Lecture 1 -- Overview of the Universe


This course covers the Universe, which is another way of saying that is covers everything. We'll start with an appreciation of naked eye astronomy --- what can you derive about the Universe, the Everything, with nothing but your eyes and your mind. This will lead us into learning about the rules of the Universe. In other words, physics. We'll learn that, of all the forces in the Universe (and there are four) the weakest, and by far the most important, is gravity. We'll learn how gravity works, how it doesn't work, and how we know that Hollywood screenwriters have never taken astronomy. Then we'll consider a much stronger force, the electromagnetic force, which is responsible for light. Unlike scientists in other disciplines, astronomers cannot perform experiments. Instead, the Universe provides us with billions of experiments going on around us all the time. Astronomers observe the experiment, figure out what the experiment is, and then what the experiement is telling you about the Universe. Astronomers cannot touch these experiments, nor hear them, nor taste them, nor smell them. Everything we know about the universe comes from light, so we will learn how light works, and the incredible amount of information it contains.

Once we learn the rules, we will step out into the Universe and consider the stars. What are stars? What are they make of? How bright are they, really? How far away are they? How can they tell us about the Universe? These are some of the questions will be address. We will learn what stars are and why they shine. We will learn how they are born. We will see how they die. And when we do this, we will come upon some of the strangest objects in the Universe: binary stars which swallow each other whole; stars as small as State College, but with the mass of the Sun; stars as large as the orbit of Mars, that has consumed all of its planets; stars that gently throw off their atmosphere and quietly retire into oblivion; stars that end their life in a tremendous explosion that can outshine 100,000,000,000 stars.

With our study of stars complete, we will step out once again and begin to consider the universe (the everthing) as a whole. We'll start out slowly, and use our knowledge of how stars work to understand star clusters. We'll find that these objects can teach us a great deal about the universe. Next, we'll step out again and focus on the great cities of stars, called galaxies. These will serve as beacons and allow us to probe out to the edge of the universe. Using these galaxies, we'll consider questions such as how big is the universe, how old is the universe, did the universe have a beginning, will the universe have an end. These are the questions that are in the forefront of astronomy today.

Finally, we turn in the other direction and look at the small corner of the Universe we call the solar system. We'll compare the properties of the various planets and Moons. We'll learn about how the solar system formed, and investigate why its planets and moons are so different from each other. Why does the Earth have water, while Venus has sulfuric acid clouds? Why is the Earth's Moon dead, while Io, the moon of Jupiter, has one volcano erupting after another. Why do Saturn (and Jupiter and Uranus and Neptune) have rings? How important are comets to the Earth's past, present, and future? And, of course, how unique are we?

The Sky

If you go out at night, you will see that we are surrounded by stars. The sky is like a big globe, a celestial sphere that surrounds us. Stars are scattered on this celestial sphere, and since evolution has trained our eye to see patterns, we naturally group sets of stars into constellations. There's nothing magic about these constellations -- different people will see different things. (As an example, the constellations of the ancient Egyptians are different from those of the American Indians, which are different from those of the Chinese, which are different from those of the Incans, etc.) However, thoughout history, the positions of the stars relative to each has remained fixed. The patterns that you see today are essentially the same as those seen by the Phoenician sailors 5000 years ago.

Not all stars, of course, appear the same. Some appear brighter, others fainter. A Greek (Hipparchus) was the first to devise a scheme to describe the brightness of stars. Bright stars were defined as stars of the first magnitude. Slightly fainter stars were called stars of the second magnitude. The faintest stars you can see with your eye (at a dark sight, with no Moon) are sixth magnitude stars. On the other hand, really bright stars can have a magnitude of zero, and some planets have negative magnitudes. In the extreme is the Sun, which has a magnitude of -26. Now imagine the earth spinning inside this globe, this celestial sphere of stars. The earth spins west to east, so as the night progresses, the stars will rise in the east, cross the meridian (the line in the sky which divides east from west), and set in the west. This is the diurnal motion of the heavens, i.e., the motion that reflects the rotation of the earth. Note that not all stars rise and set. If you were to spin around in a room, the patterns on the walls would come into and go out of your view, but the light on the ceiling would stay fixed. Thus, there can be circumpolar stars, i.e., stars that never rise or set. The behavior of the stars depends on where you are. If you are standing at the North Pole, all stars are circumpolar. However, at the North Pole, you are never able to see the stars that are located over the southern areas of the earth, i.e., stars that are below the celestial equatior. Conversely, a person on the equator would eventually see all the stars of the celestial sphere, but aside from a pole star, which would be on the horizon, none of the stars would be circumpolar. By luck, there is a moderately bright star positioned almost precisely at the north celestial pole. This is Polaris. By luck, there is no bright star positioned over the South Pole of the earth.