Download Earth Science 25.2A : Stellar Evolution

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Astronomical spectroscopy wikipedia, lookup

P-nuclei wikipedia, lookup

Aquarius (constellation) wikipedia, lookup

Ursa Minor wikipedia, lookup

Corvus (constellation) wikipedia, lookup

Perseus (constellation) wikipedia, lookup

Cygnus (constellation) wikipedia, lookup

Cassiopeia (constellation) wikipedia, lookup

Timeline of astronomy wikipedia, lookup

Hipparcos wikipedia, lookup

Observational astronomy wikipedia, lookup

Lyra wikipedia, lookup

International Ultraviolet Explorer wikipedia, lookup

Extraterrestrial life wikipedia, lookup

Dialogue Concerning the Two Chief World Systems wikipedia, lookup

Formation and evolution of the Solar System wikipedia, lookup

Planetary habitability wikipedia, lookup

Rare Earth hypothesis wikipedia, lookup

Geocentric model wikipedia, lookup

CoRoT wikipedia, lookup

Ursa Major wikipedia, lookup

Stellar evolution wikipedia, lookup

Star formation wikipedia, lookup

Stellar kinematics wikipedia, lookup

Future of an expanding universe wikipedia, lookup

Dyson sphere wikipedia, lookup

Theoretical astronomy wikipedia, lookup

Star wikipedia, lookup

H II region wikipedia, lookup

Star of Bethlehem wikipedia, lookup

Cygnus X-1 wikipedia, lookup

Transcript
Earth Science 25.2A : Stellar Evolution
The Birth
and
Evolution
of Stars
5/11/2010
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.
5/11/2010
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.
5/11/2010
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.
5/11/2010
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
light.
 This large red object is called a
protostar; a developing star that is
not yet hot enough to attain nuclear
fusion.
5/11/2010
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.
5/11/2010
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
gravity.
 When this balance is reached, the
star becomes a stable main sequence
star.
 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.
5/11/2010
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.
5/11/2010
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
years.
5/11/2010
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
giant.
5/11/2010
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.
5/11/2010
Earth Science 25.2 : Stellar Evolution
 As the core contracts, it grows
hotter by converting
gravitational heat into heat
energy.
 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.
5/11/2010
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
carbon.
 So a red giant consumes both
helium and hydrogen to produce
energy.
5/11/2010
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
giant.
 More massive stars will pass
through this stage even more
rapidly.
 The force of gravity will again
control the star’s destiny as it
squeezes the star into a much
smaller, denser piece of matter.
5/11/2010
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
theory.
 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
sizes
and mass.
5/11/2010
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
time.
 Consequently, these small,
cool, red stars may remain
on the main sequence for up
to 100 billion years.
5/11/2010
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
hydrogen.
 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
dwarf.
5/11/2010
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
dwarfs.
5/11/2010
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.
5/11/2010
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
stage.
 If one of the stars nearest to
Earth produced a supernova, it
would be brighter than our sun.
5/11/2010
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
apart.
 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.
5/11/2010
Earth Science 25.2 : Stellar Evolution
 A supernova event is thought to
be triggered when a massive star
consumes most of it’s nuclear
food.
 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
inward.
 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
supernova
event.
5/11/2010
Earth Science 25.2 : Stellar Evolution
Click on the following
link to view a short
animation showing the
life of our sun
http://janus.astro.um
d.edu/astro/stars/Su
nsLife.html
Part B: continued
tomorrow as 25.2B
5/11/2010