White dwarfs are stars residues

What is a star?

A star is a object similar to our sun, which shines through nuclear reactions that occur in the center.
With the exception of the Sun, the stars appear to the naked eye as a bright, glittering due to atmospheric turbulence, without immediate apparent motion relative to other fixed objects in the sky. All the stars are considerably more distant from Earth than the Sun.
The nearest star, Proxima Centauri, is located about 4 light years of the solar system, nearly 250 000 times farther than the Sun. The mass of a star is the order of 10³⁰ kg and its radius of about a few million miles.
The power radiated by a star like the Sun is about 10²⁶ watts. Stars form due to the contraction of a nebula of gas and dust under the influence of gravity. If the heating of the material is sufficient, it will trigger the cycle of nuclear reactions in the heart of the nebula to form a star. The energy released by these reactions is then sufficient to stop its contraction due to the radiation pressure generated.

The number of stars in the universe is estimated between 10²² and 10²³. Apart from the Sun, the stars are too faint to be observable in daylight.

Image: Birth of a star image made from data of X-ray telescope Chandra (blue) and data from the Spitzer infrared telescope (red and orange). At about 4000 light years from Earth lies RCW 108, a region of the Milky Way, where star formation is active where the presence of clusters of young blue stars in the picture. That we see born, yellow in the center of the image is deeply rooted in a cloud of molecular hydrogen.

White dwarfs

 * White dwarfs are stars off residues. This is the penultimate stage of evolution of stars whose mass is between 0.3 and 1.4 times that of the Sun.
The density of a white dwarf is very high: a white dwarf of one solar mass has a radius of about that of Earth.
The high density of matter that quantum effects are becoming predominant and it is said that matter is in a state of degeneration.
The diameter of the white dwarf no longer depends on its temperature, but mainly depends on its mass: the greater the mass, the greater its diameter is small. However, there is a value above which a white dwarf can not exist, it is the Chandrasekhar limit.
Beyond this mass, the pressure due to electrons is insufficient to compensate the gravity and the star continues its contraction to become a neutron star.

Image: The planetary nebula NGC designated 2440, contains one of the hottest white dwarf stars known. The white dwarf can be seen as a bright point near the center of the photo. Our Sun will eventually become a white dwarf, but in 5 billion years. The false-color image above cons was post-processed by Forrest Hamilton. NGC 2440: Cocoon of a New White Dwarf Credit: H. Bond (STScI), R. Ciardullo (PSU), WFPC2, HST, NASA