The Universe in the Classroom

An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time


The Sun is one of hundreds of billions of stars that make up a magnificent grouping of stars we call the Milky Way galaxy. A number of these stars are now known to have planets, just the way the Sun does. Some stars show evidence of being much older than the Sun, and some are just gathering together from the raw material of the galaxy.

One of the nicest things about the universe is that it sends its waves of information to us at the fastest possible signal speed, the speed of light. This is an amazing 300,000 kilometers per second (or 670 million miles per hour in units your student may be more familiar with). The other stars are so far away, however, that even at this speed, light from the next nearest star takes 4.3 years to reach us. And it takes light over 100,000 years to cross the Milky Way galaxy. (The distance light travels in one year, about 9.5 trillion km, is called a light-year and is a useful unit of measurement for astronomy. We can then say that the nearest star is 4.3 light-years away.)

Despite these distances, the stars are so bright that we receive enough light (and other radiation) from them to learn a great deal about how they work and how long ago some of them formed.

The Andromeda Galaxy
The Andromeda Galaxy

Beyond the Milky Way lies the realm of the other galaxies. Our largest telescopes reveal billions of other galaxies (collections of billions of stars) in every direction we look. The Milky Way shares its cosmic neighborhood with several dozen other galaxies, but only one that is bigger than we are. That one, the great galaxy in the constellation of Andromeda, is 2.4 million light-years away. The light we see from it tonight left it 2.4 million years ago, when our species was just beginning to establish a fragile foothold on the surface of planet Earth. Some galaxies are so far away that their light takes over ten billion years to reach us.

As we shall see below, astronomers do not quote such mind-boggling distances or times idly. During the 20th century, they developed techniques for measuring the distances to stars and galaxies and establishing the vast scale of the universe in which we find ourselves.

In similar ways, they have also found ways of establishing the scale of cosmic time. These measurements show that the universe had its beginnings in a very dense, hot state we call "the Big Bang" about 10 to 15 billion years ago. The Sun and the Earth formed from the "raw material" gas and dust in the Milky Way galaxy some 4.5 to 5 billion years ago. The earliest evidence we have for living things on Earth goes back to about 3.5 billion years ago.

On this scale, everything with which we are normally concerned is recent indeed. Here is an interesting thought experiment. Suppose we were to compress the entire history of the universe from the Big Bang to today into one calendar year. On that scale, the dinosaurs would have flourished a mere few days ago, and the life-span of a person would be compressed to a tenth of a second. To see this worked out in more detail, see the "cosmic calendar" activity in our activity listing below.


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An Ancient Universe - Table of Contents

Home | Introduction | The Universe: An Overview | The Process of Science | The Ancient Universe - The Age of the Expanding Universe - The Age of the Oldest Stars - The Age of Light From Distant Galaxies - The Age of the Chemical Elements | The Changing Universe - Changes in the Solar System - Changes in Stars - Changes in the Universe | Science and Religion | Resource Guide | Activities

© Copyright 2001, American Astronomical Society. Permission to reproduce in its entirety for any non-profit, educational purpose is hereby granted. For all other uses contact the publisher: Astronomical Society of the Pacific, 390 Ashton Ave., San Francisco, CA 94112.

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