The Universe in the Classroom

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


c) The Age of Light from the Most Distant Galaxies: The "Time Machine" Effect

Astronomers can measure the distances to other galaxies from their apparent size or brightness, and in many other ways. These distances are so great that billions of years are required for their light to reach us. Thus we are actually seeing these galaxies not as they are today, but as they were billions of years ago.

Light travels at 299,792.458 kilometers per second (300,000 km/sec is a convenient approximation). During the last century, this number has been measured with exquisite accuracy, and found to be constant. But even at the extraordinary speed of 300,000 kilometers per second (186,000 miles per second), light takes considerable time to reach us from distant objects. Light from the Sun, for instance, takes eight minutes to reach us, so that we see the Sun as it was eight minutes ago. Similarly, we see the stars in the nighttime sky as they were decades, centuries and even thousands of years ago.

An example of the "time machine effect" in everyday life is to listen for the slower sound of thunder which accompanies a lightning flash; if the thunder follows the lightning by 10 seconds, then it is about 3 km away; if the thunder and the lightning are simultaneous - the storm must be right on top of us! Another example: for spacecraft exploring the outer solar system, it takes many hours for their radio signals (which travel at the speed of light) to reach the Earth; the spacecraft cannot be controlled remotely from Earth because the communication time would be too long. This is why the spacecraft's instructions must be carried in its on-board computer.

The galaxies, however, are so distant that their light may take billions of years to reach us. So when we look deeply into space we are looking into the past, across vast gulfs of time. When we study distant galaxies, we find that their stars are still being born from the loose gas from which the galaxies formed. When we study even more distant galaxies, we see them as they were 10 billion or more years ago. In these long-ago galaxies, we find that the stars are just beginning to form.

The "Hubble Deep Field" is a 10-day time exposure made by the Hubble Space Telescope. Almost every object in this image is a distant galaxy, seen as it was in the past - at times up to 10 billion years ago. It is from images such as this that we can unravel the history of the universe and determine its age.


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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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