Starry Monday at Otterbein

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Starry Monday at Otterbein
Welcome to

• Astronomy Lecture Series
• -every first Monday of the month-
• February 6, 2006
• Dr. Uwe Trittmann

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Todays Topics

• Lifecycle of Stars
• The Night Sky in February

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On the Web

• To learn more about astronomy and physics at
  Otterbein, please visit
• http//www.otterbein.edu/dept/PHYS/weitkamp.asp
  (Observatory)
• http//www.otterbein.edu/dept/PHYS/ (Physics
  Dept.)

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The Lifecycle of the Stars
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Reminder Hertzsprung-Russell-Diagrams

• 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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Reminder 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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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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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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The Fundamental Problem in studying the stellar
lifecycle

• 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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Theory and Experiment

• Theory
• Need a theory for star formation
• Need a theory to understand the energy production
  in stars ? make prediction how bight stars are
  when and for how long in their lifetimes
• Experiment observe how many stars are where when
  and for how long in the Hertzsprung-Russell
  diagram
• ? Compare prediction and observation

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Nuclear Fusion is the energy source of the Stars

• 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

• Start 4 2 protons ? End Helium nucleus
  neutrinos
• Hydrogen fuses to
  Helium

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Fusion is NOT fission!

• In nuclear fission one splits a large nucleus
  into pieces to gain energy
• Build up larger nuclei ?Fusion
• Decompose into smaller nuclei ?Fission

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Check Solar Neutrinos

• 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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Theory of Star Formation

• A stars existence is based on a competition
  between gravity (inward) and pressure due to
  energy production (outward)

Heat
Gravity
Gravity
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Star Formation Lifecycle

• Stage 1 Contraction of a cold interstellar cloud
• Stage 2 Cloud contracts/warms, begins radiating
  almost all radiated energy escapes
• Stage 3 Cloud becomes dense ? opaque to
  radiation ? radiated energy trapped ? core heats
  up

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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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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
• 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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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
• Size several Suns

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Stage 9 The Red Giant Stage

• Luminosity huge ( 100 Suns)
• Surface Temperature lower
• Core Temperature higher
• Size 70 Suns (orbit of Mercury)

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Lifecycle

• Lifecycle of a main sequence G star
• Most time is spent on the main-sequence (normal
  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 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 and
  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
• 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
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The Night Sky in February

• Long nights, early observing!
• Winter constellations are up Orion, Taurus,
  Gemini, Auriga, Canis Major Minor ? lots of
  deep sky objects!
• Saturn at its brightest

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Moon Phases

• Today (Waxing Gibbous, 67)
• 1/ 12 (Full Moon)
• 1 / 21 (Last Quarter Moon)
• 1 / 27 (New Moon)
• 3 / 6 (First Quarter 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 January
• Saturn

Mars Moon
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South-West

• Plejades
• Mars in Aries / Taurus

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Due North

• Big Dipper points to the north pole

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West 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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Zenith

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

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South

• The Winter Constellations
• Orion
• Taurus
• Canis Major
• Gemini
• Canis Minor

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The Winter Hexagon

• Sirius
• Procyon
• Pollux
• Capella
• Aldebaran
• Rigel

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East

• Saturn near Praesepe, an open star cluster

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Mark your Calendars!

• Next Starry Monday March 6, 2005, 7 pm
• (this is a Monday
  )
• Observing at Prairie Oaks Metro Park
• Friday, May 5, 900 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.