Astrophysics is Easy!: An Introduction for the Amateur Astronomer by Mike Inglis

Astrophysics is Easy!: An Introduction for the Amateur Astronomer by Mike Inglis

Author:Mike Inglis
Language: eng
Format: epub, pdf
Tags: Springer, ISBN-13: 9781852338909


Stars

101

So, a Sirius will burn hydrogen in its core for about 1/6 54 × 1010 years, or about

1.5 billion years.

On the other hand, a main-sequence star with a mass of 0 5 M will have a lifetime

of:

1

=

1√ = 1 = 5 66solar lifetimes

0 52 5

0 52 0 5

0 177

which is about 56 billion years.

3.9 Red Giant Stars

Although the amount of hydrogen in a star’s core is vast, it is not infinite, and

so, after a very long time, the production of energy will cease when the central

supply of hydrogen is used up. Throughout the length of time that nuclear fusion

has been taking place, the hydrogen has been transformed into helium, by way

of the proton-proton chain, and without this source of energy, the star uses

gravitational contraction to supply its energy needs. Thus, the core will start to

cool down, which means that the pressure also decreases, with the result that the

outer layers of the star begin to weigh down on the core and compress it. This

has the effect of causing the temperature within the core to rise again, and for

heat to flow outward from the core. Note that although a tremendous amount of

heat is formed now, it is not due to nuclear reactions but to gravitational energy

being converted into thermal energy.

In a relatively short time, astronomically speaking, the region around the

star’s hydrogen-depleted core will become hot enough to begin nuclear fusion of

hydrogen into helium, in a thin shell around the core, in a process called shell

hydrogen-burning. This is shown in Figure 3.12.

The core will consist of helium, but the outer layers are hydrogen rich. The

shell, where energy production occurs, is relatively thin. [Note: figure to scale.]

For a star like the Sun, this hydrogen-consuming shell develops almost immedi-

ately from the moment nuclear fusion stops in the core, and so the supply of

energy is more or less constant. For massive stars, there can be an interval of

perhaps a few thousand years to a few million years from the end of the core

nuclear-fusion phase to the beginning of the shell hydrogen-burning phase.

The new supply of energy, and thus heat, has the effect of causing the rate

of shell hydrogen-burning to increase, and so it begins to eat further into the

surrounding hydrogen. The helium that is the by-product of the hydrogen fusion

in the shell falls to the center of the star, where, along with the helium already

there, it heats up as the core continues to contract and increase its mass. In the

case of, say, a 1 M star, the core will be compressed to as much as one-third

of its original size. The result of this core compression is an increase in the

temperature, from about 15 million K to nearly 100 million K.

Now, most of what has happened in this stage of a star’s life has occurred

inside of it and so was invisible to our eyes. Nevertheless, it does have effects



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