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Galaxies

CLASSIFICATION, FORMATION, & EVOLUTION

Unit 4: Cosmology – Mrs. Paul - JCHS

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What are the three major types of galaxies?

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Hubble Ultra Deep Field

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Galaxy Formation

  • Matter originally filled all of space almost uniformly
  • Gravity of denser regions pulled in surrounding matter
  • Denser regions contracted, forming protogalactic clouds, where hydrogen and helium formed
  • Supernova explosions from the first stars kept much of the gas from forming stars

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Formation of a Spiral Galaxy

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Formation of an Elliptical Galaxy

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Stellar Birthrate in Galaxies

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Check-In

  1. Which type of galaxy creates more stars at the beginning of its life?
    • Elliptical
  2. Which type of galaxy is likely to have more older stars?
    • Elliptical
  3. In which type of galaxy would you be more likely to find new stars now?
    • Spiral

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Galaxy Formation

  • This picture of galaxy formation is incomplete
  • Mergers, collisions, and interactions between galaxies are very important in their formation, particularly in the early stages of the Universe

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Galaxy Formation

  • Stars collide with each other very rarely because the distance between neighboring stars is about 10 million times the diameter of a star
  • Galaxies collide with each other quite frequently because distance between neighboring galaxies is about 20 times the diameter of a galaxy

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Expansion of the Universe

  • The Universe is expanding
  • This means that the Universe used to be smaller
  • In the early stages of the Universe
    • there were more galaxies
    • they were closer together
    • therefore, they interacted more

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Galactic Evolution

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Galactic Evolution

  • Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another
    • Interactions can rip stars out of galaxies, producing tidal tails (a thin, elongated region of stars and interstellar gas that extends into space from a galaxy)
    • Interactions can disturb gas in and between galaxies, producing starbursts

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The Mice

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Galactic Evolution

  • Low-speed, head-on, encounters between galaxies result in mergers
    • individual stars in the two galaxies pass harmlessly by each other
    • a few stars attain high speed and are lost to intergalactic space
    • most of the stars are traveling slowly enough to form a single large elliptical galaxy
    • the merging process takes hundreds of millions of years, so merging galaxies are often caught in progress

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Colliding Galaxies

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Galactic Evolution

  • Although stars don't collide when two galaxies merge, the much larger gas clouds do
    • the clouds in the two galaxies slam into each other violently
    • shock waves from the collision run through the clouds and trigger the collapse of dark nebulae to form stars
    • if the two colliding galaxies are rich in gas, their merger will cause a burst of star formation

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Cartwheel Galaxy

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Galactic Evolution

  • The merger of spiral galaxies produces elliptical galaxies
    • spiral galaxies are orderly, while elliptical galaxies are not
    • when two spiral galaxies collide, they become chaotic
    • giant elliptical galaxies continue to grow by “cannibalizing” smaller galaxies

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Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.

© 2015 Pearson Education, Inc.

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Check-In

  1. In a younger universe, were there more or less galactic interactions?
    • more
  2. True or False: When galaxies merge, the stars crash into each other
    • False
  3. Two spiral galaxies collide to form a(n)…
    • Elliptical galaxy
  4. What happens when a large galaxy merges with a small galaxy?
    • Galactic cannibalism

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The Fate of the Milky Way