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ZatsWhy (Answers) - Way Outer Space

Are planets and stars made out of atoms?
Why do the planets rotate around the sun?

Are planets and stars made out of atoms?
Why sure, Victoria, the earth and the stars are all made out of atoms. In fact, most atoms are made inside the stars. Stars are really big balls of gases, mostly hydrogen, that have come together and grown so big that the gases on the inside of the star get pushed together with a lot of pressure. This pressure gets so intense that the hydrogen atoms start to smash together to form helium atoms. When the hydrogen atoms start to make helium atoms, the reaction gives off a lot of energy, which only makes the whole reaction continue even more. Soon the big ball of gas is giving off a lot of energy in the form of light and heat. We see this energy as a little twinkling star because we’re so far away. If we were closer, though, we’d see that most of these stars are just like our own sun. (And if we could see our sun from far, far away in space, it would look like a star.) The star can go on like this, making helium atoms out of hydrogen atoms, for a long time. When the star runs out of hydrogen, the pressure in the star tries to keep the reaction going by smashing other atoms together to make still more types of atoms. The bigger the atoms the star tries to make, the harder and harder it is to keep the reaction going. Some stars just burn out, but others BLOW UP. When a star blows up (becomes a “nova”), the star make s many, many atoms of all different types and scatters the atoms out into space. These atoms might drift around for a long, long time until they become part of another star, or part of a planet (like the earth) or part of something on a planet (like you or me.) Great question, Victoria, as you can tell, it’s a BIG one.
by Jeff | Sun Dec 2 21:27:20 PST 2001 | Back to Top

Why do the planets rotate around the sun?
You don’t ask many small questions, do you, Joanna? The reason the planets rotate around the sun brings up some BIG ideas in the part of science called “physics.” This is going to be a long answer, so get ready! Physics is the study of matter and energy, especially in terms of force and movement. Your question concerns “matter,” “force” and “movement” so let’s talk about what those words mean. “Matter” refers to everything anywhere that is made up of atoms. Anything in gas, liquid or solid form is made up of atoms. In fact, scientists call gas, liquid and solid “states of matter” because they are all made up of atoms and are all types of matter, just in different forms. (When something is made up of matter, scientists also say that it has “mass.” Put another way, anything made up of atoms is made up of “matter” and the more atoms in it, the more “mass” it has.) The sun is made up of atoms. It’s actually an enormous ball of gas, mostly hydrogen, much, much bigger than any of the planets. The planets are made up of atoms, too. Some are solid (like Earth) and others are mostly gas or gas with a liquid or liquid and solid center (like Jupiter or Saturn).

BIG IDEA #1: the sun and the planets are made up of matter. Here’s the next big idea. Scientists use the word “force” to describe an influence on something that produces a change in movement or shape or has some other effect on the thing the force acts on. For instance, the movement of your foot produces a force on a ball when you kick it. The ball changes shape a little right when you kick it and (if you kick it hard) it changes its direction and speed of movement a lot. Here’s a big idea about matter and force: everything made up of atoms (every bit of matter) is pulling on every other bit of matter in the universe! Not only that, the more matter something has, the bigger the pull it has on everything else. This force of everything with matter pulling on everything else with matter is called the force of gravity. We usually only think of gravity as the force that makes things fall to the earth. When you let go of something (say a cookie), the force of gravity takes over and the cookie and the earth pull on each other, trying to get closer. Because the earth is so big, the pull the little cookie has on the earth is very small. The pull of the cookie on the earth is so small and the earth moves so little, we don’t notice it. But pull the huge earth has on the little cookie is much, much greater than the pull the cookie has on the earth. We see the cookie respond to the force of gravity – when we see it fall, we are actually seeing the effect of the force of gravity or the “pull” the earth is having on it. One more thing about gravity: it’s strong when the two things with matter are close together, and weaker the farther away they are from each other. The force of gravity between the earth and the cookie we dropped is very strong because the cookie and the earth are only a few feet from each other, not a few miles or a few hundred miles or a few thousand miles.

BIG IDEA #2: The planets and the sun are pulling on each other through the force of gravity and the sun’s pull is really strong because it has so much mass. Are you with me so far? Okay, here’s the last big idea before we put it all together. When we see something moving in the world around us (when scientists talk about something moving, they say that it is “in motion’), it usually only keeps going if we do something to make it keep going. For instance the ball that we kicked in our gravity example will stop unless we kick it again before it stops. Put another way, something in motion only stops or changes its direction because a force makes it stop or change direction. The ball stops because rolling on the ground little by little pulls away the energy we gave it with our kick. If we kick it again, we can speed it up and make it go in a new direction. The cookie we dropped stops because it hits the ground and has nowhere else to go. If we could get out in space where there’s no ground to slow things down and nothing blocking the way, a ball or a cookie or a planet that we kick or drop or put in motion some other way will just keep going in a straight line until something makes it stop or change direction. “Momentum” is the science word for the big idea that anything with matter keeps moving straight unless something makes it stop or change direction.

BIG IDEA #3: If a planet is in motion, it will continue to move in a straight line unless some force makes it stop or change direction. Okay, with all that in mind, here’s why the planets rotate around the sun. Gravity makes gases and dust and rock and other matter in space pull together to form the planets and the stars, like our sun. The gravity puts the planets in motion toward the sun because, like the cookie going toward the earth, the planets are much smaller so they do the moving we would notice. If the planet is close to the sun and not moving very fast, the planet would crash into the sun, the same way the cookie “crashes” into the earth when we drop it. But if the planet is moving fast or is far enough away from the sun that the pull of gravity is not so strong, the sun would only make it stray away from its straight line motion a little bit. The planet’s straight-line direction would curve a little bit but the force of the sun’s gravity wouldn’t be enough to make it stop or crash into the sun. After curving a bit, the planet would keep going out into space. But there’s a third possibility. Sometimes the pull of the sun’s gravity, the speed of the planet and the distance between the sun and a planet is just right so that the planet doesn’t fall into the sun and doesn’t sail out into space. Instead, the pull of gravity is enough to bend the planet’s straight-line path all the way around into a sort of a circle around the sun. This is a very special effect of gravity: the sun is pulling on the planet enough to keep it from going on it’s way but not enough to pull it in all the way. The momentum of the planet keeps it moving and the force of the sun’s gravity keeps it from getting away. The balance between these two forces causes the planets to rotate around the sun. Whew! That’s a lot to understand, but these ideas are fun and exciting, too. Great question, Joanna.
by Jeff | Wed Jan 30 11:56:14 PST 2002 | Back to Top

What is WhyizZat all about?

WhyizZat is a fun site to learn about science and ask science questions. My name is Jeff, and I have been interested in science since I was in about the 4th grade. I like all kinds of science subjects, including how our bodies work, what makes up the world around us, what the stars and planets are like. I'm not a scientist myself, but I've learned a lot about science and what I don't know I can usually find out.

- Jeff Koppelmaa