Life Cycle of a Star
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We now know that stars move through a
complex life cycle – they are created, live
extremely long lives and then expire.
By studying different stars in various
stages of development, astronomers have
now established a detailed process for
their life cycle.
Protostar and the Nebula
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A nebula is a cloud of dust and gas,
composed primarily of hydrogen (97%)
and helium (3%). Gravity causes this dust
and gas to “clump” together.
Protostar and Nebula
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A protostar is a star in its embryonic
stage, and although it glows due to the
release of gravitational energy, it is not
yet hot enough to produce nuclear
reactions within its center.
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As the protostar continues to collapse due
to gravity, it will attract more atoms and
continually increase in mass and density.
The increased density and gravity will
cause the core temperature to eventually
rise.
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The falling atoms of gas speed up as they
reach the center. As they speed up they
collide with each other and heat up.
Why is hydrogen so reactive?
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This is why……
Hydrogen has only 1 electron in its outer
most shell. In order for it to be “stable” it
needs another electron.
For example: water = H2O
Watch the board…..so you can understand
why elements react to other elements.
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In order to achieve life as a star, the
protostar will need to achieve and
maintain equilibrium.
What is equilibrium? It is a balance, in this
case a balance between gravity pulling
atoms toward the center and gas pressure
pushing heat and light away from the
center.
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Gas pressure depends upon two things to
maintain it: a very hot temperature (keep
those atoms colliding!) and density (lots of
atoms in a small space).
There are two options for a protostar at
this point:
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Option 1: If a critical temperature in the core of
a protostar is not reached, it ends up a brown
dwarf. This mass never makes “star status.”
Nebula = Protostar = Brown Dwarf
Option 2: If a critical temperature in the core of
a protostar is reached, then nuclear fusion
begins. We identify the birth of a star as the
moment that it begins fusing hydrogen in the
core into helium.
Star is born!
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A star is a luminous globe of gas
producing its own heat and light by
nuclear reactions (nuclear fusion).
Hydrogen fuses with helium in the core.
Medium-Sized Stars
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The main factor that shapes the evolution
of a star is how much mass it began with.
In other words…how big the core is! A
star’s life can take different paths.
A star will spend most of its life as a main
sequence star.
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Once the star’s core has been changed
from hydrogen to helium, the core begins
to shrink.
As it shrinks, the core starts to heat up
again causing the outer hydrogen shell to
expand and cool. Red Giant
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As the Red Giant continues to use up its
fuel, the pressure released does not equal
the pressure of the core.
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Gravity wins and the star collapses inward.
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The star now becomes a tiny White Dwarf.
White Dwarf
The matter squeezed into a white dwarf is
extremely dense. Still burns, and gives off
light.
 Once “fuel” is used up. The star dies and
turns into a Black Dwarf or (dead star).
Nebula = Protostar = Main sequence = Red
Giant = White Dwarf = Black Dwarf
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Massive Stars
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More mass than medium stars and
continue the same life-cycle until they
become Red Giants or Supergiants.
Once nuclear fusion stops within the
massive star, the energy is released in the
form of a Supernova.
Supernova
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The dust and gases from the supernova
forms a new nebula which may form new
stars.
The core of the star will explode
(depending on the size) and form into a
Neutron Star.
Black Holes
Stars that have the mass of 10 x or more
than our sun may end up as a black hole.
 The core is so massive that it is swallowed
up by its own gravity.
Nebula = Protostar = Main sequence = Red
Super Giant = Supernova a) neutron star
b) black hole
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