White Dwarfs.

White Dwarfs. Corpses of Dead Stars.

The length of the Star's life changes drastically depending on their total mass. Really massive stars burn hot and fast, dying violently in supernova explosion a few years after birth. About 97% of all stars will end their existence as a white dwarfs.

Source:NASA, ESA and G. Bacon (STScI), ESA/Hubble

White Dwarf is a small astronomical object consisting of degenerate matter, emitting, among others, visible radiation. It arises after the nuclear reactions in a star of low or medium mass. Few massive stars do not achieve enough conditions to ignite helium in fusion reactions during their evolution and white helium dwarfs are formed from them. Medium massive stars burn helium to give white carbon or oxygen-carbon dwarfs. The remains of stars with masses in the range of 4-8 solar masses are white dwarfs with an admixture of oxygen, neon and magnesium.

Small stars (called red dwarfs) burn out over trillions of years until they eventually quietly turn into White Dwarfs. Medium-sized stars like our Sun are more interesting. This image of sun can be presented as a huge pressure cooker that fuses hydrogen into helium in its core through its gravity. The fusion of elements releases extreme amount of energy, that pushes outwards and stabilizes the star, keeping it in a delicate balance. When the Sun is old, the hydrogen in the core is exhausted, and the Sun begins to burn helium into heavier elements.

When the process is over more than half of the Sun's mass is lost into space as a spectacular planetary nebula millions of kilometers across. What remains is it former core, a White Dwarf (Star Corpse).

While its former self was about 100 times its diameter, after implodion it's only about as big as Earth, but still with about half of its former mass. This means it's extremaly dense and it's surface gravity is over 100,000 times higher than Earth's, and if we tried to land on it we would be immediately be compressed. So life around White Dwart is vely unlikely.

The first known white dwarf was the 40 Eridani B star, discovered in 1783 by William Herschel. In 1862 Alvan Graham Clark discovered the very weakly glowing comrade Sirius (alpha Canis Majoris).

It was not until the twentieth century that it was found Sirius' companion, known as Sirius B, has a surface temperature of around 25,000 K. Its brightness is much smaller than Sirius A, hence its surface must be much smaller.

Soon other white dwarfs were discovered and realized that they often appear in our Galaxy.

White Dwarfs shine so much longer than other star types. It results from this that they are very, very hot, up to 40 times hotter than our Sun, ranking among the hottest objects in the universe, but they are not incredibly active. All the heat insie of them is trapped and has nowhere to go. Only on its outer layer can it escape into space. Space is mostly empty, so heat can't be transferred by conduction, and the only way energy can escape is by radiation. This is so inefficient that white dwarfs will take trillion of years to cool down. They might be the last source of light and energy in a changing universe.

There are three types of white dwarfs:

• White helium dwarf (0.08 - 0.4 Sun's mass at earlier stages of evolution) - during its evolution it was unable to create conditions necessary for helium combustion in thermonuclear reactions.
• White carbon-oxygen or carbon dwarf (0.4-4 Sun's mass) - burned helium at earlier stages of evolution.
• White oxygen-neon-magnesium dwarf (4-8 Sun's mass) - burned helium and coal at earlier stages of evolution.

According to some estimates white dwarfs might shine as long as 100 billion billion years, so long that probably no regular stars will shine anymore.

References:

[1]http://www.ptma.szczecin.pl/prelekcje/2014_04_03_Ewolucja_gwiazd_Tadeusz_Smela.pdf
[2]http://users.camk.edu.pl/bcz/wyklad/wyklad11/wyklad11.pdf
[3]http://www.astrouw.edu.pl/~kiraga/Dydaktyka/Astr_t_1/w12.pdf
[4]http://astronomia.shoutwiki.com/wiki/Bia%C5%82y_karze%C5%82