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So, 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.

Interactive Lab

This activity shows what happens to different size stars at the beginning of their life cycles.



A Matter of Mass

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 have observed.

Five minutes 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 star lives?


    Choose from the following hypotheses regarding length of star life:

    1)  The bigger a star is, the longer it will live.
    2)  The smaller a star is, the longer it will live.

    Now, for whichever hypothesis you chose, type a 1-3 sentence explanation for why you think this is so.

To find out if you are correct, read the following explanation carefully

    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.



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