Stars are formed in nebulae, clouds of dust and gas found in spiral galaxies. Dense parts of these clouds undergo gravitational collapse, compressing to form a rotating gas globule. As the globule collapses over thousands to millions of years due to gravity and pressure, the increasing temperature and rotation cause it to form a central core and surrounding protoplanetary disk. Once the core reaches temperatures over 27 million degrees and nuclear fusion begins, it becomes a stable main sequence star.
4. Stars are formed in
nebulae, interstellar
clouds of dust and
gas (mostly
hydrogen). These
stellar nurseries are
abundant in the
arms of spiral
galaxies.
6. In these stellar
nurseries, dense parts
of these clouds
undergo gravitational
collapse and compress
to form a rotating gas
globule.
8. The globule is cooled by emitting radio
waves and infrared radiation. It is
compressed by gravitational forces and
also by shock waves of pressure from
supernova or the hot gas released from
nearby bright stars. These forces cause
the roughly-spherical globule to collapse
and rotate. The process of collapse takes
from between 10,000 to 1,000,000
years.
10. A Central Core and a Protoplanetary Disk:
As the collapse proceeds, the temperature
and pressure within the globule increases, as the
atoms are in closer proximity. Also, the globule
rotates faster and faster. This spinning action
causes an increase in centrifugal forces (a radial
force on spinning objects) that causes the globule
to have a central core and a surrounding flattened
disk of dust (called a protoplanetary disk or
accretion disk). The central core becomes the star;
the protoplanetary disk may eventually coalesce
into orbiting planets, asteroids, etc.
12. Protostar:
The contracting cloud heats up due to friction
and forms a glowing protostar; this stage lasts
for roughly 50 million years. If there is enough
material in the protostar, the gravitational
collapse and the heating continue.
If there is not enough material in the protostar,
one possible outcome is a brown dwarf (a large,
not-very-luminous celestial body having a mass
between 1028 kg and 84 x 1028 kg).
14. A Newborn Star:
When a temperature of about
27,000,000°F is reached, nuclear fusion
begins. This is the nuclear reaction in
which hydrogen atoms are converted to
helium atoms plus energy. This energy
(radiation) production prevents further
contraction of the star.
15. Young stars emit jets of intense radiation that
heat the surrounding matter to the point at
which it glows brightly. These narrowly-focused
jets can be trillions of miles long and can travel
at 500,000 miles per hour. These jets may be
focused by the star's magnetic field.
The protostar is now a stable main sequence
star which will remain in this state for about 10
billion years. After that, the hydrogen fuel is
depleted and the star begins to die.
16. Life span:
The most massive stars have
the shortest lives. Stars that are 25
to 50 times that of the Sun live for
only a few million years. Stars like
our Sun live for about 10 billion
years. Stars less massive than the
Sun have even longer life spans.