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The Life Cycle of a Star
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nebula
All stars begin
as a big cloud
of dust and
gas (mostly
hydrogen)
called a nebula.
Gravity pulls the
particles in the
nebula closer
together and the
cloud gets hotter
and denser.
protostar
The increase in
pressure means
that particles
collide more
often. When the
particles brush
past each other
they cause friction.
This increases the
thermal energy
store of the
protostar, causing
it to emit light.
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S
ur
red giant
mass
ive th
an ou
r Sun
main sequence star
Eventually, hydrogen nuclei gain
enough energy that they start to
fuse when they collide. This is called
nuclear fusion. The star is now
known as a main sequence star.
The heat from the centre of the star
causes it to expand. The expansion
is equal to the gravitational pull
towards the centre and therefore,
this part of the lifecycle is the most
stable. The larger the star, the hotter
and brighter it will be and the faster
it will move through this stage.
red supergiant
Eventually, the larger nuclei start to fuse to make
even heavier elements. All naturally occurring
elements are produced in this way.
This causes an increase in the thermal energy
store at the core of the star which makes the star
rapidly expand.
Stars about the same size as our sun will form a
red giant. Stars much bigger than our sun will
form a red supergiant. Nuclei as large as iron
can fuse here.
white dwarf
black dwarf
A white dwarf is the leftover core of an average
sized star after it has finished its red giant stage and
collapsed. Fusion is not occurring at this point. White
dwarf stars are very bright and very hot because all
of their energy is condensed into a small area.
Over time, this energy is emitted
into the surroundings. The star
gets dimmer and dimmer until
it turns into a brown dwarf, and
then a black dwarf.
neutron star
supernova
The fusion of large nuclei in the red supergiant causes the core to get denser.
When it runs out of fuel, the core is so heavy that its gravitational force pulls the
outer layers of the star in towards the centre. The shockwaves resulting from
this collapse cause the star to explode in a supernova.
Elements heavier than iron are produced and the explosion distributes them
throughout the universe. All naturally occurring elements larger than iron have
come from a supernova.
black hole
At the centre of a supernova, the particles form an
incredibly dense core. Most supernovas collapse to form
neutron stars. Fusion does not occur at this stage.
The largest supernovas form black holes. Gravity in a
black hole is so strong that not even light can escape.