<span class="SFPTagline"> From SCIFIPEDIA </span>

A Star is a astronomical object usually visible as a small bright point of light in Earth's night sky. Thousands of stars and distant galaxies are visible from Earth's night sky. The Sun is also a star, the closest star to Earth, and provides the Earth with light and energy. Its proximity to Earth results in its appearance as a large ball in Earth's sky. The sun, a yellow star, is considered a small star, though not the smallest known type. Stars vary widely in size, luminosity, color, and power output. The smallest stars are neutron stars of incredible density. One of the largest known nearby stars are Rigel and Antares, which are, respectively, 78 and 776 times larger than Sol.

Though Earth's own star is the only one in our system, observations of other star systems indicate that this is less common. More common are binary systems or even more complex multiple star systems like Proxima Centauri, Sol's closest neighbor, which is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B.

Stars cluster together in galaxies of varying size and shape, separated by vast stretches of empty space. Earth's galaxy, the Milky Way, contains more than 100 billion stars and there may be more than 100 billion galaxies in the universe for a possible total of 100 billion trillion stars. Of this incomprehensible number, only approximately 3,000 stars are visible from Earth without the assistance of a telescope or binoculars.

Stars, like people, have a beginning, an end, and several stages in between. They develop from large clouds of swirling gas comprised mainly of hydrogen, helium, and smaller quantities of other elements. Under the force of gravity, the cloud collapses inwards, becoming warmer as the pressure increases. The process eventually slows and the inner portions become a proto star. The proto star becomes a true star during a process known as nuclear fusion, when the pressure and temperature at the core of the proto star becomes high enough for the joining of two atomic nuclei to produce a larger nucleus. When the two nuclei fuse, their mass becomes energy. The fusion process produces heavier and heavier elements and the temperature necessary for further processes increases.

The life cycle of a star and what kind of star is produced is largely determined by its initial mass. Single stars, like our sun, have more straightforward lives and tend to outlive more massive stars, whose hydrogen is used up more rapidly. When an intermediate mass star, like our sun, uses up its hydrogen fuel, it's core no longer produces fusion energy and begins to collapse under the force of gravity, causing hydrogen fusion in the outer regions. The significant energy production causes the star to expand to vast proportions, cooling in the process and becoming redder. It also becomes brighter, transforming into a red giant.

Contents 1 Types of Stars 2 Commonly known Stars 3 Also see 4 External Links

Types of Stars

Yellow dwarf

Red Dwarf

White Dwarf

Brown dwarf

Red giant

Blue giant

Super giant

Neutron star

Pulsar

Binary stars

Commonly known Stars

Betelgeuse

Polaris

Sirius

Also see

Star System

External Links

• Star at NASA.gov