The Birth and Death of Stars

If the cloud contracts, gas pressure, temperature, centrifugal force, and angular momentum - a measure for the speed of rotation - rise. This is how ions (atoms which have lost or gained one or more electrons) and magnetic fields can develop. Little by little, a cloud weighing up to 1000 times the mass of our sun disintegrates into smaller amounts of gas condensing in large portions and attaining high densities inside the cloud's active centers. The contraction accelerates and creates a protostar enclosed by gas and dust - a preliminary stage to a sun. Because of the surrounding envelope raining down onto the protostar, the young star shines in the range of infra-red wavelength. These rays can be determined by very sensitive instruments, mostly by satellite observatories in orbit around earth, but they are not visible to the human eye.

Gliese 623b
Gliese 632b

When the heat that is created by the gravitational pressure inside of the star rises and the dull envelope of gas and dust has disappeared, the protostar finally shines in the visible spectrum. If pressure and temperature in the core are high enough (several million Kelvin), nuclear fusion starts: hydrogen becomes helium. One condition for this process is that the protostar must have a mass of at least one tenth that of the sun. Otherwise, instead of a young star, there will only exist the infra-red radiation caused by its contraction. These small stars which send out infra-red light are known as "brown dwarfs".

A Brown Dwarf

Gravitation vs. Inner Pressure

Brown Dwarfs, because of their small mass, were prevented from becoming suns. They are, a cross between a sun and a planet. The pressure created by radiation and temperature represents a counter-force to gravitation in the young sun. It keeps the size of the star constant for up to several billion years. Size and lifespan of a star depend on the star's mass: the heavier, the bigger, the more quickly burning, the more short-lived. Vast stars starve fast.

After nuclear fusion has set in for the first time, it will not stop for a long time - for a period of millions and perhaps even billions of years. When fusing nuclei, mass is transformed to energy according to Einstein's popular equation, E=mc². The bigger a sun is, the more mass it has, and the more energy can be produced in a given period of time. By this time, helium accumulates in the center of the sun. If the temperatures there are high enough, helium is fused to heavier elements, too. Starting with hydrogen, carbon, nitrogen, oxygen, up to iron can be fused in the core of a star. Most of the known elements have originated from within the stars. And most of the atoms our environment consists of are remnants of stellar explosions - we are actually "children of the stars".

If we take a closer look at the processes that take place in a sun's inner core, we are able to determine that several envelopes develop around the sun's core. These envelopes consist of different types of atoms. The exisiting helium core inside will not impede the fusion of hydrogen to helium, the p-p-process.

The p-p-process

The 3-alpha process

Similar to an onion chopped in half and viewed as a cross-section, an envelope of hydrogen wherein fusion takes place continues to develop around the core. When this hydrogen-burning envelope migrates outward, the helium core shrinks, and temperature and size of the star will change. It blows up and cools down, and eventually becomes a red giant. The helium core becomes larger and larger, making it heavier. Sometimes an explosion will happen: a helium flash, whose ultimate result is a re-contraction of the star.

Both of these states, blowing-up and contracting, can occur several times until the star swells to a red supergiant and an essential part of the helium is transferred to carbon by what is known as the 3alpha-process. This is the final state for stars that containing less than 1/2 times the mass of our sun.

An expanding and contracting star