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)

    Element    Symbol    Percentage

   Hydrogen        H        78

   Helium        He        20

   Oxygen        O          0.8

   Carbon        C          0.3

   Nitrogen        N          0.2

   Neon        Ne          0.2

   Nickel        Ni          0.2

   Silicon        Si          0.04

   Sulfur        S          0.04

   Iron        Fe          0.04

Mass of the Sun = 2 x 10³⁰ kg!!!
(Read as two times ten to the thirty kilograms)
2,000,000,000,000,000,000,000,000,000,000 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
Radiative Diffusion.
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 Convection.
Like boiling water: hot stuff moves up, gives away it's heat and then sinks back down to get more heat and start all over again.

THE EXTERIOR:

The Photosphere: The "surface" of the Sun

Temperature is about 6000 K

Two Main Features:

Granulation:
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.
Sunspots:
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:
The magnetic 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).

Sunspots are also associated with solar flares and prominences.

The "Grand-Daddy" Prominence!
Solar Prominence in Action!
These flares do have some effects on the Earth (like the aurorae, the fall of SkyLab).

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 Corona).

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!
1) Combustion?
If the Sun were made of coal, the Sun could last only 10,000 years.
Not Really Good Enough!

2) Gravitational Contraction?
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.
Not Really Good Enough!

3) Nuclear Fusion?
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.
Nuclear reactors produce energy this way.
2) Nuclear Fusion
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 take place:

¹H + ¹H ==> ²H + e⁺ + neutrino
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 less mass 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 neutrinos.

THE SOLAR NEUTRINO "PROBLEM"

What are Neutrinos or "ghost particles"?
1) They carry energy (and maybe a little mass).
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!

How many neutrinos reach the Earth?
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!

Neutrino Detectors:

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.

Predicted hit rate -- about 1 each day.

Experiment hit rate -- about 1 every 3 days.

Theory and experiment disagree!
==>> A serious problem!

The gallium experiment (Gallex) find a similar result:

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.

Light and Radiation.
Basic Properties of Stars.
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