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Stellar Evolution
Star birth in the Eagle Nebula Courtesy of the Space Telescope Science Institute
Introduction
• Human lifetimes vs. ages of stars
• How do we know . . .?
– Humans via pictures
• In one day, take pictures of people, then
piece together human behavior &
history; similar to finding the life history
of stars
• Theories tested, modified, some
completely rewritten
• Many questions remain
unanswered
The Birth of a Star
Nebulae = more than
one nebula
• Vast clouds of gas in
space
• Mainly hydrogen
• Disturbance
– Colliding with other
clouds
– Blast from nearby
supernova explosion
The Birth
of a Star
GRAVITY RULES!!
The Birth of a Star
Rotating cloud collapses in
on itself
As center of the cloud
becomes more dense,
collapse accelerates due
to increased gravitational
attraction between gas
particles
Collapsing clouds mark the
formation of a protostar
Particles far
apart don’t exert
much gravity on
each other
The same particles, now
closer together, exert more
(not yet a true star; no nuclear
gravitational force on each
reactions occurring yet)
other
•
The
Birth
of
a
Star
As the clouds continue to
collapse it begins to warm
up
• When the gas particle
collides with the center of
the cloud,
Out here, the gas particle
has both kinetic & potential
energy
– it loses kinetic energy
because it slows down
– It loses potential energy
because it isn’t so far away
from the middle of the
cloud.
• This energy turns into
HEAT
Center of
gas cloud
The Birth of a Star
• Warming occurs slowly
at first
• Center begins to glow,
dim to bright
• When central
temperature is high
enough (~15 000 K, ~15
273 C) nuclear reactions
can begin
• Protostar has now
become a true star
As the temperature
increases, these
hydrogen particles
move faster.
Eventually, they move
so fast that when they
collide they’ll stick
together.*
A helium nucleus has
been formed!
When this
“sticking” (fusion)
occurs, a bit of
mass is converted
to energy as in
E = mc2
*OK, really . . .
The Birth of a Star
• Stars can form from
extremely large
interstellar clouds
that have
fragmented into
smaller clouds.
• These clusters of
stars are called . . .
Star clusters (!)
• Ex: The Pleiades
(Seven Sisters)
The Birth of a Star
• The haze (“nebulosity”) is
part of the original gas
cloud that’s left over.
• How long does formation
take?
– Small low mass stars can
take billions of years to
form
– More massive stars can
completely form in a few
hundred thousand years
Main Sequence
•
•
•
•
Star has settled into the
most stable part of its life
Converts hydrogen to
helium (H => He)
Next step depends on the
mass of the star
Three different examples of
stars:
1. Stars similar to our Sun
2. Stars several times more
massive than the Sun
3. HUGE HUMONGOUS stars,
VERY massive
The Life of a Sun-like Star
• Will remain on the main sequence (H to He) for
about 10 billion years
• As more He is produced, temperature increases and
core contracts
– We see this as an increase in brightness
– Temperature not high enough to sustain He to C fusion
• Central core then expands
as more He is produced
• Star expands, becoming a
Red Giant
• Our sun, as a red giant, will
be as large as Earth’s
present orbit
The Life of a Sun-like Star
• Over thousands of years, the
star’s central region shrinks &
heats up.
• Outer regions are pushed
away
• We see:
– a small, dense central star
– surrounded by expanding shell
of gas
• The star is now a planetary
nebula
The Life of a Sun-like Star
• The object seen at the center of the gas cloud
is the core of the original star
• Still very hot (~100 000 C)
• Gradually cools & contracts to become a white
dwarf
• Cools even more to become a black dwarf; not
much bigger than Earth, but much more dense
The Life of a Sun-like Star
The Life of a Star Several Times
More Massive Than the Sun
• Enters main
sequence (H to He
process) at a higher
temperature than
smaller stars
• Core is hotter than
smaller stars,
causing faster
“aging”
• After all H is
converted to He, He
is fused into carbon
(requires 100 million
degrees)
The Life of a Star Several Times
More Massive Than the Sun
• After all the He is used, C fuses into neon (requires 500
million degrees)
• As each element is used up, star becomes a red giant.
• . . . And so forth, as long as temperatures are high
enough to fuse that particular element
• As particles that are colliding get larger, much more
heat (energy) is needed to get them to stick together
The Life of a Star Several Times
More Massive Than the Sun
• When an iron core is
formed:
– Reactions STOP
– Iron fusion requires HUGE
amounts of energy
– Eventually, cools to white
dwarf, then black dwarf
stage
– Different than smaller
star’s fate because
different elements will
compose the core
The Life of HUGE Stars
• As with all other stars, follows main
sequence
• If the star is still large (>1.4 Suns) when
the core becomes iron, a supernova
results
The Life of HUGE Stars
•Within seconds of running out of nuclear fuel, the
HUGE gravitational force (remember, large mass =
large gravity) attracts all of the atmosphere into the
core.
http://ircamera.as.arizona.edu/
NatSci102/movies/corcoll3.gif
The Life of HUGE Stars
• As particles fall to the core they lose kinetic & potential
energy and more HEAT results
• This heat triggers nuclear fusion in the outer layers,
and the resulting explosion is the supernova.
• The energy released can fuse iron and other heavier
elements, up to uranium.
The Life of HUGE Stars
•
“This next image is one of the most
spectacular views of 1987A yet
acquired by the HST. The single large
bright light is a star beyond the
supernova environs. Around the central
supernova is a single ring but
associated with the expansion of
expelled gases are also a pair of rings
further away that stand out when
imaged at a wavelength that screens
out much of this bright light.”
Courtesy http://rst.gsfc.nasa.gov/Sect20/A6.html
The Life of HUGE Stars
• The death of the largest stars results in
a core more dense than anything we
know on earth
• This core has such a large gravitational
force that light cannot escape it.
• . . . Hence the name, black hole
• Picture here
Caption: In this image, X-ray contours
are overlaid on an optical image. The
X-ray contours and the colors in the
optical image represent brightness
levels of the X-ray and optical
emission, respectively. When viewed
with an optical telescope this galaxy,
located 2.5 billion light years from
Earth, appears normal. But the
Chandra observation discovered an
unusually strong source of X rays
concentrated in the central regions of
the galaxy. The X-ray source could be
another example of a veiled black
hole associated with a Type 2
Quasar. This discovery adds to a
CXO 0312 Fiore P3 (CXOUJ031238.9growing body of evidence that our
765134): A possible Type 2 quasar veiled
black hole.(Credit: X-ray: NASA/CXC/SAO; census of energetic black hole
Optical: ESO/La Silla)
sources in galaxies is far from
complete.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
From http://chandra.harvard.edu/photo/2000/0312/0312_hand.html
Some artists’ conceptions of a
black hole
•
The Life of HUGE Stars
• How do we know a
black hole exists?
• Evidence
– Strong x-ray emissions
from charged particles
accelerating REALLY
fast
– Gravitational lensing
• Light from stars is bent
when a black hole is
between us & the stars
• Usually form in binary
star systems
We are all made of stars . . .
For next time: read through Chapter 20, sections 2 &
3 and answer the questions at the end of each
section. Quiz next time!