This week, we shall look at one of the
largest single objects in the universe, red giant stars. Red giant stars
are large stars, often larger than the orbit of the planet Mars in our solar
system,and our sun will one day be
such a red giant.
If you read the last article in this column, you know that red stars have to be cool, and that is the reason for the color. Perhaps the most famous of the red giants is the star Betelguese, in the constellation Orion. In fact, this is a true super giant, with a diameter many times that of the Sun. Another famous red super giant is the star Aldebaran, the glowing red eye in the constellation of Taurus, the bull.
Red giants form from the remains of most stars, at least for a while, and are a remarkable machine in the clockwork of the universe. Our Sun has been shining now for, it is estimated, some 5 billion years.
The method by which the Sun shines is the process of nuclear fusion; converting atoms of hydrogen into helium. This method, which warms the Earth, causes the breeeze, and nourishes all life on Earth, is also the secret to the hydrogen bomb. The Sun, like most stars, remains stable because of the balance between two forces; the pull of gravity, trying to force the star to collapse inwards, and the push of the nuclear forces inside, trying to explode the star outwards. As long as these forces are balanced, a star will remain stable, and continue to shine, usually at a steady pace, often for billions of years.
If a star were to ever shrink, for some strange reason, the pressures in the core of the star would increase, thereby increasing the number of nuclear reactions in the core, pushing the star back outwards. Conversely, if a star were to increase in size, the temerature would drop, slowing down the nuclear reactions, and making the star shrink once again. But what happens when the fuel runs out? What changes happen to the star when gravity finally wins?
To learn the answer to these questions, first we must understand a little about the inside of a star like the Sun. A star like the Sun is not well mixed. The sun is more like an onion, with different layers of the star having different properties. What we see is only the outside layer, the photosphere. Inside the core (which, contrary to what seems like logic, would be seen by human eyes to be pitch dark) is the ashes of the Sun's nuclear fires, helium atoms, existing in a state known as degenerative matter. Degenerative matter is a type of gas, which looks just like a metal.
When a star runs out of hydrogen fuel in it's core, it starts to contract, pushing the degenerate helium together. Before it can get hot enough to fuse itself, however, the contracting core turns gravitational energy into heat, and the hydrogen surronding the core, once too cool to be turned to helium, is heated to a point where it too, begins to fuse. The fusion of hydrogen just outside the core produces far more energy than the star needs to offset the pull of gravity, thus, the star expands. The core, no longer producing energy, contracts, and increases the gravitation at the core, thereby bringing more fuel in for the raging nuclear fires, and the star expands even more.
When this happens
to the Sun, about 5 billion years from now, it will expand to a size about
equal to the orbit of the Earth. Mercury will be destroyed, Venus will be
gone, and the Earth itself either consumed in the fires of the Sun, or heated
to thousands of degrees, ending all life forever on the Earth.
When stars reach a temperature of 100,000,000 degrees c (180,000,000 degrees f), the helium ash itself begins to fuse, and the mechanism that once sustained the steady state of the stars becomes operational once again. This is how the energy from red giants is formed. A star may remain a red giant for millions of years. After the helium fuel runs out? Then, the star once again collapses, and, depending on it's mass, will form either a white drawf, a nuetron star, or even a black hole.
Before we begin to explore those ends, however, next week we will look at the other type of giant star, the blue giants. As you look to the south east sky, and see the constellation Orion, glimpse at what would be his right shoulder if he were facing you. That is the star Betelguese. As you look at it, remember that one day our Sun will be such a star, and that will finally cause the end of the world.
Clear skies, and good viewing.
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