*Billions of galaxies make up the universe, which, according to the big bang model, formed between 10 & 20 billion years ago.

After today’s lecture, you should be able to:

-Tell what a galaxy is and describe the various types of galaxies.

-Explain the origin of the universe according to the big bang model.

A) Galaxies

Galaxy = a collection of stars, planets, gases, dust, and satellites (moons)

  • Objects within a galaxy move through space together as a whole unit.
  • Held together by gravity.
  • At least 1 billion galaxies are in the universe. (May be 50-100 billion!)
  • Each galaxy is made-up of billions of stars, wh/in turn makes-up most of the visible mass of the universe!
  • Almost every light you see in the night sky is in the Milky Way Galaxy, the galaxy we live in!
    • Our sun is just 1 star among billions!
    • Our solar system is located on the edge of the galaxy, wh/spans 100,000 lit-yrs!
  • Northern Hemisphere = see Andromeda Galaxy (2 million light years away)
  • Southern Hemisphere = see Magellanic Clouds

– Our 2 nearest galactic neighbors; Can be seen without a telescope.

B) Types of Galaxies

  • Classified by their shape.
  1. 1. Elliptical Galaxies = shaped like a  flattened football
  • Most do not rotate (spin)
  • Contain less material than other types of galaxies
  • May be older and more condensed.

2. Spiral Galaxies = shaped like a flat disc with a bulge in the center

  • Long, twisted arms that rotate around the center like a pin wheel
  • Usually contain large amounts of gas and dust
  • Ex.- The Milky Way Galaxy

3. Barred Spiral Galaxies = arms are attached to a straight-bar shape

  • Bar is much brighter than the arms

4. Irregular Galaxies = vary greatly in size and shape

  • Least common type of galaxy
  • Ex.- Magellanic Clouds

C) Other Star Groups

  • Local Group = cluster of several galaxies (Most galaxies are in these.)

  • Star Cluster = areas where many stars are close together
    • Pleiadez = grouping of 7 stars (fall/winter)
    • Binary Star = a pair of stars
      • More numerous than single stars.

D) Constellations

  • Stars that appear to be in patterns as seen from Earth.  (A product of peoples’ imaginations!)
  • Big Dipper (Ursa Major), Orion, etc.
  • 88 constellations in all
  • Help form a picture of the night sky and tell the season.

E) Expansion of the Universe

  • Big-Bang Theory = How the universe began; Occurred 10-20 billion years ago.
    • All matter in the universe was packed into a giant sphere of H.
    • It exploded, spreading matter and gases outward.
    • As matter cooled, the force of gravity pulled particles together to form galaxies, stars, etc.
    • Main evidence to support this = Most galaxies are moving away from each other (redshifting)!

2. How do scientists know the universe is expanding?

Doppler Effect = apparent change in wavelength due to an object’s position

  • Moving away = appear to have a longer wavelength (redshift)
  • Standing still = no change in wavelength

Moving closer = appear to have a shorter wavelength (blueshift

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