Average Star Life (vB)
Overview
A star's life is determined almost exclusively by its mass. Large stars (above about three solar
masses) live much shorter and violent lives than most other, average-sized stars (such as our
sun). Even the processes by which they create energy are different.
This page details what occurs during the life of the latter type of star.
Energy Production During Normal Life
Average-sized stars create energy by three main processes of hydrogen fusion. The basic idea is
to combine light atoms into heavier ones, but the mass of the heavier ones is slightly less than the
sum of the lighter ones. The extra mass is lost as energy and radiated into space; the energy is in
Einstein's equation of
.
The following process is known as the PPI chain and it occurs approximately 69% of the time in
an average star (it is also depicted in the picture below it):
In the picture to the right, two protons join together to form a deuterium nucleus, which is also
known as "heavy water." A positron and a neutrino are released as by-products. The deuterium
nucleus is bombarded by another proton, creating a helium-3 nucleus. The by-product of this is a
photon in the form of a gamma ray (a very high-energy form of light). Then, the helium-3
nucleus in bombarded by another helium-3 nucleus, creating a normal helium-4 nucleus. The byproduct of this are two protons, which are free to start the whole process over again. The positron
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Average Star Life (vB)
will be destroyed and form another gamma ray; the energy from this in the form of gamma rays
is radiated out of sun's core.
A second hydrogen fusion process that occurs approximately 30.907% of the time in an average
star is called the PPII chain. After line 2 of the PPI chain:
The third - and much more rare - form of fusion occurs only 0.093% of the time in an average
star and is called the PPIII chain. After line 1 of the PPII chain:
Compositional Changes
In a star such as the sun, this process lasts for time scales of about 10 billion years before the star
begins to die. However, before that happens, all of the nuclear processes that have been
occurring act to change the chemical composition of the star, which causes it to alter some
properties.
As the hydrogen is turned to helium, the mean molecular weight of the core rises. Given the
Ideal Gas Law - if the mean molecular weight rises, then the outward pressure drops, and if the
pressure drops, then the core contracts, so the density rises. This must happen for the star to stay
in hydrostatic equilibrium.
As the core contracts half the gravitational energy released by contraction must go to heating the
star. And, as the heat rises, energy production rises as
. The end result of all this is
for the star to become more luminous as it ages. For example, the sun is currently about 140% as
luminous now as it was when it began fusion about 5 billion years ago.
All these changes build up, and eventually, the core runs out of hydrogen to fuse. When this
happens, the star moves off the Main Sequence and begins its journey to death.
Questions:
1. What is the main factor that determines a stars evolutionary track?
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Average Star Life (vB)
2. What mass is considered to be “large” in this text compared to the mass of the sun?
3. Calculate this mass (recall from the mass of the Sun from the handout on Star birth)
4. How do average size stars, such as our sun, produce energy?
5. Use the table below to describe the 3 stages of the PP chain. If you have just had Stars
Galaxies and Cosmology – you know this well – please help those around you!
6. If the product of the 2nd step has less mass than the original components, where is the
missing mass?
7. How will average stars composition change over the course of its lifetime?
8. Why does it do this? What is hydrostatic equilibrium?
9. Approximately how long does it take?
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