General Properties

• Absolute visual magnitude MV = 4.83

• Central temperature = 15 million 0K

X = 0.73, Y = 0.25, Z = 0.02

• Initial abundances:

• Age: ~ 4.52 Gyr

• Spectral type G2V

• Effective (surface) temperature = 5770 0K

• Absolute bolometric magnitude Mbol = 4.76

The Sun

The Sun’s Interior Structure

Temp, density and pressure decr. outward

Energy generation via nuclear fusion

Energy transport via radiation

Energy transport via convection

Flo

w o

f en

erg

y

Photosphere

Interior Structure of the SunSite of main energy production

dL/d

r

r/R00.50.40.30.20.1

Interior Structure of the SunMass composition

Mas

s fr

actio

n

r/R00.50.40.30.20.1 0.6

0.1

0.6

0.80.7

0.50.4

0.3

0.2

11H

42He

32He (x100)

The Sun’s Atmosphere

Only visible during solar eclipses

Apparent surface of the sun

Hea

t F

low

Solar interior

Temp. incr. inward

Granulation

… is the visible consequence of convection

Sun Spots (I)

Sun Spots (II)

Visible Ultraviolet

Cooler regions of the photosphere (T ≈ 4000 K).

Active Regions

Solar Activity, seen in soft X-rays

Magnetic Fields in Sun Spots Magnetic fields on the photosphere can be

measured through the Zeeman effect

→ Sun Spots are related to magnetic activity on the photosphere

Sun Spots (III)

Related to magnetic activity.

Magnetic field in sun spots is about 1000 times stronger than average.

In sun spots, magnetic field lines emerge out of the photosphere.

Magnetic North Poles

Magnetic South Poles

Magnetic Loops

Magnetic field lines

Mass ejection from the sun often follow magnetic field loops.

The Solar Cycle

11-year cycle

Reversal of magnetic polarity

After 11 years, North/South order of leading/trailing sun spots is reversed

=> Total solar cycle = 22 years

The Sun’s Magnetic Cycle

After 11 years, the magnetic field pattern becomes so

complex that the field structure is re-arranged.

→ New magnetic field structure is similar to the

original one, but reversed!

→ New 11-year cycle starts with reversed magnetic-field

orientation

The Solar Cycle (II)

Maunder Butterfly Diagram

Sun spot cycle starts out with spots at higher latitudes on the sun

Evolve to lower latitudes (towards the equator) throughout the cycle.

The Maunder MinimumThe sun spot number also fluctuates on

much longer time scales:

Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

Prominences

Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

Eruptive Prominences

(Ultraviolet images)

Extreme events, called coronal mass ejections (CMEs) and solar flares,

can significantly influence Earth’s magnetic field structure and cause

northern lights (aurora borealis).

Eruptive Prominences

(Ultraviolet images)

Solar Aurora

Sound waves

produced by a solar

flare

~ 5

min

utes

Coronal mass ejections

The Chromosphere

Chromospheric structures visible in H emission

Region of sun’s atmosphere just above

the photosphere.

T: 4400 K → 25,000 K

n: 1011 cm-3 → 107 cm-3

Absorption and emission lines from singly ionized metals (He

II, Fe II, Si II, Cr II, Ca II)

The Chromosphere (II)

Spicules = filaments of

hot gas, visible in H

emission.

Streams extend up to ~ 10,000 km above photosphere

The Transition Zone

h ~ 2300 – 2600 km above photosphere

Transition from moderate to high ionization

T ~ 25,000 → 106 K

n ~ 107 → 105 cm-3

Observe selective heights in (UV) emission lines of ionized metals

Ly → ~ 20,000 K

CIII 977 → ~ 90,000 K

O VI 1032 → ~ 300,000 K

Mg X 625 → ~ 1,400,000 K

The Solar Corona

Very hot (T ≥ 106 K), low-density (n ≤ 105 cm-3) gas

Active vs. Quiescent

Corona

Near Solar Maximum

Near Solar Minimum

Coronal Holes

X-ray images of the sun reveal coronal holes.

These arise at the foot points of open field lines

and are the origin of the solar wind.