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					        Chapter 12 Stellar Evolution

I. Main-Sequence Stars
    A. Stellar Models
    B. Why is there a Main Sequence?
    C. The Upper End of the Main Sequence
    D. The Lower End of the Main Sequence
    E. The Life of a Main-Sequence Star
    F. The Life Expectancies of Stars

II. Post-Main-Sequence Evolution Giant stage
     A. Expansion into a Giant
     B. Degenerate Matter
     C. Helium Fusion
     D. Fusing Elements Heavier than Helium
          Chapter 12 Stellar Evolution
III. Evidence of Evolution: Star Clusters
     A. Observing Star Clusters
     B. The Evolution of Star Clusters

IV. Evidence of Evolution: Variable Stars
    A. Cepheid and RR Lyrae Variable Stars
    B. Pulsating Stars
    C. Period Changes in Variable Stars
The HR diagram & a protostar
The HR diagram & a protostar


                 Protostar takes form
                 but is hidden within
                 dusty gas cloud
Still enshrouded in opaque ―cocoons‖ of dust => barely
visible in the optical, but bright in the infrared
The HR diagram & a protostar

      Protostar contracts & starts heating up;
      gravitational    pe is converted to
      thermal          energy
The HR diagram & a protostar

     Fusion starts --- protostar turns
     into a star
The HR diagram & a protostar

                            Main Sequence star




The rate of fusion in the
core is high enough for
radiation to balance
contraction.
 Hydrogen
  Fusion


Gravitational
  collapse
Two forces balance each other: Pressure = Gravity


                                    Gravitational
                                    equilibrium:

                                    Energy produced
                                    at core creates an
                                    OUTWARD force
                                    against the
                                    gravitational force
                                    which pushes IN
From Protostars to Stars


                     The Birth Line:
                     Star emerges
                     from the
                     enshrouding
                     dust cocoon
From Protostars to Stars
                       Higher-mass
                       stars evolve more
                       rapidly from
                       protostars to
                       stars
 What is the MOST important mechanism in stars?

               Nuclear fusion
       (which depends on pressure and temperature)



What is the MOST important characteristic of stars?

                     Mass
     How is a star’s mass connected
           to nuclear fusion?
   Since mass is IMPORTANT, how do we
   examine stars relative to their mass?

         Define mass ranges:

  Low mass stars: less than 2 MSUN

Intermediate stars: between 2 & 8 MSUN

 High mass stars: greater than 8 MSUN
       Definition is not random.
       Definition is founded in physics.
                     Role of Mass
• A star’s mass determines its entire life story because
  it determines its core temperature

• High-mass stars with >8MSun have short lives,
  eventually becoming hot enough to make iron, and
  end in supernova explosions

• Low-mass stars with <2MSun have long lives, never
  become hot enough to fuse carbon nuclei, and end
  as white dwarfs

• Intermediate mass stars can make elements
  heavier than carbon but end as white dwarfs
High
       Intermediate
           mass


                 Low mass




                            Brown
                            Dwarfs
                 Brown Dwarf Stars
Originally called black dwarfs these substellar objects were
             first theorized of in the early 1960s


                               Minimum Mass of
                              Main-Sequence Stars
                                Mmin = 0.08 Msun
                              (80 times that of Jupiter )




A binary brown dwarf system found in the Orion Nebula.
Brown dwarfs are stars that, because they lack sufficient
mass, do not to achieve nuclear fusion.

Image courtesy: Space Telescope Science Institute
    Maximum Masses of Main-Sequence Stars
                Mmax ~ 100 solar masses
a) More massive clouds fragment into smaller pieces
during star formation.             b) Very massive
                                     stars lose mass in
                                     strong stellar winds




                       Example: Eta Carinae: Binary system
                       of a 60 Msun and 70 Msun star
                       Dramatic mass loss; major eruption in
                       1843 created double lobes
                     Stars of higher mass
                      are more luminous
Mass                       and short lived
 &
AGE




       Stars of lower mass
       are less luminous and long lived
                           High presure
                           High temperature
  Mass                     High rate of fusion
   &
 CORE
(fusion)

           lower presure
           lower temperature

           low rate of fusion
How does a star’s mass affect nuclear fusion?

             The larger the MASS,
the larger the PRESSURE and TEMPERATURE
             of the core of the star

               which increases
            the nuclear fusion rate

 which in turn determines the star’s lifetime.
          HOW DO WE KNOW THIS?
Star Clusters and Stellar Lives
                 • Our knowledge of the
                   stellar evolution comes
                   from comparing
                   mathematical models
                   of stars with
                   observations
                 • Star clusters are
                   particularly useful
                   because they contain
                   stars of different mass
                   that were born
                   about the same
                   time & are at the
                   same distance
The Source of Stellar Energy
         (fusion)
   Proton - Proton cycle
                      Start with:
                       4 protons

                      Finsh with:
                      4He nucleus

                     2 gamma rays
                       2 positrons
                       2 neutrinos
                   LOTS of energy!
           The Source of Stellar Energy
                    (fusion)
                   CNO cycle



                                      Start with:
                                             Protons
                                      Finsh with:
                                      4He nucleus

Takes place in stars that are         gamma rays
slightly more massive than the sun,    positrons
                                       neutrinos
a more powerful energy generation
                                      LOTS of energy!
mechanism than the PP cycle
In 25 words or less …..
            Remember ?


        Defined mass ranges:

  Low mass stars: less than 2 MSUN

Intermediate stars: between 2 & 8 MSUN

 High mass stars: greater than 8 MSUN
                        Remember ?
                Solar Thermostat




Decline in core temperature      Rise in core temperature
causes fusion rate to drop, so   causes fusion rate to rise, so
core contracts and heats up      core expands and cools down
  Life as a Low Mass Star
Moving off the Main Sequence
  Life as a Low Mass Star
Moving off the Main Sequence
                 Observations of star
                   clusters show that a star
                   becomes larger, redder,
                   and more luminous after
                   its time on the main
                   sequence is over
  Life as a Low Mass Star
Moving off the Main Sequence
                     CORE
 Life as a Low Mass Star 1
Moving off the Main Sequence

                         In the core:
                  Hydrogen           He
                   H burning continues in a
                   SHELL around the core.

                  What happens to
                  gravitational equilibrium?
                 He Core + H-burning shell
                 produce more energy than
                 needed for pressure support
                   Envelope expands


                    Envelope cools
            Earth’s Fate




Sun’s radius will grow to near current
         radius of Earth’s orbit
Sun will expand beyond Earth’s orbit!
 Life as a Low Mass Star 2
Moving off the Main Sequence
                     CORE
Evolution off the Main Sequence:
  Expansion into a Red Giant



                       H burning continues
                       in a shell around the
                       core.

                     Heat from shell burning
                     radiates OUT and IN



                      Core heats up until the
                       temperature reaches
                           100,000 K.
 Life as a Low Mass Star 2
Moving off the Main Sequence
                        CORE
                  H-burning shell keeps
                  dumping He onto the core.
                  He-core gets denser and
                  hotter until the next stage
                  of nuclear burning can
                  begin in the core:
                  He fusion through the


                  “Triple-Alpha Process”
                  4He   + 4He  8Be + g
                  8Be   + 4He  12C + g
                 Helium Fusion
                 “Triple-Alpha Process”




Helium fusion requires higher temperatures than hydrogen
fusion—larger charge leads to greater repulsion

Fusion of two helium nuclei doesn’t work, so helium fusion
must combine three He nuclei to make carbon
 Life as a Low Mass Star 3
Moving off the Main Sequence
                     CORE
 Life as a Low Mass Star 3
Moving off the Main Sequence
                     CORE
                  The core becomes hot
                  enough to fuse
                  helium into carbon.

                  Helium burning star:
                  Helium fusion begins with
                  a FLASH!

                  The core then expands,
                  slowing the rate of
                  hydrogen shell
                  burning and the star’s
                  outer layers shrink.
H burning core




                 He burning core
            Earth’s Fate




                     He burning core




Sun’s radius will grow to near current
         radius of Earth’s orbit
Thermal Pressure
 The main form of pressure in most stars
       Depends on heat content
       which comes from the nuclear fusion in the core.



Is there anything else that keeps a star from collapse?

     Yes --- pressure due to degeneracy
                Helium Flash
• In a low-mass red giant electron degeneracy
  pressure supports core (no thermal pressure)

• Core temperature rises rapidly when helium
  fusion begins. But because of the
  degeneracy, the core DOES NOT inflate.




• Helium fusion rate skyrockets until thermal
  pressure takes over and expands core again
             Degeneracy Pressure
10 people; lots of chairs   Take away the chairs,
                            leave 9 or less chairs.
                            Still 10 people.




Laws of quantum mechanics prohibit two
electrons from occupying same state in same
place
Pressure doesn’t depend on the heat content
Degenerate Matter
            Matter in the He core has
            no energy source left.
            Thermal pressure is ot
            enough to resist and
            balance gravity to resist
            and balance gravity
            Matter assumes a new
            state, called

            degenerate matter:
            Pressure in degenerate
            core is due to the fact that
            electrons can not be
            packed arbitrarily close
            together and have small
            energies.
Life Track after Helium Flash
                       • Observations
                         of star
                         clusters
                         agree with
                         those models

                       • Helium-
                         burning stars
                         are found in
                         a horizontal
                         branch on
                         the H-R
                         diagram
 Life as a Low Mass Star 4
Moving off the Main Sequence
                      CORE

                          Red Giant
                      Horizontal Branch
                Double shell-burning red giant:
                         Helium shell burning
                         begins around the
                         inert carbon core
                         after the core helium
                         is exhausted.
                         The star enters 2nd
                         red giant stage
                         with fusion in shells
                         of hydrogen and
                         helium.
 Life as a Low Mass Star 5
Moving off the Main Sequence
                     CORE

                        Red Giant
                    Horizontal Branch

                   Planetary nebulae:
                   The dying star
                   expels its outer
                   layers and leaves
                   behind the exposed
                   inert core.
Sun will expand beyond Earth’s orbit!
      Double Shell Burning
Helium fusion stops--> Carbon in the core
Helium fuses into Carbon in a shell around the
  carbon core
H fuses to He in a shell around the helium layer
                                He -->C
                      C         H -->He



This double-shell burning stage never reaches
  equilibrium—fusion rate periodically spikes
  upward in a series of thermal pulses
With each spike, convection dredges carbon up
  from core and transports it to surface
Life as a Low Mass Star -- 6
                     Planetary nebulae:
                     The dying star
                     expels its outer
                     layers and leaves
                     behind the exposed
                     inert core.



                          Core of star
Planetary Nebulae
               • Double-shell
                 burning ends
                 with a pulse
                 that ejects the
                 H and He into
                 space as a
                 planetary
                 nebula

               • The core left
                 behind
                 becomes a
                 white dwarf
• Double-shell
  burning ends with
  a pulse that ejects
  the H and He into
  space as a
  planetary nebula

• The core left
  behind becomes a
  white dwarf
Life as a Low Mass Star -- 7
             White dwarf:
             The remaining white dwarf is
             made primarily
             of CARBON and OXYGEN
             because the low mass
             star never grows hot enough
             to produce heavier
             elements.
A diamond weighing 10 billion trillion trillion carats
is at the heart of a dead white dwarf star nicknamed
Lucy in this conception by an artist at the
Harvard-Smithsonian Center for Astrophysics
                                                   Lucy in the Sky is a Diamond
The largest diamond ever found is not on Earth, but faraway across the galaxy. It's the burned out corpse of a star named BPM
37093 only about 50 lightyears away from Earth in the region of the sky we refer to as the constellation Centaurus. The white
dwarf star is a chunk of crystallized carbon that weighs 5 million trillion trillion pounds. That would equal a diamond of 10 billion
trillion trillion carats.

Lucy. After it was discoverd in 2004, astronomers nicknamed the space diamond Lucy after the Beatles song Lucy In The Sky
With Diamonds. Lucy, also known as BPM 37093 and V*886 Cen, is the 886th variable star in the constellation Centaurus.

Star of Africa. By comparison, the largest such precious stones on Earth are the 545-caret Golden Jubilee Diamond and the 530-
carat Great Star of Africa. The Golden Jubilee Diamond was found in 1985 and is in Thailand's Royal Palace as part of the crown
jewels. The Great Star of Africa was found in 1905 and is in the Tower of London as part of the Crown Jewels of England.

White dwarf. A white dwarf is the hot cinder left behind when a star uses up its nuclear fuel and dies. It is made mostly of carbon
and oxygen. and surrounded by a thin layer of hydrogen and helium gases. The Sun's diameter is 870,000 miles (1.4 million km).
Lucy is tiny at a mere 2,500 miles (4,000 km) diameter. The Sun is 109 times the diameter of Earth. Lucy is only about 2/3rds the
size of Earth. That's tiny for a star. However, Lucy's mass is about the same as our Sun. That's a lot of weight in a tiny ball.
While Lucy is a dead star now, it used to shine like our Sun. Lucy is very dim now, shining with only 1/2000th of the Sun's visual
brightness.

What is Lucy? Lucy is the most massive pulsating white dwarf currently known. Like other white dwarfs, Lucy probably is
composed mostly of carbon and oxygen created by the past thermonuclear fusion of helium nuclei. Lucy has a very thin
atmosphere of hydrogen and helium. The atmosphere of our Sun is mostly hydrogen and helium. Astronomers say that, similarly,
our Sun will deplete its nuclear fuel and die in another five billion years, and then become a white dwarf like Lucy. Then, about two
billion years after that, the cinder Sun will be a similar diamond.

How do they know? Astronomers had suspected since the 1960s that the interiors of white dwarfs would be crystallized and
Lucy seems to confirm that. In its death throws, the core of a star like Lucy or our own Sun becomes exposed and slowly cools
down over time. Such a star begins to pulsate when the core surface temperature drops to about 12,000 degree. By comparison,
the Sun's core temperature now is about 27,000,000°F (15,000,000°C). Its surface temperature is about 11,000°F
(6,000°C). Lucy pulsates like a giant gong. Its internal pulsations are something like seismic waves inside Earth. Astronomers
measured the pulsations to figure out Lucy's carbon interior was solidified (crystallized). Astronomers measured the pulsations
hidden in Lucy's interior in the same way geologists use seismographs to measure earthquakes inside Earth.

Where to look. Lucy is not visible from Earth with the unaided eye. It must be viewed with a telescope and is best seen from
Earth's Southern Hemisphere during March-June. www.spacetoday.org/images/DeepSpace/Stars/LucyDiamondStarWhiteDwarf.jpg
                Small mass stars
     – H fusion in core (main sequence)
     – H fusion in shell around contracting core
       (red giant)
     – He fusion in core (horizontal branch)
     – Double-shell burning (red giant)
     – Ejection of H and He in a planetary
       nebula leaves behind an inert white dwarf
• Fusion ends in a low-mass star because the core
  temperature never grows hot enough for fusion of
  heavier elements (some He fuses to C to make
  oxygen)
• Electron degeneracy pressure supports the white
  dwarf against gravity
Life as a High Mass Star

      > 8 MSUN
        Life as a High Mass Star
       Moving off the Main Sequence

Main sequence: 4 H             1 He
                Uses CNO cycle
                NOT proton proton cycle



Later stages of high-mass stars are similar to
  those of low-mass stars:
   Hydrogen core fusion (main sequence)
   Hydrogen shell burning (supergiant)
   Helium core fusion (supergiant)
  Life as a Low Mass Star
Moving off the Main Sequence
 Life as a High Mass Star
Moving off the Main Sequence
                       Supernova




   M > 8 Msun
Iron is dead end for fusion because
nuclear reactions involving iron do not
release energy

(Fe has lowest mass per nuclear particle)
                 Supernova Explosion




• A large mass star is degenerate
• Core electron degeneracy pressure goes away because
  electrons combine with protons, making neutrons and
  neutrinos neutron degeneracy
• Neutrons collapse to the center, forming a neutron star
Energy and neutrons released in supernova
explosion enable elements heavier than iron to
form, including Au and U
                              Supernova remnant

                              • Energy released
                                by collapse of core
                                drives outer layers
                                into space

                              • The Crab Nebula
                                is the remnant of
                                the supernova
                                seen in A.D. 1054
           Supernova 1987A




•   The closest supernova in the last four
    centuries was seen in 1987
    Rings Around Supernova 1987A




•   The supernova’s flash of light caused rings
    of gas around the supernova to glow
       Impact of Debris with Rings




•   More recent observations are showing the
    inner ring light up as debris crashes into it
           High mass stars

– They are similar to the life stages of a
  low-mass star

– Higher masses produce higher core
  temperatures that enable fusion of
  heavier elements

– Iron core collapses, leading to a
  supernova
How does a star’s mass
determine its life story?
How are the lives of stars with
close companions different?
Remember that half the stars are binary
The binary star Algol consists of
a 3.7 MSun main sequence star and
a 0.8 MSun subgiant star.

What’s strange about this pairing?

How did it come about?
Stars in Algol are close
enough that matter can
flow from subgiant
onto main-sequence
star
           BIG star            little star




BIG star evolves fast   little star evolves slower
                 MASS TRANSFER




BIG star becomes a red giant and expands
Because of gravity, the BIG star loses mass to little star
little star is bigger; big star is a subgiant
                 Summary

Mass determines how high a star’s core
 temperature can rise and therefore
 determines how quickly a star uses its
 fuel and what kinds of elements it can
 make

Stars with close companions can exchange
  mass, altering the usual evolution of stars
             Stellar Evolution Concepts


1.   Mass rules
2.   Variety of fusion reactions
3.   Helium Flash
4.   Degeneracy
End of chapter 12

				
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