The White Dwarf

Wanna Know one of the heaviest element around the space?

The White Dwarf

Birth

All large mass stars collapse either to black hole or take a path through neutron star to black hole. But the medium & small stars have a different phenomenon. Suppose our sun(medium sized star in comparison to other stars) has hydrogen within it’s layers i.e the core. 

Sun's Internal

As the temperature is extremely hot in the core so hydrogen fuses to helium which further increases the temperature and pressure builds up which opposes the gravitational force that is pushing the star from the outer layer to collapse it. These two forces (the pressure(force) from inside and the outer gravitational force) balance each other and star maintains it’s size. Now when the Sun uses up all the hydrogen by burning it to helium , the temperature decreases which results in decreasing the pressure which pushes the star outward , and gravitational force takes the edge and star starts collapsing. When the star gets a little contracted the temperature in it rises and little remaining hydrogen burns gain to helium and pressure is created which opposes the gravitational force and pushes the outer layer outward and Sun gets more bigger(big enough to swallow mercury) and this object is called “Red Giant).

Now as the Sun expanded , the energy was released which decreased the temperature , but the core was still hot enough to burn all the helium to carbon and other heavy elements.

Now when all this helium burns to carbon (1 billion years process) , the core isn’t hot enough to burn the carbon , so once again the pressure opposing the gravitational force decreases and gravity takes over and contracts the star. As the sun contracts energy gets released which makes the an envelope around the star(expanded) , and now the ‘Red Giant’ has grown even larger than before with it’s radius greater than earth’s orbit. As the size will be humongous , it’s mass will be huge which will be unstable for the star to handle , and the star will start to lose mass until it blows off it’s outer layers and the thing remaining behind will be the core of the star which is called “The White Dwarf”. This white will be surrounded by shell of gases(expanded) known as ‘planetary nebula’. Some of these planetary nebulas can be seen through backyard telescope.

 

Planetary Nebula 

All these white dwarfs are extremely hot(up to 100,000 Kelvins) so it takes billion years to cool down , and loses it’s luminosity afterwards and turns to black dwarfs

 White Dwarf’s properties:

·         Some of white dwarfs emit dwarf emit low energy , soft X-ray and even strong UV rays that have been observed.

·         A white dwarf’s mass is about half of the mass of the sun and it’s size is comparable to the size of earth and it’s average density is about ‘1x10^9 kg/m^3’.

·         First whte dwarf was discovered around 1844. Actually it is companion star to ‘Sirius’ in constellation Canis Major. A scientist in 1844(Friedrich Bessel) noticed that Sirius has back and fourth motion and it was orbiting a slight black object. In 1863 Alvan Clark noticed that this companion(around which Sirius was orbiting) was actually a white dwarf and this white dwarf was named as Sirius B. 

Sirius B(The white dwarf)

·            The Hubble Space Telescope with 2.4 meter detected 75+ dwarfs in global cluster M4 in constellation Scorpius in August , 1995 . As M4 is located 7000 light years away from earth and is 14 million years old. Though telescopes aren’t the only source to detect white dwarfs ,as a HZ43(white dwarf) was detected by X-ray satellite(ROSAT) too.

Inside of a white dwarf

As the white dwarf isn’t able to create internal pressure(because all the fusion ceased) so gravity acts on it to contract it until all the electrons of atoms smash together.(According to Pauli’s exclusion principle no two electrons with same spin can occupy same single energy level , so when the gravity pushes the dwarf inward all the energy levels get filled(the density is much higher and electrons are much closer(degenerate matter*))).

So once the star is degenerate( all the energy levels get filled) gravity can’t compress the dwarf( as it is pushing the electrons where they can’t go) so the white dwarf survives by quantum mechanical rules.

This degenerate matter is extremely strange because the more mass it will have the smaller the dwarf will be. This is because the more mass the dwarf will possess the more electrons will be jammed up and will apply pressure outward to prevent star from collapsing from the force of gravity. But there is a limit to the increase of this mass which is known as ‘Chandrasekar’s Limit’ . When the Chandrasekar’s limit(1.4 of the solar mass(solar mass: 2.765 x 10^30 kg)) is crossed than the white dwarf’s electron degenerate pressure isn’t enough to handle the mass and gravity acts on it and dwarf get’s converted to black hole or neutron star.