March/April 1999

How Stars Work

by Dr. Frank Bash

Even though stars are very large and very hot, they are relatively simple. There are some complications when they form and some complications when they die. For long periods of a star's life, between its birth and death, it changes very slowly. The changes are so slow that astronomers who study stars consider a star's mass, size, temperature, and luminosity to be constant.

Stars come in a variety of sizes and colors, but they all shine because they are hot. Their color — white, red, orange, blue-white — directly measures their temperature. The temperature of a star's surface indicates how much energy each square foot radiates. The energy radiated by each square foot multiplied by the number of square feet on the star's surface gives the star's luminosity. The star's luminosity measures how much energy it radiates per second, just like the number stamped on the top of a light bulb gives the amount of energy the light bulb radiates into space per second.

Luminosity is not the same as brightness. A 100-watt light bulb has a constant luminosity but can appear less bright the farther away it is. A dim star may be dim because it is far away, small, or has a low temperature — or all three.

The surface temperature of the Sun is about 10,000 degrees Fahrenheit and its diameter is approximately 100 times the diameter of Earth. The hottest stars are about 10 times the Sun's temperature and the coolest ones are about half the Sun's temperature. The size of a star can change drastically as it forms or when it is dying, but during most of its life, a star's size ranges from about 20 times the Sun's diameter to about one-tenth the Sun's diameter.

A star must replace the energy it radiates into space. A star could expand out of control or collapse on itself if the forces of gravity and internal gas pressure are not balanced at each point in the star. The temperature of a star increases from the surface down to the core. Core temperatures, which vary depending on the type of star, are so high and the gas pressures so enormous that no solid or liquid can exist. A star's core is composed of gas only. The Sun's core temperature is more than 26 million degrees Fahrenheit, and the gas is compressed to a density about 12 times the density of lead.

Astronomers long knew the Sun's surface temperature, luminosity, and size before they had figured out its energy source. Geological evidence, among other clues, suggests that the Sun has been stable for five billion years. A piece of coal having the Sun's mass and producing its luminosity would be consumed in 300,000 years, so the Sun can”t be fueled by burning coal or by any other chemical reaction.

The discovery of nuclear reactions on Earth allowed astronomers to solve the source of the Sun's energy. Initially the discovery that the fission (splitting) of heavy atoms such as uranium and plutonium produces large amounts of energy seemed to solve the problem. Chemical analysis of starlight indicates that hydrogen, the simplest atom, is by far the most abundant chemical element in stars, including the Sun. Only small amounts of uranium and plutonium are present in stars. Clearly, fission is not powering the stars, but another type of nuclear reaction is. Enormous amounts of energy can be created through fusion, in which small atoms such as hydrogen are combined into heavier elements.

Stable stars like the Sun fuse hydrogen atoms into helium. To keep its temperature constant and replace the energy it shines into space, the Sun converts almost 700 million tons of hydrogen into helium every second. The hydrogen in the Sun's core will last about another five billion years.

A star spends most of its life "burning" hydrogen, combining four atoms of hydrogen to make one atom of helium and energy. When the hydrogen is exhausted, the "burning" stops, the star cools, the pressure drops, and the star begins to collapse. The energy of the collapse heats the core, sometimes enough to restart fusion, this time with helium combining to form carbon. The star settles back into a new stable phase, though it will be shorter than the previous one because helium is less plentiful than hydrogen.

The cycle of cooling, collapsing, and the fusion restarting with the byproduct of the previous cycle may continue until the star has converted its core to iron. The star will never be as efficient at making energy as it was when its core was mostly hydrogen. One ton of hydrogen when converted to helium produces more energy than the fusion of one ton of any other element. The fusion process stops with iron. Elements heavier than iron only produce energy through fission, and stars do not switch from fusion to fission.

Fusion reactions also explain where carbon and heavier elements come from. The carbon atoms in the ink on this paper and in your eyes with which you read these words were at some time in the past, between the creation of the universe and the birth of the Sun and Earth, forged in the heart of a star. When that star died, it polluted the neighborhood with its fusion byproducts. The Sun, Earth, and the rest of the solar system formed from the hydrogen, carbon, and other elements in that polluted gas cloud. So those stars, whose carbon polluted our neighborhood, made us possible.

Dr. Frank Bash is the director of the University of Texas McDonald Observatory.

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