Astronomy Ch 20 The Universe

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Astronomy Ch 20 The Universe

• What is the universe?
• Sum of all space, matter, and energy past,
  present, future
• http//hubblesite.org/explore_astronomy/hubbles_un
  iverse_unfiltered/
• Where do we live in the universe
• Our Solar System, Milky Way Galaxy, Local Group
  Cluster, Virgo Super Cluster

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Astronomy Ch 20 The Universe

• Section 1 The Life and Death of Stars
• What Are Stars? Stars are huge spheres of very
  hot gas that emit light and other radiation.
  Stars are formed from clouds of dust and gas, or
  nebulas, and go through different stages as they
  age. (p. 693)

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• Stars have influenced cultures for thousands of
  years by helping people mark the passage of time
  and by providing markers for navigation in the
  night sky.

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• Stars are born in a nebula (left). After billions
  of years, most stars become old and lose their
  outer layers of gas. As a star begins to die, it
  may become a planetary nebula (right).

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light-year

• Stars are located at various distances from
  Earth. We use the light-year (ly) to describe the
  distance from Earth to far-away objects such as
  stars and galaxies. A light-year is the distance
  that light travels in one year, or about 9.5
  1012 km.

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Stars are powered by nuclear fusion reactions

• .
• A star is a huge sphere of very hot hydrogen and
  helium gas that emits light. A star is held
  together by the enormous gravitational forces
  that result from its own mass. Inside the core of
  a star, the pressure is more than a billion times
  the atmospheric pressure on Earth. The
  temperature in this incredibly dense core is
  hotter than 15 million kelvins.

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Energy from a nuclear fusion reaction in the core
may take tens of thousands of years to reach a
stars surface. When the energy reaches the
surface, the energy is released into space as
electromagnetic radiation
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• Studying Stars
• The telescope allowed astronomers to study stars
  in more detail for the first time.
• Greeks noticed that stars had color and divided
  stars by their apparent brightness
• Astronomers did not learn about the nature of
  stars until the optical telescope was invented. 
• As science and technology have improved,
  telescopes have become more powerful and have
  thus allowed us to see more. 

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• Some stars appear brighter than others.
• The brightness of a star depends on the stars
  temperature, size, and distance from Earth.
• The brightest star in the night sky is Sirius in
  the constellation Canis Major. Sirius appears so
  bright because it is close to Earth, only about
  8.7 light-years away. The surface temperature of
  Sirius is about 10,000 K.
• The suns surface is only 6,000 K, but the sun is
  so close to Earth that it prevents us from seeing
  other stars during the day.

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We learn about stars by studying energy. Stars
produce various types of electromagnetic
radiation from visible light to X rays to radio
waves. Scientists use optical telescopes to
study visible light and radio telescopes to study
radio waves emitted from astronomical
objects. Earths atmosphere blocks other
wavelengths, so telescopes in space are used to
study a wider range of the electromagnetic
spectrum than telescopes on the ground can
detect.  
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Section 1 The Life and Death of Stars

• The Life Cycle of Stars In a way that is similar
  to other natural cycles, stars are born, go
  through stages of development, and eventually
  die. (p. 698)

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This graph shows the intensity of light at
different wavelengths for the sun and for two
other stars.
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The color spectra of most stars contain dark
lines. Dark lines are caused by gases in the
outer layers of the stars that absorb the light
at these wavelengths. Astronomers match the
dark lines in starlight to the known lines of
elements found on Earth to determine what
elements make up a star.. 
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Stars change in many ways over their life cycle.
Small and medium stars are born in giant nebulas
of dust and gas and often end up as white dwarfs
billions of years later. The images in this life
cycle are not shown to scale.
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• Massive stars
• supernova -If the core that remains after such a
  supernova has occurred has enough mass, the
  remnant can become a
• neutron star. Neutron stars are small in
  diameter, but they are very massive. Just a
  teaspoon of matter from a neutron star would
  weigh more than 100 million tons on Earth.
  Neutron stars can be detected as
• pulsars, or spinning neutron stars that pulsate
  radio waves.If the leftover core is great enough,
  it will collapse to form something elsea
• black hole, which consists of matter so massive
  and compressed that nothing, not even light, can
  escape its gravitational pull. Because no light
  can escape a black hole, black holes cannot be
  seen directly. Black holes can, however, be
  detected indirectly by observing the radiation of
  light and X rays from objects that revolve
  rapidly around them.

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Section 2 The Milky Way and Other Galaxies

• Galaxies
• While the nearest stars are a few light-years
  away from Earth, the nearest galaxy to our own is
  millions of light-years from Earth.   A galaxy is
  a collection of millions to billions of stars.
  The deeper scientists look into space, the more
  galaxies they find. There may be more than 100
  billion galaxies. If you counted 1,000 galaxies
  per night, it would take 275,000 years to count
  all of them. 

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• Gravity holds galaxies together.
• Without gravity, everything in space might be a
  veil of gas spread out through space. But because
  of gravity, clouds of gas come together and
  collapse to form stars. As the first stars in a
  galaxy age, they throw off gas and dust or become
  supernovas. New stars then form. The gas, dust,
  and stars form galaxies because of gravity.
• Just as Earth revolves around the sun because of
  gravity, the solar system revolves around the
  center of the galaxy because of gravity. It takes
  our solar system about 226 million years to
  complete one orbit of our galaxy.

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• Galaxies are often found together in clusters.
• Galaxies are not spread out evenly through space.
  Galaxies are grouped together and bound by
  gravity in clusters like the cluster shown in
  Figure 3. The Milky Way galaxy and the Andromeda
  galaxy are two of the largest members of the
  Local Group, a cluster of more than 30 galaxies.
  New members of the Local Group, are being
  discovered as more telescopes, such as the Hubble
  Space Telescope that is shown in Figure 2, become
  available.Clusters of galaxies can form even
  larger groups called superclusters. A typical
  supercluster contains thousands of galaxies that
  contain trillions of stars in individual
  clusters. Superclusters can be as large as 100
  million light-years across. They are the largest
  known structures in the universe.

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Section 2 The Milky Way and Other Galaxies

• Types of Galaxies
• Galaxies can be divided into three major types
  spiral, elliptical, and irregular.   The three
  types of galaxies have many stars, but differ in
  structure.
• We live in the Milky Way galaxy.

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• The Milky Way is a spiral galaxy.
• Our galaxy is a huge spiraling disk of stars,
  gas, and dust. This gas and dust is called
  interstellar matter
• The Milky Way has a huge bulge in the center that
  contains primarily old red stars
• The spiral arms contain young blue stars

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Section 2 The Milky Way and Other Galaxies

• How Galaxies Change Over Time By studying closer
  galaxies that might be similar to ancient ones,
  scientists can slowly piece together the puzzle
  of how galaxies evolve. (p. 706)

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Section 3 Origin of the Universe

• What Is the Universe? The universe consists of
  all space, matter, and energy that existsnow, in
  the past, or in the future. (p. 708)

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• We see the universe now as it was in the past.
• Astronomers need large units of measure to
  express distances. A light-year, which is
  approximately 9.5 1012 km, Remember that while
  a year is a unit of time, a light-year is a unit
  of distance.
• It takes time for light to travel in space. When
  we say the sun is 8 light-minutes away, we are
  expressing not only its distance from Earth, but
  also that we see the sun as it was 8 min ago. We
  never see it as it is in the present.
• When we see distant objects, we see them as they
  were when they were younger.
• Astronomers can compare how galaxies age by
  looking at many galaxies at various distances and
  thus at different ages.

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Section 3 Origin of the Universe

• What Happened at the Beginning? Scientists
  theorize that the universe formed during a
  cataclysmic event known as the big bang. (p. 710)

scientists estimate that the universe is about
13.7 billion years old.
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• In 1965, scientists Arno Penzias and Robert
  Wilson were making adjustments to a new radio
  antenna that they had built.
• They could not explain a steady but very dim
  signal from all over the sky in the form of
  radiation at microwave wavelengths. They finally
  realized that the signal was the cosmic
  background radiation predicted by the big bang
  theory.

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• Radiation dominated the early universe.
• According to the big bang theory, immediately
  after the big bang, the universe was extremely
  hot and made up of pure energy. There was a
  period of rapid expansion that caused the energy
  to cool and allowed sub-atomic particles, such as
  protons, electrons, and neutrons, to form. The
  first stars were born about 400 million years
  after the big bang.

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• Expansion implies that the universe was once
  smaller.
• Most galaxies are moving away from each other.
• Long ago, the entire universe might have been
  contained in an extremely small space.

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• During the past 100 years, other theories for the
  origin of the universe have been proposed. A few
  are still being studied, but the big bang theory
  is the one that is best supported by the current
  evidence including the cosmic background
  radiation and observations of the movement of
  distant galaxies

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Section 3 Origin of the Universe

• Predicting the Future of the Universe Scientists
  use their increasing knowledge of the universe to
  hypothesize what might happen to the universe in
  the future. (p. 715)

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• They depend on a mixture of theories and precise
  observations of very faint and distant objects.
• These observations depend on technology, such as
  telescopes
• New space telescopes that collect infrared
  radiation and X rays are being built and
  launched.
• Data in these regions of the electromagnetic
  spectrum may provide important clues about the
  beginning and future of the universe.

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• One example of new, more sensitive technology is
  the Chandra X-Ray Observatory, which was launched
  into orbit around Earth in 1999.
• This observatory can take photographs in the
  X-ray part of the electromagnetic spectrum. The
  presence of X rays indicates matter at
  temperatures of more than one million degrees.

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• According to Einsteins theory of relativity,
  mass curves space, much in the same way that your
  body curves a mattress when you sit on it.
• In 1919, observations of a total solar eclipse
  showed that Einstein was correct. In the
  direction of the sun, stars that could be seen
  only during the eclipse were in slightly
  different positions than expected.

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• The future of the universe is uncertain.
• The universe is still expanding, but it may not
  do so forever. The combined gravity of all of the
  mass in the universe is also pulling the universe
  inward, in the direction opposite to the
  expansion. The competition between these two
  forces leaves three possible outcomes for the
  universe
• 1.The universe will keep expanding forever.
• 2.The expansion of the universe will gradually
  slow down, and the universe will approach a limit
  in size.
• 3.The universe will stop expanding and start to
  fall back on itself.