Stellar Evolution (Star Life-Cycle)

Basic Structure Mass governs a star’s temperature, luminosity, and diameter. In fact, astronomers have discovered that the mass and the composition (makeup) of a star determine nearly all its other properties.

The balance between gravity squeezing inward and pressure from nuclear fusion and radiation pushing outward, called hydrostatic equilibrium, must hold for a star to be stable. Otherwise, the star would expand or contract.

Fusion Fusion reactions involving elements other than hydrogen can occur. Once a star’s core has been converted into helium, the helium may fuse to form carbon if the temperature is high enough. At even higher temperatures, carbon can react with helium to form oxygen, then neon, then magnesium, then silicon, and finally, iron.

Life Cycle

1. All stars form in much the same manner as the Sun did. The formation of a star begins with a cloud of interstellar gas and dust called a nebula.

2. The nebula collapses on itself as a result of its own gravity. As the cloud contracts, its rotation forces it into a disk shape with a hot condensed object at the center called a protostar.

Eventually, the temperature inside a protostar becomes hot enough for nuclear fusion reactions to begin. 3. Once this reaction begins, the star becomes a main sequence star (stable) because it then has sufficient internal heat to produce the pressure needed to balance gravity.

What happens during a star’s life cycle depends on its mass. For example, as a star like the Sun converts hydrogen into helium in its core, it gradually becomes more luminous because both the core density and temperature rise slowly and increase the reaction rate. It takes about 10 billion years for a star with the mass of the Sun to convert all of the hydrogen in its core into helium.

After millions to billions of years (depending on their initial masses), stars run out of their main fuel - hydrogen. Once the supply of hydrogen in the core is gone, nuclear processes occurring there cease. Without the outward pressure generated from these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward. Just as during formation, when the material contracts, the temperature and pressure increase. This newly generated heat temporarily counteracts the force of gravity, and the outer layers of the star are now pushed outward. The star expands to larger than it ever was during its lifetime - a few to about a hundred times bigger.

4. The star has become a red giant. Afterwards, when the helium in the core is all used up, the star is left with a core made of carbon.

Sun Sized Stars A star of the Sun’s mass never becomes hot enough for carbon to react, so the star’s energy production ends at this point. The outer layers expand once again and are driven off entirely. This shell of gas is called a planetary nebula. It has nothing to do with planets, despite its name.

In the center of a planetary nebula, the core of the star becomes exposed as a small, hot object about the size of Earth. The star is then a white dwarf made of carbon.

After a white dwarf cools enough to no longer emit light or heat, it will become a black dwarf. Scientists, however, have discovered that the time required for a white dwarf to cool enough to become a black dwarf is most likely longer than the age of the Universe. Therefore, scientists believe that no black dwarfs exist, yet.

Massive Stars After the red giant phase, massive stars contract again allowing the core to become hot enough to fuse elements into iron. When this occurs the star’s mass is so immense, it can no longer support it causing a sudden, violent, collapse. At this moment, the entire outer portion of the star is blown off in a massive explosion called a supernova. This explosion creates elements that are heavier than iron and enriches the universe.

Some massive stars will then have what is called a neutron star remaining. A neutron star has a mass of times the Sun’s mass, but a radius of only 10km!

Pulsars Most neutron stars will form pulsars which are neutron stars that rotate rapidly and emit a beam of electromagnetic radiation.

A pulsars magnetic poles (where the beam of radiation is emitted) and it’s rotational axis are not exactly the same. This results in a “pulsing” look to the star from our perspective here on Earth. v=VgGXaIU0lTU&list=TLo9gX g6qecwQ

Some stars are too massive even to form neutron stars. The mass of the star is so great that the core of the star simply continues to collapse, compacting matter into a smaller and smaller volume. The small, but extremely dense, object that remains is called a black hole because its gravity is so immense that nothing, not even light, can escape it.