Your students may have seen photos from the Hubble Space Telescope, showing entire galaxies in a single image. One of the most striking features of photos like those are the never-ending stars—even outside the main structure of the pictured galaxy, there are still stars everywhere you look. Astronomers and astrophysicists tell us that there are, essentially, infinite stars in the universe.
The stars we can see from Earth are just a tiny fraction of a fraction of a fraction of a fraction of a fraction of all the stars that exist. And, most stars are significantly larger than our Sun (BTW—if your students don’t know that the Sun is a star, you have failed them super hard). With all those stars out there, bigger and brighter than our sun, why isn’t the sky constantly lit up like an Xmas tree?
A Simple Explanation
This handy dandy Demo Science science demo will help you explain to your students (or whoever) how distance affects a star’s apparent brightness. You only need two items for this one: a flashlight (the bigger the better) and a wall. If you don’t have a wall available while you’re teaching class, something is severely askew. Maybe look into the “no walls” thing before you do anything else.
Turn out the lights in your classroom, but make sure you have enough light to navigate the room by without crashing into desks and such. Let Kevin know that, dim though the light is, you can still see him picking his nose—enough already, Kev!
Stand in the middle of the room and shine your flashlight against the wall of your choosing. Instruct students to observe changes in the light pattern on the wall as you slowly walk toward the wall. What see ye, children?
Distance Does It, Dude
As you get closer to the wall, the light pattern thrown by your flashlight will become brighter and more concentrated. This is because the light has less distance over which to spread out. When you’re further from the wall, the light from your torch continues to spread out until it hits the wall. Starlight behaves the same way.
Two different stars may give off the same amount of light, but will have different apparent brightness because they’re different distances from Earth. The farther out a star is, the more its light will spread out, and less of that light will reach us. This results in a lower magnitude of brightness. If every star was the same distance away, they’d all appear as uniform little dots in the night sky.