The Life Cycles of Stars
A star's life cycle is determined by its mass. The larger the mass, the shorter
the life cycle. A star's mass is determined by the amount of matter that is
available in its nebula, the giant cloud of gas and dust in which it is born. Over time, gravity pulls the hydrogen gas in the nebula together and it
begins to spin. As the gas spins faster, it heats up and is known as a
protostar. Eventually the temperature reaches 15,000,000 °C
and nuclear fusion occurs in the cloud's core. The cloud begins to
glow brightly. At this temperature, it contracts a little and becomes
stable. It is now called a main sequence star and will remain in this
stage, shining for millions or billions of years to come.
As the main sequence star glows, hydrogen in the core is converted into
helium by nuclear fusion. When the hydrogen supply in the core begins
to run out, the core becomes unstable and contracts. The outer shell of
the star, which is still mostly hydrogen, starts to expand. As it
expands, it cools and glows red. The star has now reached the red giant
phase. It is red because it is cooler than it was in the main sequence
star stage and it is a giant because the outer shell has expanded
outward. All stars evolve the same way up to the red giant phase. The
amount of mass a star has determines which of the following life cycle
paths it will take after the red giant phase.
Throughout the red giant phase, the hydrogen gas in the outer shell
continues to burn and the temperature in the core continues to increase.
At 200,000,000 °C the helium atoms in the core fuse to form
carbon atoms. The last of the hydrogen gas in the outer shell is blown
away to form a ring around the core. This ring is called a planetary
nebula. When the last of the helium atoms in the core are fused into
carbon atoms, the medium size star begins to die. Gravity causes the
last of the star's matter to collapse inward and compact. This is the
white dwarf stage. At this stage, the star's matter is extremely dense.
White dwarfs shine with a white hot light. Once all of their energy is
gone, they no longer emit light. The star has now reached the black
dwarf phase in which it will forever remain.
Once massive stars reach the red giant phase, the core temperature
increases as carbon atoms are formed from the fusion of helium atoms.
Gravity continues to pull carbon atoms together as the temperature
increases forming oxygen, nitrogen, and eventually iron. At this point,
fusion stops and the iron atoms start to absorb energy. This energy is
eventually released in a powerful explosion called a supernova. A
supernova can light up the sky for weeks. The temperature in a
supernova can reach 1,000,000,000 °C. The core of a massive star
that is 1.5 to 4 times as massive as our Sun ends up as a neutron star
after the supernova. Neutron stars spin rapidly giving off radio waves.
If the radio waves are emitted in pulses (due to the star's spin), these
neutron stars are called pulsars. The core of a massive star that has 8
or more times the mass of our Sun remains massive after the supernova.
No nuclear fusion is taking place to support the core, so it is
swallowed by its own gravity. It has now become a black hole which
readily attracts any matter and energy that comes near it. Black holes
are not visible. They are detected by the X-rays which are given off as
matter falls into the hole.
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