24.2

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25.3 The Universe
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Do Now

• What is the name of our galaxy? What type of
  galaxy is it?

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Do Now

• What is the name of our galaxy? What type of
  galaxy is it?
• Milky Way and spiral galaxy

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Key Word

• Irregular

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Vocab Words

• Big Bang Theory
• Galaxy
• Galaxy Cluster
• Hubbles Law

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The Universe

• Galaxies are groups of stars, dust, and gases
  held together by gravity.
• There may be more than 100 billion stars in the
  Milky Way galaxy alone.
• Our galaxy looks milky because the solar system
  is located within a flat disk the galactic
  disk.
• We view it from the inside and see stars in every
  direction.

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The Milky Way
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The Milky Way Galaxy

• When astronomers began to survey the stars
  located along the plane of the Milky Way, it
  appeared that equal numbers lay in every
  direction.
• That doesnt necessarily mean that the solar
  system is in the center, its just like walking
  into a forest a few feet, looking around and
  seeing trees in every direction we arent in the
  center, just not on the edge.

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Size of the Milky Way

• Its hard to study the Milky Way Galaxy with
  optical telescopes because large quantities of
  interstellar matter block our vision.
• With the aid of radio telescopes, scientists have
  determined the structure of our galaxy.
• The Milky Way is a large spiral galaxy whose disk
  is about 100,000 light-years wide and about
  10,000 light-years thick at the nucleus.
• As viewed from Earth, the center of the galaxy
  lies beyond the constellation Sagittarius.

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Milky Way
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Structure of the Milky Way

• Radio telescopes reveal that the Milky Way has at
  least three distinct spiral arms, with some signs
  of splintering.
• The sun is positioned in one of these arms about
  2/3 of the way from the center, at a distance of
  about 30,000 light-years.
• The stars in the arms of the Milky Way rotate
  around the galactic nucleus.
• The most outward arms move the slowest, and the
  ends of the arms appear to trail behind.
• Our solar system orbits the galactic nucleus
  about every 200 million years.
• Surrounding the galactic disk is a nearly round
  halo made of thin gas and numerous clusters of
  stars.

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Types of Galaxies

• In the mid-1700s, German philosopher Immanuel
  Kant proposed the fuzzy patches of light
  scattered among the stars were actually distant
  galaxies like the Milky Way.
• Today, we know that the universe includes
  hundreds of billions of galaxies, each containing
  hundreds of billions of stars.
• Several basic types
• Spiral
• Elliptical
• Irregular
• The big difference in addition to shape and size,
  is the age of the stars.

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Types of Galaxies
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Spiral Galaxies

• Just like the Milky Way, spiral galaxies are
  usually disk-shaped, with a somewhat greater
  concentration of stars near their center.
• Composed mainly of young (out in arms) and old
  stars (in the center).
• The major difference between spiral galaxies is
  how the arms are situated in the galaxy.
• Barred vs. Regular
• Barred stars arranged in the shape of a bar and
  the ends spiraling. Recent evidence shows us that
  the Milky Way may be a barred spiral galaxy.
• 10 of all galaxies are barred spiral
• 20 of all galaxies are regular spiral

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Spiral Galaxies
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Elliptical Galaxies

• About 60 of all galaxies are classified as
  elliptical galaxies.
• Contain old stars.
• Range in shape from round to oval.
• Most are small, but we have found some rather
  larger ones about 200,000 light-years in
  diameter.
• There are no spiral arms in this type of galaxy.

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Elliptical Galaxies
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Irregular Galaxies

• Only 10 of all galaxies are irregular galaxies.
• Composed mostly of young stars.
• The best known irregular galaxies, the Large and
  Small Magellanic Clouds, are easily seen without
  a telescope.
• These were named after the explorer Ferdinand
  Magellan, who observed them when he sailed around
  the Earth in 1520.
• They are our nearest neighbors at about 150,000
  light-years away.

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Irregular Galaxies
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Galaxy Clusters

• When we discovered that stars were found in
  groups, scientists began to think if it was
  possible for galaxies to be in groups as well.
• These groups are called galaxy clusters.
• Some may contain thousands of galaxies.
• Our own cluster, the Local Group, contains at
  least 28 galaxies.
• 3 spiral, 11 irregular and 14 elliptical.
• Galaxy clusters also make up huge groups called
  superclusters.
• May be the largest entities in the universe.

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Galaxy Clusters
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The Expanding Universe

• Remember from the Doppler effect, when a source
  is moving away, its light appears red.
• Red Shift discovery made in 1929 by Edwin
  Hubble. Basically uses the ideas of the Doppler
  Effect to explain how galaxies that have the
  greatest red shifts are the most distant.
  (indicates the universe is expanding as more and
  more galaxies have red shifts)
• Hubbles Law States that galaxies are
  retreating from us at a speed that is
  proportional to their distance. This works with
  the red shift idea.

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The Expanding Universe
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The Big Bang

• Any theory about the origin of the universe must
  account for the fact that all distant galaxies
  are moving away from us.
• The concept of an expanding universe led to the
  widely accepted big bang theory.
• According to this theory, the universe began as a
  violent explosion from which the universe
  continues to expand, evolve, and cool.
• The big bang theory states that at one time, the
  entire universe was confined to a dense, hot
  supermassive ball. Then, about 13.7 billion
  years ago, a violent explosion occurred, hurling
  this matter in all directions.
• Marks the beginning of the universe, all matter
  and space were created at that instant.

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The Big Bang Theory
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Supporting Evidence

• Scientists have gathered substantial evidence
  that supports the Big Bang Theory.
• The Red Shift of galaxies that you read about
  earlier indicates that the universe is still
  expanding.
• Cosmic background radiation.

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The Big Crunch?

• There are many different views on how the
  universe will end.
• It will last forever.
• The outward expanding of galaxies will slow and
  eventually stop.
• The reverse of the Big Bang, and it will be the
  big crunch where there will be gravitational
  contraction and the galaxies would begin to
  collapse.

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Group Challenge Question

• Imagine that you are scientists studying the
  birth of stars in a spiral galaxy. Which part of
  the galaxy would you study? Explain your answer.