1. Chapter 8: Stars By Sydney Bullock & Abby Swanager

2. Stars The different colors of stars tells how hot the stars are. All stars are made up of different elements in the form of gases. Some gases can be hotter than others making the stars different colors.

3. Differences in Brightness The brightest stars in the sky are called first- magnitude stars. The dimmest stars are called sixth-magnitude stars. Positive numbers represent dim

4. Motion of stars Daytime and nighttime are both caused by Earth’s rotation The Earth’s tilt and revolution around the Sun cause the seasons. During each season, the Earth faces a different part of the sky at night. Because of Earth’s rotation, the Sun appears to move across the sky. In fact, at night you can observe that the whole sky is rotating above us.

5. The Actual Motion of Stars Because of stars being distant, their actual motion is hard to see. If you could put thousands of years into one hour, a star’s movement would be obvious.

6. The Life Cycle of Stars

7. The Beginning and End of Stars A star enters the first stage of it’s life cycle as a ball of gas and dust. Gravity pulls the gas and dust into a sphere. As the sphere becomes denser, it gets hotter and the hydrogen changes to helium in a process called nuclear fusion

8. Different types of Stars Stars can be classified by their size, mass, brightness, color, temperature, spectrum, and age. After a star forms, it enters the second and longest stage of its life cycle known as the main sequence. After the main-sequence stage, a star can enter the third stage of their life cycle.

9. A Tool for Studying Stars In 1911, a Danish astronomer named Ejnar Hertzsprung compared the brightness and temperatures of stars on a graph. Two years later, American astronomer named Henry Norris Russell made some similar graphs. Although these astronomers used different data, they had similar results.

10. A Tool for Studying Tools (continued) The combination of their ideas is now called the Hertzsprung-Russell Diagram, or HR Diagram. The HR Diagram is a graph that shows the relationship between a stars’ surface temperature and its absolute magnitude. The modern HR Diagram is shown below.

11. HR Diagram This is the modern HR Diagram.

12. When Stars Get Old Although stars may stay on the main sequence for a long time, they don’t stay their forever. Average stars like the Sun, become red giants and then white dwarfs. Massive stars use their hydrogen much faster than stars like the sun do.

13. Types of Galaxies There are many different types of galaxies. Edwin Hubble, the astronomer for whom The Hubble Space Telescope is named, began to classify galaxies, mostly by their shapes in the 1920s. Here are a couple of galaxies, spiral galaxies, Milky Way, elliptical galaxies, and irregular galaxies.

14. The Big Bang Theory With the discovery that the universe is expanding, scientists started wondering what it would be like to watch the formation of the universe in reverse. The universe would appear to be contracting, not expanding. All matter would eventually come together at a single point.