Solar Interior, Lecture notes of Astrophysics

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Solar Interior

The P-­‐P Chain in the SUN

Step 1: p + p ⇒ D + e

+

e

Step 2: D + p ⇒

3

He + γ (5.49 MeV)

Step 3:

3

He +

3

He ⇒

4

He + 2p (12.86 MeV)

Net: 4p + 2e

-

4

He + 6 γ + 2 ν

e

(~ 26 MeV)

(Where 1 MeV = 10 6 eV = 1.6 x 10

- J)

e

+

+ e

-

⇒ 2 γ (1.02 MeV)

Need two of Step 1 & 2 to have one of Step 3 (21.02MeV) + (25.49MeV) + (1*12.86MeV) = 25.88 MeV

The Solar Core:

• Extends out to 0.2 Rsun
• Contains ~50% of the Sun’s Mass.
• Is bounded (loosely) by the point where temperature and density are too low to support P-P fusion.
• Contains ~2% of the Sun’s Volume.

The Radiative Zone:

• Extends from 0.2-0.7 Rsun
• Contains ~48% of the Sun’s Mass.
• The radiative zone is bounded by the point where temperature and density are low enough to permit atoms to hold some of their electrons.
• Contains ~32% of the Sun’s Volume.
• Contains free electrons and atomic nuclei (plasma)

The Convective Zone:

• Extends from 0.7-1.0 Rsun
• Contains ~2% of the Sun’s Mass.
• The convective zone is bounded by the point where Light Directly Escapes From the Solar Atmosphere (The Visible Surface or Photosphere ).
• Contains ~66% of the Sun’s Volume.
• Some of the electrons in this region are bound to nuclei.

Water Lead

The Standard Model for the Sun

Deepest Ocean Trench

Doppler Effect Red shiLing of absorpOon lines Sound Waves Light Waves

• These Cells Vent Solar Energy like Water Boils. Each one Lasts for ~10 Minutes.
• Cell Structure Exists on Many Scales in the Sun, with Detectable Regions up to 10 5 km Across. Time and Size Scales in the Convective Zone:

Granules are convecOon

cells about the size of

Texas (121,000 km

2

image shows 1% of sun

surface.

Each delivers equivalent

to 1000 yrs of Hoover

Dam energy in 5

minutes

Time and Size Scales in the Convective Zone:

Caught in a Box: Sound waves reflect from the top of the Sun’s atmosphere without penetrating.

• The photosphere is very diffuse and sound doesn’t travel well.
• The change in density and the inability of the wave to penetrate further leads to an internal reflection.
• The wave goes back in the direction that it came from, but the interaction moves the photosphere up and down. Incoming wave Outgoing wave down welling upwelling 14

Helioseismology: The study of solar sound wave oscillations is called helioseismology.

• We use the same technique on the Earth!
• Earthquakes make the entire planet ring.
• By looking at where, when, and what type of earthquake waves reach different parts of the planet, we can determine the structure of the Earth’s interior!
• At the Sun we can do the same thing. We can also use them to probe the back side of the Sun.

Each ’Harmonic ’ of the Sun carries specific information about the interior. What does Helioseismology tell us?:

• How does pressure , density , temperature , and composition change with radius in the sun?
• Predict what is coming h_p://gong.nso.edu/data/farside/^17

The Solar Atmosphere: The base of the solar atmosphere is called the ‘Photosphere’ because this is where nearly all the light energy comes from.

• Photosphere: The photosphere is really just the visible ‘surface’ of the sun. It is the thin (~300 km thick) altitude range where convective cells break and the Sun’s blackbody radiation is emitted.
• It’s 6000K, and produces the bulk of the light from the Sun. Sharp edge is an illusion. Negative hydrogen ions (H with an extra electron) here absorb all light from below and re-emit it.

The Chromosphere:

• Chromosphere: The chromosphere is a diffuse region from 300 to 10,000 km above the photosphere.
• Most of the chromosphere is hotter than the photosphere and reaches up to 20,000K.