Stellar Evolution

Nebula

Nebulas are clouds of interstellar gas and dust. The gravitational attraction that material in a nebula has for itself can eventually make the nebula coalesce into a denser object called a protostar.

Protostar

A protostar forms when a dense region occurs in a cloud of interstellar material. Surrounding material is gravitationally attracted to this dense spot and condenses into a more compact form. As the material becomes more dense, the pressure and temperature at the center of the protostar increase.

Sun

A protostar becomes a star when its inner temperature reaches 10 million degrees C (18 million degrees F). This high temperature triggers nuclear reactions in the core of the protostar. Hydrogen atoms fuse together to form helium, releasing a tremendous amount of energy in the process. This energy fuels the new star and radiates outward in the form of light and heat.

Red Giant

A star evolves into a red giant when its hydrogen fuel begins to run out. The core of the star shrinks as it nears the end of its hydrogen, but the outer layers of the star expand as the star begins to burn hydrogen in the layer surrounding its core. A red giant can grow to be 100 to 1,000 times bigger in diameter than the Sun, but its surface is relatively cool, making it glow with a red color instead of blue or white.

Supergiant

Very massive stars go through a second expansion after exhausting their hydrogen fuel and become supergiants. Supergiants form when helium atoms in the core of massive red giants fuse to form carbon. These nuclear reactions release a second wave of energy and cause the red giant to expand into a supergiant.

Planetary Nebula

A planetary nebula forms when a red giant that is six to eight times more massive than the Sun runs out of fuel. The star becomes unstable and collapses under its own weight, ejecting its outer layers into space. This halo of ejected gas and dust forms the planetary nebula.

Supernova

Supernovas occur when very massive stars, more than eight times the mass of the Sun, use up their fuel and collapse under their own weight. The collapse leads to an explosion that sends a shock wave through space, which is followed by a shell of material from the star's atmosphere. The supernova may completely destroy the star, or it may leave the core of the star intact.

White Dwarf

A white dwarf forms when a red giant uses up its fuel and collapses. Such a star is extremely compact and continues to glow because of energy trapped in its core. White dwarfs can be as small as Earth yet contain as much as 70 percent of the mass of the Sun.

Black Dwarf

A white dwarf eventually releases all of its trapped energy and cools off to become a black dwarf. Black dwarfs are much more massive than planets, but they are too cold to give off light.

Neutron Star

A neutron star forms from the core of star that has undergone a supernova. The atoms in the star condense and collapse until two of the particles that compose atoms, protons and electrons, fuse to form neutrons. The star is no longer composed of complete atoms, only of neutrons.

Black Hole

When an extremely massive star supernovas, the gravitational pull within the remaining core may be strong enough to cause even neutrons to collapse. The core condenses into a black hole, an object so dense that it attracts everything around it, including light. The volume of the black hole may be smaller than that of a typical city on Earth, but its mass is more than ten times that of the Sun.

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