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Introduction to Stellar Evolution
13.7 billion years ago, the Universe as we know it came
to being as a result of what we call the “Big Bang”.
At that time the Universe contained only protons and
electrons.
Eventually those protons and electrons combined to
form Hydrogen and Helium.
Where did everything else come from?
How Do Stars Shine?
Stars shine by nuclear fusion: the process of extracting
energy from the fusion of lighter elements into heavier
elements.
Helium is made from 4 Hydrogen atoms, but one
Helium atom is slightly less massive than the 4
Hydrogen atoms combined.
Nuclear Fusion/Hydrostatic Equilibrium
The “extra” mass is converted into energy
according to Einstein’s famous formula:
2
E = mc
Fusion generates
heat, which keeps
stars from
collapsing under
their own weight.
Next we are
going to talk
about the
changes that
stars undergo
with time.
But before that
let’s look at the
intrinsic
differences
between stars.
The 30 closest stars to the Sun:
Based on
their
brightness,
color, etc.,
we can
group stars
into various
classes.
Stellar Brightness/Distance
• Ancient astronomers assumed that all
stars were the same distance away.
• We now know that the stars are not all
the same distance away.
• A star’s distance from the Earth affects
how bright it appears to be.
• Farther stars appear fainter; closer stars
appear brighter
The Inverse Square Law
Move 2x as far
from a light and it
gets 4x dimmer.
The light is simply
spread out over a
larger area.
Luminosity vs. Brightness
Therefore, two stars
that appear equally
bright might be a
closer, dimmer star
and a farther, brighter
one:
Group Question
Which star is
brightest?
a)
b)
c)
d)
A
B
A and B are equal
Can’t tell
A
B
Which star is the
most luminous?
a)
b)
c)
d)
A
B
C
Can’t tell
C
Stellar Temperature/Color
The color of a star is indicative of its temperature. Red
stars are relatively cool, while blue ones are hotter.
Stellar Colors/Temperature
The color of an object
is inversely
proportional to
temperature.
If we know the color
of a star, we know the
temperature.
Stars: Color vs. Temperature
Red star
Yellow star
Blue star
The Hertzsprung-Russell
(H-R) Diagram
Any plot of (intrinsic)
brightness versus color or
temperature is called an
HR diagram.
Classes of Stars
Stars aren’t scattered
randomly on the HR
diagram, rather they
fall in certain clumps.
In our neighborhood,
about 90% of stars lie
on the “main
sequence”; 9% are
“red giants” and 1%
are “white dwarfs”.
Main Sequence Stars
• Also called dwarfs
• Normal, run-of-the-mill stars
• About 90% of nearby stars are MS
• In H-R Diagram: band from upper left
(bright and hot) to lower right (faint
and cool)
• Sun is a MS star (type G2)
• Cool MS stars are more common than
hot MS stars.
Giants
• MUCH bigger than the Sun (10 to 100x)
• Red Giants – Coolest (~4000 K) giant
stars; appear very red
• Supergiants – Both bigger and brighter
than the average giant
White Dwarfs
• MUCH smaller than the Sun
• Very hot, but not very bright
• Actually remnants of dead or
dying normal stars
OK, now that we know that
there are hot, blue massive stars
and cooler, red less massive
stars, let’s talk about what
happens to them over time.
Star Formation and Lifetimes Tutorial
Hydrostatic Equilibrium
Fusion keeps stars from collapsing under
their own weight. Pressure from the
outflowing hot gas balances the pressure
of gravity.
This process is called
hydrostatic equilibrium
Stellar Evolution
At first, all stars process
hydrogen into helium.
In low-mass stars this
can take billions of
years.
The Death of Stars
Eventually, as hydrogen in the core is consumed,
the star begins to die.
But stars don’t go down easy!
Its evolution from then on depends very much on
the mass of the star:
Low-mass stars go quietly
High-mass stars go out with a bang!
Evolution of a Low-Mass Star
As the fuel in the core is used up, the core
contracts; when it is used up the core begins to
collapse.
That collapse
releases gravitational
energy and the layer
just outside the core
heats up.
Hydrogen begins to
fuse outside the core:
Evolution of a Low-Mass Star
Initial giant stage: the
red giant branch -shell hydrogen fusion
As the core continues to
shrink, the outer layers of
the star expand and cool.
The star, now a red giant,
is as big as the orbit of
Mercury.
Despite its cooler
temperature, its luminosity
increases enormously due
to its large size.
Evolution of a Low-Mass Star
Next giant stage: the
horizontal branch - core helium
burning
Once the core temp has
risen to 100,000,000 K,
the helium in the core
starts to fuse into carbon
Helium begins to fuse
extremely rapidly; within
hours the enormous
energy output is over,
and the star once again
reaches equilibrium
Evolution of a Sun-like Star, Cont
As the helium in the core fuses to carbon, the
core becomes hotter and hotter, and the helium
burns faster and faster.
Soon all the helium in
the core has been
converted to carbon.
Again the core
contracts and
collapses, releasing
gravitational energy.
Evolution of a Low-mass Star
Final giant stage:
asymptotic giant
branch (supergiant
stage) -- shell helium
fusion
Helium is fused in
the first shell,
hydrogen in the
next.
Interior of an Old Low Mass Star
Death of a Low-Mass Star
• Low mass stars are not massive enough to
turn carbon and oxygen cores into heavier
elements (not hot enough)
• These stars eject most of their outer layers
• Only the core is left, which “lights-up” the
gas that the star has been ejecting – causing
a planetary nebula
Death of a Low-Mass Star
The core continues to contract, but never gets hot enough to
burn Carbon. Meanwhile, the outer layers of the star
expand to form a planetary nebula.
The Helix Nebula
NGC 6826 and 7027
Death of a Low-Mass Star
Even after the nebula
has dispersed into
space, the core
remains. It is extremely
dense and extremely
hot, but quite small.
This stage is called a
white dwarf.
White dwarfs are very
hot, but not very
luminous.
Group Question
The force of gravity acts to make a star
a)
b)
c)
d)
Larger
Smaller
Cooler
None of these
Group Question
More massive
white dwarfs are
______ compared
with less massive
white dwarfs.
1) hotter
2) smaller
3) larger
4) cooler
5) identical in size
Group Question
More massive
white dwarfs are
______ compared
with less massive
white dwarfs.
1) hotter
2) smaller
3) larger
4) cooler
5) identical in size
Chandrasekhar showed that more mass will squeeze a
white dwarf into a smaller volume, due to electron
degeneracy pressure.
White Dwarfs
White Dwarf – the burned out carbonoxygen core of a “dead” star
• Supported by electron degeneracy pressure
• Less than 1.4x MSun (Chandrasekhar Limit)
• Form from MS stars between 0.8x and 8x MSun
A White Dwarf
The small star
Sirius B is a whitedwarf companion of
the much larger and
brighter Sirius A:
As a white dwarf
cools, its size does
not change much; it
simply gets dimmer
and dimmer
The Hubble Space Telescope has detected
white dwarf stars (circled) in globular clusters:
Low-Mass Star Summary
Giants – after core hydrogen burning
• First: shell hydrogen burning (normal giants)
• Second: core helium burning (horizontal
branch giants)
• Third: shell helium burning (supergiants)
• Most go through an unstable variable stage
White Dwarfs – collapsed remnants of lowmass stars.
Stellar
Evolution
Sequence
(Low-Mass
Stars)
Universe Video
Ch 2. 3:50 – 7:30. 11:44- eoc
Evolution of High-Mass Stars
High-mass stars, like all stars, leave the Main
Sequence when there is no more hydrogen fuel
in their cores.
The first few events are similar to those in lowermass stars – first a hydrogen shell, then a core
burning helium to carbon, surrounded by heliumand hydrogen-burning shells.
But instead of stopping at Carbon, a star of more
than 8 solar masses can fuse elements far
beyond carbon in its core, leading to a very
different fate.
Evolution of High-Mass Stars
Stars more massive
than the Sun follow
very different paths
when leaving the Main
Sequence:
High Mass Star Cores
Star has layers of lighter and lighter elements, like
an onion.
High Mass Stars Continued
• Eventually the core is composed entirely
of Iron (Fe)
• Iron is heaviest element that can be
created by fusion
• Core now supported only by electron
degeneracy pressure
• Once core becomes more than 1.4x MSun,
the core collapses
Supernovae
Core collapses until the density reaches the
neutron degeneracy limit
Core collapse comes to a sudden halt and
bounces back (core bounce)
Outward moving core slams into infalling
outer layers of star
Star is blown to smithereens (supernova)
Elements heavier than iron are formed
during the explosion.
Supernova Explosions
A supernova is a one-time event – once it
happens, there is little or nothing left of the
progenitor star.
There are two different types of supernovae,
both equally common:
Type I -- results from excess mass being
dumped onto a white dwarf from a binary
companion
Type II -- the “normal” death of a high-mass
star.
Type I vs. Type II Supernovae
Supernova Remnants
The supernova explosion lights-up the gas
and dust that the star has already ejected,
creating a kind of nebula – a supernova
remnant.
The Gum Nebula
Supernova 1987A
The Cycle of Stellar Evolution
Star formation is
cyclical: stars form,
evolve, and die.
In dying, they send
heavy elements
into the interstellar
medium.
These elements
then become parts
of new stars.
You are made of star dust.