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Nebular Theory
History Behind the Theory:
First developed in the 18th century, it was argued that a nebula, a cloud of gas and dust left over from a supernova, began to collapse
and rotate under gravity into a disk which would eventually become our solar system. Over the years, new theories were proposed
and disregarded regarding the details of how then planets and sun were formed. It wasn’t until the 1970’s when a Soviet
astronomer developed the most recent and widely accepted model of the formation of the solar system called the Nebular Theory
or Nebular Hypothesis. This new theory explains how planets could form at along the accretion disk, while most of the mass was
collected in the center to form a sun.
READ through the text above. Highlight the key points.
READ through pages 2-4, titled: “Formation of the Solar System.”
IDENTIFY which of the 6 steps on the 1st Page of the text, belong in the 6 boxes below.
NUMBER, ILLUSTRATE, TITLE, and write down a couple FACTS for these 6 key steps that took place during the
formation of the solar system. These steps should CORRELATE with the PowerPoint notes on the previous day.
5. NUMBER, ILLUSTRATE, and TITLE box 7 on the next page reflecting what the “Solar System” looks like now with
“Inner Planets” and “Outer Planets.”
6. NUMBER, ILLUSTRATE, and TITLE box 8 on the next page reflecting “Moons” that orbit around planets.
What is the Frost Line?
Rock and metals can form anywhere it is cooler than about 1300 K. Carbon grains and ices can only form where the gas
is cooler than 300 K. The frost line is the dividing line where inner and outer planets can form. This is related to the
distance to the in the solar nebula from the central protsun where it is cold enough for hydrogen compounds, such as
water, ammonia, and methane to condense into solid ice grains. Depending on density, that temperature is estimated to
be about 150 K. The frost line of the Solar System is around 5 AU.
Graph the frost line on a graph where the y-axis is Temperature (K) with increments from bottom to top of:
20, 50, 100, 200, 500, 1000, 2000.
The x-axis reflects distance (AU) with the following increments from left to right:
.20, .50, 1.0, 2.0, 5.0, 10.0, 20.0, 30.0, 40.0.
You can use the graph paper available in class.
NEITHER axis needs to be scaled to accuracy, meaning you can separate each mark equally.
Make SURE you label your axis with the correct title and unit.
Read the above paragraph, highlight key points, and draw the vertical “Frost Line” at the correct point
through the x-axis.
Mercury = 1100 K
Venus = 700 K
Earth = 550 K
Mars = 400K
Jupiter = 150 K
Saturn = 90 K
Uranus = 50 K
Neptune = 35K
Pluto = 25K
Mercury = .387 AU
Venus = .723 AU
Earth = 1.000 AU
Mars = 1.524 AU
Jupiter = 5.203 AU
Saturn = 9.523 AU
Uranus = 19.208 AU
Neptune = 30.0874 AU
Pluto = 39.746 AU
Formation of the Solar System
Main goal: Understand the reason for the different sizes, compositions, and orbital and rotational
motions of planets (Terrestrial, Jovian) and minor objects (Pluto and similar objects, asteroids,
comets), including exceptions to the general patterns. The theory should produce a scenario similar
to the one for other stellar systems.
Stages of Star System Formation
Starting point: A cloud of interstellar gas and dust, the "solar nebula";
Most of it (98%) is hydrogen and helium, but it includes atoms and dust
grains of heavier material, formed in previous generations of stars.
Onset of formation: The nebula is already thicker than the average
interstellar region, and possibly part of a chaotic region of starbirth;
Because of some disturbance that compresses it, such as a supernova
explosion, it starts a gradual process of collapse.
Contraction: The cloud starts collapsing under its own gravity; over
100,000 years, it shrinks down to 100 AU, heats up (thermal energy),
and compresses in the center.
Accretion disk: The matter around the center spins up and flattens into
a disk, while heat vaporizes the dust.
Protostar: Forms in the center, when the core becomes opaque; later
will become the Sun. (The gas orbiting the protostar in some cases may
start to compress under its own gravity, producing a double star.)
Condensation: The disk radiates away its energy and cools off; some
gas condenses into tiny dust grains of metal, rock and, far enough from
the forming star, outside the "frost line", ice (differentiation).
Planetesimals: Dust grains stick to each other (ice helps) and sweep
their paths, forming larger particles; this accretion goes on until the
particles become the size of boulders or small asteroids, which attract
matter with their gravity.
Protoplanets: The larger particles' growth accelerates. They collide and
accumulate all of the solid matter close to their own orbit. In 100,000 to
20,000,000 yr, the protoplanets' size is large asteroid/lunar size in the inner
solar system, and several times the Earth's size in the outer solar system
(lower temperature).
Solar wind: After about 1,000,000 yr, it sweeps away the leftover gas. If a
protoplanet is already large enough, its gravity pulls in the surrounding gas,
and it becomes a gas giant (leftover gas and dust around it condenses into
moons); if not, it remains a rocky or icy body.
Overview: After a billion years of clean-up and meteoritic
bombardment, you end up with ten or so planets, in stable
orbits; The protostar turned into a star when the core became hot
Catastrophes: Needed to explain specific isolated features and
exceptions. The planets, their surfaces and atmospheres may be
heavily modified by the last, big collision they experience.
Examples: For Earth, our Moon and the presence of water,
brought by comets (the original Earth could not have retained
it); Also, composition of Mercury, Venus' rotation, Uranus' tilt.
Debris: Some planetesimals remain in the asteroid belt (a
would-be planet, if not for Jupiter) and the Kuiper belt; others
are thrown outwards by "gravity assist" during close encounters
(Oort cloud); Some dust remains in a dust disk in the plane of
the solar system; we see it from the zodiacal light it scatters..
How big is it? Pluto's orbit at 40 AU, Kuiper Belt between 30
and 100 AU or so, the Oort Cloud extends out to 50,000100,000; The nearest star is at about 300,000.
What Evidence Do We Have?
Earth and Moon rocks: They can be dated using their
radioactive elements; The oldest ones are about 4.5 billion years
Meteorites: The oldest objects in our solar system are 4.57-Gyr
old, mm-sized grains found in some meteorites; Some even give
us evidence that a star exploded in our neighborhood around the
time the solar system formed, and the Sun may have been part
of a cluster.
Exploration and experiments: Spacecraft have been sent to
observe asteroids made of "primitive rock" (like NEAR, with
asteroid Eros, and Hayabusa, to asteroid Itokawa) and comets
(Rosetta), and collect samples of solar wind (Genesis);
Conditions have been recreated in a Space Shuttle flight.
Solar neighborhood: Its configuration also shows evidence for
some kind of past explosion; For example, we seem to be inside
a bubble with walls about 70 light years away; Further away, we
can see other (proto)planetary systems where the process is
happening right now.
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