Too Hot to Handle
You might remember an instance when an adult told you never to look directly at the sun. But if you had to guess, what color would you say the sun is? Maybe you have watched the sun as it was setting in the sky. Sometimes it looks as though it is on fire, especially when it is shining through the clouds. The reason it looks that way is because the sun is on fire. Can you guess how hot the fire at the center of the sun is? It is more than 25 million degrees on the Fahrenheit scale! That's 250 thousand times hotter than the hottest summer day in Arizona. But what may surprise you even more is there are many stars in the universe that are thousands of times hotter than that. Have you ever wondered if it would be better if our sun's temperature were just a little warmer or cooler? Scientists believe that it is just right for us. The other planets in our solar system are good examples of what our world would be like if we were any closer or farther from the sun — or if the sun's temperature could be changed. Some stars are much hotter than the sun, which makes them burn up much faster. Fortunately, the sun is a cooler star that should be burning for many billions of years.
Out of the Dust
What would you expect to find in something called a nebula? If you guessed stardust, you were right! A nebula is misty or cloudy type of body that's filled with the gas and dust that was left in the universe after an old star or a supernova exploded. Something as simple as a bump from another explosion can cause gas and dust to start spinning around and picking up more material, resulting in this mass called a nebula.
Have you ever crushed the wrapper from a stick of gum? And did you notice that before you did that, the wrapper was several inches long, but after you have applied pressure, it was smaller than a pea? As a nebula is picking up more material, its gravity starts increasing and enormous pressure is placed on the center. It eventually becomes so hot that one particle of gas combines with another in a process called fusion!If you have ever watched a forest fire on TV, you know that a small spark can soon become an enormous fire. The fuel for the stars comes from hydrogen gas and it burns more quickly and much hotter than wood does. This star furnace can burn for millions or billions of years. But the hotter the fire is, the less time it burns. After it has used up all its hydrogen, the nebula may collapse inward, creating a very small star. Or it might explode outward, making another nebula filled with gas and dust. What do you think will happen next?
See for Yourself
If you visit an observatory, ask your tour guide to show you some of the many nebulae (that's the plural of “nebula,” meaning many of them) that are scattered throughout the universe — such as the Horsehead, the Crab, and the Rosette. Some of them you can see because another star's light illuminates their dust, but astronomers have to guess where some of the other ones are because they are blocked by the light coming from other stars around them!
Nebulae can be both light and dark. If a star is shining on the nebula's dust or is inside it, it will appear light. Sometimes a nebula will block the light from a star that is behind it, making it appear dark or black.
I can be a solid, a liquid, or a gas. I am very important to the planet Earth and everything that lives on it.
Gone But Not Forgotten
Astronomers have a name and a measurement for the amount of sunlight that never warms the earth because it is reflected back into the atmosphere. They call this unused sunlight albedo.