Our Sun

Question 1

Sun Facts

• Surface Temperature
• Central Temperature
• Mass
• Current Composition

Answer 1

• Surface temp = 5800K
• Central temp = 10⁷ K
• Mass = 10³⁰ kg
• Current composition:

1. 92.1% H
2. 7.8% He

Question 2

Energy Production - Overview

Answer 2

Energy is produced by fusion in which, simply, hydrogen is converted into helium: •4H →He + energy

• (n.b. other fusion processes take place as well, some of which are more important for other types of stars)
• Because the total mass afterwards is less than the total mass before, then energy is released because: E = mc²

Question 3

The Proton-Proton Chain: Step 1

Answer 3

Step 1:

• Two protons (hydrogen nuclei, ¹H) interact producing:

1. The hydrogen isotope deuteron (²H)
2. A neutrino, ν
3. A positron, e⁺

• Subsequently the e⁺ annihilates with an e- producing two gamma rays, γ

Question 4

The Proton-Proton Chain: Step 2

Answer 4

Step 2

• The ²H nucleus from the first step collides with a third proton.
• A helium isotope ³He is formed and another gamma-ray photon is released.

Question 5

The Proton-Proton Chain: Step 3

Answer 5

Step 3

• Two ³He nuclei collide
• A different helium isotope with two protons and two neutrons ⁴He is formed and two protons are released.

Question 6

The Proton-Proton Chain: Pictorial Summary

Answer 6

Question 7

Why doesn’t the Sun collapse under its own weight?

Answer 7

Maintaining the balance:

• Because the Sun is not a solid object it retains it shape through a balance of forces.
• Gravity is trying to crush it
• Fusion is producing escaping energy which counteracts gravity

Note that this balance applies throughout the Sun, even though the actual temperatures and pressures will change enormously with radius.

Question 8

Temperature/Density vs distance from the centre of the Sun - graph

Answer 8

Question 9

Sun - Cross Section

Answer 9

Question 10

Reason for the Suns surface moving up and down

Answer 10

• The Sun “rings” at a range of frequencies.
• Typical periods are around 5 minutes and result in the sun’s surface moving up and down about 10m.

Question 11

How do we study the Solar Fusion process?

Answer 11

Solar neutrinos

• If we wish to check on the solar fusion process, then we need to use a neutrino telescope.
• This is because heat energy takes about 1 million years to reach the surface and so we need something more direct
• Neutrinos escape at the speed of light and reach the earth in 8 minutes.

Question 12

Photosphere - General Information

Answer 12

• The outer layer from which all visible photons arise.
• Note the presence of Sun Spots (see later) and the effect called Limb Darkening (next slide).

Question 13

Limb Darkening - General Information

Answer 13

• Because of scattering off H- ions, the visible photons can only travel a fixed distance inside the sun.
• Remember temperature is falling with radius and since F=σT4 a small change in T makes a big change in F.

Question 14

Solar Granules - General Information

Answer 14

• Solar granules are convective cells about 1000km wide.
• Doppler imaging allows us to map out the rising and falling parts of the suns surface.

Question 15

Chromospheres - General Information

Answer 15

• This may be considered to be the solar atmosphere. Very tenuous, and much hotter (25000K) than the photosphere (6000K).
• Normally invisible to the human eye except during Total Eclipses.

Question 16

Solar Atmosphere - Information and Components

Answer 16

• This may be considered to be the solar atmosphere. Very tenuous, and much hotter (25000K) than the photosphere (6000K).
• Normally invisible to the human eye except during Total Eclipses
• The three components of the solar atmosphere.
• Nb the photosphere is only 400 km thick.

Question 17

Solar Atmosphere - Temperature profile

Answer 17

• Temperature profile through the solar atmosphere.
• The high temperatures in the corona are due to particles being rapidly accelerated along magnetic field lines. (Remember, temperature is a measure of how fast the atoms are moving)

Question 18

The Corona - Is it smooth?

Answer 18

• The corona is far from smooth as this UV image shows.
• Coronal holes are even more visible in the X-ray band.

Question 19

Sunspots - What are they?

Answer 19

• Regions of relative coolness in the photosphere.
• Remember the Stefan-Boltzman Law : radiated flux = σT^4
• They have intense magnetic fields (0.3Tesla) - some 5000 times stronger than the earth’s magnetic field – and this can be measured directly using the Zeeman effect
• Mapping the Sun’s magnetic field. Sunspots are normally made up of north and south magnetic polarities

Question 20

Solar Rotation

Answer 20

• Period = 25d at the equator and 35d at the poles.
• This is known as differential rotation.

Question 21

Sunspot Cycle

Answer 21

• The 11 year sunspot cycle. The most recent maximum was in 2011.

Question 22

Butterfly Diagram - average latitude of Sunspots

Answer 22

The “Butterfly” diagram shows that the average latitude of sunspots varies throughout the solar cycle

Question 23

Differential Rotation and Sunspots

Answer 23

• Differential rotation leads to magnetic field entanglement producing sun spot groups. Differential rotation within the sun:
• Surface and convective zones show strong differential rotation.
• However, deeper in, the radiative zone seems to rotate like a solid body

Question 24

Structures associated with the Sun’s magnetic field

Answer 24

• An Hα image of the active sun (656 nm).
• All three structures shown are associated with strong magnetic fields.

Question 25

Solar Prominence and Coronal Mass Ejection (CME)

Answer 25

• A huge solar prominence seen by the SOHO spacecraft in the UV.
• Material in the prominence reaches temperatures of millions (K).
• This material is often ejected out into interplanetary space.

Question 26

The Earth’s Climate

Answer 26

• The relationship between solar activity and the earth’s climate is a subject of intense research.
• It is believed that even a 1% change in the amount of solar energy could have serious effects, perhaps producing ice ages.