Starry Monday at Otterbein

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Starry Monday at Otterbein
Welcome to

• Astronomy Lecture Series
• -every first Monday of the month-
• November 7, 2005
• Dr. Uwe Trittmann

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Todays Topics

• Classification of Stars
• The Night Sky in November

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Feedback!

• Please write down suggestions/your interests on
  the note pads provided
• If you would like to hear from us, please leave
  your email / address
• To learn more about astronomy and physics at
  Otterbein, please visit
• http//www.otterbein.edu/dept/PHYS/weitkamp.asp
  (Obs.)
• http//www.otterbein.edu/dept/PHYS/ (Physics
  Dept.)

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Classification of Stars

• We can classify stars by many categories
• Name
• Position
• Constellation
• Distance
• Color
• Temperature
• Size
• Brightness
• Spectra
• Features double stars, variable stars,

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How Stars Got Their Names

• Some have names that go back to ancient times
  (e.g. Castor and Pollux, Greek mythology)
• Some were named by Arab astronomers (e.g.
  Aldebaran, Algol, etc.)
• Since the 17th century we use a scheme that lists
  stars by constellation
• in order of their apparent brightness
• labeled alphabetically in Greek alphabet
• Alpha Centauri is the brightest star in
  constellation Centaurus
• Some dim stars have names according to their
  place in a catalogue (e.g. Ross 154)

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Positions of Stars

• The Celestial Sphere
• An imaginary sphere surrounding the earth, on
  which we picture the stars attached
• Axis through earths north and south pole goes
  through celestial north and south pole
• Earths equator
• Celestial equator

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Celestial Coordinates

• Earth latitude, longitude
• Sky
• declination (dec) from equator,/-90
• right ascension (RA) from vernal equinox, 0-24h
  6h90
• Examples
• Westerville, OH 40.1N, 88W
• Betelgeuse (a Orionis) dec 7 24
  RA 5h 52m

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But Whats up for you

• Observer Coordinates
• Horizon the plane you stand on
• Zenith the point right above you
• Meridian the line from North to Zenith to south

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depends where you are!

• Your local sky
• your view depends on your location on
  earth

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Constellations of Stars

• About 5000 stars visible with naked eye
• About 3500 of them from the northern hemisphere
• Stars that appear to be close are grouped
  together into constellations since antiquity
• Officially 88 constellations
  (with strict boundaries for
  classification of objects)
• Names range from mythological (Perseus,
  Cassiopeia) to technical (Air Pump, Compass)

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Constellations of Stars (contd)

• Orion as seen at night Orion as
  imagined by men

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Constellations (contd)

• Orion from the side
• ?Stars in a constellation are not connected in
  any real way they arent even close together!

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Distances to the Stars

• Parallax can be used out to about 100 light years
• The parsec
• Distance in parsecs 1/parallax (in arc seconds)
• Thus a star with a measured parallax of 1 is 1
  parsec away
• 1 pc is about 3.3 light years
• The nearest star (Proxima Centauri) is about 1.3
  pc or 4.3 lyr away
• Solar system is less than 1/1000 lyr

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Our Stellar Neighborhood
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Scale Model

• If the Sun a golf ball, then
• Earth a grain of sand
• The Earth orbits the Sun at a distance of one
  meter
• Proxima Centauri lies 270 kilometers (170 miles)
  away
• Barnards Star lies 370 kilometers (230 miles)
  away
• Less than 100 stars lie within 1000 kilometers
  (600 miles)
• The Universe is almost empty!
• Hipparcos satellite measured distances to nearly
  1 million stars in the range of 100 pc
• almost all of the stars in our Galaxy are more
  distant

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Brightness

• A measure of the apparent brightness
• Logarithmic scale
• Notation 1m.4 (smaller ?brighter)
• Originally six groupings
• 1st magnitude the brightest
• 6th magnitude the dimmest
• The modern scale is more complex
• The absolute magnitude is the apparent magnitude
  a star would have at a distance of 10 pc 2M.8

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Electromagnetic Spectrum
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Three Things Light Tells Us

• Temperature
• from black body spectrum
• Chemical composition
• from spectral lines
• Radial velocity
• from Doppler shift

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Black Body Spectrum (gives away the temperature)
Peak frequency

• All objects - even you - emit radiation of all
  frequencies, but with different intensities

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Measuring Temperatures

• Find maximal intensity
• ? Temperature (Wiens law)

Identify spectral lines of ionized elements ?
Temperature
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Wiens Law

• The peak of the intensity curve will move with
  temperature, this is Wiens law
• ? T const. 0.0029 m K
• So the higher the temperature T, the smaller
  the wavelength ?, i.e. the higher the energy of
  the electromagnetic wave

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Luminosity and Brightness

• Luminosity L is the total power (energy per unit
  time) radiated by the star
• Apparent brightness B is how bright it appears
  from Earth
• Determined by the amount of light per unit area
  reaching Earth
• B ? L / d2
• Just by looking, we cannot tell if a star is
  close and dim or far away and bright

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Measuring the Sizes of Stars

• Direct measurement is possible for a few dozen
  relatively close, large stars
• Angular size of the disk and known distance can
  be used to deduce diameter

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Sizes of Stars

• Dwarfs
• Comparable in size, or smaller than, the Sun
• Giants
• Up to 100 times the size of the Sun
• Supergiants
• Up to 1000 times the size of the Sun
• Note Temperature changes!

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Star Systems Binary Stars

• Some stars form binary systems stars that orbit
  one another
• visual binaries
• spectroscopic binaries
• eclipsing binaries
• Beware of optical doubles
• stars that happen to lie along the same line of
  sight from Earth
• We cant determine the mass of an isolated star,
  but of a binary star

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Visual Binaries

• Members are well separated, distinguishable

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Spectroscopic Binaries

• Too distant to resolve the individual stars
• Can be viewed indirectly by observing the
  back-and-forth Doppler shifts of their spectral
  lines

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Eclipsing Binaries (Rare!)

• The orbital plane of the pair almost edge-on to
  our line of sight
• We observe periodic changes in the starlight as
  one member of the binary passes in front of the
  other

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Spectral Classification of the Stars

• Class Temperature Color Examples
• O 30,000 K blue
• B 20,000 K bluish Rigel
• A 10,000 K white Vega, Sirius
• F 8,000 K white Canopus
• G 6,000 K yellow Sun, ? Centauri
• K 4,000 K orange Arcturus
• M 3,000 K red Betelgeuse

Mnemotechnique Oh, Be A Fine Girl/Guy, Kiss Me
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Spectral Lines Fingerprints of the Elements

• Can use spectra to identify elements on distant
  objects!
• Different elements yield different emission
  spectra

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Origin of Spectral Lines

• Atoms electrons orbiting nuclei
• Chemistry deals only with electron orbits
  (electron exchange glues atoms together to from
  molecules)
• Nuclear power comes from the nucleus
• Nuclei are very small
• If electrons would orbit the statehouse on I-270,
  the nucleus would be a soccer ball in Gov. Bob
  Tafts office
• Nuclei made out of protons (el. positive) and
  neutrons (neutral)

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• The energy of the electron depends on orbit
• When an electron jumps from one orbital to
  another, it emits (emission line) or absorbs
  (absorption line) a photon of a certain energy
• The frequency of emitted or absorbed photon is
  related to its energy
• E h f
• (h is called Plancks constant, f is
  frequency)

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Hertzsprung-Russell-Diagram

• Hertzsprung-Russell diagram is luminosity vs.
  spectral type (or temperature)
• To obtain a HR diagram
• get the luminosity. This is your y-coordinate.
• Then take the spectral type as your x-coordinate.
  This may look strange, e.g. K5III for Aldebaran.
  Ignore the roman numbers ( III means a giant
  star, V means dwarf star, etc). First letter is
  the spectral type K (one of OBAFGKM), the arab
  number (5) is like a second digit to the spectral
  type, so K0 is very close to G, K9 is very close
  to M.

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Constructing a HR-Diagram

• Example Aldebaran, spectral type K5III,
  luminosity 160 times that of the Sun

L
1000
Aldebaran
160
100
10
1
Sun (G2V)
O B A F G K M
Type
0123456789 0123456789 012345
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The Hertzprung-Russell Diagram

• A plot of absolute luminosity (vertical scale)
  against spectral type or temperature (horizontal
  scale)
• Most stars (90) lie in a band known as the Main
  Sequence

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Hertzsprung-Russell diagrams

• of the closest stars of the brightest stars

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Mass and the Main Sequence

• The position of a star in the main sequence is
  determined by its mass
• ?All we need to know to predict luminosity and
  temperature!
• Both radius and luminosity increase with mass

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Stellar Lifetimes

• From the luminosity, we can determine the rate of
  energy release, and thus rate of fuel consumption
• Given the mass (amount of fuel to burn) we can
  obtain the lifetime
• Large hot blue stars 20 million years
• The Sun 10 billion years
• Small cool red dwarfs trillions of years
• ?The hotter, the shorter the life!

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Preview Stellar Lifecycle

• Next Starry Monday
• How stars are born and die
• What makes stars shine
• Planetary nebulae are dead stars!
• and much more

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The Night Sky in November

• Back to standard time -gt earlier observing!
• Autumn constellations are up Cassiopeia,
  Pegasus, Perseus, Andromeda, Pisces ? lots of
  open star clusters!
• Mars at opposition
• Saturn is visible later at night

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Moon Phases

• Today (New Moon, 36)
• 11 / 8 (First Quarter Moon)
• 11 / 15 (Full Moon)
• 11 / 23 (Last Quarter Moon)
• 12/ 1 (New Moon)

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Today at Noon

• Sun at meridian, i.e. exactly south

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10 PM

• Typical observing hour, early November
• Mars
• Uranus at meridian
• Neptune

Moon
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South-East

• Plejades
• Mars at its brightest in Aries

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West

• The summer triangle is still hanging on

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Due North

• Big Dipper points to the north pole

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High up the Autumn Constellations

• W of Cassiopeia
• Big Square of Pegasus
• Andromeda Galaxy

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Andromeda Galaxy

• PR Foto
• Actual look

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South-East

• High in the sky
• Perseus and
• Auriga
• with Plejades and the Double Cluster

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South-West

• Planets
• Uranus
• Neptune
• Zodiac
• Capricorn
• Aquarius

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Mark your Calendars!

• Next Starry Monday January 9, 2005, 7 pm
• (this is a Monday
  )
• Observing at Prairie Oaks Metro Park
• Friday, November 18, 730 pm
• Web pages
• http//www.otterbein.edu/dept/PHYS/weitkamp.asp
  (Obs.)
• http//www.otterbein.edu/dept/PHYS/ (Physics
  Dept.)

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Mark your Calendars II

• Physics Coffee is every Wednesday, 330 pm
• Open to the public, everyone welcome!
• Location across the hall, Science 256
• Free coffee, cookies, etc.

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Its Nuclear Fusion !

• Atoms electrons orbiting nuclei
• Chemistry deals only with electron orbits
  (electron exchange glues atoms together to from
  molecules)
• Nuclear power comes from the nucleus
• Nuclei are very small
• If electrons would orbit the statehouse on I-270,
  the nucleus would be a soccer ball in Gov. Bob
  Tafts office
• Nuclei made out of protons (el. positive) and
  neutrons (neutral)

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Nuclear fusion reaction

• 4 hydrogen nuclei combine (fuse) to form a helium
  nucleus, plus some byproducts
• Mass of products is less than the original mass
• The missing mass is emitted in the form of
  energy, according to Einsteins famous formulas
  
• E mc2
• (the speed of light is very large, so there is a
  lot of energy in even a tiny mass)

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Further Reactions Heavier Elements
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Could We Use This on Earth?

• Requirements
• High temperature
• High density
• Very difficult to achieve on Earth!

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Nuclear Fission

• Problems limited fuel supply, dangerous
  byproducts, expensive technology, limited
  lifetime of power plant due to radiation

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The Solar Neutrino Problem

• We can detect the neutrinos coming from the
  fusion reaction at the core of the Sun
• The results are 1/3 to 1/2 the predicted value!
• Possible explanations
• Models of the solar interior are incorrect
• Our understanding of the physics of neutrinos is
  incorrect
• Something is horribly, horribly wrong with the
  Sun
• 2 is the answer neutrinos oscillate

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Homework Luminosity and Distance

• Distance and brightness can be used to find the
  luminosity
• L ? d2 B
• So luminosity and brightness can be used to find
  Distance of two stars 1 and 2
• d21 / d22 L1 / L2 (since B1 B2
  )

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Stars II - Lifecycle
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The Fundamental Problem

• We study the subjects of our research for a tiny
  fraction of its lifetime
• Suns life expectancy 10 billion (1010) years
• Careful study of the Sun 370 years
• We have studied the Sun for only 1/27 millionth
  of its lifetime!

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Suppose we study human beings

• Human life expectancy 75 years
• 1/27 millionth of this is about 74 seconds
• What can we learn about people when allowed to
  observe them for no more than 74 seconds?

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Star Formation

• A stars existence is based on a competition
  between gravity (inward) and pressure due to
  energy production (outward)
• Stage 1 Contraction of a cold interstellar cloud
• Lasts about 2 million years
• Central temperature about 10 K
• Size tens of parsecs

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Star Formation (contd)

• Stage 2 Cloud contracts/warms, begins radiating
  almost all radiated energy escapes
• Duration 30,000 years
• Temperature 100 K at center, 10 K at surface
• Size about 100 times that of the solar system
• Stage 3 Cloud becomes dense ? opaque to
  radiation ? radiated energy trapped ? core heats
  up
• Duration 100,000 years
• Temperature 10,000 K at center, 100 K at
  surface
• Size the solar system

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Example Orion Nebula

• Orion Nebula is a place where stars are being born

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Orion Nebula (M42)
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Protostellar Evolution

• Stage 4 increasing temperature at core slows
  contraction
• Luminosity about 1000 times that of the sun
• Duration 1 million years
• Temperature 1 million K at core, 3,000 K at
  surface
• Still too cool for nuclear fusion!
• Size orbit of Mercury

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The T Tauri Stage

• Stage 5 (T Tauri)
• Violent surface activity
• high solar wind blows out the remaining stellar
  nebula
• Duration 10 million years
• Temperature 5?106 K at core, 4000 K at surface
• Still too low for nuclear fusion
• Luminosity drops to about 10 ? the Sun
• Size 10 ? the Sun

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Possible T Tauri Stars
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Jets from T Tauri Stars
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Path in the Hertzsprung-Russell Diagram

• Stages 1-5

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Observational Confirmation

• Preceding the result of theory and computer
  modeling
• Can observe objects in various stages of
  development, but not the development itself

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A Newborn Star

• Stage 6 Temperature and density at core high
  enough to sustain nuclear fusion
• Duration 30 million years
• Temperature 10 million K at core, 4500 K at
  surface
• Size slightly larger than the Sun
• Stage 7 Main-sequence star pressure from
  nuclear fusion and gravity are in balance
• Duration 10 billion years (much longer than all
  other stages combined)
• Temperature 15 million K at core, 6000 K at
  surface
• Size Sun

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Path in the Hertzsprung-Russell Diagram

• The new-born stars hops onto the main sequence

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Mass Matters

• Larger masses
• higher surface temperatures
• higher luminosities
• take less time to form
• have shorter main sequence lifetimes
• Smaller masses
• lower surface temperatures
• lower luminosities
• take longer to form
• have longer main sequence lifetimes

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Failed Stars Brown Dwarfs

• Too small for nuclear fusion to ever begin
• Less than about 0.08 solar masses
• Give off heat from gravitational collapse
• Luminosity a few millionths that of the Sun

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Main Sequence Lifetimes

• Mass (in solar masses) Luminosity
  Lifetime
• 10 Suns 10,000
  Suns 10 Million yrs
• 4 Suns
  100 Suns 2 Billion
  yrs
• 1 Sun
  1 Sun 10 Billion yrs
• ½ Sun
  0.01 Sun 500 Billion yrs

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Why Do Stars Leave the Main Sequence?

• Running out of fuel

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Stage 8 Hydrogen Shell Burning

• Cooler core ? imbalance between pressure and
  gravity ? core shrinks
• hydrogen shell generates energy too fast ? outer
  layers heat up ? star expands
• Luminosity increases
• Duration 100 million years
• Temperature 50 million K (core) to 4000 K
  (surface)
• Size several Suns

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Stage 9 The Red Giant Stage

• Luminosity huge ( 100 Suns)
• Core contains 25 of the stars mass and
  continues to shrink
• Strong stellar winds eject up to 30 of stars
  mass from surface
• Duration 100,000 years
• Temperature 100 ? 106 K (core) to 4000 K
  (surface)
• Size 70 Suns (orbit of Mercury)

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Lifecycle

• Lifecycle of a main sequence G star

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The Helium Flash and Stage 10

• The core becomes hot and dense enough to overcome
  the barrier to fusing helium into carbon
• Initial explosion followed by steady (but rapid)
  fusion of helium into carbon
• Lasts 50 million years
• Temperature 200 million K (core) to 5000 K
  (surface)
• Size 10 ? the Sun

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Stage 11

• Helium burning continues
• Carbon ash at the core forms, and the star
  becomes a Red Supergiant

• Duration 10 thousand years
• Central Temperature 250 million K
• Size gt orbit of Mars

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Stage 12

• Inner carbon core becomes dead it is out of
  fuel
• Some helium and carbon burning continues in outer
  shells
• The outer envelope of the star becomes cool
  enough for atoms to recombine with electrons, and
  becomes opaque solar radiation pushes it outward
  from the star
• A planetary nebula is formed

Duration 100,000 years Central Temperature 300
? 106 K Surface Temperature 100,000 K Size 0.1
? Sun
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Planetary Nebulae

• Eye of God Nebula

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Cats Eye Nebula
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Wings of the Butterfly Nebula
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• The Ring Nebula (M57)

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• Eskimo Nebula

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• Stingray Nebula

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• Ant Nebula

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Stage 13 White Dwarf

• Core radiates only by stored heat, not by nuclear
  reactions
• core continues to cool and contract
• Temperature 100 ? 106 K at core 50,000 K at
  surface
• Size Earth
• Density a million times that of Earth 1 cubic
  cm has 1000 kg of mass!

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Stage 14 Black Dwarf

• Impossible to see in a telescope
• About the size of Earth
• Temperature very low
• ? almost no radiation
• ? black!

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Evolution of More Massive Stars

• Gravity is strong enough to overcome the electron
  pressure (Pauli Exclusion Principle) at the end
  of the helium-burning stage
• The core contracts until its temperature is high
  enough to fuse carbon into oxygen
• Elements consumed in core
• new elements form while previous elements
  continue to burn in outer layers

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Evolution of More Massive Stars

• At each stage the temperature increases
• ? reaction gets faster
• Last stage fusion of iron does not release
  energy, it absorbs energy
• ? cools the core
• ? fire extinguisher

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Neutron Core

• The core cools and shrinks
• nuclei and electrons are crushed together
• protons combine with electrons to form neutrons
• Ultimately the collapse is halted by neutron
  pressure
• Most of the core is composed of neutrons at this
  point
• Size few km
• Density 1018 kg/m3 1 cubic cm has a mass of
  100 million kg!

Manhattan
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Formation of the Elements

• Light elements (hydrogen, helium) formed in Big
  Bang
• Heavier elements formed by nuclear fusion in
  stars and thrown into space by supernovae
• Condense into new stars and planets
• Elements heavier than iron form during supernovae
  explosions
• Evidence
• Theory predicts the observed elemental abundance
  in the universe very well
• Spectra of supernovae show the presence of
  unstable isotopes like Nickel-56
• Older globular clusters are deficient in heavy
  elements

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Review The life of Stars