
• About 60% of all
galaxies
• Most are smaller
than spiral galaxies;
however, they are
also the largest
known galaxies
Galaxies and Galactic Clusters
3.Irregular galaxy
•
•
•
•
Lacks symmetry
About 10% of all galaxies
Contains mostly young stars
e.g., Magellanic Clouds
Galaxies and Galactic Clusters
Galactic cluster
• Group of galaxies
• Some contain thousands of galaxies
• Local Group
–Our own group of galaxies
–Contains at least 28 galaxies
• Supercluster
–Huge swarm of galaxies
–May be the largest entity in the universe
Galaxies and Galactic Clusters
Galaxies and Galactic Clusters
• Galactic Interactions
– Collisions between galaxies
– Driven by one galaxy’s gravity disturbing another
– A large galaxy may engulf a dwarf satellite galaxy
– Two dwarf satellite galaxies are currently merging
with the Milky Way
• Two galaxies of similar size may pass through
one another without merging
– Interstellar matter will likely interact
– Triggers an intense period of star formation
• In 2 to 4 Ga, 50% probability that Milky Way
and Andromeda Galaxies will collide and merge
Galaxies and Galactic Clusters
Focus Question 16.7
What property does the universe possess that
will determine its final state?
The Big Bang Theory
Doppler effect
• Change in wavelength due to motion
–Movement away stretches the wavelength
• Longer wavelength
• Light appears redder
–Movement toward “squeezes” the
wavelength
• Shorter wavelength
• Light shifted toward the blue
The Big Bang Theory
The Big Bang Theory
Doppler effect
• Amount of the Doppler shift indicates the
rate of movement
–Large Doppler shift indicates a high velocity
–Small Doppler shift indicates a lower velocity
The Big Bang Theory
Most galaxies exhibit a red Doppler shift
• Far galaxies
–Exhibit the greatest shift
–Greater velocity
• Discovered in 1929 by Edwin Hubble
• Hubble’s Law
–Recessional speed of galaxies is proportional to
their distance
• Accounts for red shifts
The Big Bang Theory
Big Bang Theory
• Accounts for galaxies moving away from us
• Universe was once confined to a “ball” that
was:
–Supermassive
–Dense
–Hot
The Big Bang Theory
• Big Bang marks the inception of the
universe
– Occurred about 15 billion years ago
– All matter and space was created
• Matter is moving outward
• Fate of the universe
– Two possibilities:
1. Universe will last forever
2. Outward expansion will stop and gravitational
contraction will follow
The Big Bang Theory
The Big Bang Theory
Fate of the universe
• Final fate depends on density of the
universe
–If density is more than the critical density,
universe will contract
–Current estimates are less than critical density
• Predicts ever-expanding,or open, universe