Energy Generation, Stellar Models, Mass: The Fundamental Stellar Property, Four Stars, Variable StarsThe nature of the stars
A star is a hot, roughly spherical ball of gas that shines as a result of nuclear fusion reactions in its core. Stars are the fundamental objects in the universe. They are the factories where elements heavier than hydrogen are formed. The radiation from a typical star like the Sun provides temperate conditions on planets like Earth where life can arise. Since the Sun is obviously the central source of energy for the earth and its many ecosystems, understanding how our star works is an important area of research. Only in the past 80 years has the answer "Why does the Sun shine?" been partially answered, and many aspects of solar and stellar behavior are still poorly understood. Research on the physics of the Sun and stars will remain fresh and challenging for many years.
Stars have been objects of human curiosity since our earliest ancestors looked skyward. Throughout history humans have told stories about the stars, formed bright stars into pictures in the sky, and, in just the past 80 years, begun to understand how stars work.
It is natural that we should be so fascinated by the stars, for we are tightly linked to them. Stars—and indeed the entire universe—are made mostly of hydrogen, the simplest and lightest element. However, our bodies are composed of many more complex elements including carbon, nitrogen, calcium, and iron. These elements are created in the cores of stars, and the final act in many stars' lives is a massive explosion that distributes the elements it has created into the galaxy, where eventually they may form another star, or a planet, or life on that planet. Understanding stars, therefore, is part of understanding ourselves.
The Sun is a stable star. Its energy output is almost constant, with only tiny variations. This energy streams out into the solar system, where it is sufficient to heat the earth, an entire planet nearly 9,000,000 mi (150,000,000 km) away. How does a ball of gas with the mass of the Sun (two million trillion kilograms) remain in a stable state like this for millions or billions of years?
Stars like the Sun exist in hydrostatic equilibrium, which means that at every point within the star, there is a balance between the weight of the material overlying that point and the gas pressure at that point. Figure 1 makes this a little clearer. Suppose you are halfway between the surface and the center of a star. Gravity attracts the star's material towards its center, so the gas between you and the surface tends to push you downward (arrow #1 in Figure 1). But the gas where you are also exerts a pressure. The gas is being heated by the energy-producing reactions going on in the star's core, and the hotter gas is, the more pressure it exerts. Trying to compress the gas is like trying to squeeze a balloon. You can't just crush a balloon down to a point, because the air inside exerts a pressure on the sides of the balloon that resists your squeezing. In just the same way, the hot gas inside a star resists the weight of the overlying material, preventing it from falling inward under the influence of gravity (arrow #2 in Figure 1).
A stable star has to have this balance between gravity and gas pressure at every point in its interior. But the closer you go to the star's center, the greater is the weight of the overlying material, in the same way that when you swim closer to the bottom of a swimming pool, the pressure of the ears becomes progressively greater.
Therefore, the gas nearer the center of the star has to be hotter, to exert a greater pressure that just counteracts the weight of all the gas above it. This is illustrated in Figure 2, where the size of the arrows shows the amount of gravity and gas pressure at different points within a star. The Sun and all other stable stars exist in this condition.
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