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The Sun and the Origin
of the Solar System
Mid-sized, G-type main sequence star
Distance: 1 AU = 150 million km away
Size: Actual radius 700,000 km = 100 Earths
Temperature, Luminosity (surface) T = 6000 K
• 99.86% of the total mass of the Solar System
• 75% hydrogen, 23% heliun
• 2% O, C, Ne, Fe
• surface areas that are darker than
surroundings (lower temperatures)
• regions of intense magnetic activity
Solar Evolution
Star (Sun) formation
Stellar Evolution
• The Sun is a middle-aged, low-mass, main-sequence
• 5 billion years ago:
– Beginning of its life on main-sequence
– Sun had 1/3 luminosity it has now.
• 5 billion years from now:
– End of its life on main-sequence
– Sun will have twice the luminosity it has now.
• When H is exhausted, core shrinks.
• Heats up
• High temperatures ignites a shell of H around
the core.
• Increased pressure drives the envelope of the
star outward.
• Creates a Red giant
– Contraction of core, raises the temperature
– Ignites He shell around the core
– Eventually the core stabilizes
– Envelope is ejected as a "planetary nebula"
– The core remains as a "white dwarf"
Solar system formation
Starting point:
– A cloud of interstellar gas and dust, the "solar
– Most of it (98%) is hydrogen and helium, includes
dust grains of heavier material, formed in previous
generations of stars.
Accretion disk
Asteroid Belt
• Small bodies in the inner solar system
• Asteroid Belt between Mars & Jupiter
• Orbits are strongly influenced by
• Made of rock, metal, or a mix of the
• >300,000 asteroidal objects
• >150,000 with good enough orbits to give
official numbers
• ~15,000 asteroids with official manes
• When you know its orbit, you can name it.
Examples of asteroid names
• Ceres (largest – 914 km)
• Eros (landed on in2001 Feb 12)
• Bach
• Beethoven
• Lennon
• McCartney
• Santana
• Clapton
Irregular shape
• Too small for gravity to make them spherical
Composition of Asteroids
• C-type: "Carbonaceous" – mostly carbonbearing materials. ~75% of all asteroids.
• S-type: "Silicaceous" - mostly of silicates
(stony or stony iron). 17% of all asteroids.
• M-type: "Metallic" - probably iron-rich
Asteroid Origins
• fragments of larger, differentiated bodies
shattered by collisions
• remnants of more primordial material that
never got differentiated
Impact with Earth
Would disrupt climates and trigger mass
• Chunks of rock & iron smaller
than asteroids orbiting the
• Sizes range from grains to
100 meters across
• Streak of light when a
meteoroid enters the Earth's
• Most are tiny grains
• Meteor showers are trails of
debris left behind by passing
• Any remnant that reaches the ground intact.
Russian Meteorite
Meteor Impacts
• About 100 tons of meteoroids hit the Earth
each day
• Most are no bigger than grains of sand or
Earth Impact Effects Program
Small bodies consisting of aggregates of ices
mixed with rock & dust
• As they approach the Sun, they heat up and
the ices sublimate (go from solid to gas):
Halley's Comet
• In 1705, Edmund Halley computed the orbit of
the great comet of 1682 using Newton's laws
• Found that orbit of 1682 comet was the same
as comets seen in 1531 & 1607.
• Predicted return in 1758.
• Seen again on Christmas day
1758, 12 years after
Halley's death
Origin of Comets
• Short-period comets are from the Kuiper Belt
• Long-period comets are from the Oort Cloud
Structure of Comets
• Nucleus:
– Dirty snowball of ices & dust
– >99% the mass of the comet
• Coma
– Bright "head" of the comet
– Low-density cloud of gas & dust sublimed off the
– Extends out to 100,000 km or more
• Comets have two tails
– Dust Tail, dusty particles swept back in a curved
path by solar radiation, white
– Ion Tail, ionized atoms & molecules swept straight
back by the solar wind, blue
Comet Orbits
• Typically have a high degree of eccentricity
Kuiper Belt
• Region of the Solar System beyond the planets
extending from the orbit of Neptune (at 30
AU) to approximately 55 AU from the Sun
Oort Cloud
• Cloud of comets which may lie roughly 50,000
AU, or nearly a light-year, from the Sun.
• Nearly a quarter of the distance to Proxima
Centauri, the nearest star to the Sun