What Does the Universe Contain?

            The earth, the moon, the sun, and the rest of the solar system, all the stars, and all other heavenly bodies are contained in what we call the universe. When you look into the sky on a clear, dark night, you can see as many as 2000 stars with your naked eyes. Most stars are many times bigger than the earth. Stars only seem small because they are very far away. In fact, some stars are so far away that it would take millions of years to get to them by spacecraft! Stars are made up of hot gases. A gas is something that has no shape of its own. Air is made up of many kinds of gases. The gases that make up a star are so hot that they give off light. Though this light shines all the time, you may not always be able to see it. Light from a star must shine through the air before you can see the star. Air is always moving. When it moves, it causes light from the star to move back and forth a bit. When the light moves, it makes the star twinkle. the color of a star depends on how hot the star is. The coolest stars are red. The hottest stars are blue-white.
            Many of these stars have been well known since ancient times. Long ago, people imagined that they saw pictures made in the sky by different groups of stars. They gave the names of kings, queens, heroes, animals, and objects to these star pictures. We still use the names that were given long ago. The groups of stars that form these pictures are called constellations. One group of stars looked like a lion to many people. This group is called Leo. Leo was a lion in an old story. Two other groups looked like dippers. These groups are called the Big Dipper and the Little Dipper.
            Most stars appear to rise and set just as the sun does. but if you watch the northern sky at night, you can find one star that seems to stand still. This is the North Star, or Polaris. It appears to stand still because it is almost directly above the North Pole. The other stars in the northern sky seem to move around the North Star in circles. As the earth revolves around the sun during the year, its dark side is turned toward different parts of the heavens. For this reason, some constellations that can be seen during one season cannot be seen during another season.
            From just seeing the stars at night, it is difficult to imagine how big or how far away they are. Our sun is large enough to hold more than a million earths. Yet it is only a medium-sized star. Two large stars, Betelgeuse and Mira, are over 400 times larger in diameter than the sun. Distances in the universe are so great that instead of measuring them in miles, astronomers measure them in light-years. A light-year is the distance that light travels in a year. It is a measure of distance rather than of time. Scientists have discovered that the speed of light is about 186,000 miles a second. So if you multiply the number of seconds in a year by 186,000, you will find that there are about 6 trillion miles in a light-year.
            Looking at the stars, you see that they do not seem to be evenly scattered. In some parts of the sky, there appear to be very few stars. But in one part, they look so close together that they seem to form a wide band of light that stretches across the sky. We call this band of light the Milky Way. Our solar system is part of the Milky Way. Powerful telescopes show that the Milky Way contains billions of stars. These stars are not crowded close together as they seem to be. Actually, they are very far apart. The stars in the Milky Way only appear close together because they are so far away and we are looking at such a large group of them. Astronomers believe that the sun with its solar system is part of a huge group of stars called a galaxy. Our galaxy is thought to contain about 100 billion stars. It is about 100,000 light-years in diameter and 10,000 light-years thick. The sun and the solar system are not in the center of the galaxy but closer to the rim or outer edge. When you look out from our galaxy in certain directions, you see comparatively few stars. But when you look the long way through the galaxy toward its rim, you see so many stars that they seem to form a solid band of light.
            Scattered among the stars in our galaxy are huge clouds of gas and dust. A cloud of this kind is called a nebula. Many nebulae glow brightly and appear through a telescope as irregular patches of light. Other nebulae are dark and we see them as black patches against the bright background of the stars. Astronomers noticed long ago that many of the patches of light they thought were nebulae had a spiral shape. Later, when better telescopes were used to look at these objects, it was discovered that they are not nebulae at all. Instead, astronomers found that they are immense groups of stars so far away that they are completely outside our galaxy. In other words, they are other galaxies like our own. Each of these galaxies contains billions of stars.

How Was the Universe Formed?

            Scientists often observe facts that are not explained by any of their theories. Then they try to develop new theories to explain these facts. Before scientists try to develop a new theory, they gather together as much information about the new fact as they can get. Scientists get this information by doing experiments and making observations. Then they have to use their imaginations to figure out different ways that all this information can be put together to make a good explanation. Sometimes a new theory does not explain all the information that has been obtained. Sometimes two or more theories will be developed, each of which explains part of what is known. And sometimes two theories will be developed, both of which explain all the information available, but which are entirely different explanations. Then the scientists must do more experiments and make more observations to see which theory best explains the new information.
            Astronomers have developed theories about the formation of the universe in much the same way as other scientists develop their theories. Of course, astronomers have difficulty observing the universe because it is so huge, and because the planets and stars are so far away. And in trying to explain the beginning of the universe they must guess about things that happened billions of years ago. but by using instruments such as telescopes, spectroscopes, and radio telescopes, astronomers have collected much valuable information. Just as valuable to them are new tools for thinking that have been developed, such as new systems of mathematics.
            At the beginning of last century, scientists used such a new system to develop a theory that the universe is expanding.  In 1922, astronomers studying the light given off by distant stars found that those stars were moving away from us. This new information agreed with the theory that our universe is expanding. Since this fact was discovered, two main theories have been developed to explain how the universe was formed. According to one of these theories, all the matter in the universe was once concentrated in a single place. Then, as the universe began to expand, this matter formed separate clouds of gas that continued to move away from each other. Then the gas in each of the clouds began to form stars, somewhat as water vapor condenses to form drops of water. This would explain why we find stars clustered together in galaxies. Some astronomers who accept this theory think that the universe will continue to expand indefinitely. Others believe that once it has expanded to a certain limit, it will start to contract. Eventually all the matter in the universe will once again be concentrated in one place. Then the process will begin all over again.
            The second theory is very different from the first. According to this theory, our universe had no beginning and will have no end. It says that as the universe expands, new matter is formed. This new matter is hydrogen which eventually forms new galaxies. So instead of thinning out as it expands, the universe stays about the same, with any given amount of space always containing about the same amount of matter. But some of the matter is new and some of it is old.
            No one knows which of these two theories is correct. Only further observations will enable scientists to decide. It may be that when new observations are made, both theories will be found to be wrong. If that is the case, a new theory will have to be developed that explains the new information that the old theories failed to explain.