Discovery Space: Stars
Stars are colossal power plants, producing energy in abundance. They are also the source of the chemical elements that make up the planets—and our own bodies.
The birth of a star
The birth of a star takes place in a gigantic cloud of gas. Stars differ in mass, color, and brightness, but share the same energy source: nuclear fusion.
The cradles of the stars are huge molecular clouds in space. Composed mainly of hydrogen, they may also contain heavier elements produced by earlier generations of stars. Thicker areas within the cloud coalesce, drawn together by their own mass. As they gradually attract more and more material, they form rotating masses. Each of these huge balls of gas is the preliminary phase of a star—a so-called protostar.
Nuclear fusion
Due to its enormous pressure, the core of a protostar becomes extremely hot. Depending on the protostar’s mass, its interior can reach up to several million degrees
Fahrenheit. At these temperatures, some of the hydrogen atoms lose their electron shells, and their unprotected nuclei may collide with each other. When this happens, they fuse to form helium nuclei, releasing large amounts of energy. The greater the mass of the protostar, the more active the nuclear reactions, until finally it shines as a new star.
The main sequence
After the nuclear fusion process has begun, the star settles into a stable form. Its interior pressure is high enough to counteract gravity, so that the forces both generated and released remain balanced.
This relatively calm period in a star’s development is known as the main sequence-the phase that our sun is currently experiencing. A star’s main sequence continues until the supply of hydrogen fuel in its core has been completely exhausted.
STAR MASSES AND ENERGY LEVELS
A star’s mass largely determines its lifespan: the larger the mass, the shorter the star’s life cycle. While a massive star starts out with a larger supply of nuclear fuel, it uses up its fuel much more rapidly, since the pressure in its core is greater. Because of this increased pressure, the core is hotter and more fusion reactions take place per second.
The amount of released energy rises dramatically, and the star shines more brightly. A star ten times as massive as the sun uses its fuel 1,000 times faster than one similar to the sun. Thus, it will shine only for some 100 million years, instead of 10 billion like our sun. Stars with eve lower masses use their energy reserves only sparingly and can shine thousands of times longer than the sun.