Section 25.3 The Universe

The Universe

25.3 The Universe

• A galaxy is a group of stars, dust, and gases held together by gravity.

• The Milky Way is a large spiral galaxy whose disk is about 100,000 light-years wide and about 10,000 light-years thick at the nucleus.  Size of the Milky Way

  
  

• Radio telescopes reveal that the Milky Way has at least three distinct spiral arms, with some splintering.

Multiple Choice

How is the galaxy held together?

• gases

• gravity

• nucleus

• stars

25.3 The Universe

Galaxies

•  Spiral Galaxies: About 30 percent of all galaxies are spiral galaxies.

• They have large diameters of 20,000 to 125,000 light-years and contain both young and old stars

• Elliptical galaxies: About 60 percent of galaxies are classified as elliptical galaxies.

• Elliptical galaxies range in shape from round to oval.

Type of Galaxy

Spiral Galaxy

Type of Galaxy

Elliptical Galaxy

Multiple Choice

The majority of the galaxies are made up of spiral galaxies.

• true

• false

Other Galaxies

• In addition to shape and size, one of the major differences among different types of galaxies is the age of their stars.

• Irregular Galaxies: Only 10 percent of the known galaxies have irregular shapes and are classified as irregular galaxies. •

• Irregular galaxies contain young stars.

Other Galaxies

• A galaxy cluster is a system of galaxies containing several to thousands of member galaxies

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The Expanding Universe

• Red shift, or a Doppler shift toward the red end of the spectrum, occurs because the light waves are “stretched,” which shows that Earth and the source are moving away from each other.

• Hubble’s law is a law that states that the galaxies are retreating from the Milky Way at a speed that is proportional to their distance.

• The red shifts of distant galaxies indicate that the universe is expanding.

The Expanding Universe

Hubble’s Law: To help visualize the nature of the universe, imagine a loaf of raisin bread dough that has been set out to rise for a few hours. As the dough doubles in size, so does the distance between all the raisins. Those objects located farther apart move away from each other more rapidly. 

The Big Bang

• The big bang theory states that at one time, the entire universe was confined to a dense, hot, supermassive ball.

• Then, about 13.7 billion years ago, a violent explosion occurred, hurling this material in all directions.

The Big Bang

• Supporting Evidence

• The red shift of galaxies supports the big bang and the expanding universe theories.

• Scientists discovered a type of energy called cosmic background radiation. Scientists think that this radiation was produced during the big bang.

The Big Bange

• The Big Crunch?

• The future of the universe follows two possible paths: 1. The universe will expand forever. 2. The outward expansion will stop and gravitational contraction will follow.

• The view currently favored by most scientists is an expanding universe with no ending point.

• It should be noted, however, that the methods used to determine the ultimate fate of the universe have substantial uncertainties