Transcript Lecture 2 - Connolly, Harold

How did the Solar System form?
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Before we can investigate this fundamental
issue of all of science we need some general
information about science and the Solar
System.
How did the Solar System form?
1. Where is our Solar System located?
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How did the Solar System form?
2. What must occur first for the Solar System to
form?
How did the Solar System form: Sun and Clouds?
• Our Solar System’s formation is the by-product
of the formation of our star, the Sun.
• The Sun rules the Solar System. It is the
largest object in our system.
• It makes up ~ 99.85% of the mass of the Solar
System and is composed mainly of H2 and He.
• The Sun is the closest star to Earth.
• It is somewhat of a mystery that the Sun is
alone.
How did the Solar System form: Sun and Clouds?
Therefore, the Sun must form first. But
how?
How did the Solar System form: Sun and Clouds?
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In the beginning…
…a molecular cloud existed in the vicinity of
our solar system.
Stars are formed within molecular clouds.
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How did the Solar System form: Sun and Clouds?
• A molecular cloud is a dark interstellar cloud
where stars and hence Solar Systems are born.
– They are composed mostly of molecular hydrogen
(H2) with carbon monoxide (CO) and organic
compounds.
– They contain dust that likely acts as shields so that
molecules exist.
– They are cold, ~10K (0K is absolute 0, -273C)
– They have masses from a few times the mass of our
star up to 107 times the mass of our star.
How did the Solar System form: Sun and Clouds?
• The Nebular Contraction Theory: a large cloud of
interstellar gas or molecular cloud collapses under the
influence of its own gravity or due to a super nova
shock wave.
– It becomes denser and hotter during contraction, forming a
star or young stellar object (YSO) at the center. A disk of gas is
produced around the young stellar object. Planets are then
formed within this disk.
• This object is known as a solar nebula and within it a
protoplanetary disk or accretion disk forms.
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We see them!
How did the Solar System form: Sun and Clouds?
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How is a star formed?
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Most of the mass of the nebula becomes concentrated into one or
more areas that become hot enough for nuclear fusion to start and
thus a protosun is formed. The internal temperature reaches some
1,000,000 K.
The number of atoms required to have accumulated is nearly 1057
atoms-more than all the sand on all the beaches of the world and all
the atoms on our entire planet!
At this point most of the collapse or contraction stops and a star is
born. This is called a protostar.
This protostar evolves over a relatively short time, contracting and
getting hotter until about 30 million years pass and finally the star
is ‘main sequence’ burning at 15,000,000 K in the core.
How did the Solar System form: Sun and Clouds?
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How is a star formed?
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Star formation is usually discussed as having 7 stages.
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Stage 1 = Interstellar cloud
Stage 2 = Collapsing cloud fragment
Stage 3 = Fragmentation ceases
Stage 4 = Protostar
Stage 5 = Protostellar evolution
Stage 6 = A newborn star
Stage 7 = The main sequence
How did the Solar System form: Sun and Clouds?
How did the Solar System form: Sun and Clouds?
How did the Solar System form: Sun and Clouds?
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How did the Solar System form: Sun and Clouds?
• How does the disk form and why?
• As the cloud continues to contract, gravity become greater and
greater. But, the cloud does not contract into a single mass.
• The collapse towards the center of the cloud proceeds very slowly
due to several other effects, including turbulence and magnetic
fields, but the most important is the conservation of angular
momentum.
How did the Solar System form: Sun and Clouds?
• As the cloud continues to contract it begins to
spin or orbit the center, which contains a YSO.
The more it contracts the faster it spins.
• Thus, a decrease in the size of a rotating mass
must be balanced by an increase in its
rotational speed. This concept is known as
angular momentum and was first determined
by Pierre Simon de Laplace, 1796.
How did the Solar System form: Sun and Clouds?
• What is angular momentum?
• Well, first we must understand momentum.
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Linear momentum = mass x velocity (or
acceleration)
• Run into a wall slowly, now run into the wall
very fast OR increase the size of the person
running into the wall.
How did the Solar System form: Sun and Clouds?
Angular momentum = mass x angular speed x radius2
• It is a measure of an object’s tendency to keep spinning,
or how much effort must be expended to stop spinning.
• Both types of momentum, according to Newton’s laws
of motion, must be conserved at all times. As long as no
external forces is active, momentum must remain
constant before, during, and after a physical change in
any object.
How did the Solar System form: Sun and Clouds?
• The fact is, dust also exists within a
molecular cloud and the contraction
theory does not account for gas-dust,
dust-gas, or dust-dust interactions.
• We know from observations that dust is
found as a component of disks.
How did the Solar System form: Sun and Clouds?
• A more up-to-date theory - Nebular
Condensation Theory. This theory
incorporates most of the Nebular Contraction
Theory and it adds one critical component,
dust from the interstellar medium.
• This dust is known as interstellar dust, similar
and in some cases the same as, circumstellar
dust.
How did the Solar System form: Sun and Clouds?
• Dust is important because it helps to cool warm
matter by radiating its heat away, thus
reducing pressure and allowing the gas portion
to collapse more easily under the influence of
gravity.
• It also acts as nuclei for the formation of larger
grains by condensation and accretion.