Two hard astronomy questions from students.

1.) What makes gravity?

The easy answer is that we don't really know what gravity is. Sir Isaac Newton described it very well, but describing is different from explaining. Newton figured out that gravity pulls between any two objects with mass. Remember that mass is the amount of stuff, or matter, in something. The greater the masses the stronger the pull. He also figured out that gravity depends on the distance between the objects, that it gets weaker as the square of the distance. That means if the moon were twice as far away, Earth would pull on it 1/4 as strongly. Three times as far and the pull would be 1/9th. This was very clever for Newton, but he didn't know what gravity is.

Albert Einstein also took a crack at it, describing it as the bending of a space-time continuum (Whoa!) by objects with mass. What he meant was that space, in more than three dimensions, is bent or warped by mass in such a way as to bring massive objects together. Again, this is a description and not an explanation. Gravity has many mysteries wrapped up in it, all of our other forces have opposites, magnets can pull or push, electricity can pull or push, do you think some day somebody, maybe you, will discover the push that comes with the pull of gravity? Or maybe you will be the one to explain gravity! I bet there is a Nobel Prize waiting for the person who does.

2.) Why does the Earth spin?

I like this question! I am tempted to say that the earth is spinning because it was spinning yesterday and it has a lot of inertia. I don't think that answers your question, though.

The earth formed at the same time as the rest of our solar system out of debris (junk) left over after a star or several stars before our sun exploded. All this stuff flying through space was massive enough that gravity started pulling it together toward one big pile in the middle. This was the birth of our sun, but it was not that simple. The solar system is big, it is huge, it is GIGANTIC! The stuff was moving around in all directions, and it would make sense that on the average, there would be no overall rotational bias, or preference to be going one way any more than any other. But just like your kitchen sink or bathtub, there is always a slight preference for one direction or another. In the solar system's case, this might have been pushed along a little by the spin of the Milky Way galaxy. As the Solar System formed, it behaved like the whirlpool in a bathtub. Distant parts that were moving very slowly went faster as they fell closer to the sun. Not only that, but the orbits of the planets as they formed were all in the same direction, just like that whirlpool. The planets are left-overs from stuff that fell but missed hitting the sun and by collisions with other stuff settled into stable orbits.

The planets themselves were formed by stuff falling in on them, and the same thing happened again. A tiny amount of spin very far away was turned into more spin as the object fell closer and hit the earth. This in turn was probably being pushed by the spin of the solar system, as nearly all the planets spin in the same direction. We think the exceptions are planets that started out in the right direction and then were smashed by asteroids or small early planets that turned them to spinning backwards. So Earth was also like a whirlpool in a drain, most of the matter that fell on the planet as it formed had a preference to be spinning in from west to east, pushing our planet to spin in that direction.

Really, the earth is spinning because it was spinning yesterday, and it has a lot of inertia. :)

Here is a question for you: When you fill a sink with water, there are currents flowing in every direction in the water. If you let it sit, eventually those currents will slow down and stop because water has friction. How long do you think you have to wait for the water to be SO still that it won't make a whirlpool when you open the drain? Try it with a sink or tub where you can open the drain without reaching into the water, which will start currents again. Keep track of your results, and record which way the whirlpool spins if you still get one.

How was the Earth formed?

How the earth formed is a long story, but I will try to make it shorter than it took for the Earth to form.

Shortly after the birth of our universe in the big bang, space was filled with energy and hydrogen atoms. (I am skipping over the question of dark matter and dark energy because I don't understand them.) Hydrogen atoms are the smallest and simplest atoms. As the Universe expanded, it cooled, and the hydrogen gas atoms formed clouds.

These clouds had gravity, and the gravity pulled the atoms closer and closer together. When atoms move apart, as in the expanding universe, they cool down. When they move together, as in these hydrogen clouds, they heat up. After a long time, they can get so close together and so hot that two hydrogen atoms smash together hard enough to fuse, or join together to make a helium atom. This releases energy, and more atoms fuse. This is the birth of a star, and it happened in places all over the universe.

Eventually the helium atoms start fusing with hydrogen atoms and with other helium atoms, and the star starts making heavier and bigger atoms like carbon, nitrogen and oxygen. This system works until our star starts making iron atoms, which are the limit for most stars. Iron is a very stable element, and difficult to fuse. Iron also poisons the star process, and when enough iron builds up, the star collapses. As the star falls in on itself, it again heats up, sometimes enough to blow itself to pieces.

This explosion, a supernova, has enough energy to create the rest of the elements that we know in nature, and blasts this stuff out all over the local part of space. This stuff is what Earth is made of. In time a new star was born, our sun. Earth is mostly iron and nickel metals, which are dense heavy materials, and at first it was like a huge pile of dust and rubble floating through space, orbiting the new star. Early in its life, Earth was smashed by something or several things huge, releasing enough energy to melt it. Iron and nickel sank towards the center, and the lighter, less dense elements floated to the surface. These materials today make up our atmosphere, oceans, earth's crust, and you and I.

How was the world made?

This is an interesting and deep question. The scientists who study this question are called cosmologists. Most cosmologists agree that our universe was born about twelve billion years ago in an event they call the big bang. While the science of the big bang is pretty well understood, it is difficult to explain, and very weird stuff. Some things that are important to know are that it resulted in a lot of energy and a lot of hydrogen gas that was spread out in lumps in the early expanding universe. It also apparently produced even more ‘dark matter’ and ‘dark energy,’ but no one knows what they are yet. Maybe you will be able to help us with that some day!

            After a few billion years, some of the lumps of hydrogen gas settled down and gravity pulled the hydrogen in the lumps closer and tighter together. This made the temperature rise; heating the gas so hot that nuclear fusion began. In nuclear fusion, atoms combine to make larger atoms. Fusion releases a lot of energy, raising the temperature of the gas balls, and creating new and heavier elements. These balls are what we call stars.

            The big bang made mostly hydrogen and I think a little bit of helium. These are the two lightest elements. Stars can cook all the elements up to elements as heavy as iron out of hydrogen and helium, but as the amount of iron increases, it slows down the action of the star. Eventually the star runs out of energy and collapses. The collapse of the star can set off another huge reaction that blows it to smithereens and creates even more elements, up to the weight of uranium. These are scattered throughout the space where the star had been.

            Our sun was born after this had already happened. It started as a lump of hydrogen, and condensed to become a star. The star had enough gravity to attract some of the remains of the earlier star, which condensed first into a ring or rings like those around Saturn, and then those condensed into the planets. Earth and all of its materials including you and me are the product of hydrogen from the big bang and everything else is from stardust. Earth condensed about four and a half billion years ago, and our sun will keep on being like a sun for Earth for at least another four and a half billion years.