1. Characteristics of Stars There are millions of stars in the universe, but they are very far apart. Because of this, distances between stars are measured in light years. Light travels at 300,000 km per second. A light year is the distance that light travels in one year. One light year = 9.5 million million km!

2. After the Sun, Earth’s closest star is Proxima Centuri. It takes light 4.2 years to reach Earth from P. Centuri. Therefore, P. Centuri is 4.2 light years away from Earth.

3. Astronomers have found a way to measure the distances to stars using parallax. Parallax is the apparent change in the position of an object when seen from different places. + ++$++$

4. Astronomers look at a star when Earth is on one side of the sun. Six months later, when Earth is on the other side of the sun, they observe the same star again. They measure how much the star appears to move in comparison to other stars. This measurement, called the parallax shift, is used to calculate how far away the star is. The less a star appears to move, the farther away it is.

5. By definition, stars are huge spheres of glowing gas. They are made up mostly of hydrogen and create energy by nuclear fusion. This is a picture of Proxima Centuri taken using a telescope.

6. Stars are classified according to their size, temperature, and brightness. I. Size - The sun is a medium sized star. Stars range in size from tiny neutron stars to supergiant stars.

7. The picture below is a supergiant star called Betelgeuse. If Betelgeuse were in our solar system, it will extend past Jupiter.

8. A white dwarf star is about the size of Earth. II. Color and Temperature - A star’s color shows its temperature. Star color ranges from red to blue.

9. Red stars are cooler than blue stars. (You will see that a blue flame is hotter than a red flame.)

10. III. Brightness - Brightness refers to how much light stars give off. A stars brightness depends on its size and temperature. How bright a star looks from Earth depends on two things: 1. Actual brightness 2. Distance from Earth

11. The brightness of a star may be described in terms of either apparent or absolute magnitude. Apparent Magnitude - brightness seen from Earth Does the Sun have a greater apparent magnitude than the stars in this picture? Explain.

12. Absolute Magnitude - the brightness a star would have if It were a set distance from the Earth

13. The Hertzsprung-Russell Diagram compares a star’s temperature to its brightness. The star in the diagonal line are called the main sequence stars. Their surface temperature increases as brightness increases.