what is a star? A star is a really hot ball of gas, with hydrogen
fusing into helium at its core. Stars spend the majority of their
lives fusing hydrogen, and when the hydrogen fuel is gone, stars
fuse helium into carbon. The more massive stars can fuse carbon
into even heavier elements, which is where most of the heavy elements
in the universe are made. Throughout this whole process is that
battle between gravity and gas pressure, known as equilibrium. Itís
crucial to keep this battle in your mind when trying to understand
how stars live and die.
The Main Sequence
Stars live out the majority of their lives in a phase termed as
the Main Sequence. Once achieving nuclear
fusion, stars radiate (shine) energy into space. The star slowly
contracts over billions of years to compensate for the heat and
light energy lost. As this slow contraction continues, the starís
temperature, density, and pressure at the core continue to increase.
The temperature at the center of the star slowly rises over time
because the star radiates away energy, but it is also slowly contracting.
This battle between gravity pulling in and gas pressure pushing
out will go on over the entire life span of the star.
What determines how long you will live? You could live a long full
life, dying of old age primarily because your old, tired body has
worn out. You could get a disease, like cancer, and that could impact
the length of your life. You could have a heart attack, be in a
car accident, or fall off a cliff on a hiking excursion. But most
people start to see health decline when their bodies cannot maintain
a good balance. Biologists call this homeostasis,
which means balance or equilibrium.
For example, some biologists believe that all individuals
die a cellular death. If your cells are starved of oxygen, for whatever
reason, they die. This happens relatively quickly too.
copyright 1994 STScI
This is an image of one of the smallest stars scientists
or less without oxygen will cause brain death in a human. Without
oxygen, the cells of the body (including your blood) become more
acidic, until eventually all of the enzymes
that cause your body to work are ďfriedĒ by the acid
levels. Once your enzymes are fried, your prognosis for recovery
is slim. Your body did not maintain its proper pH, or acid balance.
Too far out of balance, and your body shuts down.
A star needs to maintain a balance too Ė but this
balance is between gas pressure and gravity. What do you think determines
the length of life of a star? Well, your hint is that itís a matter
of mass. What has mass got to do with it?
Well, hereís some logic to help you figure it out.
If a star has a small mass, it has fewer atoms to maintain at equilibrium.
If a star has a large mass, it has more atoms to keep at equilibrium.
Do you think being bigger is better when it comes to how long a
Choose from the following hypotheses regarding length of star
To find out if you are correct, read the following explanation
Larger stars have more fuel, but they
have to burn (fuse) it faster in order to maintain equilibrium.
Because thermonuclear fusion occurs at a faster rate in massive
stars, large stars use all of their fuel in a shorter length
of time. This means that bigger is not better with respect
to how long a star will live. A smaller star has less fuel,
but its rate of fusion is not as fast. Therefore, smaller
stars live longer than larger stars because their rate of
fuel consumption is not as rapid.