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Earth Science 25.2A : Stellar Evolution
The Birth
of Stars
Earth Science 25.2 : Stellar Evolution
 Determining how stars are born,
age and die can be difficult because
the life of a star can span billions
of years.
 However, by studying stars of
different ages astronomers have
been able to piece together the life
and evolution of a star.
Earth Science 25.2 : Stellar Evolution
The Birth of a Star:
 The birthplaces of stars are dark,
cool interstellar clouds.
 These nebulae are made up of dust
and gases.
 In the Milky Way, nebulae consist
of 92% hydrogen, 7% helium, and
less than 1 % of the remaining
heavier elements.
 For some reason not fully
understood by scientists, some
nebulae become dense enough that
they begin to contract, growing
smaller yet denser as they shrink.
Earth Science 25.2 : Stellar Evolution
 A shock wave from another nearby
star may trigger this contraction.
 Once this process begins to occur,
gravity squeezes particles in the
nebulae pulling each and every
particle toward the center.
 As the nebulae shrinks and grows
denser in it’s center, this
gravitational energy increases and
is converted into heat energy.
Earth Science 25.2 : Stellar Evolution
Protostar Stage:
 The initial contraction of gases
spans a million years time.
 As time passes, the temperature of
this gas body slowly rises enough to
radiate energy from it’s surface in
the form of long-wavelength red
 This large red object is called a
protostar; a developing star that is
not yet hot enough to attain nuclear
Earth Science 25.2 : Stellar Evolution
 During the protostar stage,
gravitational contraction continues:
slowly at first than more rapidly.
 This collapse causes the core of
the protostar to heat much more
intensely than the outer layer.
 When the core of a protostar has
reached about 10 million degrees K,
pressure within it is so great that
the nuclear fusion of hydrogen
begins, and a star is born.
Earth Science 25.2 : Stellar Evolution
 Heat from hydrogen fusion causes
the gases to increase their motion.
 This in turn causes an increase in
outward gas pressure.
 At some point, this outward pressure
exactly balances the inward pull of
 When this balance is reached, the
star becomes a stable main sequence
 A stable main-sequence star is
balanced between two forces:
gravity, which is trying to squeeze it
into a smaller space, and gas
pressure, which is trying to expand it.
Earth Science 25.2 : Stellar Evolution
Main Sequence Stage:
 From this point on in the
evolution of a main-sequence
star until it’s death, the
internal gas pressure
struggles to offset the
unyielding force of gravity.
 Typically, the hydrogen
fusion continues for a few
billion years and provides the
outward pressure required to
support the star from
gravitational collapse.
Earth Science 25.2 : Stellar Evolution
 Different stars age at
different rates.
 Hot, massive, blue stars
radiate energy at such an
enormous rate that they
deplete their hydrogen fuel
in only a few million years.
 By contrast, the least
massive main-sequence stars
may remain stable for
hundreds of billions of years.
 A yellow star, such as our
sun, remains a main-sequence
star for about 10 billion
Earth Science 25.2 : Stellar Evolution
 An average star spends 90
percent of it’s life as a
hydrogen burning mainsequence star.
 Once the hydrogen fuel in
the star’s core is depleted, it
evolves rapidly and dies.
 However, with the exception
of the least massive red
stars, a star can delay it’s
death by fusing heavier
elements and becoming a
Earth Science 25.2 : Stellar Evolution
Red Giant Stage:
 The red giant stage occurs
because the zone of hydrogen
fusion continually moves outward,
leaving behind a helium core.
 Eventually, all the hydrogen in
the star’s core is consumed.
 While hydrogen fusion is still
occurring in the star’s outer
shell, no fusion is taking place in
the inner core.
 Without a source of energy, the
core no longer has enough
pressure to support itself against
the inward force of gravity and
the core thus begins to contract.
Earth Science 25.2 : Stellar Evolution
 As the core contracts, it grows
hotter by converting
gravitational heat into heat
 Some of this energy is radiated
outward, increasing hydrogen
fusion in the star’s outer shell.
This energy in turn heats and
expands the star’s outer shell.
 The result is a giant body
hundreds to thousands of times
bigger than the original mainsequence size.
Earth Science 25.2 : Stellar Evolution
 As the star expands, it’s
surface cools, which explains
the star’s reddish appearance.
 During expansion, the core
continues to collapse and heat
until it reaches 100 million K.
 At this temperature, it is hot
enough to convert helium to
 So a red giant consumes both
helium and hydrogen to produce
Earth Science 25.2 : Stellar Evolution
 Eventually, all the usable
nuclear fuel in these giants will
be consumed.
 The sun, for example, will spend
less than a billion years as a
 More massive stars will pass
through this stage even more
 The force of gravity will again
control the star’s destiny as it
squeezes the star into a much
smaller, denser piece of matter.
Earth Science 25.2 : Stellar Evolution
Burnout and Death of a Star:
 Most of the events of
stellar evolution we have
looked at so far are well
documented. What we will
look at now is based more on
 We do know that all stars,
regardless of their size,
eventually run out of fuel
and collapse due to gravity.
 With this in mind, let us look
at the final stages of a
variety of stars of different
and mass.
Earth Science 25.2 : Stellar Evolution
Death of a low-mass star:
 As we see in the figure at
right, stars less than one
half the mass of our sun
consume their fuel at a low
rate and live a very long
 Consequently, these small,
cool, red stars may remain
on the main sequence for up
to 100 billion years.
Earth Science 25.2 : Stellar Evolution
 Because the interior of a
low-mass star never
reaches high enough
temperatures and
pressures to fuse helium,
it’s only energy source is
 So low-mass stars never
evolve into red giants.
 Instead, they remain as
stable main-sequence
stars until they consume
their hydrogen fuel and
collapse into a white
Earth Science 25.2 : Stellar Evolution
Death of medium-Mass Stars:
 Our star, the sun, is a
medium mass star. As we
see in the table at right,
all stars with masses
similar to our sun evolve in
much the same way.
 During their red giant
phase, sun-like stars
consume hydrogen and
helium fuel at a fast rate.
 Once this fuel is
exhausted, these stars
also collapse into white
Earth Science 25.2 : Stellar Evolution
 During their collapse from red
giants to white dwarfs, mediummass stars are thought to cast
off their bloated outer shell,
creating an expanding round
cloud of gas.
 The remaining hot, central white
dwarf heats the gas cloud,
causing it to glow.
 These often beautiful gleaming
spherical clouds are called
planetary nebulae.
Earth Science 25.2 : Stellar Evolution
Death of Massive Stars:
 In contrast to sun-like stars,
stars with masses 4 times that
of our sun have relatively short
life spans.
 These stars end in a brilliant
explosion called a supernova.
 During a supernova, a star
becomes millions of times
brighter than it’s prenova
 If one of the stars nearest to
Earth produced a supernova, it
would be brighter than our sun.
Earth Science 25.2 : Stellar Evolution
 Supernovas are very rare.
 None have been observed in
our galaxy since the invention
of the telescope, although
Tycho Brahe and Galileo each
recorded one about 30 years
 An even larger supernova was
recorded in 1054 by ancient
Chinese astronomers.
 Today, the remnant of this
great supernova is thought to
be the Crab nebulae.
Earth Science 25.2 : Stellar Evolution
 A supernova event is thought to
be triggered when a massive star
consumes most of it’s nuclear
 Without a heat engine to
generate the gas pressure
required to maintain it’s balance,
it’s immense gravitational field
causes the star to collapse
 This implosion, or bursting
inward, is huge, resulting in a
shock wave that moves outward
from the star’s interior.
 This shock wave destroys the
star and blasts the outer shell
into space, generating the
Earth Science 25.2 : Stellar Evolution
Click on the following
link to view a short
animation showing the
life of our sun
Part B: continued
tomorrow as 25.2B