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THE LIFE CYCLE OF STARS
Stars are born in nebulae. Huge
clouds of dust and gas collapse under
gravitational forces, forming protostars.
These young stars undergo further
collapse, forming main sequence stars.
Stars expand as they grow old. As
the core runs out of hydrogen and then
helium, the core contacts and the outer
layers expand, cool, and become less
bright. This is a red giant or a red super
giant (depending on the initial mass of
the star). It will eventually collapse and
explode. Its fate is determined by the
original mass of the star; it will become
a black dwarf, neutron star, or black
hole.
THE BIRTH OF STARS
Accretion Disk: Stars are formed in nebulae, interstellar clouds of dust and gas (mostly hydrogen). These
stellar nurseries are abundant in the arms of spiral galaxies.
In these stellar nurseries, dense parts of these clouds undergo gravitational collapse and compress to form a
rotating gas globule.
The globule is cooled
by emitting radio waves
and infrared radiation. It is
compressed by
gravitational forces and
The Eagle
also by shock waves of
nebula, a stellar
pressure from supernova
nursery
or the hot gas released
illuminated by
from nearby bright stars.
ultraviolet light
These forces cause the
which is
roughly-spherical globule
emitted from
to collapse and rotate. The
the newborn
process of collapse takes
stars.
from between 10,000 to
1,000,000 years.
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.
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-veryluminous celestial body having a mass between 1028 kg and 84 x 1028 kg).
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.
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.
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.
THE DEATH OF SUN-LIKE STARS
(with a mass up to 1 1/2 times that of the Sun)
A stars expands as it grows old. As the core runs out of hydrogen and then helium, the core contacts and the
outer layers expand, cool, and become less bright; this is a red giant.
After expanding and reaching the enormous red giant phase, the outer layers of the star continue to expand.
As this happens, the core contracts; the
helium atoms in the core fuse together,
forming carbon atoms and releasing
energy. The core is now stable since
the carbon atoms are not further
compressible.
Now the outer layers of the star
start to drift off into space, forming a
planetary nebula (a planetary nebula
has nothing to do with planets).
The star loses most of its mass to
the nebula. The star cools and shrinks;
it will eventually be only a few
thousand miles in diameter!
The star is now a white dwarf, a stable star with no nuclear fuel. It radiates its left-over heat for billions of
years. When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star (perhaps replete
with diamonds, highly compressed carbon).
NOVA
A nova is a white dwarf star that suddenly increases in brightness by several magnitudes. It fades very slowly.
A White dwarf
star: (circled) The Egg nebula: a
in the globular planetary nebula formed a
cluster M4.
few hundred years ago.
THE DEATH OF HUGE STARS
(from 1.5 to 3 times the mass of the Sun)
Betelgeuse, a
red supergiant
in Orion.
Supernova
SN1987A: the
beginning of a
supernova.
When huge stars grow old, they become even more enormous red supergiants (as their core fuses all the
hydrogen into helium). Their core shrinks, becoming hotter and denser. With these changes, different nuclear
processes occur; fusion now produces heavier elements (this temporarily stop the core's shrinking).
Eventually this core collapses (in an instant). As the iron atoms are crushed together
in this gravitational collapse, the core temperature rises to about 100 billion degrees.
The repulsive electrical forces between the atoms' nuclei overcomes the gravitational
forces, causing a massive, bright, short-lived explosion called a supernova. During
the explosion, shock waves, blow away the star's outer layers.
Supernova N132D: 3,000
years after a supernova,
ejecting stellar material
If the star's remaining mass is between 1 1/2 to 3 times the mass of the Sun, it (including oxygen-rich
will collapse into a small, dense neutron star (about ten miles in diameter,
gas) in luminescent
about 1.4 times the mass of the Sun, with an extraordinarily strong magnetic shock fronts. It is located
field, and rapid spin).
in the Large Magellanic
Cloud (169,000 lightIf the star's remaining mass is greater than three times the mass of the Sun, the years from Earth).
star contracts tremendously and becomes a black hole (incredibly dense with
a gravitational field so strong that even light cannot escape).
The next stage depends on the star's remaining mass:


EVOLVED STAR
An evolved star is an old star that is near the end of its existence. Its nuclear fuel is
mostly gone. The star loses mass from its surface, producing a stellar wind (gas that is
ejected from the surface of a star).
The Crab nebula: the
remnant of a
supernova in A.D.
1054. It has a rapidly
spinning neutron star.
THE DEATH OF GIANT STARS
When huge stars grow old, they become even more enormous red supergiants (as their core fuses all the
hydrogen into helium). Their core shrinks, becoming hotter and denser. With these changes, different nuclear
processes occur; fusion now produces heavier elements (this temporarily stop the core's shrinking).
Eventually this core collapses (in an instant). As the iron atoms are crushed together in this gravitational
collapse, the core temperature rises to about 100 billion degrees.
The repulsive electrical forces between the atoms' nuclei overcomes the gravitational forces, causing a massive,
bright, short-lived explosion called a supernova. During the explosion, shock waves, blow away the star's outer
layers.
The next stage depends on the star's remaining mass:


If the star's remaining mass is between 1 1/2 to 3 times the mass of the Sun, it will collapse into a small,
dense neutron star (about ten miles in diameter, about 1.4 times the mass of the Sun, with an
extraordinarily strong magnetic field, and rapid spin).
If the star's remaining mass is greater than three times the mass of the Sun, the star contracts
tremendously and becomes a black hole (incredibly dense with a gravitational field so strong that even
light cannot escape).
EVOLVED STAR
An evolved star is an old star that is near the end of its existence. Its nuclear fuel is mostly gone. The star loses
mass from its surface, producing a stellar wind (gas that is ejected from the surface of a star).