*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.
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. 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.
- 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