Stars form due to the weaker force but more widespread in the universe: the
severity.
The universe is dotted with huge piles of dust and gas, called "nebulae"; in
Following their spontaneous movements sometimes they begin to rally around nuclei
heavy, inevitably, begin to exert a gravitational pull on
rest of the dust and gas surrounding, attracting just as the Earth exerts
a gravitational pull on the apple falling from the tree or on us that
we remain with our feet on the floor instead of getting lost in space. Here it is
then triggered the mechanism that can lead to the birth of a star.
The gravitational pull exerted by these gas concentrations, mainly
hydrogen (the element by far the most abundant in the universe), with the passing of
hundreds of thousands of years and then become so strong as to attract a lot of material;
so much matter means so much weight, so much compression, then an increase
soaring temperatures (a gas is compressed more it is hot, due to friction
stronger which is exercised between the molecules that compose it. It is the same mechanism
which raises the temperature of the air in conditions of Foehn, ie wind
descendant that compresses air masses falling down the mountain slopes!
While the mechanism contrary, the rarefaction of the gas-air with a relative decrease
Thermal, is that which, through different processes, leads to the condensation of the clouds
or to the phenomenon that in proportion ago on average colder than the ground!).
When the temperature of the proto-star has reached a particularly
high, here they are enough to trigger the so-called mergers thermo-nuclear:
"Thermo" because it depends on the heat reached, "nuclear" because affecting the nuclei
of hydrogen atoms that, being the most massive of the atoms, they begin to
scindersi then transformed into helium, gas slightly heavier.
I recommend reading this thread in my forum
http://www.ilforumditutti.net/t186p10-astrologia-astronomia-fisica-e-astrofisica