1. Chapter 3

2. The Scale of the Universe  Astonomers deal with very large numbers on a regular basis. For this reason, they use scientific notation to make things easier to see, write, say and understand.  How do we know how far away the stars are?  Astronomers use parallax to determine the distance the of stars in space. By observing the apparent changes in position of the stars, they can calculate these distances.

3. The Light-Year A light-year is the distance that light can travel in one year. A light-year is equivalent to 9.46 trillion kilometers (5.87849981 x 10 12 miles) Astronomers use light years because they often deal with very large distances. The closest star to our solar system is Alpha Centauri, which is 4.3 light years away What does a light-year measure? DISTANCE!!

4. Star Systems and Galaxies  To us, the earth seems huge. But in relative to the rest of our solar system, it is a tiny speck.  Our solar system is just a speck compared to our galaxy.  Our galaxy is just a speck compared to the universe.

5. Stars vs. Planets  A star is a giant ball of gas, primarily hydrogen and helium  A planet is an object that orbits a star, is large enough to have become rounded by its own gravity and has cleared the area of its orbit.  Planets of the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune  Made mostly of gas as opposed to solids and liquids, most stars are much larger and hotter than planets!!  If the sun were hollow, you could fit 100 Earths inside o f it!

6.  A solar system contains a star and the planets and other objects that revolve around the star.  Not all planets are made of the same materials. Some planets are made mostly of rocky and metallic materials, but others are mostly liquid and gas.  The outer planets are much larger than the inner planets.

7.  Our solar system has only one star. But most stars are members of groups of 2 or more stars, called star systems.  Star systems that have two stars are called binary stars

8.  Many stars belong to a larger grouping of stars called star clusters. All the stars in a particular cluster formed around the same time and are about the same distance from Earth  It was recently discovered that star clusters may have planets orbiting them.  A galaxy is a huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity. Our galaxy is about 100,000 times bigger than our solar system.

9. The 3 Major Types of Galaxies  Spiral Galaxies: appear to have a bulge in the middle and arms that spiral outward, like pinwheels.  The arms contain gas, dust, and many bright, young stars  The milky way is a spiral galaxy

10. The 3 Major Types of Galaxies  Elliptical Galaxies- look like round or flattened balls. These galaxies contain billions of stars but have little gas and dust between the stars. Most elliptical galaxies contain only old stars

11. The 3 Major Types of Galaxies  Irregular Galaxies have no regular shape. They are usually smaller than other types of galaxies. Generally they have many bright, young stars and lots of gas and dust Left: Henize 2-10 Right: Zwicky 18

12. One more type of galaxy  Quasars are young galaxies with huge black holes at their centers. Gases spin around the black hole, heats up and glows.

13. Were you paying attention?

14. CHECK  What is scientific notation?  How is it important to astronomers?  What does a light-year measure?

15.  Imagine if the sun were an binary star. How would the sky look different?  What do all the stars in a star cluster have in common?  They formed around the same time and are about the same distance from Earth.

16.  Stars are made up of …  GAS  Stars are usually ______ than planets  BIGGER  A large grouping of stars is called a…  STAR CLUSTER  Which type of galaxy is the milky way?  SPIRAL