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THE CHANGING UNIVERSE: EVOLUTION HAPPENS!
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.
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.
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