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Stars, Galaxies, and the Universe
Chapter 30
Earth and Space Science
1
Analyzing Starlight
• Nuclear fusion is the combination of light
atomic nuclei to form heavier atomic nuclei
• Astronomers learn about stars by analyzing
the light that the stars emit.
• Starlight passing through a spectrograph
produces a display of colors and lines called a
spectrum.
2
Analyzing Starlight
• All stars have dark-line spectra.
• A star’s dark-line spectrum reveals the star’s
composition and temperature.
• Stars are made up of different elements in
the form of gases.
• Scientists can determine the elements that
make up a star by studying its spectrum.
3
The Compositions of Stars
• Scientists have learned that stars are made
up of the same elements that compose
Earth.
• The most common element in stars is
hydrogen.
• Helium is the second most common element
in star.
• Small quantities of carbon, oxygen, and
nitrogen are also found in stars.
4
The Temperatures of Stars
• The temperature of most stars ranges from
2,800˚C to 24,000˚C.
• Blue stars have average surface temperatures
of 35,000˚C.
• Yellow stars, such as the sun, have surface
temperatures of between 5,000˚C and
6,000˚C.
• Red stars have average surface temperatures
of 3,000˚C.
5
The Sizes and Masses of Stars
• Stars vary in size and mass.
• Stars such as the sun are considered
medium-sized stars.
• Most stars visible from Earth are
medium-sized stars.
6
Stellar Motion
• Two kinds of motion
– Actual Motion
– Apparent Motion
7
Apparent Motion of Stars
• The apparent motion of stars is the motion
visible to the unaided eye.
• Apparent motion is caused by the movement
of Earth.
• The rotation of Earth causes the apparent
motion of stars sees as though the stars are
moving counter-clockwise around the North
Star.
• Earth’s revolution around the sun causes the
stars to appear to shift slightly to the west
every night.
8
Spot Question
• Why does Polaris appear to remain
stationary in the night sky?
• Polaris is almost exactly above the pole
of Earth’s rotational axis, so Polaris
moves only slightly around the pole
during one rotation of Earth.
9
Circumpolar Stars
• Some stars are always visible in the
night sky. These stars never pass below
the horizon.
• In the Northern Hemisphere, the
movement of these stars makes them
appear to circle the North Star.
• These circling stars are called
circumpolar
10
Circumpolar Stars
• The stars of the little dipper are
circumpolar for most observers in the
Northern Hemisphere.
• At the pole all visible stars are
circumpolar.
• As you move off the pole fewer and
fewer circumpolar stars exist.
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Actual Motion of Stars
• Most stars have several types of actual
motion.
• Stars rotate on an axis.
• Some stars may revolve around another star.
• Stars either move away from or toward our
solar system.
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Actual Motion of Stars
• The spectrum of a star that is moving toward
or away from Earth appears to shift, due to
the Doppler effect.
• Stars moving toward Earth are shifted slightly
toward blue, which is called blue shift.
• Stars moving away from Earth are shifted
slightly toward red, which is called red shift.
13
Actual Motion of Stars
Doppler Effect
• The spectrum of a star that is moving toward or
away from Earth appears to shift, as shown in the
diagram below.
14
Distances to Stars
• Distances between the
stars and Earth are
measured in light-years.
• light-year the distance
that light travels in one
year.
– about 9.5 trillion kilometers
(5.8 trillion miles).
15
Distance to Stars
How big is the universe?
• Proxima Centauri is about 4.3 lightyears from the earth.
– The light produced by Proxima Centauri
takes about 4.3 years to reach earth.
– Light from the sun reaches the earth in
about 8 minutes.
• This fact suggests that the universe is
incomprehensibly large.
16
Measuring Distances to the Stars
•
Stellar parallax, the extremely slight
back-and-forth shifting in a nearby
star's position due to the orbital motion
of Earth.
– The farther away a star is, the less its
parallax.
– Parallax angles are very small.
17
Stellar parallax
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Another method
o
• The Parsec: 1 of Parallax angle
– A unit used to express stellar distance is =
to about 3.2 light-years.
– 30.4 trillion kilometers (18.56 trillion
miles).
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20
Stellar Brightness
• Three factors control the brightness of a
star as seen from Earth:
– size (how big),
– temperature (how hot),
– distance from Earth (how far away).
21
Stellar Brightness
• Magnitude is the measure of a star's
brightness.
• Apparent magnitude is how bright a star
appears when viewed from Earth.
• Absolute magnitude is the "true" brightness if
a star were at a standard distance of about
32.6 light-years.
• The difference between the two magnitudes
is directly related to a star's distance.
22
Apparent magnitude
• The lower the number of the star on the
scale shown on the diagram below, the
brighter the star appears to observers.
• The sun has an apparent magnitude of –
26.8
• All other objects are dimmer.
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End of Section 1
• Answer Questions 1-6 on page 780.
24
Classifying Stars
• One way scientists classify stars is by plotting the
surface temperatures of stars against their luminosity.
• The H-R diagram is the graph that illustrates the
resulting pattern.
• Astronomers use the H-R diagram to describe the
life cycles of stars.
• Most stars fall within a band that runs diagonally
through the middle of the H-R diagram.
• These stars are main sequence stars.
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H-R Diagram - History
•A useful astronomical tool which plots stellar
temperature (color) against luminosity.
•Independently invented by Henry Russell in 1913 &
Ejnar Hertzsprung in 1905 through the study of true
brightness and temperature of stars.
•Useful for studying properties & life cycles of stars:
• Mass, Luminosity, Surface Temperature, Age
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H-R Diagram
27
Don’t bother copying…
• Stellar temperature/color also gives rise to
“Spectral Classes.”
–
–
–
–
–
–
–
O (> 30,000 K).
B (10,000 – 30,000 K).
A (7,000 – 10,000 K).
F (6,000 – 7,000 K).
G (5,000 – 6,000 K) – the sun!
K (4,000 – 5,000 K).
M (< 4,000 K).
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The Hertzsprung-Russell diagram
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H-R Diagram cont.
• Stars located in the upper-right position of an H-R
diagram are called giants, luminous stars of large
radius.
• Supergiants are very large.
• Very small white dwarf stars are located in the
lower-central portion of an H-R diagram.
• Ninety percent of all stars, called main-sequence
stars, are in a band that runs from the upper-left
corner to the lower-right corner of an H-R diagram.
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H-R Diagram
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Points of Note
• Stars spend 90% of their lives on Main
Sequence
• Main Sequence stars are burning only
Hydrogen
• High mass stars live fast, die young:
• 20 Solar Mass Star
- 10 Million Years
• Sun
- 10 Billion Years
• Red Dwarf
- >100 Billion Years
32
Differences Between High Mass and Low
Mass Stars
• Stars that are more massive than the Sun have
stronger gravitational forces.
• These forces need to be balanced by higher
internal pressures.
• These higher pressures result in higher
temperatures which drive a higher rate of fusion
reactions.
• The Hydrogen within the core of a high mass star
therefore gets used up much faster than in the Sun
and “ages” faster.
• Low mass stars “age” slower.
33
Star Formation
• A star beings in a nebula.
• As gravity pulls particles of the
nebula closer together, the
gravitational pull of the particles on
each other increases.
• As more particles come together,
regions of dense matter begin to build
up within the cloud.
34
Nebula
• New stars are born out of enormous
accumulations of dust and gases, called
nebula, that are scattered between existing
stars. Nebula comes from the Latin for “cloud”.
The Orion Star Forming Complex
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Interstellar Matter
36
Dark Nebula
• When a nebula is not
close enough to a bright
star to be illuminated, it
is referred to as a dark
nebula.
• Horsehead Nebula is a
dark nebula.
37
Bright Nebula
• A bright nebula glows
because the matter is close to
a very hot (blue) star.
• Emission nebulae: derive
their visible light from the
fluorescence of the ultraviolet
light from a star in or near the
nebula.
38
Bright Nebulae
• Reflection nebulae:
relatively dense dust
clouds in interstellar
space that are
illuminated by
reflecting the light of
nearby stars.
39
Stellar Lifecycles
• The process by which
stars are formed and use
up their fuel.
• What exactly happens to
a star as it uses up its
fuel is strongly
dependent on the star’s
mass.
The Orion Nebula - Birthplace of stars
40
Protostars
• Gravity within a nebula compacts it to
form a flattened disk.The disk has a
central concentration of matter called a
protostar.
• The protostar continues to contract and
increase in temperature for several
million years and becomes plasma.
41
The Birth of a Star
• A protostar’s temperature continually increases until
it reaches about 10,000,000°C.
• At this temperature, nuclear fusion begins.
• The process releases enormous amounts of energy.
• The onset of nuclear fusion marks the birth of a star.
Once this process begins, it can continue for billions
of years.
42
A Delicate Balancing Act
• As gravity increases the pressure on the matter
within the star, the rate of fusion increase.
• In turn, the energy radiated from fusion reactions
heats the gas inside the star.
• The outward pressures of the radiation and the hot
gas resist the inward pull of gravity.
• This equilibrium makes the star stable in size.
43
The Main-Sequence Stage
• Energy continues to be generated in the core
of the star as hydrogen fuses into helium.
• A star that has a mass about the same as the
sun’s mass stays on the main sequence for
about 10 billion years.
• Scientists estimate that over a period of
almost 5 billion years, the sun has converted
only 5% of its original hydrogen nuclei into
helium nuclei.
44
Leaving the Main Sequence
•
•
When almost all of the hydrogen atoms
within its core have fused into helium atoms
the core of the star contracts because of
gravity.
As the temperature rises the last of the
hydrogen atoms fuse and send energy into
the outer shell.
45
Giant Stars
• A star enters its third stage when
almost all of the hydrogen atoms within
its core have fused into helium atoms.
• A star’s shell of gases grows cooler as it
expands. As the gases in the outer
shell become cooler, they begin to glow
with a reddish color. These stars are
known as giants.
46
Supergiants
• Main-sequence stars that are more massive
than the sun will become larger than giants
in their third stage.
• These highly luminous stars are called
supergiants.
• These stars appear along the top of the H-R
diagram.
• Despite the high luminosity these stars are
relatively cool.
47
The Final Stages of a Sunlike Star
• When all the helium has been used up,
the fusion will stop.
• With no energy available the star will
enter its last stages.
48
Planetary Nebulas
• As the star’s outer gases drift away,
the remaining core heats these
expanding gases.
• The gases appear as a planetary
nebula, a cloud of gas that forms
around a sunlike star that is dying.
49
The Sun’s Planetary Nebula
• When it runs out of
Helium fuel it
begins to contract
and heat up.
• The Sun increases
its luminosity.
• The outer layers of
the Sun expand,
cool and redden
again.
• The outer layers of the
Sun start streaming
away from the core.
• This material forms a
nebula surrounding
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the Sun.
White Dwarfs
• As a planetary nebula disperses,
gravity causes the remaining matter in
the star to collapse inward.
• A hot, extremely dense core of matter a white dwarf - is left.
• White dwarfs shine for billions of years
before they cool completely.
51
Novas and Super novas
• When a star explosively brightens, it is called
a nova (new star). Excessively large
explosions are called supernovas.
• During the outburst, the outer layer of the star
is ejected at high speed.
• After reaching maximum brightness in a few
days, the nova slowly returns in a year or so
to its original brightness.
52
Novas and Supernovas
• Some white dwarfs revolve around red
giants. When this happened, the gravity of
the whit dwarf may capture gases from the
red giant.
• As these gases accumulate on the surface of
the white dwarf, pressure begins to build up.
• This pressure may cause large explosions.
These explosions are called novas.
53
Supernova
• Stars more than
three times the
mass of the Sun
terminate in a
brilliant explosion
called a supernova.
54
The Final Stages of Massive Stars
• The result of a star that exploded in 1054 AD.
• This spectacular supernova explosion was recorded
by Chinese and (quite probably) Anasazi Indian
astronomers.
The Crab Nebula
55
Supernovas in Massive Stars
• Massive stars become supernovas as part of
their life cycle.
• After the supergiant stage, the star
collapses, producing such high temperatures
that nuclear fusion begins again.
• When nuclear fusion stops, the star’s core
begins to collapse under its own gravity.
This causes the outer layers to explode
outward with tremendous force.
56
Neutron Stars
• Stars more massive than the sun do not
become white dwarfs.
• After a star explodes as a supernova, the
core may contract into a neutron star.
• A star that has collapsed under gravity to the
point that the electrons and protons have
smashed together to form neutrons
57
Pulsars
• Variable stars fluctuate in brightness.
• Some neutron stars emit a beam of radio
waves that sweeps across space and are
detectable here on Earth.
• These stars are called pulsars. For each pulse
detected on Earth, we know that the star has
rotated within that period.
58
Black Holes
Supernovae events can produce
small, extremely dense (A peasized sample of matter would
weigh 100 million tons)
neutron stars, composed
entirely of subatomic particles
called neutrons; or even smaller
and more dense black holes,
objects that have such immense
gravity that light cannot escape
their surface.
59
Section 3: Star Groups
• We can only see some of the trillions of stars
that make up the universe.
• Most of the ones we see are within 100 lightyears of Earth.
• In the constellation Andromeda there is a
hazy region that is actually a collection of
stars that are 2 million light-years from Earth.
60
Dividing Up the Sky
• In 1930, astronomers around the world
agreed upon a standard set of 88
constellations which the sky has been
divided in order to describe the
locations of celestial objects.
• You can use a map of the
constellations to locate a particular star.
61
Naming Constellations
• Many of the modern names we use for
the constellations come from Latin.
• Some constellations are named for real
or imaginary animals, such as Ursa
Major (the great bear) or ancient gods
or legendary heroes, such as Hercules
or Orion.
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The Constellation Orion
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Multiple-Star Systems
• Over half of all observed stars form multiplestar systems.
• Binary stars are pairs of stars that revolve
around each other and are held together by
gravity.
• In star systems that have more than two
stars, two stars may revolve rapidly, while a
third star revolves more slowly at a greater
distance from the pair.
64
Spot Question
• What percentage of stars are in
multiple-star systems?
• More than 50% of all stars are in
multiple-star systems.
65
Star Clusters
• Sometimes, nebulas collapse to form groups
of hundreds or thousands of stars called
clusters.
• Globular clusters have a spherical shape and
can contain up to 100,000 stars.
• An open cluster is loosely shaped and rarely
contains more than a few hundred stars.
66
Galaxies
• Galaxies are the major building blocks
of the universe. Astronomers estimate
that the universe contains hundreds of
billions of galaxies.
• A typical galaxy, such as the Milky Way,
has a diameter of about 100,000 lightyears and may contain more than 200
billion stars.
67
Types of Galaxies
• Galaxies are classified by shape into three main
types.
• A spiral galaxy has a nucleus of bright stars and
flattened arms that spiral around the nucleus.
• Elliptical galaxies have various shapes and are
extremely bright in the center and do not have spiral
arms.
• An irregular galaxy has no particular shape, and is
fairly rich in dust and gas.
68
Galaxy Types
• Spiral galaxies are
typically disk-shaped
with a somewhat
greater concentration of
stars near their centers,
often containing arms of
stars extending from
their central nucleus.
(30% of all galaxies)
69
Galaxy Types
• Elliptical galaxies are
the most abundant
type, 60% of all
galaxies, which have an
ellipsoidal shape that
ranges to nearly
spherical, and lack
spiral arms.
70
Galaxy Types
• Irregular galaxies,
which lack symmetry
and account for only
10% of the known
galaxies.
71
Our Galaxy
• The Milky Way Galaxy is a large, diskshaped, spiral galaxy about 100,000 lightyears wide and about 10,000 light-years
thick at the center.
• There are three distinct spiral arms of
stars, with some showing splintering.
• The Sun is positioned in one of these arms
about two-thirds of the way from the
galactic center, at a distance of about
30,000 light-years.
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The Milky Way Galaxy
73
Quasars
• Quasars appear as points of light,
similar to stars.
• Quasars are located in the centers of
galaxies that are distant from Earth.
• Quasars are among the most distant
objects that have been observed from
Earth.
74
Section 4: The Big Bang Theory
• The study of the origin, structure, and
future of the universe is called
cosmology.
• There are many scientific theories and
un scientific theories to the origin and
evolution of the universe.
75
Hubble’s Observations
• Cosmologists and astronomers can
use the light given off by an entire
galaxy to create the spectrum for that
galaxy.
• Edwin Hubble used galactic spectra to
uncover new information about our
universe.
76
The Doppler Effect
• By applying the Doppler Effect (the apparent
change in wavelength of radiation caused by
the motions of the source and the observer)
to the light of galaxies, galactic motion can be
determined.
• Large Doppler shift indicates a high velocity
• Small Doppler shift indicates a lower velocity
• It was soon realized that an expanding
universe can adequately account for the
observed red shifts.
77
Doppler cont.
• Most galaxies have Doppler shifts toward the
red end of the spectrum, indicating increasing
distance.
• The amount of Doppler shift is dependent on
the velocity at which the object is moving.
78
Doppler cont.
• Because the most
distant galaxies have
the greatest red shifts,
Edwin Hubble
concluded in the early
1900s that they were
retreating from us with
greater recessional
velocities than more
nearby galaxies.
79
The Expanding Universe
The “Raisin Bread” Theory of an
Expanding Universe
80
The “Big Bang” Theory
• The belief in the expanding universe led to the widely
accepted Big Bang Theory of the origin of the
universe.
• According to this theory, the entire universe was at
one time confined in a dense, hot, super massive
concentration.
• About 20 billion years ago, a cataclysmic explosion
hurled this material in all directions, creating all
matter and space.
• Eventually the ejected masses of gas cooled and
condensed, forming the stellar systems we now
observe fleeing from their place of origin.
81
Cosmic Background Radiation
• Astronomers believe that cosmic
background radiation formed shortly
after the big bang.
• The background radiation has cooled
after the big bang, and is now about
270°C below zero.
82
Ripples in Space
• Maps of cosmic background radiation
over the whole sky show ripples.
• These ripples are irregularities caused
by small fluctuations in the distribution
of matter in the early universe, and may
indicate the first stages in the formation
of the universe’s first galaxies.
83
A Universe of Surprises
Dark Matter
• Analysis of the ripples in the cosmic
background radiation shows that the matter
that humans, the planets, the stars and the
matter between the stars makes up only 4%
of the universe.
• About 23% of the universe is made up of a
type of matter that does not give off light but
that has gravity. This type of matter is called
dark matter.
84
A Universe of Surprises
Dark Energy
• Most of the universe is made up of an
unknown material called dark energy.
• Scientists think that dark energy acts as a
force that opposes gravity.
• Many scientists think that some form of
undetectable dark energy is pushing
galaxies apart.
85
The End
86