lecture11

Introduction to modern astronomy 11
島袋隼⼠(Hayato Shimabukuro)（云南⼤学、
SWIFAR）
©GETTYIMAGES

Sun（太阳）
•The sun is the most familiar star for us
•The sun provides us with energy that powers weather,
climate, and life on Earth.
•Without the sun, we would not exist.
•The size of the sun is 700,000km (cf. Earth 6,300km)
•The weight of the sun is kg(cf. Earth )
2 × 1030
6.0 × 1024
kg

Structure of the sun
⽇冕
光球层
⾊球层
过渡区
对流层
辐射区
⽇珥
耀斑
太阳⻛

Temperature of the sun
•The temperature of the sun depends on place.
•The core of the sun is hottest (~1600万K)
•The chromosphere (atmosphere) of the sun is
~1万K
•The temperature of the corona is ~100万K
•The temperature of the flare is ~2000万K
•The surface of the sun is 6000K

The size of the sun is around earth!
The sunspot
•We can see “Sunspot (太阳⿊⼦)”. The size of sunspot is around 10’000km.
•The sunspot's temperature(~4500K) is lower than other solar surfaces (~6000K).
•We expect that the sunspot is generated by solar magnetism(太阳磁场)

The solar magnetism
•As I explained in the previous slide, the sunspot is caused by solar magnetism.
•The sun has a magnetic field similar to the earth. In particular, the magnetic field in the
sunspot is much stronger than the earth’s magnetic field.

The solar magnetism
•If there are sunspot pairs, magnetic field lines are generated.
•The gas at the surface of the sun interacts with the magnetic field. Thus, we can observe
flare(耀斑) along the magnetic field.

•The temperature of the sun depends on place.
•The core of the sun is hottest (~1600万K)
•The chromosphere (atmosphere) of the sun is
~1万K
•The temperature of the corona is ~100万K
•It is very strange that the temperature of the corona (100万K) is much larger than
chromosphere (1万K)
•The temperature of the flare is ~2000万K
•The surface of the sun is 6000K

•Generally, we feel warm when we have a distance from the stove.
~100度
•But, in the case of the sun, you get burned because the temperature of the corona is 100 times
larger than the chromosphere
~10000度
~20度
~100度

⾊球层⽇冕
温度
到光球层的距离
•Apart from the sun’s surface, the temperature rapidly increases.
•We have not fully understood why
the temperature rapidly increases.
But, we expect it relates to the
magnetic field.
•The magnetic energy transforms to
temperature?

What is the amount of energy of the sun?
•We receive energy from the sun.
•In earth, we receive the energy about 1400
per second from the sun.
W/m2
(W:watt )
[kg ⋅ m2 ⋅ s−3]
•The total amount of energy(Luminosity,光度) emitted from the sun is W per second.
4 × 1026
•Typical family consumes about 4000kW per year.
The sun is a very powerful source of energy!

solar luminosity
solar mass
= 2 × 10−4
W/kg.
•The sun’s luminosity is W and the mass of the sun is kg.
4 × 1026
2 × 1030
•Thus, the solar luminosity by the solar mass is
This is not much energy
(e.g.)
•If you burn wood, the luminosity per
kg is around ~ W. times
larger than the sun!
103
106
− 107
•In the case of humans, the luminosity per
kg is ~ 2W. Humans (per kg) are much
brighter than the sun!

solar luminosity
solar mass
= 2 × 10−4
W/kg.
4 × 1026
2 × 1030
103
106
− 107
(e.g.)

solar luminosity
solar mass
= 2 × 10−4
W/kg.
4 × 1026
2 × 1030
What is the difference between the sun and
wood?
103
106
− 107
(e.g.)

solar luminosity
solar mass
= 2 × 10−4
W/kg.
4 × 1026
2 × 1030
wood?
•The sun continues burning ~ 5 billion years
(50亿年). Woods cannot continue burning for
such a long time.
103
106
− 107
(e.g.)

solar luminosity
solar mass
= 2 × 10−4
W/kg.
4 × 1026
2 × 1030
wood?
•The sun continues burning ~ 5 billion years
(50亿年). Woods cannot continue burning for
such a long time.
•How does the sun generate energy for
such a long time?
103
106
− 107
(e.g.)

How does the sun generate energy?
•The energy in the sun is generated by nuclear fusion(核聚变)
Example of nuclear fusion
nucleus(原⼦核)1+原⼦核2→原⼦核3+energy(能量)
Proton（质⼦） + Deuteron（氘⼦）
> Proton（质⼦）+ Positron（正电⼦）+ neutrino（中微⼦）
(One of the nuclear fusion in the Sun.)

How does the sun generate energy?
•The energy in the sun is generated by nuclear fusion(核聚变)
Example of nuclear fusion
Proton（质⼦） + Deuteron（氘⼦）
> Proton（质⼦）+ Positron（正电⼦）+ neutrino（中微⼦）
This energy provides the sun with energy.
(One of the nuclear fusion in the Sun.)

How much energy is generated by nuclear fusion?
E = mc2
Einstein(爱因斯坦)
Energy(能量)
Mass（质量）
Speed of light（光速度）
c = 3 × 108
m/s
•If we can transform 1kg’s material to the energy, we can generate
E = (1kg)2
× (3 × 108
)2
= 9 × 1016
J
•The atomic bomb dropped on Hiroshima used only 876g of
uranium out of 64kg.

Nuclear reaction in the Sun
•The basic nuclear reaction in the Sun is 质⼦-质⼦链(proton-proton chain)
4质⼦(proton) 4个氦(Helium)+2个中微⼦(Neutrino)+能量
→
或者
4(1
H) → 4
He + 2ν + energy

→
或者
1. Two protons combine to form
deuterium(氦核) and positron(正电⼦).
4(1
H) → 4
He + 2ν + energy

→
或者
2. Resulting positrons annihilate(湮
灭) with electrons(电⼦) and release
energy in the form of gamma-ray.
4(1
H) → 4
He + 2ν + energy

→
或者
2. Resulting positrons annihilate(湮
灭) with electrons(电⼦) and release
energy in the form of gamma-ray.
3.Deuteron（氦核） combines with
protons to form Helium3（氦）
4(1
H) → 4
He + 2ν + energy

→
或者
4(1
H) → 4
He + 2ν + energy
4.Finally 2 Helium3 combines to form
Helium4.

→
或者
4(1
H) → 4
He + 2ν + energy
4.Finally 2 Helium3 combines to form
Helium4.
In the process of the proton-proton
chain, the energy is released in the
form of a gamma-ray and the sun
burns by this energy.

How much is the life-time of the sun?
•The sun generates energy . How long can sun continue nuclear fusion?
∼ 4 × 1026
J/s
E = mc2 c = 3 × 108
m/s
• If 1kg of hydrogen atom generates helium by nuclear fusion, totally 0.7% of mass is
consumed (7g). Then, if the total mass of sun( ) is consumed by nuclear fusion,
how much energy[J] is generated?
2.0 × 1030
kg

∼ 4 × 1026
J/s
E = mc2 c = 3 × 108
m/s
2.0 × 1030
kg
E = Δmc2
= 2 × 1030
kg × 0.007 × (3 × 108
m/s)2
∼ 1.3 × 1045
J

∼ 4 × 1026
J/s
E = mc2 c = 3 × 108
m/s
2.0 × 1030
kg
E = Δmc2
= 2 × 1030
kg × 0.007 × (3 × 108
m/s)2
∼ 1.3 × 1045
J
τ =
1.3 × 1045
J
4 × 1026J/s
∼ 3.3 × 1018
s ∼ 950亿年

∼ 4 × 1026
J/s
E = mc2 c = 3 × 108
m/s
2.0 × 1030
kg
E = Δmc2
= 2 × 1030
kg × 0.007 × (3 × 108
m/s)2
∼ 1.3 × 1045
J
τ =
1.3 × 1045
J
4 × 1026J/s
∼ 3.3 × 1018
s ∼ 950亿年
•But, only 10% of mass of sun is used for nuclear fusion. Thus, the life-time of the sun
is roughly 95亿年 (The sun currently has spent 46亿年).

Does nuclear reaction really happen?
•It is difficult to get evidence that nuclear reaction really happens by the electromagnetic wave.
4(1
H) →4
He + 2ν + energy
•Instead of the observation of electromagnetic waves, astronomers tried to detect the neutrino
as evidence of the nuclear reaction in the sun.
→
Super Kamiokande
•We observed the neutrino from the sun.
>It is evidence of the nuclear reaction in
the Sun!
A Nobel Prize was awarded for this discovery.

Summary
• The sun is the most familiar star for us.
• We learn the structure and activity of the sun.
• The energy in the sun is generated by nuclear reactions.
• We detected neutrino as the evidence of the nuclear
reactions in the sun.

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