This is the process which transforms a cloud of interstellar material & gases
into a star. This process has been extensively studied & largely understood.
As a protostar's gas cloud collapses under the weight of its own gravity it
will condense into a large mass at the center of a large accretion disk.
The whole system spins faster & faster to conserve angular momentum.
The accretion disk separates, in ways not fully understood, into a number of
concentric rings, which eventually accrete into proto-planets.
The rapidly growing evidence of extra solar planetary systems seems to indicate
that planetary systems are quite common.
As the central proto-star concentrates its pressure
and temperature increase rapidly until the collisions between individual
atoms, especially those of hydrogen are sufficiently violent to cause
nuclear reactions to occur. Once this starts the star has become a fusion
nuclear reactor & it can produce astounding amounts of energy for a long time.
Stars initially burn hydrogen. While that fuel lasts the heat produced creates
enough of a back pressure stopping the star's gravitational collapse.
This nuclear combustion transforms atoms of hydrogen into atoms of
helium and other light elements. Once the hydrogen is used up the gravitational
collapse resumes leading to yet higher pressure & temperature.
Eventually
the condition for helium burning are reached and a new series of nuclear
reactions starts. This, in turn, creates a back pressure which halts the
gravitational collapse. This cycle is repeated a number of times while
producing ever heavier elements. As we saw in the cosmological process
the original Big Bang produced basically only hydrogen (80%) and
helium (20%) all other elements are essentially produced in the nuclear
furnaces we call stars.
These cycles are repeated until most of the star
material has been transmuted into iron or elements close to it. In fact
iron atoms have the highest binding energy per atomic weight in the whole
table of elements. I.e. iron is the most stable element. Atoms lighter than
iron have less binding energy because the fewer neutron & protons cause fewer
bonds in the nucleus. Atoms above iron tend to have increasing electrical
repulsion because of the large number of protons. The fact that iron is
the most stable element means that once the star has converted nearly
all its fuel to iron or its neighbors it ceases to burn as a nuclear
reactor. I.e. the star dies.
We now have a very elegant and complete theory of star life & death.
That allows us to predict how long a given star will live & what kind of
death it will experience. The key factor is the mass (weight) of the star.
If we indicate with the number one the mass of our Sun. Stars with core
mass, after the helium flash, (Google: Helium flash) of 1.4 solar masses
(Chandrasekhar limit)
or less will simply collapse into a hunk of very heavy rock which
will continue to radiate for a while (white dwarf) and eventually go cold
& stop all radiation (black or brown dwarf).
Stars with core mass greater
than 1.4 solar masses will collapse to a super dense state of neutrons
(neutron star).
Stars with core mass greater than 2.0 solar masses will overcome even the
nuclear
force which keep neutrons & protons from coalescing. They become so dense that
they punch a hole in the structure of space-time & disappear from our universe
altogether. These holes are called "Black Holes" since anything that happens
to fall in them never comes back. This applies to light as well.
There is some speculation that such black holes could be expanding universes
in other dimensions, but of course we have no proof of such eventuality.
One can further speculate that our own universe may be a black hole in
some other universe.
Hard evidence is accumulating that black holes exist at the center of all
or at least most galaxies. Black holes are a cutting edge research subject.
The theory of star evolution predicts specific life expectancies for each
different type or class of star. The key factors to determine the type of
a star are first & foremost its mass a secondary factor is its composition
which in turn depends on whether the star is a first generation (built up
from the materials left over from the Big Bang) or later generations.
The star class names can be remembered by the phrase:"Oh Be A Fine Girl and Kiss
Me". I.e. O, B, A, F, G, K, M. O are the largest stars M the smallest. Os burn
fast and furious & emit a blue light. O stars last only a few tens of
million years. A planet around such a star would have NO chance of evolving
life since there would not be enough time. Our Sun is a G star and is predicted
to have a lifetime of about 10 billion years, it is now about 4.5 billion years
old. Smaller stars have even longer lives. The life span
of a star varies dramatically with its mass.
These predictions of star lifespans agree very well with the age of Earth & the
Moon as measured from the radioactive decay in Earth & Moon rocks. In fact
the eldest Earth rocks are 3.8 billion years old & the Moon's have an age
of 4.5 billion years.
We can count on a long stretch of almost 5 billion year with the Sun being
stable and maintaining the ideal conditions we have here on Earth. At the
end of that span the Sun will swell up to a size that will engulf and
vaporize Mercury, Venus, Earth/Moon & possibly Mars as a red giant. The red
giant will eventually collapse into a white dwarf.
To put this in perspective the whole of human evolution has taken about
5 million years. We could wipe ourselves out & there would still be time
for a thousand new experiments to evolve on Earth an intelligent & hopefully
wise race of beings from the remaining stock of higher animals.
Nuclear Evolution