the sun is a star 2

7/28/2019 The sun is a star 2

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19-Apr-13

IESO

The Sun is a Star

Part 1


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Formation of theSolar System


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Formation of the Solar System

The Formation of the Solar System

What properties must a planetary formation theory

explain?

1. It must explain the patterns of motion of the present solar system

(last week).

2. It must explain why planets form into 2 groups.

3. It must explain the huge existence of asteroids and comets.

4. It must allow for possible exceptions to the rules.

The theory may be able to be used on other solar systems in the Galaxy


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Evolutionary Theories

All evolutionary theories have their start with Descartesswhirlpool or vortex theory proposed in 1644.

Using Newtonian mechanics, Kant (in 1755) and thenLaplace (around 1795) modified Descartess vortex to arotating cloud of gas contracting under gravity into a disk.

The Solar Nebular Hypothesis is an example of anevolutionary theory.


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Catastrophic Theories Catastrophic theory is a theory of the formation of the solar

system that involves an unusual incident such as the collisionof the Sun with another star.

The first catastrophic theory - that a comet pulled material

from the Sun to form the planets - was proposed by Buffon in1745.

Otherclose encounter hypotheses have been proposed too.

Catastrophic origins for solar systems would be quite rare(relative to evolutionary origins) due to the unusual nature ofthe catastrophic incident.


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Solar Nebula Hypothesis

Origin of the Solar Nebula

Galactic recycling

Most of the universe started as Hydrogen and Helium. All otherheavy elements (loosely called metals by astronomers) were

formed in stars When stars die they release much of the content into space

While this has been going on for 4.6 billion years, only 2% of all thehave been converted to metals

Evidence from other gas clouds

All new systems that we can observed formed within interstellar clouds,such as the Orion Nebula


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Towards a Solar Nebula Hypothesis A supernovae shock wave likely triggered the events which led to the

birth of our solar system

The nebular cloud collapsed due the force of gravity on the cloud.But the cloud does not end up spherical (like the sun) because there

are other processes going on: HeatingThe cloud increases in temperature, converting gravitational

potential energy to kinetic energy. The sun would form in the center wheretemperatures and densities were the greatest

Spinningas the cloud shrunk in size, the rotation of the disk increases(from the conservation of angular momentum).

Flatteningas cloud starting to spin, collisions flattened the shape of thedisk in the plane perpendicular to the spin axis


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Testing the Model

If the theory is correct, then we should see disks

around young stars

Dust disks, such as discovered around beta-

Pictoris or AU Microscopii, provide evidence that

conditions for planet formation exist around many

Sun-like stars.


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The Formation of Planets As the solar nebula cooled and flattened into a disk some 200 AU in diameter,

materials began to freeze out in a process called condensation (changing froma gas to a solid or liquid).

The ingredients of the solar system consist of 4 categories (with % abundance): Hydrogen and Helium gas (98%)

Hydrogen compounds, such as water, ammonia, and methane (1.4%) Rock (0.4%)

Metals (0.2%)

Since it is too cool for H and He to condense, a vast majority of the solar nebuladid not condense

Hydrogen compounds could only condense into ices beyond the frost line, whichlay between the present-day orbits of Mars and Jupiter


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Building the Terrestrial Planets In the 1940s, Weizscher showed that eddies would form in a rotating gas cloud

and that the eddies nearer the center would be smaller.

Eddies condense to form particles that grow over time in a process calledaccretion.Materials such and rock and metal (categories #3 and #4).

These accreted materials became planetesimals which in turn sweep up smaller

particles through collision and gravitational attraction. These planetesimals suffered gravitational encounters which altered their orbits

caused them to both coalesce and fragment. Only the largest planetesimals grewto be full-fledged planets.

Verification of this models is difficult and comes in the form of theoreticalevidence and computer simulations.


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Building the Jovian Planets

Planetesimals should have also grown in the outer solarsystem, but would have been made of ice as well as metaland rock.

But Jovian planets are made mostly of H and He gas

The gas presumably was captured by these ice/rock/metalplanetesimals and grew into the Jovian planets of today.


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A Stellar wind is the flow of nuclear particles from astar.

Some young stars exhibit strong stellar winds. If theearly Sun went through such a period, the resultingintense solar wind would have swept the inner solar

system clear of volatile (low density) elements,molecules and compounds.

The giant planets of the outer solar system would thenhave collected these outflowing gases.


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An object shrinking under the force of gravity heats up.High temperatures near the newly formed Sun (protosun)

will prevent the condensation of more volatile (low

density) elements. Planets forming there will thus be made

of nonvolatile, dense material.

Farther out, the eddies are larger and the temperatures

cooler so large planets can form that are composed of

volatile elements (light gases).


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Problem: The total angular momentum of the planets isknown to be greater than that of the Sun, which shouldnot occur according to conservation laws (i.e. the presentSun is spinning too slowly).

Solution: As the young Sun heated up, it ionized the gas

of the inner solar system. The Suns magnetic field then swept through the ions in the

inner solar system, causing ions to speed up.

As per Newtons third law, this transfer of energy to the ionscaused the Sun to slow its rate of rotation.


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Explaining Other Clues

Over millions of years the remaining planetesimals fellonto the moons and planets causing the cratering we seetoday. This was the period ofheavy bombardment.

Comets are thought to be material that coalesced in theouter solar system from the remnants of small eddies.

The Asteroid belt formed from debris that could notcoalesce into a planet due to the gravitational influenceof Jupiter


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The formation of Jovian planets and its moonsmust have resembled the formation of the solar

system. Jupiter specifically:

Moons close to Jupiter are denser and contain fewerlight elements;

Moons farther out decrease in density and increase in

heavier elements.


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The Exceptions to the Rule

Captured Moonssatellites which go the opposite waywere likely captured. Most of these moons are small are

lie far away from the planet.

Giant impactsmay have helped form the Moon and

explain the high density of Mercury and the Pluto-Charon

system. Furthermore, the unusual tilts of Uranus and

Venus can also be explained by giant impacts.


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Solar System Destiny

The nebular hypothesis accounts for all major features in the solarsystem

It does not account foreverything, however

It probably took about a few tens of million of years, about 1% ofthe current age of the solar system

The solar system was probably not completely predestined from thecollapse of the solar nebula, though the initial were orderly andinevitable

The final stage of accretion and giant impacts were fairly random innature and made our solar system unique


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Radioactivity

Radioactivity Certain isotopes (elements which contain differing number of

neutrons) are not stable and will decay into two or more lighter

elements

The time it takes for half of a given isotope to decay is calledthe half-life

By noting what percentage a rock (or human body) has left of a

radioactive element can enable us to estimate the age of that

object. This process is called radioactive dating. SeeMathematical Insight 8.1


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Radioactivity

Radioactivity - examples Potassium-40 decays into Argon-40 with a half-life of 1.25 billion years

Since Argon-40 is an inert gas, it is very unlikely to have formed inside a rock

as the solar nebula condensed, so it must have formed via decay

Uranium-238, after a series ofdecays, turns into Lead-206 with half-life of

4.5 billion years Lead and Uranium have very different chemical behaviors

Some minerals have nearly no lead to begin with, so when uranium is mixed

with lead, we can assume that the lead formed via decay


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Radioactivity

RadioactivityThe general formula for the age of a radioactive material is

(see Mathematical Insight 8.1):

10

10

log

1log

2

half

current amount

original amount

t t


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Radioactivity

Earth rocks, Moon rocks, and meteorites The oldest Earth rocks date back to 4 billion years and some

small grains go back to 4.4 billion years. Moon rock brought

back from the Apollo mission date as far back as 4.4 billion

years. These tell us when the rock solidified, not when the planet formed

The oldest meteorites, which likely come form asteroids, are

dated at 4.55 billion years, marking the time of the accretion of

the solar system


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The End

the sun is a star 2

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