Astronomy 100 -- The Nearest Star: The Sun
THE SUN -- Our Closest Star
Diameter is about 100 times that of Earth's.
So 1,000,000 Earths would fit inside!
Mass = 300,000 times the Earth's mass.
Distance from the Earth is:
150,000,000 km or
8.3 light-minutes or
1 Astronomical Unit (AU)
Surface Temperature = 6100 K
Age of the Sun = 4.6 Billion Years!
Composition: 98% Hydrogen and Helium.
COMPOSITION OF THE SUN (by mass)
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Mass of the Sun = 2 x 1030 kg!!!
(Read as two times ten to the thirty kilograms)
Average Density of the Sun = 1.4 grams/cc
(just a little higher than water)
THE STRUCTURE OF THE SUN
- THE INTERIOR:
- The Core:
Core Temperature = 15,000,000 K
About 2 percent of Sun's volume
Contains 1/2 of Sun's mass
ALL of the Sun's Energy is produced here!
- The Radiative Zone:
This is the largest region in our Sun
Energy passes through this region from the core outward by
Photons will only pass very slowly (diffuse) through
the Radiative Zone, as they suffer many collisions.
It takes about 200,000 years
for a photon to bounce its way through this zone!
- The Convective Zone:
This is outer 20 percent of the Sun's interior.
Energy (photons) is brought to the Sun's surface by
Like boiling water: hot stuff moves up, gives away it's heat
and then sinks back down to get more heat and start all
- THE EXTERIOR:
- The Photosphere: The "surface" of the Sun
Temperature is about 6000 K
Two Main Features:
The granulation is the `boiling' at the surface due
to hot material rising to the surface and cool material sinking
below the surface. These cells are about the size of Texas.
Dark spots in the photosphere and typically about the size of the
Earth. The temperature in the center is about 4000 K. So, they
are actually red in color, but appear
black against the bright photosphere.
Sunspots are about the size of the Earth and are regions of
very strong magnetic fields.
The Sunspot Cycle:
The number of sunspots increases and decreases over an
11 year cycle! (Peaks are 11 years apart).
This cycle has been observed from the 1700's.
Here is what we think is happening:
field lines of the Sun start out nice and straight (few sunspots),
but because the middle of the Sun rotates faster than the polar
regions, the magnetic field lines get become wrapped up
(lots of sunspots). When they become too tightly wrapped,
The lines "snap" and new,
straight magnetic field lines form (few sunspots).
- The Chromosphere
Thin, cooler transparent (nearly clear) layer above the
Photosphere. Temperature T = 5000 K (though regions get down as
low as 4000 K and as high as 1,000,000 as one gets close to the
Red Light picture of the chromosphere.
Daily Images of the Sun
Great H-Alpha Image of the Sun
- The Corona
Very tenuous (the density is very low), very hot, outer region
of the Sun's atmosphere.
The temperature is about 2,000,000 K
Can be seen during solar eclipses.
The Source of the Sun's Energy
What are the possibilities?
Any energy source must have able to produce energy for as long as
the age of the Earth, 4.6 billion years!
If the Sun were made of coal, the Sun could last only
2) Gravitational Contraction?
Not Really Good Enough!
As the Sun contracts (shrinks) in size, energy is released.
If the Sun contracts by 10 meters in radius each year,
this would provide energy for only 100 million years.
3) Nuclear Fusion?
Not Really Good Enough!
Fusion provides energy through Einstein's formula
E = mc2.
The mass of the Sun could provide
enough energy for many billions of years.
That's Good Enough!!
Nuclear Fission and Fusion
Iron is the most stable element in Nature.
All atoms want to become part of an iron atom.
This can happen by two processes.
1) Nuclear Fission
Heavy atoms, such as uranium, split into
lighter atoms. "Radioactivity." The energy
used in holding the heavy atom together is
released as a photon.
2) Nuclear Fusion
Nuclear reactors produce energy this way.
Light atoms combine to form a heavier atom and
release energy (a photon). Difficult to get the
nuclei of the atoms close enough to bind. Need high
temperature and a high density of nuclei, as in the
core of the Sun.
Source of energy for the hydrogen bomb.
The Proton-Proton (P-P) Cycle
The PROTON-PROTON CYCLE,
or P-P Cycle, is a nuclear fusion
reaction that combines 4 hydrogen nuclei (which are protons)
into a single helium nucleus. These are the reactions that
1H + 1H ==> 2H + e+
2H + 1H ==> 3He + photon
3He + 3He ==> 4He + 1H +1H + photon
Here is a diagram.
For each 3He made, 3 1H are used. So when
the 2 3He combine (a total of 6 1H),
2 1H come back out when the 4He is made.
What are the results of the P-P Cycle?
1) Energy is released.
2) The composition of the core is changed.
This energy comes from the fact that 1 helium nucleus has
than the 4 hydrogen nuclei that made it
(by about 0.7%). That missing mass is converted to energy
(those emitted photons) via Einstein's wonderful equation:E = mc2
Since c2 is the speed of light squared, a little
mass can make a lot of energy. To produce the energy emitted by
the Sun, the Sun has to convert 4 million tons of mass to energy
each second!! The Sun has enough mass to produce this amount of
energy for 9 to 10 billion years!
How do we know the Sun uses the P-P cycle?
The only direct method to test this is to look at
THE SOLAR NEUTRINO "PROBLEM"
What are Neutrinos or "ghost particles"?
1) They carry energy (and maybe a little mass).
How many neutrinos reach the Earth?
2) They travel at (or very close to) the speed of light.
3) They do not like to interact with matter.
4) Postulated to exist before their discovery.
5) MANY, MANY, MANY are created in the P-P cycle.
Neutrinos are created in the core of the Sun, but unlike
the photons, they can pass straight out of the Sun.
They will reach the Earth 8 minutes after they are created!
If we can detect them,
we can "see" into the core of the Sun!
A LOT!! 300,000,000,000,000 (3 x 1014) pass
through your body each second (lying down). Yet,
in your lifetime, NOT ONE NEUTRINO will hit anything
in your body!
Some elements, such as chlorine and gallium, have a
chance of being hit by a neutrino.
The first neutrino "telescope":
100,000 gallon tank of C2C4
in a deep mine.
The gallium experiment (Gallex) find a similar result:
- Predicted hit rate -- about 1 each day.
- Experiment hit rate -- about 1 every 3 days.
- Theory and experiment disagree!
==>> A serious problem!
Not enough neutrinos are being
detected from Sun's core!
Much research is being done to understand this. Neutrinos are more complex
than first thought. The "best" theory now suggests neutrinos can "flucuate".
The Sun produces only 1 type of neutrino, but there are
three types or "flavors" of neutrinos. Physicists are working on
understanding if neutrinos can change "flavor". If so, this could
explain why we only detect 1/3 of the neutrinos expected.
Basic Properties of Stars.
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