Historical introduction

The Earth orbits a star, the Sun, at a distance of 140 million km, and the distance to the next closest star, α-Centauri, is more than 4 · 1013 km. The Sun is one star in our galaxy, the Milky Way. The Milky Way has 1011 stars and the distance from the Sun to its center is 2.5 · 1017 km; it is one galaxy in a large group of galaxies, called the Local Group and the distance to the next nearest group, called the Virgo Cluster, is about 5 · 1020 km. The Universe is made up of a vast number of clusters and superclusters, stretching off into the void for enormous distances. How can we learn anything about what's out there, and how can we understand its nature?

We can't expect to learn anything about distant galaxies, black holes or quasars, or even the nearest stars by traveling to them. We can maybe explore our own solar system but, for the foreseeable future, we will learn about the Universe by using telescopes, on the ground and in space.

The principal methods of astronomy are spectroscopy and imaging. Spectroscopy measures the colors of light detected from distant objects. The strengths and wavelengths of spectral features tell us how an object is moving and what is its composition. Imaging tells us what an object looks like. Because distant stars are so faint, the critical characteristic of a telescope used for spectroscopy is its light-gathering power and this is determined principally by its size, or “collecting area.” For imaging, the critical characteristic is its resolution.