The Sun

Our Star

Please press “1” to test your transmitter.

General Properties

• Average star

• Central temperature = 15 million 0K

• 333,000 times Earth’s mass

• 109 times Earth’s diameter

• Consists entirely of gas (av. density = 1.4 g/cm3)

• Only appears so bright because it is so close.

• Surface temperature = 5800 0K

Which parts of the sun can only be seen during a total solar eclipse?

1. Prominences

2. The solar corona

3. Sun spots

4. 1 and 2

5. All of the above.

Structure of the Sun

Only visible during solar eclipses

Apparent surface of the sun

Hea

t F

low

Solar interior

Temp. incr. inward

The Sun’s Interior Structure

Temp, density and pressure decr. outward

Energy generation via nuclear fusion

Energy transport via radiation

Energy transport via convection (explained soon)

Flo

w o

f en

erg

y

Photosphere

Do we have a direct view of the sun’s energy source?

1. Yes, because the sun is just a transparent gas ball.2. Yes, because most of the energy is produced very

close to the surface.3. Yes, because the sun’s center is so bright that the light

is shining through any material.4. No, because the sun has a non-transparent solid

surface.5. No, because the radiation produced in the center is

scattered around many times on its way towards the surface.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

How is energy produced in an H bomb?

1. (Chemical) Burning of hydrogen.2. Nuclear fusion of hydrogen into heavier

elements.3. Nuclear fission of hydrogen.4. Nuclear fission of heavier elements into

hydrogen.5. Nuclear fission of heavier elements into

elements heavier than hydrogen.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

Energy generation in the Sun:Fusion of Hydrogen into Helium

Basic reaction:

4 1H → 4He + energy

4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He.

Energy gain = m*c2

= 0.43*10-11 J

per reaction.

Need large proton speed ( high temperature) to overcome

Coulomb barrier (electromagnetic repulsion between protons).

Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own

gravity.

T ≥ 107 K = 10 million K

How do we know that the sun is made mostly of Hydrogen?

1. Space probes have taken samples of solar material and analyzed it.

2. The sun’s spectrum shows strong emission lines from Hydrogen.

3. The sun’s spectrum shows strong absorption lines from Hydrogen.

4. Hydrogen is very easily flammable; this explains the sun’s brightness.

5. Nonsense! The sun is actually made mostly of Nitrogen and Oxygen.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

Absorption Lines

Analyzing absorption spectra• Each element produces a specific set of

absorption (and emission) lines.

By far the most abundant elements in the Universe

• Comparing the relative strengths of these sets of lines, we can study the composition of gases.

Which Hydrogen Lines appear in visible light?

1. The Balmer Lines (from/to the first excited state)

2. The Balmer Lines (from/to the ground state)

3. The Lyman Lines (from/to the first excited state)

4. The Lyman Lines (from/to the ground state)

5. The Einstein Lines (from/to the second excited state)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

The Balmer Lines

n = 1

n = 2

n = 4

n = 5n = 3

H H H

The only hydrogen lines in the visible wavelength range.

Transitions from 2nd to higher levels of hydrogen

2nd to 3rd level = H (Balmer alpha line)2nd to 4th level = H (Balmer beta line)

…

The Cocoon Nebula (H emission)

Energy TransportEnergy generated in the sun’s center must be transported to the surface.

Inner layers:

Radiative energy transport

Outer layers (including photosphere):

Convection

Bubbles of hot gas rising up

Cool gas sinking down

Gas particles of solar interior-rays

Granulation

… is the visible consequence of convection

Which every-day phenomenon is another example of convective

energy transport?

1. Gas bubbles rising up in a soda drink.

2. Gas bubbles rising up in boiling water.

3. Giant waves moving onto the sea shore.

4. Earthquakes.

5. All of the above.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

Which every-day phenomenon is another example of radiative

energy transport?

1. The heat of a bonfire warming you when you’re sitting close to it.

2. Heating food in the microwave oven.3. The air around a light bulb heating up when

the light is on.4. All of the above.5. None of the above.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

The Sun’s Interior Structure

Temp, density and pressure decr. outward

Energy generation via nuclear fusion

Energy transport via radiation

Energy transport via convection (explained soon)

Flo

w o

f en

erg

y

Photosphere

Animation

Very Important Warning:

Never look directly at the sun through

a telescope or binoculars!!!

This can cause permanent eye damage – even blindness.

Use a projection technique or a special sun viewing filter.

Sun Spots

Visible Ultraviolet

Cooler regions of the photosphere (T ≈ 4240 K).

Active Regions

Considering that sunspots are cooler regions on the photosphere with a temperature of ~ 4240 K, how

would you think a sunspot would appear if you could put it on the night sky without the sun surrounding it?

1. It would be invisible.2. It would glow very faintly, similar to the faint red glow

of the eclipsed moon.3. It would appear moderately bright, comparable to the

brightest stars.4. It would appear very bright – even brighter than the

full moon.5. It would be almost as bright as the sun itself.

Solar Activity, seen in soft X-rays

What can we infer from the fact that we see the gas above active regions (sun spots)

mostly in ultraviolet light and X-rays?

1. The gas must be very dense.2. The gas must be very dilute.3. The gas must be very hot.4. The gas must be very cold.5. The gas must consist mostly of Helium.

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 Field Lines

Magnetic North Pole

Magnetic South Pole Magnetic

Field Lines

Mass ejections from the sun often follow magnetic field loops.

The Solar Cycle

11-year cycle

Reversal of magnetic polarity (during solar minima)

After 11 years, the direction of magnetic fields is reversed

=> Total solar cycle = 22 years

Solar Maxima

Full 22-year cycle

Are we currently near a solar maximum or a solar minimum?

0 of 5

1. Maximum.

2. Minimum.

The Solar Corona

Very hot (T ≥ 1 million 0K), low-density gas

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)