Constellations, Galaxies, and the Universe

*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