Stars

Stars are born in the interstellar medium by the gravitational collapse of gas and dust within interstellar molecular clouds which have mass many times greater than the mass of a single star. Starbirth results from Gravitational Instability, followed by

Gravitational Collapse.

Most stars in the sky that appear to be single are actually made up of two or more objects revolving around each other. Double stars are called "double" even when three or more stars are present. In some double-star systems, the stars periodically block each other as they orbit, making the total brightness we see vary. These particular binary star systems are examples of eclipsing binaries. Some stars vary in brightness all by themselves. Sometimes a star actually periodically changes in size and therefore brightness. These stars are called variable stars.

A few stars fall in the lower left portion of the H-R diagram, below the main sequence.

An artist's impression of Sirius A and Sirius B. S...

English: The oldest star of our Galaxy: HE 1523-09...

This diagram shows the predicted main sequence lif...

Just as giant stars are larger and brighter than main-sequences stars, these stars are smaller and dimmer. These smaller, dimmer stars are hot enough to be white or blue-white in color and are known as white dwarfs. White dwarf stars are only about the size of Earth. They represent stars with about the mass of the Sun that have burned as much hydrogen as they can. The gravitational force of a white dwarf's mass is pulling the star inward, but electrons in the star resist being pushed together. The gravitational force is able to pull the star into a much denser form than it was in when the star was burning hydrogen. The final stage of life for all stars like the Sun is the white dwarf stage.

Stars convert hydrogen to helium to produce light. As time progresses, the heavier helium sinks to the center of the star, with...