Download Section 4 Galaxies and the Universe

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Stars and Galaxies T5
DISCUSSION QUESTION:
If the Sun is too young to have formed carbon and iron on its own, how did these elements get
into the Sun? These elements are part of old, dead stars that have been recycled into the Sun
Section 4 Galaxies and the Universe
A. Galaxy—gravity holds together a large collection of stars, gas, and dust
1. Earth’s galaxy is Milky Way which is part of a galaxy cluster named the Local Group
2. Spiral galaxies—spiral arms wind out from inner section; some have barred spirals with
stars and gas in a central bar
3. Elliptical galaxies—large, three-dimensional ellipses; most common shape
4. Irregular galaxies—smaller, less common galaxies with various different shapes
B. The Milky Way Galaxy—usually classified as a spiral galaxy
1. May contain one trillion stars
2. About 100,000 light-years wide
3. Sun orbits galaxy’s core every 225 million years
C. Theories on the origin of the universe
1. Steady state theory—universe has always existed just as it is now
2. Oscillating model—universe expands and contracts repeatedly over time
D. Universe is expanding
1. Doppler shift—light changes as it moves toward or away from an object
a. Starlight moving toward Earth shifts to blue-violet end of spectrum
b. Starlight moving away from Earth shifts to red end of spectrum
2. All galaxies outside the Local Group indicate a red shift in their spectra indicating they are
moving away from Earth
E. Big Bang Theory—holds that universe began 13.7 billion years ago with huge explosion
that caused expansion everywhere at the same time
1. Galaxies more than 10 billion light-years away give information about a young universe
2. Whether the universe may eventually stop expanding and begin contracting is unknown
DISCUSSION QUESTION:
What is common among all galaxies? They are expanding.
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