1. Ch 25 Sec. 4
Stars and Galaxies

2. Galaxies
 Galaxies are large groups (billions) of
stars, gas and dust bound by gravity.
 3 kinds
 Spiral
 Elliptical
 Irregular

3. Spiral Galaxy
 Spiral arms wind
out from inner
section
 Some have
barred spirals
with stars and
gas in a central
bar

4. Elliptical Galaxy
 Large and three dimensional
 Most common shape

5. Irregular Galaxy
 Smaller
 Less common
 Various different
shapes

6. The Milky Way
 It is the name of our Galaxy.
 It has spiral shape.
 It contains one trillion stars.
 Sun is one of the star in Milky way and
orbits the galaxy's core every 225 million
years.

7. THE UNIVERSE
 The Universe contains billions of galaxies
with each galaxy containing billions of
stars.
 Universe is always evolving. Everything
that exits in the universe is not permenant.
New stars are being born while existing
stars will eventually die.

8. Origin of The Universe
Two Models proposed
Steady State Theory- the universe has
always existed as it is now
Oscillation Model- universe expands
and contracts repeatedly over time

9. Edwin Hubble
• discovered :
1. galaxies are moving away
from us & each other
2. relationship between
distance to a galaxy & its
speed

10. Hubble’s law
Farther away galaxy is faster it is moving away from us
Doppler Shift- Light changes as it moves toward or away
form an object.
Starlight moving toward Earth shifts to blue-violet end of
spectrum
Starlight moving away from Earth shifts to red end of
spectrum

11. Big Bang Theory
 Universe began 13.7 billion years ago with
a huge explosion that caused expansion
everywhere at the same time.
 Hubble shows over 1,500 galaxies at a
distance more than 10 Billion light yrs
away showing a young universe
 Whether the universe will continue to
expand or begin to contract is unknown.

12. Video
 http://www.youtube.com/watch?v=PV0ACIykxQI