The Universe in the Classroom

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


b) Changes in Stars

One of the great discoveries of modern science is that stars (like people) live only a measurable life-time and then die. Although the lives of the stars are enormously longer than the span of a human life, we can learn about the life story of the stars by studying them at many different stages in their life cycle, from birth to death. As an analogy, imagine that a hypothetical race of aliens visited the Earth for an hour or two, and had to make observations to piece together the life cycle of humans. Studying one human being or even three or four in that short time would hardly give them much useful information.

The trick would be to examine as many humans of different types as possible and then deduce the different stages in our lives. For example, a few of them might visit a maternity ward, and see humans in a stage just before or after birth. They might even see a birth in progress. Others in the same hospital might witness the stages just before and after death. Some out on the street would observe people of various ages: young ones with their parents, old ones with their children, teenagers and adults in various groupings.

The Pleiades Star Cluster
The Pleiades Star Cluster

Similarly, astronomers (able to glimpse any given star for only a "moment" of its long existence) must examine many stars and hope to find some in each stage of its life. And we have been able to do exactly that - we have found young stars near the "maternity wards" of gas and dust where they are born. We can observe stars like our own Sun, which are in the stable "adult" stage of their lives. (A good number of such sun-like stars nearby are surrounded by one or more planets, just like the Sun is.) We can see red giant stars in "mid-life crisis", bloated by changes deep within. And studying stellar corpses called white dwarfs and neutron stars, we observe the after-effects of stellar death.

The slow processes of stellar life and death can be deduced from groupings of stars called star clusters, groups of stars which are born together and live out their lives as a group. A good example of such a group is the beautiful Pleiades cluster, which can be seen in the fall and winter sky. In such a cluster, different stars go through their lives at different paces, and we can find stars that started together, but are now in very different stages of their lives.

Changes in how stars live their lives can be observed directly in a special class of stars called "pulsating variable stars"; the North Star - Polaris - is one example. This star expands and contracts in rhythmic fashion, every 4 days. But as it slowly swells with age, it becomes larger, and the regular expansion and contraction take measurably longer.

What do we learn from studying the stars in different stages (and by simulating their behavior and physics on high-speed computers)? We find that stars evolve from one form to another - from energetic youngsters, to stable adults, to bloated giants, and on to death and becoming a corpse. We note (because some stars explode) that new generations of stars include some of the materials produced by previous generations and that the number of more complex atoms in the universe is slowly growing. We have good evidence that our Sun (with its planets) was not among the first stars the universe produced, but formed later from materials enriched by the deaths of previous generations.

This is a key idea in astronomy - that the evolution of the stars gradually changes the make-up of the cosmos. The stars are not mere backdrops to our existence on Earth - creatures as complex as we are could not have evolved on Earth without the materials that earlier generations of stars contributed to the cosmic "element-pool." And the Sun itself will not last forever, but will someday die. In the process, it will eventually expand and make life on Earth impossible, quite independent of what we humans do.


<< previous page | next page >>

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.

back to Teachers' Newsletter Main Page