Lecture 4

Star Formation

The origin of the Solar System (thanks to Jorge Piekarewicz)

• formation of Milky Way

  • about one billion years after Big Bang, first stars and galaxies formed from primordial H and He

  • in most massive stars, heavier elements (including those necessary for life: C, N, and O) were ``cooked'' by nuclear fusion

  • within a few million years, these massive stars died in supernovae, spewing heavy elements

  • about 3 billion years after Big Bang, Milky Way started forming from light and heavy elements

    

  • Milky Way has diameter of 100,000 light-years and over 100 billion stars

  • in one spiral arm, the so-called ``Orion-Cygnus'' arm, lies the Sun, an inconspicuous G2 V star

• formation of the Sun

  We know Stars form from disks because we see these disks around other stars when they are young. Here is a picture (the best ever taken) of a young binary star with a ring that might be forming into a binary solar system. My old graduate student (Dan Potter) took this picture with the Gemini 8 meter telescope in Hawaii. Look here to see some of more very sharp images that I've taken in the past when I was working in Hawaii.

  

  • just like other stars, Sun formed when gravity pulled together cloud of interstellar gas (movie)

    

  • about 4.5 billion year ago, rotating ball collapsed into thin disk (movie)

  • Protosun (cloud of gas that became Sun) located in center

    Stellar Nucleosynthesis is the process but which stars generate energy. The core of a star is so hot and dence that atoms "FUSE" together into heavier elements. For example: H + H -> He or He + He + He -> C (movie of the fusion of Carbon)

    Once Carbon is made the way is clear to higher mass elements: He + C -> O or C + C -> Mg

    This is the process by which all of the elements larger than Lithium were produced from the original H, He from the Big Bang. So it is completely correct to state that we are each made of "star stuff".

• origin of planets
  • in the disk, solid material accumulates into larger and larger particles

  • particles (aka planetesimals) can be clustered into clumps of up to a few kilometers across (movie)

  • Protoplanets then form slowly by accretion of planetesimals of similar composition

  • after Protoplanet forms, accumulation of heat from radioactive decay of short-lived elements melts planet, allowing materials to differentiate (to separate according to their density)

  • differentiation causes heavy metals, such as iron and nickel, to settled in core, while lightest silicates float to surface

    

  • important clue to origin and formation of Solar System is the clear division of planets into two categories

    • terrestrial planets (Mercury, Venus, Earth, and Mars) lie in inner Solar System and are small, dense, rocky worlds

    • Jovian (Jupiter, Saturn, Uranus, and Neptune) lie in outer Solar System -- beyond asteroid belt -- and are large, gaseous, low-density worlds

    • Pluto does not fit either category very well -- it is small, like terrestrial planets, but lies far away from Sun and has low density just like Jovian planets

      Relative Sizes of Objects in Solar System

  • formation of Jovian planets
    • far from Sun, in coldest regions of space, icy planetesimals survived (movie)

    • at first, planetesimals were closely packed

    • over time, some of them collided gently enough to coalesce into larger objects, while others collided so fast that they were further broken up (movie)

    • in outer Solar System, four very large masses formed (four Jovian planets)

    • all of these planets grew to have masses of at least 15 times Earth's mass

    • once planet has grown to such large mass, it has strong enough gravitational pull to begin capturing gas directly from solar nebula, the material from which Sun formed, to form thick atmosphere of gas

      • solar nebula is 74% Hydrogen, 24% Helium, and 2% of heavier elements

      • thus, Jovian worlds are low-density worlds rich in hydrogen and helium

    • these large planets grew disks of their own, out of which moons condensed, leading to formation of minute solar systems (Jupiter has well over a dozen moons!)

      

      For an animation of Jupiter's day, click here.

  • formation of terrestrial planets
    • in the warmer regions near Sun, early planetesimals made of rock and metal, materials cooked billions of years ago in cores of massive stars

    • too many collisions for large planets to form

    • of four terrestrial planets, Earth is most massive (only 1/320 times the mass of Jupiter), Venus (with 82% percent of Earth's mass) is next, and then Mars (10% of Earth's mass) and Mercury (5% of Earth's mass) are next

    • surfaces were under constant bombardment that inhibited their growth

    • thus, terrestrial planets are not large enough to exert large pull on solar nebula and are poor in hydrogen and helium

    • even if terrestrial planets had hydrogen and helium, proximity to Sun would heat gases and cause them to escape

    • terrestrial planets are dense worlds -- with low hydrogen and helium content - and they are composed mostly from 2% of heavier elements contained in solar nebula

      

• evolution of Solar System
  • once density of gas in Protosun was sufficient for nuclear burning, Sun became a luminous object and cleared nebula as pressure from its light and solar wind pushed material out of Solar System

  • planets helped to clean up by absorbing some planetesimals and ejecting others

  • once Solar System was mostly clear of debris, planet building ended

  • eventually, most planetesimals were destroyed in collisions and or ejected to remote parts of Solar System (movie)

  • some of these smaller bodies, however, settled in asteroid belt between Mars and Jupiter, while a few were captured as moons of planets

  • OR, some moons may have formed from collisions of planets (movie)

  • Solar System is still filled with smaller bodies, such as satellites (moons), asteroids, and comets

    

    

  • rings around giant planets, such as Saturn's, are probably result of stray planetesimals being torn apart by gravity when they ventured too close to planet (movie)

    

    

    Jupiter's Rings

    Uranus's Rings

    For an animation of Voyager's flight by Saturn, click here.

    For a movie of Saturn's rings rotating, click here.

  • Venus, Earth and Mars acquired their atmospheres at later stage in formation of Solar System
    • most likely scenario is from outgassing (from gas blown out of volcanos)

    • on Earth, oxygen, essential to animals, was produced by plants breaking down CO₂

  • today, all solid surfaces scarred by craters from meteorite impacts (movie)

    

    

    

To read some more about the origin of the Moon, go to the Hand-Outs and Reference Materials page.

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