The Universe in the Classroom

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


d) The Age of the Chemical Elements

Supernova 1987A
Supernova 1987A

Just after the Big Bang, the universe was made almost entirely of the simplest elements: hydrogen and helium. We have confirmed this by looking at galaxies really far away - and thus long ago. And, indeed, they have greater proportions of hydrogen and helium. The other chemical elements were formed later -- some in nuclear reactions in the cores of stars, others when the most massive stars ended their lives in gargantuan explosions that astronomers call a supernova. (A spectacular supernova was observed in 1987 in a galaxy very close to ours. Astronomers actually observed some of the newly-formed elements emerging in this explosion.)

Some isotopes (forms of the element with different numbers of neutrons in the nucleus) of these elements are radioactive; they change into other isotopes at a rate that can be measured accurately in the laboratory. As time goes on, less and less of the original or "parent" isotope is left and more and more of the product or "daughter" isotope can be found all around it. By comparing the amount of the parent isotope to that of the daughter isotope, astronomers can determine how long it has been since the radioactive parent isotope formed. In this way, astronomers have determined that, although some radioactive isotopes (such as the ones produced by the 1987 supernova) are recently formed, the oldest radioactive isotopes in the universe are 10-20 billion years old.

The same radioactive dating technique allows us to measure the ages of the oldest rocks on Earth, on the Moon, and in meteorites, chunks of rock from space that land on Earth. Such dating experiments have shown that the age of the solar system (the Sun and its planets) is about 4.5 billion years, as we mentioned above. The universe is a lot older than our little neighborhood. More recently, the same technique has even been used to confirm the ages of stars.

The key thing to notice is that all of the independent estimates of the age of the universe are in remarkable agreement - our best estimate being about 14 billion years, give or take a 10 percent measurement uncertainty. That strengthens astronomers' view that the universe, the galaxies, and the stars are truly ancient, and not recent creations. There are other less direct ways of estimating the ages of these objects, and the age of the solar system, and they too are in agreement.


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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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