The%20Memphis%20Astronomical%20Society%20Memphis,%20Tennessee,%20USA

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TheMemphis Astronomical SocietyMemphis,
Tennessee, USA
GREETINGSfrom
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Christian Brothers UniversityMemphis,
Tennessee, USA
and from
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to the
Mornington Peninsula Astronomical
Society Frankston, Victoria, Australia
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STELLAREVOLUTIONDr. William J. Busler19
July, 2006
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Ejnar Hertzsprung 1873 - 1967
Henry Norris Russell 1877 - 1957
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HERTZSPRUNG RUSSELL DIAGRAM

• About 1910, Ejnar Hertzsprung in Denmark and
  Henry Norris Russell in the U.S. (Princeton)
  independently tried to see if there was any
  correlation between the absolute magnitude (or
  luminosity) of stars and their spectral type (or
  temperature).
• They plotted the spectral type (O through M)
  along the x-axis, i.e., decreasing temperature.
• On the y-axis, they plotted the absolute
  magnitude (decreasing upwards) or luminosity
  (increasing upwards).

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• Hertzsprung- Russell Diagram

Blue White Y/W Yellow
Orange Red
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HERTZSPRUNG RUSSELL DIAGRAM

• Hertzsprung and Russell found that most stars,
  rather than being randomly distributed, were
  concentrated in a band from upper left (hot and
  luminous) to lower right (cool and dim).
• There were also lesser concentrations of stars in
  the upper right corner (cool and luminous) as
  well as at the lower left (hot and dim).
• Finally, there were a few stars scattered in
  other areas between the band and the upper
  right across the top and down the left side.

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• H-R Diagram
• The main band of stars running from upper left
  to lower right was called the main sequence,
  since they thought stars evolved in that
  direction.
• This erroneous theory gave rise to the early
  and late designations for spectral sub-classes.

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• H-R Diagram
• The stars in the upper right quadrant were
  called red giants and red super-giants, since
  they must have a huge surface area in order to be
  so luminous at such a low temperature.

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• H-R Diagram
• By a similar line of reasoning, the stars in the
  lower left-central area were called white
  dwarfs.
• They must be very small if they appear dim while
  having a rather high surface temperature.

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• Hertzsprung-Russell Diagrams for the nearest
  (mostly dim) and the brightest stars (many not on
  the main sequence).

Blue White Yellow Red
Blue White Yellow Red
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• H-R Diagram
• How are all these types of stars related??
• This will be
• our topic this
• evening!

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STAR FORMATION I. Dark Nebulae

• Star formation begins in the dark nebulae, such
  as the Horsehead Nebula in Orion or the Coal
  Sack in the Southern Cross.
• Dark nebulae are found in the spiral arms of
  galaxies.
• They obscure the light from stars and bright
  nebulae behind them. (Formerly considered
  holes in the sky.)
• Dark nebulae are also known as giant molecular
  clouds.

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The Milky Way in Sagittarius Astrophotograph by
David Talent / NOAO
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• Coal Sack in The Milky Way in Crux
• Astrophotograph by Hans Vehrenberg

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Astrophotograph by Philip Perkins

• The Horsehead Nebula in Orion

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• NGC 7331 in Pegasus
• Astrophotograph by George Greaney

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• M33 in Triangulum
• Astrophotograph by George Greaney

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STAR FORMATION I. Dark Nebulae

• Composition Mostly hydrogen (95) and helium
  (3) traces of other elements (2), in
  2nd-generation stars and beyond.
• Temperature Only a few degrees Kelvin (?K)
  close to absolute zero.
• Size Several hundred light-years in diameter.
• Other examples Dark lanes in the Lagoon,
  Trifid, and Orion Nebulae.

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• M8, The Lagoon Nebula in Sagittarius
• Astrophotograph by Mark Sibole

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• M20,
• The Trifid
• Nebula
• In
• Sagittarius
• Astrophotograph by
• David Hanon

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• M42, The Great Nebula in Orion
• Astrophotograph by David Hanon

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• North
• America
• Nebula
• in
• Cygnus
• Astrophotograph
• by
• Philip Perkins

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STAR FORMATION II. Condensation

• The force of gravity between molecules in the
  cloud, along with turbulence, causes condensation
  to begin at the most concentrated points.
  (Remember The temperature is very low!)
• This process is accelerated, even triggered, by
  supernova shock waves and wind or light
  pressure from other stars perhaps gravitational
  waves from the rotating nucleus of the galaxy.

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STAR FORMATION II. Condensation

• This process is analogous to the condensation of
  raindrops within clouds.
• As condensation progresses, the nebula becomes
  fragmented into Bok globules, which are a few
  light-years in diameter.
• Within each globule, accelerated gravitational
  collapse generates heat.
• The globules are not hot enough to glow, but they
  emit infrared radiation.

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STAR FORMATION III. Protostars

• When the Bok globules reach 100?K
  (-173?C), they are called protostars.
• While still surrounded by gas and dust from the
  nebula, they are cocoon stars.
• As the protostar continues to contract, it
  becomes hotter, and eventually begins to glow.

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STAR FORMATION III. Protostars

• As the protostar approaches the Main Sequence, it
  ejects the leftover cocoon material into
  bipolar beams, forming a Herbig-Haro object.
• If the protostar is a type G, K, or M, it is now
  a T Tauri star ? still not on the Main
  Sequence, but directly visible.
• New stars seem to form in chains or loops there
  may be dozens to hundreds within a nebula.

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STAR FORMATION III. Protostars

• The time required for a globule to become a T
  Tauri star depends on its mass
• A 1-solar-mass star takes about 50 million years
  to get near the Main Sequence.
• A 10-solar-mass star goes through the contraction
  process more rapidly, taking about 200,000 years.
• A protostar considerably less massive than the
  Sun, e.g., one which will become a red dwarf, may
  take hundreds of millions of years to contract.

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Pre-Main-Sequence evolutionary tracks of stars of
different masses
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The young open star cluster NGC 2264 in
Monoceros. Notice in the H-R diagram that most of
the smaller stars have not yet reached the Main
Sequence, while the more massive stars have.
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STAR FORMATION IV. Emission Nebulae

• Dark nebulae evolve into emission nebulae as the
  new stars forming inside heat them to the point
  of glowing.
• New protostars excite the hydrogen gas to emit
  red light.
• Frequently, new stars are seen embedded in a
  glowing nebula.
• Examples Lagoon Nebula (M8 in Sagittarius)
  Orion Nebula (M42).

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• As an excited hydrogen atom returns to its ground
  state, it emits the extra energy in the form of a
  photon with a certain wavelength.

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• Each energy transition within an atom gives rise
  to a photon of a particular wavelength.

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• M8, The Lagoon Nebula in Sagittarius
• Astrophotograph by Mark Sibole

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• M42, The Great Nebula in Orion
• Astrophotograph by David Hanon

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M16, the Eagle Nebula in Serpens, NGC 6611 As
trophotograph by David Malin Anglo-Australian
Observatory
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STAR FORMATION V. Reflection Nebulae

• Eventually, nearly all of the dust and gas in the
  emission nebula has been incorporated into the
  new stars.
• Thermonuclear fusion begins in the new stars,
  marking the beginning of their life
• 4 H ? He Energy.

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STAR FORMATION V. Reflection Nebulae

• Starlight reflects off the remaining wisps of
  nebulosity, creating a reflection nebula.
• Stars emit continuous spectra, rather than the
  bright-line spectra of emission nebulae.
• Therefore, a reflection nebula has the same
  continuous spectrum as the nearby star whose
  light is being reflected.
• Examples Trifid Nebula (M20 in Sagittarius)
  Pleiades (M45 in Taurus).

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• M20, the
• Trifid Nebula
• in
• Sagittarius
• Astrophotograph
• by
• Philip Perkins

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• IC 405, The Flaming Star Nebula in Auriga
• Astrophotograph by David Hanon

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• M45, The Pleiades in Taurus
• Astrophotograph by George Greaney

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The Horsehead Nebula in Orion, IC
434 Astrophotograph by Chuck Vaughn
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STAR FORMATION VI. Open Star Clusters

• Finally, all traces of the original nebula are
  gone.
• Most of the gas and dust has been incorporated
  into the new stars.
• The remainder of the nebulosity is dispelled by
  the heat and light from the new stars.
• All that remains is a new open (galactic) star
  cluster.
• Examples Beehive (M44 in Cancer), Hyades
  (Taurus), Jewel Box (Crux).

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• M44, The Beehive Star Cluster in Cancer
• Astrophotograph by Robert Gendler

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• NGC 4755,
• The Jewel Box
• Star Cluster in Crux
• Astrophotograph by David Malin
• (Anglo-Australian Observatory)

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• NGC 3293, Open Star Cluster in Carina
• Astrophotograph by David Malin (Anglo-Australian
  Observatory)

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STELLAR EVOLUTION VI-a. Globular Star Clusters

• Globular star clusters probably form in the same
  way, with some differences
• Globular clusters were formed at the time our
  Galaxy was forming, not continuously, as with
  open clusters.
• Globular clusters are located in a halo
  surrounding the center of the Galaxy, rather than
  in the spiral arms.
• Examples Omega Centauri,
  M13 (Hercules).

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• The Globular Star Cluster Omega Centauri
• Astrophotograph by David Hanon

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• M13, Globular Cluster in Hercules
• Astrophotograph by Tim Puckett

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MAIN-SEQUENCE STARS

• After new stars are formed, they spend most of
  the rest of their lives on the Main Sequence.
• On the average, one new star is formed each year
  in the Milky Way galaxy.
• The exact point of a stars location on the Main
  Sequence depends upon its color (temperature) and
  luminosity.
• As long as a star remains on the Main Sequence,
  it converts its hydrogen into helium by
  thermo-nuclear fusion, which releases heat and
  light.
• The energy released by TNF exactly balances the
  collapsing force of the stars gravity.

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At every point inside a star, there is a
hydrostatic equilibrium, or balance between the
force of gravity and the pressure from heat.
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Overall Reaction 4 1H1 ? 2He4 2 1e0
2 ? 2 ?
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MAIN-SEQUENCE STARS

• The lifetime of stars on the Main Sequence,
  once TNF begins, depends upon their mass and
  their luminosity.
• Lifetime (?)
  ? 1010 years.
• This formula is reasonable, since it gives the
  Sun its known lifetime of 10 billion years.
• However, the luminosity of a star depends upon
  its mass, since its mass generates the gravity
  which crushes the core, causing TNF.
• I.e., the greater a stars mass, the more rapidly
  TNF occurs, and the greater its luminosity.

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• The mass - luminosity relationship
• Lsolar ? MSolar3.5
• Example If M 2, L 23.5 11.

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MAIN-SEQUENCE STARS

• Therefore, the lifetime of stars on the Main
  Sequence really only depends upon their mass.
• Substituting the mass-luminosity expression into
  the lifetime (?) equation
• Lifetime (?) ? 1010 years
  .
• Alternatively, ? ? 1010
  years .

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MAIN-SEQUENCE STARS

• Examples
• Sun ? 10 billion years its life expectancy
  is now half-over.
• Rigel L 60,000 ? 4 million
  years.
• Red Dwarf M 0.7 ? Msun L 0.35
• Lifetime (?) ? 1010
  years 20 billion years.
• Note that this is greater than the known age of
  the Universe therefore, all the red dwarf stars
  ever formed are probably still on the Main
  Sequence.

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MAIN-SEQUENCE STARS

• As individual stars in a star cluster reach the
  end of their lifetimes (TNF of hydrogen), they
  begin to leave the Main Sequence.
• The most massive (blue giant) stars are the first
  to leave the upper-left end of the Main Sequence
  will then be devoid of stars.
• Gradually, stars farther down and to the right
  peel off of the Main Sequence, moving upward
  and to the right on the Hertzsprung-Russell
  diagram.
• As a result, the age of a star cluster can be
  estimated by noting the point on the Main
  Sequence above which there are no stars left.

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• H-R Diagram for several star clusters

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• H-R Diagram for the fairly young Double Cluster
  in Perseus
• Note the presence of some blue giants as well as
  red supergiants

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• H-R Diagram for the old open cluster M67 in
  Cancer
• Note the absence of stars more luminous than the
  Sun on the Main Sequence

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INTERMISSION
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FINAL STAGES I. Red Dwarfs

• After new stars are formed, they spend most of
  the rest of their lives on the Main Sequence,
  converting their hydrogen into helium, which
  releases heat and light.
• At the end of their lifetimes (about 20 billion
  years), TNF stops in red dwarf stars.
• They will then slowly cool to darkness, becoming
  black dwarfs. (Their cores had been at about 10
  million ?K during TNF.)
• Recall that the Universe is not old enough for
  this to have happened yet.

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FINAL STAGES II. Solar-Type Stars

• At the end of their lifetimes (about 10 billion
  years), moderately small stars (such as the Sun)
  will have consumed about 10 of their hydrogen.
• The core therefore contains mostly He and some
  other heavier elements at a temperature of about
  20 million ?K.
• The first stage of TNF (4 H ? He Energy) then
  stops.
• But then the core ignites (the helium flash),
  and helium burning begins, starting with the
  triple alpha process

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Overall Reaction 3 2He4 ? 6C12 2 ?
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FINAL STAGES II. Solar-Type Stars

• The core fuses helium into carbon and heavier
  elements, all the way to iron.
• These TNF reactions release even more energy than
  in the first stage.
• The core temperature increases to about one
  trillion ?K.
• This causes the stars diameter to increase by a
  factor of 100 to 200.

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Structure of a 1-solar-mass red giant star,
showing the concentric shells in the core (about
the size of the Earth)
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FINAL STAGES II. Solar-Type Stars

• The stars outer layers, hundreds of millions of
  miles from the core, cool down the star has
  become a red giant.
• Although the red giant stars surface is cooler
  than when it was on the Main Sequence, its
  tremendous expansion actually increases its
  luminosity.
• This causes the stars position on the H-R
  diagram to shift up and to the right.

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FINAL STAGES II. Solar-Type Stars

• When the Sun becomes a red giant, the inner
  planets will drastically change.
• The atmospheres will be stripped away, and all
  volatile materials (including water) will be
  boiled off.
• No life will be possible under these conditions.
• The Earth will be glowing a dull red like the Sun
  itself the Sun will occupy about 75? in the sky.

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FINAL STAGES II. Solar-Type Stars

• Mercury and Venus will be engulfed by the outer
  layers of the Sun they will eventually spiral
  into its core.
• The expansion process takes place so rapidly that
  it causes the star to cast off its surface
  layers, forming a shell of gases known as a
  planetary nebula.
• Examples Dumbbell Nebula (M27 in Vulpecula),
  Ring Nebula (M57 in Lyra).

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FINAL STAGES II. Solar-Type Stars

• Gravitational contraction of the red giant star
  begins again, causing it to become hotter and
  more yellow, while maintaining its same
  luminosity.
• In other words, the star begins to move leftward
  across the upper middle of the H-R diagram.
• While doing so, it becomes unstable, pulsating in
  brightness and color.
• These Cepheid variables (and other types) are
  interesting in their own right.

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FINAL STAGES II. Solar-Type Stars

• Most intrinsic variables operate on a valve-like
  principle
• When the star is smaller, it is brighter and
  hotter, but its outer layers are opaque to
  radiation.
• This causes the star to expand, cool down, and
  become dimmer.
• However, the expanded star is transparent to
  radiation, which allows more energy to escape.
• This permits the star to shrink, causing the
  cycle to be repeated.

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FINAL STAGES II. Solar-Type Stars

• Eventually, such a star will gradually shrink,
  maintaining its white-hot temperature, but
  decreasing greatly in size and luminosity.
• In other words, the star becomes a white dwarf.
  (Example Sirius B.)
• A white dwarf is comparable to the Earth in size,
  but its density is about 6000 tons/ft3. (The
  stars volume has decreased by a factor of a
  million most of its mass is still there.)
• After further cooling, the star becomes a black
  dwarf -- a cosmic cinder. (Probably, none exist
  yet.)

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Unstable Variable
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• M97, The Owl Nebula in Ursa Major
• NOAO Astrophotograph

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• M27, The Dumbbell Nebula in Vulpecula
• Astrophotograph by Bob and Janice Fera

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• NGC 6543, The Cats Eye Nebula in Draco
• Hubble Space Telescope Astrophotograph

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The Helix Nebula in Aquarius, NGC 7293 Astroph
otograph by David Malin Anglo-Australian
Observatory
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• M57, The Ring Nebula in Lyra
• Astrophotograph by Chris Vedeler

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FINAL STAGES III. Sirius-A-Type Stars

• White Main-Sequence stars such as Sirius A
  probably evolve in the same manner as the Sun,
  but about 10 times faster.
• Their mass is about 3 solar masses.
• Their luminosity is about 25.
• Therefore, the lifetime of such a star on the
  Main Sequence is about one billion years.
• After leaving the Main sequence, Sirius-type
  stars become Red Giants, eject planetary nebulae,
  become intrinsic variables, and wind up as White
  Dwarfs.

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FINAL STAGES IV. Blue Giant Stars

• Blue Giant stars (e.g., Rigel, Spica) spend only
  about 4 million years on the Main Sequence while
  burning hydrogen.
• When helium burning begins, the star becomes a
  Red Supergiant (e.g., Antares, Betelgeuse), about
  350 million miles in diameter -- larger than the
  orbit of Mars.
• Eventually, TNF can no longer produce energy.
  Synthesizing elements heavier than iron actually
  requires energy.

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Structure of a 25-solar-mass red supergiant,
showing the concentric shells in the core (about
the size of the Earth)
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FINAL STAGES IV. Blue Giant Stars

• When fusion of elements heavier than iron begins,
  rapid gravitational collapse ensues, taking only
  a few hours.
• After the Supergiant collapses, its core
  rebounds, colliding with the infalling outer
  portions of the star.
• This produces what is known as a Type II
  Supernova explosion.

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Red Super-giant
White Dwarf
Red Giant
Type I-a
Type II
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FINAL STAGES IV. Blue Giant Stars

• This is rare, occurring only once every few
  hundred years in our Galaxy.
• Supernovae have been recorded in 1054 A.D. (-6th
  magnitude), 1572 (Tychos star, -4.1), and 1604
  (Keplers star, -2.2).
• When we observe a supernova in another galaxy, it
  frequently outshines the entire galaxy of a
  hundred billion stars.

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David Malin / AAO
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David Malin / AAO
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FINAL STAGES IV. Blue Giant Stars

• The Crab Nebula Story
• Early on the morning of July 4th, 1054 A.D.,
  Chinese astronomers noted the presence of a
  guest star in the constellation we call Taurus.
• The new star became so bright (-6th magnitude)
  that it outshone Venus and could be seen in broad
  daylight for several weeks.
• Before it faded from view, the Chinese carefully
  noted its position among the stars.
• Hundreds of years later, after the invention of
  the telescope, we looked back at that same place
  in the heavens, and this is what we saw ...

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• M1, The Crab Nebula in Taurus
• Astrophotograph by David Hanon

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FINAL STAGES IV. Blue Giant Stars

• The shock wave moving outward from the explosion
  fuses the elements in the outer part of the star,
  synthesizing all the heavy elements.
• These newly-formed elements are then blown into
  interstellar space, where they mix with dark
  nebulae and become incorporated into a new
  generation of stars.
• Some of these new stars probably have planets,
  which are contaminated with heavy elements.
• The Earth, whose lighter elements have been
  driven off by the Sun, is now made up mainly
  of these elements from star dust.

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Vela Supernova Remnant Astrophotograph by David
Malin Anglo-Australian Observatory
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• The Veil Nebula in Cygnus
• Astrophotograph by Jerry Lodriguss

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• The Veil Nebula in Cygnus (NGC 6992)
• Astrophotograph by Bob and Janice Fera

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• The Veil Nebula in Cygnus 52 Cygni Region
• Astrophotograph by Bob and Janice Fera

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• Horsehead Nebula in Orion
• Astrophotograph by Bob and Janice Fera

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• Proplyds (Protoplanetary Disks)
• in the Trapezium Region
• Hubble Space Telescope Image

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• Several Proplyds with T-Tauri Stars
• Hubble Space Telescope Images

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• NASA Photograph

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FINAL STAGES IV. Blue Giant Stars

• The inner portion of the collapsing star
  (supernova) is compressed by the shock wave at a
  tremendous pressure and temperature.
• The protons and electrons in the atoms of the
  core are fused into neutrons neutrinos are also
  released.
• 1H1 -1e0 ?? 0n1 ?.
• This compressed core thereby becomes a neutron
  star, only a few miles in diameter. (Very small
  star or very large atom!)
• Its density exceeds one billion tons/inch3.

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Red Super-giant
White Dwarf
Red Giant
Type I-a
Type II
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FINAL STAGES IV. Blue Giant Stars

• Due to the conservation of angular momentum, if
  the red supergiant star was rotating slowly, the
  resulting neutron star (being much smaller) will
  rotate rapidly.
• If a rapidly-rotating neutron star has hot
  spots on its surface, it will send out beams of
  radiation like a lighthouse.
• If we are in the line of sight of these
  searchlight beams, the neutron star will appear
  to be emitting pulses of radio waves, X-rays, or
  even visible light.
• Such an object is known as an LGM or pulsar.

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M1, The Crab Nebula in Taurus Astrophotograph by
David Malin, Anglo-Australian Observatory
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FINAL STAGES IV. Blue Giant Stars

• Further collapse of a neutron star is possible,
  if its mass exceeds 2 or 3 solar masses.
• The neutrons themselves collapse under the
  intense gravitational pressure.
• As the volume of the star goes to zero, its
  density goes to infinity. It is now a
  singularity -- a point in space.
• The escape velocity is greater than the speed of
  light therefore, its own light cant escape.
• Such an object is known as a black hole.

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FINAL STAGES IV. Blue Giant Stars

• Although the collapsed star is a point in space,
  it is surrounded by an imaginary spherical shell,
  inside of which vesc gt c.
• These points of no return are at the
  Schwarzschild radius or event horizon.
• The Schwarzschild radius depends directly on the
  mass of the black hole A 3-solar-mass black
  hole has a Schwarzschild radius of 9 km.
• As more material (e.g., from a companion star)
  falls into a black hole, its mass increases so
  does its Schwarzschild radius.

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• Evolutionary track of lightweight (solar) stars

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• Evolutionary track of heavyweight
• (e.g., 15-solar-mass) stars

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T H E E N D