Transcript The Small Objects. The Sun.

Lecture 35
Small Objects in the Solar System.
The Sun.
Chapter 17.4  17.7
• Asteroids and Comets
• Our Own Star
Remnants of the Solar Nebula
Small bodies remain virtually unchanged since
their formation 4.5 billion years ago
They carry history of the Solar system in their
compositions, locations, and numbers.
Asteroid means starlike, a rocky leftover
Comet means hair (Greek), an icy leftover
Meteor – a thing in the air (falling star)
Meteorite – any piece of rock than fell to the
ground from the sky
Asteroids
Undetectable to the naked eye (a.k.a. minor planets)
The first asteroid Ceres was discovered by Piazzi in
1800 in an attempt to find the missing planet
between Mars and Jupiter
Ceres has a diameter of ~500 km (~ half of Pluto)
There are more than 100,000 asteroids larger than 1
km in diameter
The asteroid belt lies between 2.2 and 3.3 A.U.
Finding Asteroids
Asteroid Vesta
Asteroid Eros
Missing planet
Origin and evolution of the Belt
The most likely reason for the belt existence is
orbital resonances
A resonance occurs if an object’s orbital period is a
simple ratio of another’s object period (1/2, 1/4,etc.)
Asteroids with resonance periods will be pushed out
of their orbits by large planets (e.g., Jupiter)
Asteroid belt
Meteor
Tunguska Meteorite
Leonid Meteor Shower
Meteorites
Meteorites are pieces of rock falling from the sky.
Seen as fireballs (sometimes with sound)
Primitive meteorites composed of a random mix
of flakes from the solar nebula, contain pure
metals, which are bound in minerals on Earth.
Processed meteorites – parts of a larger object.
Some resemble the Earth’s core, others are
similar to the Earth’s crust and mantle.
Primitive Meteorites
Comets
Comets are basically dirty snowballs where ice
mixes with rocky dust.
Their mean size is a few kilometers across.
Comets change appearance when they approach
the Sun.
The comet body is called nucleus.
Sublimating ices create coma.
A tail pointing away from the Sun appears.
There are two tails: plasma tail and dust tail.
Comet Orbits
The Origin of Comets
Comet Hale-Bopp
Sun Grazing Comets
Pluto
Pluto was discovered in 1930 by Claude Tombaugh.
Its radius is 1,195 km and mass is 0.0025 Earth mass.
Its orbital period is 248 years.
It was closer to the Sun than Neptune in 1979 – 1999.
At aphelion it is 50 AU from the Sun.
Its moon Charon was discovered in 1978.
Pluto and Charon
Pluto and Charon
Summary of Small Objects
Asteroids and comets are the best evidence of how
the solar system formed
The small bodies are significantly affected by
planets gravity.
Impacts with comets and meteorites are spectacular
events, but may even alter life.
Pluto has more similarities with the Kuiper belt
comets.
Why Does the Sun Shine?
Ancient view: a hot, glowing rock of the size of
Massachusetts
Early XIX century: cooling or chemical reactions
Enough energy for a few thousand years
Late XIX century: gravitational contraction
Enough energy for ~25 million years
XX century: nuclear fusion
Can shine for ~10 billion years
Current State of the Sun
The Sun’s size is stable, maintained by a balance
between the force of gravity and gas pressure.
This balance is called hydrostatic or gravitational
equilibrium.
Gravitational equilibrium implies that the pressure
increases with depth.
This makes the Sun extremely hot and depth in its
core.
Gravitational Equilibrium
Big Picture of the Sun
The Sun contains 70% hydrogen, 28% helium, and
2% heavier elements.
The total power output (luminosity) is 3.8 1026 W.
1Watt = 1 joule/second
The radius is 700,000 km.
The mass is 2 1030 kilograms
(300,000 times more massive than the Earth).
The surface temperature is 5,800 K
Nuclear Fusion
Nuclear fusion is the process of combining nuclei
to make a nucleus with a greater number of
protons and neutrons.
Particles in a nucleus are held together by the
strong force.
This is the only force that can overcome the
electromagnetic repulsion between two positively
charged nuclei.
Hydrogen Fusion in the Sun
The proton-proton chain
Solar Neutrino
Neutrino is a subatomic particle.
It is a by-product of the solar proton-proton cycle.
It barely interacts with anything.
Counts of neutrino coming from the Sun are
crucial to test our knowledge about solar physics.
Neutrino observatories use huge amounts of
different substances to detect nuclear reactions
with neutrino.
So far theory predicts more neutrino than is seen.
Observations of Solar Neutrino
The Super Kamiokande Experiment
How does the Light Comes Out?
Photons are created in the nuclear fusion cycle.
They collide with other charged particles and
change their direction (random walk).
They also decrease their energy while walking.
It takes ~10 million year to get outside.
The random bouncing occurs in the radiation zone
(from the core to ~70% of the Sun’s radius).
At T<2 million K, the convection zone carries
photons further towards the surface.
Sunspots and Other Solar Activity
Sunspots have T~4,000 K, cooler than the 5,800 K
surrounding plasma.
Sunspots are kept together by strong magnetic fields.
Usually sunspot appear in pairs connected by a loop
of magnetic field lines.
The loops rising into the chromosphere or corona
may appear as solar prominences.
Solar flares are events releasing a lot of energy
where magnetic field lines break.
Sunspot Close-Up
The Sunspot Cycle
Observations of the Sun since the beginning of
the telescopic era revealed that the number of
sunspots gradually rises and declines.
An average period is 11 years (from 7 to 15 years).
The magnetic fields in sunspots reverse their
direction when a cycle is over.
No sunspots were observed in 16451715, when a
Little Ice Age took place in Europe and America.
The Sunspot Cycle
Summary of the Sun
The Sun shines with energy generated by fusion of
hydrogen into helium in its core.
Gravitational equilibrium determines the Sun’s
interior structure and maintains a steady nuclear
burning rate.
The Sun is the only star near enough to study it in
great detail.