The Sun

Heat Transport in the Sun

• Conduction• Convection• Radiative Diffusion

Discussion

Which would you rather do, put your hand in an oven at 450 degrees F or put you hand on a 450 degree F stove top? Why is there a difference, aren’t both the same temperature?

Radiative Diffusion

Involves the movement of energy via photons, but not of material.

Radiative zone – inner 71 percent of the Sun’s interior.

Takes a typical photon 170,000 years to reach the Convective zone. Each time a photon is absorbed it loses energy.

Convection

Hot material flows from hotter regions to cooler regions. In the Sun convection causes currents of hot gas to flow up toward the surface and cooler gases to flow down – no net movement of material.

Convective Zone – outer 29 percent of Sun’s interior.

Why the transition?

The radiative zone is hot enough that most elements are completely ionized. Think about electrons in the mirror.

The bottom of convective zone is cool enough for heavy atoms to regain electrons which can then absorb light and heat up.

Discussion

What happens to the bottom of the convection zone as it absorbs light from the radiative zone?

Discussion

Will observations of the properties of the photons emitted by the Sun reveal much information about the interior of the Sun? Why or why not?

Solar Neutrinos

Produced in nuclear reaction that convert neutrons to protons.

Do not interact much with matter. So How do we detect them?

Neutrino Telescopes

HOMESTAKE – 615 ton tank of 378 thousand litters of cleaning fluid (C2Cl4) buried 1.5 km in a mine shaft. Once every two to three days a neutrino would hit a Cl atom and convert it a radioactive Ar atom.

The Solar Neutrino Problem

The HOMESTAKE experiment operated for 25 years and detected only about 1/3 of the predicted number of neutrinos.

Two ways to fix the Solar Neutrino Problem

1. Our models of the Sun are wrong.2. Our understanding of the neutrino is

wrong.

Discussion

What do we need to do to get less neutrinos from our model of the Sun?

Helioseismology

The Sun vibrates like a bell. We can measure these vibrations at any position on the surface of the Sun using the Doppler shift. As the surface moves toward us it is blue shifted, as it moves away from us it is red shifted.

Helioseismology

All the local oscillations on the Sun are driven by sound waves that echo back and forth through the Sun.

The speed of sound waves depends on the temperature and density of the material it passes through.

GONG – Global Oscillation Network Group

SOHO – Solar and Heliospheric Observatory

Bottom Line

The solar oscillations indicate that our current models of the density and pressure in the Sun’s interior are correct to 0.2 percent.

Super Kamiokande

50,000 tons of purified water buried 1 km in a zincmine in Japan.

Super Kamiokande

50,000 tons of purified water

The Outer Layers of the Sun

• The Photosphere• The Chromosphere• The Corona• The Heliosphere

The Photosphere

The “surface” of the Sun. That part of the solar atmosphere from which most of the light we see is emitted.

The outer atmosphere of the Sun is transparent, we can see through it. The surface we see is the point at which the Sun’s atmosphere becomes opaque.

Properties of the Photosphere

0 99.5 4465 0.0068 0.23

100 97 4780 0.017 0.54

200 89 5180 0.039 1.2

300 64 5840 0.083 2.1

400 4 7610 0.16 3.1

Depth (km) % of light Temp (K) Pressure (atm) Density (g/cm3 10-7)

Discussion

Why do we say the Sun has a surface temperature of 5800 K when clearly, the temperature of the photosphere changes with depth? How do you think we got this particular value?

Why is the Photosphere so Opaque?

Hydrogen atoms in the solar atmosphere can acquire a second electron. This weekly bound extra electron easily absorbs different wavelengths of light.

Discussion

Why is the edge of the Sun darker than the center?

Photospheric Granulation

Mottled appearance – bright areas surrounded by dark lanes.

Typically 700 to 1000 km in diameter , they persist for only 5 to 10 minutes.

Granulation

Discussion

What do you think causes the granulation? What is the difference between the light part and the dark lanes?

Supergranules

As with the granules, hot gas rises in the center, spreads out and sinks back into the Sun. But supergranules are much larger, typically 35,000 km in diameter, they move slower, 0.4 km/s, and last on the order of a day.

Supergranules

Sunspots

Dark, irregular spots in the Sun’s photosphere.

Can last from hours to months.

Can have diameters as large as 50,000 km.

Often occur in groups of 2 to 100 individual spots.

Discussion

Why do sunspots appear dark?

Hint: The umbra appears red and the penumbra appears orange when isolated from the rest of the Sun’s surface.

Sunspots and Stefan-Bolztmann

4flux from umbra 4300

0.3flux from photosphere 5800

K

K

Thus, sunspots emit less than a third of the light of the photosphere.

Magnetic field lines

Charged particles can’t cross magnetic field lines.

Cooler, neutral atoms in the Sun’s photosphere can pass freely.

How can you tell whether or not something is magnetic?

Discussion

The Zeeman Effect

A magnetic field interacts with the spin of the electron in an orbital. The spin either lines up with the magnetic field or is anti aligned with the magnetic field.

These states have slightly different energies, causing a single spectral line to split into two.

Spectrograph slit

Zeeman effect

Discussion

If the electron in the hydrogen atom can only have two spins, aligned and anti-aligned, why do you think there are three absorption lines?

Sunspot pairs have opposite polarity

In the hemisphere with the Sun’s magnetic north pole, the leading spot is a north pole and following spot is a magnetic south pole.

Polarity reverses in opposite hemisphere

Leading spot will be south pole and following spot will be north pole in hemisphere with the Sun’s magnetic south pole.

Today’s Sun

Visible light Magnetogram

Differential Rotation

Sun is made of gas. Thus it does not all have to rotate at the same rate!

The equator of the Sun rotates faster than the poles. Sidereal rotation period at equator is about 25 days, while at the poles the sidereal rotation rate is about 35 days.

The Radiative Zone rotates as a solid body!

The radiative zone rotates with a sidereal period of 27 days at all latitudes.

Only the convection zone rotates differentially.

Differential rotation

Discussion

How do you think we know the rotation rate of the radiative and convective zones, when we can’t see them. We can only see the 400 km of the photosphere.

Discussion

Differential rotation creates a tremendous amount of shear as one layer slides past the other.

A similar thing happens in Earth’s atmosphere when a cold air mass meets a warm air mass. What is the result in Earth’s atmosphere?

The Sunspot Cycle

1. The number of sunspots varies with an 11 year period (on average).

Sunspot cycle

2. The average latitude of the sunspots varies

After minimum, the sunspots reappear at higher latitudes (+/- 30 degrees) and then migrate toward the equator as maximum nears.

3. Sunspot minimum corresponds to the magnetic reversal of the Sun

At minimum there are few sunspots because the Sun’s magnetic field disappears during reversal. The north magnetic pole becomes a south magnetic pole. The polarity of the leading and trailing spots also changes.

Thus, the sunspot cycle is actually 22 years.

Magnetic-dynamo model

Discussion

If the chromosphere is so much hotter (10,000 k) than the photosphere, why does it not shine brighter than the photosphere?

Why do you think it is a purplish-red color?

Calcium K line at 393.4 nm

What makes the chromosphere so hot?

The chromosphere is heated from above by the solar corona.

Spicules occur at the boundaries of super granular cells