Download The Life Cycles of Stars

PowerPoint Created by: Alexander J. Hawkins
Information documented from DK Smithsonian UNIVERSE Definitive Visual
Guide
 As human beings go through cyclic lives, maturing from
birth to maturity to old age, stars also follow a series of
stages from their creation until death. Stars follow varying
sequences of change depending greatly on their solar
mass, or their gravitational weight (measured in 1.9891 ×
10 (30th) kilograms per 1 solar mass). Regardless of
whether a star is a low-mass, moderate-mass, or highmass star, they are all born from interstellar clouds of gas
that collapse under the pressure of gravity, enter a long
period of stability called the main sequence, and die off to
become another celestial body forever changed. However,
what happens during a star’s existence dictates what
unique paths it follows. As a result, our night sky is filled
with distinct bodies of light, all stars that have
experienced a different story in their lives.
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Stars are born in cold interstellar clouds of gas that
drift through space. Depending on how cool the cloud
is in temperature, the gaseous clump, composed
mainly of hydrogen, is less protected from a
gravitational collapse. At lower temperatures, the
cloud’s hydrogen atoms collect together to form
hydrogen molecules. Once, the cloud grows to
surpass a certain mass, and experiences a
gravitational disturbance, sometimes caused by
supernovae, it will begin to collapse into itself. As this
occurs, fragmented pieces of the cloud of varying
mass and sizes separate to form protostars, the
earliest form of a star.
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Once protostars are formed out of an interstellar cloud, the stellar
objects continue to collapse, causing central temperatures and
internal pressure to build up. Depending on the mass of a
protostar, temperature and pressure levels increase, so it can be
stated that temperature and pressure are higher with higher mass
protostars. If a protostar has a size less than 0.08 solar masses,
then the temperature and pressure at its core will not reach high
enough levels for nuclear reactions to begin, allowing it to reach
adolescence, and the star will become a brown dwarf star.
However, if a protostar surpasses 0.08 solar masses, the gas that
had clustered to form the protostar begins to rotate around the
star, increasing in speed as it draws near the stellar body, being
pulled slowly in, until a ring of stellar material is formed around
the protostar. Until entering its main sequence, the protostar
demonstrates unstable movements and reactions, ex. Rapid
rotations, strong stellar winds, etc.
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With protostars with a mass over 0.08 solar masses, the internal
pressure and temperature of the protostar will meet the
requirements needed for nuclear reactions within the star to start.
With this, the pressure of the stellar body will stabilize to balance
gravity, classifying the protostar as a official star. After entering
the main sequence, with the new star in a stable condition, the
rings of extra material rotating around the star will begin to cool in
temperature. As this happens elements within the disks will begin
to condense, sticking to one another. Small pieces of material then
join larger clumps, and the process continues until the balls of
matter are the size of a planet. Of course, planet formation may
take time, for while the clumps of material are still warm, other
fragments impacting them may cause the piece to split apart
again, making it so that planets are not permanently formed until
they have cooled enough. Any excess, loose material from the
star’s formation, after cooling without becoming a planet, become
comets, asteroids, or trails of gas.
Forming Planets Revolving Around a Parent
Star
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For 90% of a star’s life it exists in a period of stability called the
main sequence. 90% of all stars in the night sky are currently in
their main sequence, since the time frame of such calmness makes
for most of a star’s life. During this time, stars expand and
contract, but at very small levels, not changing dramatically in
activity. Temperature and pressure levels remain mostly constant,
with little differentiations over the course of the billions of years a
star stays in the main sequence. However, depending on the initial
mass of a star, the time a star follows the main sequence varies
(more massive stars exit the main sequence sooner due to their
faster burning of fuel in comparison to small stars). At the end of a
star’s main sequence, their solar mass dictates what path they will
follow in the last leg of their lives, and even the outcome of their
death.
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Low-Mass Stars: Any star half or less the mass of
our sun is considered a low-mass star.
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Sun-Like Stars: Any star with equal or
approximately equal mass as our sun is considered
a sun-like star.

High-Mass Stars: Any star with a much greater
mass than our sun is considered a high-mass star.
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As most stars follow, low-mass stars eventually
burn, or deplete, their hydrogen fuel in their
cores. Once this happens, a low-mass star will
convert its atmosphere slowly to helium instead
of hydrogen, causing it to collapse; a similar trait
among low-mass, sun-like, and high-mass stars.
However, due to low-mass stars’ inferior mass,
their internal pressure and temperature levels in
its core can not reach the point of helium
burning. This then causes the star to slowly cool
down and loose luminance until the star fades
into a black dwarf.
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Star grows in size as its hydrogen layer is burned
away.
Star begins to collapse and shrink as its hydrogen
fuel dissipates.
Star continues collapsing due to its inability to
produce helium burning.
Star grows so small and cold that only a gaseous
pressure contradicts gravity.
Minuscule, dark star progressively fades away.
After losing most of its regular pressure and
temperature, having decreased in size
tremendously, the low-mass star turns into a dim
black dwarf star.
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Sun-like stars, as can be concluded, have a similar mass as our solar
system’s sun. After exiting the main sequence, such stars begin to use up
all of their remaining hydrogen in their cores until the quantity of
hydrogen available becomes depleted. Upon the occurrence of this, sunlike stars begin their process of hydrogen shell burning, where the
hydrogen in their atmosphere begins to burn away, increasing their size
until they become a red giant star. Red giants are massive stars that a
sun-like star transforms into nearing the end of its life, which eventually
sheds its outermost layers, becoming a planetary nebula. Over time, the
planetary nebula builds up pressure and temperature at its core, causing
helium burning to reactivate, and the star to expand once more. Soon
after, the planetary nebula collapses into a white dwarf (slightly more
illuminant and hot than a black dwarf), and then a black dwarf after it
cools furthermore.
Note: Scientists have predicted that this is the likely path of our current
sun, which is relative in size to other stars that have had similar timelines.
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Star grows to become a red giant as hydrogen
burning causes an increase in the size, of the star.
Red giant star’s outer layers of hydrogen and helium
are released from the star, forming a planetary
nebula.
Star within planetary nebula begins to expand due to
helium burning, triggered by high temperature and
pressure levels in the core of the star.
Star collapses inside planetary nebula after its
helium shell is burned away, causing it to cool down
into a white dwarf star.
New white dwarf star gradually fades and cools until
it becomes a black dwarf star.
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It is a common misnomer that our sun is large enough for a supernova (an
extreme release of stellar material and heat) to occur in its future. In truth, the
only type of star that can undertake such an explosion is a high-mass star, a star
with a greater solar mass than that of our sun. In astronomy, the higher a star is
in mass, the more times it will go into a period of expansion and contraction. The
mass of a star, in addition, decides the temperature of a star’s core each time it
goes into a period of flexion. Depending on the stage in a star’s development,
various elements are formed within the core of the star, with the heaviest
sustainable material being iron, when a massive star forms an iron core.
However, any elements heavier and denser than iron cannot be produced
internally by stellar bodies (stars). The only way to create such substances is
through a supernova explosion, which can create elements such as gold in the
process. Once most high mass stars expand due to hydrogen burning, they
become supergiant stars, with a mass greater than any other form of star, which
produces heavy elements such as iron (some result in changing into red giant
stars). However, instead of calmly settling into becoming a black or white dwarf
star, high-mass red giant or supergiant stars collapse violently resulting in
supernovae. After ejecting new, heavy elements into space, supernovae create
either a neutron star or black hole that create extreme gravitational pull.
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Star begins hydrogen burning, growing in size as its hydrogen reserves
are lowered.
High pressure and heat cause the high-mass star to turn into either a
red giant star or supergiant star.
Red giant star/supergiant star creates heavy elements, such as iron,
inside of itself through nuclear reactions at its core.
After reaching the point in which the red giant/ supergiant star has
made the heaviest element it can form, iron, the star collapses and
explodes into a supernova, producing even denser elements with more
weight in the process.
In the aftermath of the supernova, the collapsed star could turn into
either a neutron star or a black hole depending on its solar mass before
the supernova.
If the high-mass star remnant is over 1.4 solar masses, it will collapse to
form a neutron star.
If the high-mass star remnant is over 3.0 solar masses, it will collapse to
form a black hole.
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Once a high-mass star reaches its stellar end point, the ultimate stage of development in a star, it either
turns into a neutron star or black hole varying on the remnant of a supernova. If this remnant is 1.4 solar
masses or larger (otherwise known as the Chandrasekhar limit), then the destroyed high-mass star will
become a neutron star. However, if the remnant surpasses 3.0 solar masses, then the collapsed high-mass
star will become a black hole.
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Neutron Stars- Neutron stars are one of the two resulting bodies created by a supernova (especially said of
type II supernovae explosions). A neutron star is an incredibly dense, compact star with a internal body
made primarily of neutrons. Much different than their parent stars, neutron stars have a crystalline outer
crust, a much stronger gravitational pull despite their small mass (usually between 0.1 and 3.0 solar
masses), and their rapid rotation. This rotation slows over time due to the loss of energy, but will
occasionally spike up again due to “starquakes”, small tremors that occur beneath the solid, thin surface of
neutron stars. Some neutron stars eject beams of radiation regularly, commonly classified as pulsars.
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Black Holes- Black holes are another resulting body created after a supernova, usually having to be greater
than at least 3.0 solar masses. When a star collapses at such a size, the stellar object becomes incredibly
small and dense, resulting in a gravitational pull so powerful that radiation (heat) or visible light can’t even
escape. Such celestial forces are classified as stellar-mass black holes, which are only able to be detected
through the effect and alterations they make to nearby objects in space; such as the light of distant objects
in space being bended by their gravitational pull, the matter sucked onto its accretion disks (rings), and the
changes they make to object movement in space. Stellar-mass black holes can be pinpointed by their high
radiation levels created by the material they suck in, allowing astronomers to carefully observe them,
despite the fact that they can hardly be seen by even telescope lenses. The most symbolic area of all black
holes, the dark middle section, is called the event horizon, where light, radiation, or any matter can no
longer escape from the black hole’s immense gravity. This area has troubled astronomers for years, for it is
unknown what is on the other side of a black hole, or even if there is one. Like neutron stars, black holes are
one of the many rare and spectacular phenomena produced by the death of a star, demonstrating that a
star’s life continues on even after its primary period of activity.
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As if nature created all things, living and non-living, alike, the stars in our
nighttime skies are not much different than the people of Earth. Stars go
through a constant cycle, being formed in a tremendous display of
growth, existing in a long period of stability, and finally being
extinguished in a remarkable eruption of gas and fire. Then, as with the
human race, stars are then born through the death of others, as the
remnants of all supernovae result in the birth of a nebula. From there,
the systematic pattern of stellar life repeats itself, producing new stars in
place of the old. Stars are an important piece of our vast universe, giving
life to planetary systems like ours, and demonstrating the basis of known
space, upon which we can study the workings and particulars of the final
frontier. All stars that reach the main sequence are formed in a similar
manner, through the formation and activation of a protostar, but what
lies between the main sequence and their stellar end points can vary.
Some stars, with lower mass dissipate quickly due to rapid cooling, and
turn into white and black dwarfs. Others with higher mass grow through
hydrogen burning, becoming supergiant stars and, eventually,
supernovae. So whenever you look up at the night sky, peering at the
twinkling stars drifting endlessly, it is important to remember those
distant, beating hearts of space.
PowerPoint Created by: Alexander J. Hawkins
Information documented from DK Smithsonian UNIVERSE Definitive Visual
Guide