ISP205 Lecture

ISP205 Lecture #17, March 13, 2001

Announcements:
1. Reminder: Homework Set 6: Due March 20 !
  But recommended as exercise for exam.
2. Bonus Point Project #2 on web site
  Your favorite planet property web link
  deadline: March 18, 2001
3. Bonus Point Project #3 on web site
  The spring break astronomy related story contest
  deadline: March 26, 2001
Meteors (picture)
1. Meteor: bright streak in the sky, produced by a small
  object (usually smaller than "pea" size) hitting earth
  being vaporized by friction in atmosphere at
  80-130 km height
2. Most meteors are produced by comet dust
3. Comets leave sometimes trails of dust.
  If earth passes through such a trail: meteor showers
  (see list of well known dust trails)
  (best viewing: after midnight, why ?)
Meteorite:
rock from space that is big enough to reach earth
1. Falls: Meteorites observed falling and recovered
  Finds: Meteorites found (identified by their properties)
2. Types of meteorites:
  1. Primitive stones (stone, undifferentiated)
  2. Differentiated stones (stone, part of differentiated body)
  3. Irons (metallic nickel-iron)
  4. Stony Irons (mix of nickel-iron grains and stone)
3. Famous meteorites:
  1. Allende (1969) (picture)
    probably several tons - shattered into many pieces
    primitive stone, grains older than solar system made
    in previous generations of stars
  2. Murchison (1969) (picture)
    Comet origin (lots of water), complex organic molecules,
    amino acids
  3. Meteorites from Mars
  4. Meteorite from Asteroid Vesta
  5. Sudbury basin: huge Nickel-Iron meteorite hit 1.8 billion
    years ago (30x60 mile crater)
    Now biggest mining operation in Canada and worlds major
    Nickel supply
  6. Hodges meteorite hit Annie Hodges Nov 30, 1954
    (~8 pound meteorite).
4. Primitive meteorites are important because they preserved
  state of solar system before/during its formation
5. Radioactive dating of primitive meteorites gives an
  age of the solar system of 4.54 billion years.
6. The composition of primitive meteorites matches
  the composition of the sun
Formation of the solar system
1. Any Theory of the formation of the solar system has to explain
  the observed features:
  1. All planets orbit counterclockwise (seen from north)
  2. All planets orbit roughly in one plane (ecliptic)
  3. Most planets and sun spin counterclockwise (seen from north)
    but Venus does not
  4. Most planets and sun spin in ecliptic
    but Uranus and Pluto do not
  5. Inner planets consist mainly of iron and rock
    Outer planets consist mainly of H, He gas and,
    further out, Methane and Water ices
  6. Age: 4.5 billion years
2. The best theory we have is the "Solar Nebula" Theory:
  (first formulated 1755 by Kant)
  Sequence of events according to that theory:
  1. A spherical, spinning nebula of gas and dust contracts
    (because of gravity and collisions)
  2. Because of angular momentum it spins faster (DEMO)
  3. Because of rapid spin it flattens into disk (DEMO)
  4. Release of potential energy heats (DEMO)
  5. Most matter concentrated in center: forms a hot gas ball
    (proto sun)
  6. Drops of matter condense in cooler outer regions
    (sequence of condensation - see picture)
  7. Drops collide and from planetesimals
    Planetesimals collide and from planets
3. Explains general features, but not:
  1. Retrograde motion of Venus, tipped spin axis of
    Uranus and Pluto, Earths moon:
    Need to assume later collisions
  2. Slow rotation of sun (24 day, 16 hours spin period).
    The sun has only 2% of angular momentum, but 99% of mass. Magnetic breaking ?
Sun - Introduction (picture)
1. Facts:
  1. The sun is a hot, bright ball of gas
  2. The sun is huge (earths fits 1.3 million times into sun)
  3. The sun is heavy (300,000 times earth)
  4. The mean density is 1.4 g/cm3
  5. The mean power output is 3.8 x 10²⁶ Watts
    (this is called "Luminosity")
  6. Of this power, the earth reach 1370 Watts/m²
  7. Overview picture and movies 1,2
2. The Photosphere
  1. Visible part of the sun (beyond sun is opaque)
    photons from the photosphere can reach earth
    photons from beneath are absorbed and reemitted
    many times before they reach the photosphere
  2. Temperature: 5800 K
  3. Spectrum: shows absorption lines (pic ?)
  4. Composition of sun: 92% Hydrogen, 7.8% Helium
    First found by Cecilia Payne-Gaposchkin 1925
3. The Chromosphere
  1. 2000-3000 km thick thinner layer on top of
    photosphere
  2. Hot ! (~10000 K) - Why ? Connecting magnetic
    field loops
  3. Emission lines of highly ionized atoms
    He was first found here
  4. Reddish color from H emission (656 nm)
  5. Visible from earth only during solar eclipse
    (H_alpha picture, He emission picture, spicules)
4. The Corona (pictures)
  1. The outermost part of the suns atmosphere
  2. Very low density, but very hot
    (several million degree Kelvin)
  3. Highly ionized ions produce unusual emission
    lines - iron that lost 12 of its 28 electrons
    was first thought to be a new element coronium
5. Solar Wind
  1. Stream of electrons and protons leaving the sun
    at high speeds (900,000 miles/hour)
  2. Reaches whole solar system
  3. Responsible for comet tails and aurorae
6. Sun Spots
  1. Photosphere has dark spots (discovered by Galileo)
    which are ~1500K cooler (picture)
  2. From splitting of absorption lines we know that
    sunspots are created when magnetic field lines
    poke out of suns surface and back in (show up
    in pairs).
    Magnetic field slows heat transport from beneath
  3. Number of sun spots varies in 11 year cycle
    (graphs)
  4. In 1680 there was the Maunder Minimum
  5. Above sunspots chromosphere shows plages
    (picture)
7. Solar Flares and Prominences
  1. Prominences: gas flows extending beyond suns
    atmosphere - sometimes as loops following magnetic
    field lines (picture)
  2. Flares are violent explosions that cause solar "storm"
    (coronal mass ejection) and radiation flash
    (last 5-10 minutes).
    They are dangerous for astronouts
    (movie)