Massive
Stars
When enormous massive clouds of hydrogen compress slowly by the force of gravity, stars are formed. The gravitational force compressing the gas gradually heats it up, increasing the kinetic energy of the hydrogen atoms. But the repulsive charge between the protons is enough to keep the atoms apart. When temperatures rise from 10 to 100 million oK, the kinetic energy of the protons (hydrogen nuclei) becomes so great that their electrostatic repulsion can no longer keep them apart. So they crash into each other! At this moment, the two hydrogen nuclei are taken over by the nuclear force, and fuse together to form helium. Because the hydrogen protons are heavier than those in helium, during the fusing process there is an excess of mass, which is converted into energy according to E=mc2 (Einstein's famous formula). So the star releases immense quantities of energy, causing it to shine! At this phase it is a yellow star.
So it continues to burn hydrogen into helium during several billion of years. The heavier the star is, the faster it'll burn it's hydrogen. When too much waste helium is built, the star cannot keep burning the small amount of remaining hydrogen. Without the energy of the nuclear furnace, the star is compressed by the force of its weight. The increasing pressure in the star, causes temperatures to rise. When the temperature is critical, the helium nuclei slam into each other fusing to form lithium and carbon. The star has decreased greatly in size but its atmosphere expands just as greatly! The star is known to be a red giant.
Then, when the helium has been burnt up, the nuclear furnace shuts down once more. Again gravity takes over, collapsing the star! It shrinks into a white dwarf with a size no more than that of a planet.
A white dwarf can be seen as a Fermi sea, or gas of electrons obeying the
Pauli Exclusion Principle:
no two electrons can occupy the same quantum state with the same quantum numbers. Thus, there is
a net repulsive force that keeps the star from collapsing down to a point!
There is little energy it can still burn. Hence, white dwarfs are not very luminous.
If the star is not too massive, it will burn the remainings of its nuclear fuel, over billions
of years, and eventually die off as a dark dwarf star!
However, if the star is massive enough, most elements in the white dwarf will fuse into heavier
elements, until reaching
iron.
From here on there is no way of fusing the heavy elements, to extract energy from the excess
mass. The nuclear furnace shuts down for good!
So the star collapses to its weight. The pressures rise tremendously and with them the
temperatures. Temperatures rise to trillions of degrees! The iron core becomes so hot, that it
collapses and the outer layer of the star is blown off, releasing the greatest burst of energy
known! The star has become a
supernova:
an exploding star.
In the explosion, vast quantities of debris are released. It eventually joins with other gases
and dust clouds, until there is enough hydrogen to form new stars!
After this tremendous explosion, a
neutron star
is left behind, about the size of a great city. Since, neutrons also obey the
Pauli Exclusion Principle,
a neutron star can also be seen as a Fermi sea. Hence, there is a repulsive force keeping the
star from collapsing to a point.
While rotating, neutron stars emit radiation and therefore are also known as
pulsars.
If the star is extremely massive, over ten times the mass of our sun, the neutron star continues
to collapse by the force of gravity! Since, the nuclear furnace has been shut down, there is no
force to counterpart gravity. So the star squeezes itself indefinitely, ending up as a
black hole!
Its density is so great that light is forced to orbit around the black hole. So the star is
black in colour!
Read more about:
Neutron Stars
Black Holes
Nebulae images from Hubble
More:
Hubble Site
Star formation in Wikipedia
The Infrared Universe
