Stars, Galaxies, and the Universe

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Stars, Galaxies, and the Universe
Chapter 30 Earth and Space Science
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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.

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Analyzing Starlight

• All stars have dark-line spectra.
• A stars dark-line spectrum reveals the stars
  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.

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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.

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The Temperatures of Stars

• The temperature of most stars ranges from 2,800C
  to 24,000C.
• Blue stars have average surface temperatures of
  35,000C.
• Yellow stars, such as the sun, have surface
  temperatures of between 5,000C and 6,000C.
• Red stars have average surface temperatures of
  3,000C.

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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.

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Stellar Motion

• Two kinds of motion
• Actual Motion
• Apparent Motion

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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.
• Earths revolution around the sun causes the
  stars to appear to shift slightly to the west
  every night.

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Spot Question

• Why does Polaris appear to remain stationary in
  the night sky?
• Polaris is almost exactly above the pole of
  Earths rotational axis, so Polaris moves only
  slightly around the pole during one rotation of
  Earth.

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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

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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.

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Actual Motion of StarsDoppler Effect

• The spectrum of a star that is moving toward or
  away from Earth appears to shift, as shown in the
  diagram below.

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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).

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Distance to Stars How big is the universe?

• Proxima Centauri is about 4.3 light-years 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.

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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.

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Stellar parallax
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Another method

• The Parsec 1o 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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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).

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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.

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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.

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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
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Dont bother copying

• Stellar temperature/color also gives rise to
  Spectral Classes.
• O (gt 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 (lt 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 - gt100 Billion Years

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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.

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Star Formation

• A star brings 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.

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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
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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.

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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.

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Bright Nebulae

• Reflection nebulae relatively dense dust clouds
  in interstellar space that are illuminated by
  reflecting the light of nearby stars.

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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 stars mass.

The Orion Nebula - Birthplace of stars
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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.

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The Birth of a Star

• A protostars temperature continually increases
  until it reaches about 10,000,000C.
• 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.

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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.

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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
  suns 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.

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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.

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Giant Stars

• A star enters its third stage when almost all of
  the hydrogen atoms within its core have fused
  into helium atoms.
• A stars 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.

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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.

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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.

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Planetary Nebulas

• As the stars 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.

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The Suns 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 the Sun.

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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.

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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.

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Novas and Supernovas

• Some white dwarfs revolve around red giants. When
  this happened, the gravity of the white 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.

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Supernova

• Stars more than three times the mass of the Sun
  terminate in a brilliant explosion called a
  supernova.

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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
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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 stars core begins
  to collapse under its own gravity. This causes
  the outer layers to explode outward with
  tremendous force.

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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

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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.

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Black Holes

• Supernovae events can produce small, extremely
  dense (A pea-sized 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.

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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
  light-years 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.

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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.

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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
  multiple-star 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.

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Spot Question

• What percentage of stars are in multiple-star
  systems?
• More than 50 of all stars are in multiple-star
  systems.

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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.

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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 light-years and may
  contain more than 200 billion stars.

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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.

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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)

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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.

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Galaxy Types

• Irregular galaxies, which lack symmetry and
  account for only 10 of the known galaxies.

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Our Galaxy

• The Milky Way Galaxy is a large, disk-shaped,
  spiral galaxy about 100,000 light-years 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
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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.

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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.

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Hubbles 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.

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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.

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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.

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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.

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The Expanding Universe

• The Raisin Bread Theory of an Expanding
  Universe

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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.

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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 270C below zero.

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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 universes first galaxies.

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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.

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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.

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