Getting Ready starry skyLook at this photograph. What do you think is the picture? You might be surprised to learn that this is a photograph of galaxies. Each galaxy is made up of billions of stars. In fact, the universe has more than 100 billion galaxies, and each galaxy has about 100 billion stars. So there are a lot of stars. People can see only a very small number of them from Earth.

Starry Night
Our sun is a huge ball of fire that radiates light. Stars are also huge balls of fire that radiate light, but they are much, much farther away, which is why they seem smaller. Like our sun, stars shine light in all directions.

Some of the light travels in the direction of Earth. Even though light travels very fast, most stars are so far away that it takes millions of years for their light to reach Earth. So when you see a star, you can see light that was produced millions of years ago. This means that when you look at the starry night sky, you actually look into the past. By the time light reaches you, the stars that produced it may have exploded or cooled down and died. It makes sense to assume that some of the stars you see do not exist anymore.

Starry Night by Vincent van Gogh

Planets, on the other hand, do not generate light. You see planets because they are relatively close to the sun, and they are lit by the sun. Light travels from the sun to the planets and then bounces off of them. Some of this light travels to your eyes here on Earth.

Use what you have learned about light and sight to draw one or more models that explain each of the following.

How do you see stars?