City of the stars
The star then has to change its whole structure. Heavy atoms
are built up from those already created, until finally, there comes a
point where internal energy producing reactions are no longer
possible and the star loses the ability to keep itself inflated against
gravitational collapse.
What happens then depends upon the initial mass of the star. In
a modest star like the Sun, the core shrinks due to gravity and this
causes layers of remaining hydrogen close to the core to react violently.
The energy produced causes the star’s outer layers to inflate, producing
what’s known as a red giant. Eventually, these outer layers are blown
away, producing what we call a planetary nebula. This is a slightly
misleading term because a planetary nebula is not really a nebula
and has nothing to do with a planet! The end product is a very small,
very dense star known as a white dwarf which shines feebly due to
continued contraction. In the end it can contract no more and becomes
a cold, dead world, still orbited by the ghosts of its remaining planets.
A white dwarf is a very dense object and a teaspoon of white dwarf
material on Earth would weigh about the same as a family saloon car!The Dog Star
The brightest star of them all in Earth’s night sky is Sirius which lies in the
constellation of the Great Dog (Canis Major). Intense though it looks, the
brilliance of Sirius largely comes about because it is actually one of our
nearest neighbours. When seen low, near the horizon, Sirius twinkles a
furious range of colours but this is simply an atmospheric effect and in
reality, the star is pure white.
Let us run over the life story of a star like our Sun. It begins by
condensing out of the material in a gaseous nebula, a process that takes
some time. As the fledgling star becomes smaller and denser under
the influence of gravity, the temperature rises until it reaches 10 million
degrees. This is where nuclear reactions begin and hydrogen atoms
combine to form helium, releasing energy and losing mass. The internal
energy helps inflate the star until an equilibrium state is reached. At this
point, gravitational collapse is balanced against internal energy release.
This is the most stable period in the life of a solar type star and can last
for thousands of millions of years. But the supply of hydrogen is not
inexhaustible, and there comes a time when it starts to run low.
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