In the year 1000 AD there were astronomers in only a few places on Earth: in Asia, particularly China, in the Middle East, and in Mesoamerica. These astronomers were aware of only six of the nine planets that orbit the Sun. Although they studied the stars, they did not know that the stars were like the Sun, nor did they have any concept of their distances from Earth. By the year 2000 AD, humanity’s horizons had expanded to include the entire universe. We now know that our Sun is but one of 100 billion stars in the Milky Way Galaxy, which is but one of about 100 billion galaxies in the visible universe. More remarkably, our telescopes have been able to peer billions of years into the past to see the universe when it was young—in one case, when it was only a few hundred thousand years old. All these observations can be interpreted in terms of the inflationary Big Bang theory, which describes how the universe has evolved since the first 10−36 seconds of cosmic time.

It is impossible to predict where astronomy will be in the year 3000 AD. But it is clear that for the foreseeable future, the defining questions for astronomy and astrophysics will be these:

  • How did the universe begin, how did it evolve from the soup of elementary particles into the structures seen today, and what is its destiny?

  • How do galaxies form and evolve?

  • How do stars form and evolve?

  • How do planets form and evolve?

  • Is there life elsewhere in the universe?

Researchers now have at least the beginnings of observational data that are relevant to all of these questions. However, a relatively complete answer exists for only one of them—how stars evolve. The development and observational validation of the theory of stellar evolution was one of the great triumphs of 20th-century astrophysics. For the 21st century, the long-term goal is to develop a comprehensive understanding of the formation, evolution, and destiny of the universe and its constituent galaxies, stars, and planets—including the Milky Way, the Sun, and Earth.

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