ASTRONOMY 

Astronomy is defined as the study of the objects that lie beyond our planet Earth and the processes by which these objects interact with one another.

We will see, though, that it is much more. It is also humanity's attempt to organize what we learn into a clear history of the universe, from the instant of its birth in the Big Bang to the present moment. In considering the history of the universe, we will see again and again that the cosmos evolves; it changes in profound ways over long periods of time.

For example, the universe made the

carbon, the calcium, and the oxygen

necessary to construct something as

interesting and complicated as you.

Today, many billions of years later, the

universe has evolved into a more

hospitable place for life. Tracing the

evolutionary processes that continue to

shape the universe is one of the most important (and satisfying) parts of modern astronomy. The ultimate judge in science is always what nature itself reveals based on observations, experiments, models, and testing. Science is not merely a body of knowledge, but a method by which we attempt to understand nature and how it behaves. This method begins with many observations over a period of time. From the trends found through observations, scientists can model the particular phenomena we want to understand. Such models are always approximations of nature, subject to further testing.

As a concrete astronomical example, ancient astronomers constructed a model (partly from observations and partly from philosophical beliefs) that Earth was the center of the universe and everything moved around it in circular orbits. At first, our available observations of the Sun. Moon. 

had to be updated by adding circle after circle to represent the movements of the planets around Earth at the center. As the centuries passed and improved instruments were developed for keeping track of objects in the sky, the old model (even with a huge number of circles) could no longer explain all the observed facts. As we will see in the chapter on Observing the Sky: The Birth of Astronomy, a new model, with the Sun at the center, fit the experimental evidence better. After a period of philosophical struggle, it became accepted as our view of the universe. When they are first proposed, new models or ideas are sometimes called hypotheses. You may think there can be no new hypotheses in a science such as astronomy-that everything important has already been learned. Nothing could be further from the truth. For example, the significance that the huge chunks of rock and ice that hit Earth have for life on Earth itself is still debated. And while the evidence is strong that vast quantities of invisible

dark energy" make up the bulk of the universe, scientists have no convincing explanation for what the dark energy actually is. Resolving these issues will require difficult observations done at the forefront of our technology, and all such hypotheses need further testing before we incorporate them fully into our standard astronomical models. Numbers in astronomy.

In astronomy we deal with distances on a scale you may never have thought about before, with numbers larger than any you may have encountered. We adopt two approaches that make dealing with astronomical numbers a little bit easier. First, we use a system for writing large and small numbers called scientific notation (or sometimes powers-of-ten notation ). This system is very appealing because it eliminates the many zeros that can seem overwhelming to the reader. In scientific notation, if you want to write a number such as 500,000,000, you express it as 5 × 108. The small raised number after the 10, called an exponent keeps track of the number of places we had to move the decimal point to the left to convert 500,000,000 to 5. 

System of Units, or SI (from the French Système International d'Unités). A common unit astronomers use to describe distances in the universe is a light-year, which is the distance light travels during one year. Because light always travels at the same speed, and because its speed turns out to be the fastest possible speed in the universe, it makes a good standard for keeping track of distances. You might be confused because a "light-year" seems to imply that we are measuring time, but this mix-up of time and distance is common in everyday life as well. For example, when your friend asks where the movie theater is located, you might say "about 20 minutes from downtown." So, how many kilometers are there in a light-year? Light travels at the amazing pace of 3 × 10 5 kilometers per second (km/s), which makes a light-year 9.46 × 10 12 kilometers. You might think that such a large unit would reach the nearest star easily, but the stars are far more makes a light-year 9.46 x 10 12

kilometers. You might think that such a

large unit would reach the nearest star

easily, but the stars are far more

remote than our imaginations might

lead us to believe. Even the nearest star

is 4.3 light-years away-more than 40

trillion kilometers. Other stars visible to

the unaided eye are hundreds to

thousands of light-years away.

Consequences of light travel time

There is another reason the speed of light is such a natural unit of distance for astronomers. Information about the universe comes to US almost exclusively through various forms of light, and all such light travels at the speed of light-that is, 1 light-year every year. This sets a limit on how quickly we can learn about events in the universe. If a star is 100 light-years away, the light we see from it tonight left that star 100 years ago and is just now arriving in our neighborhood.