ASTR 113 – 003
Lecture 02
Spring 2006
Feb. 01, 2006
Introduction To Modern Astronomy II
Review (Ch4-5): the Foundation
Star (Ch18-24)
Galaxy (Ch 25-27)
Cosmology (Ch28-39)
Extraterrestrial Life (Ch30)
1.
2.
3.
4.
5.
6.
7.
Sun, Our star (Ch18)
Nature of Stars (Ch19)
Birth of Stars (Ch20)
Evolution of Stars (Ch21)
Death of Stars (Ch22)
Neutron Stars (Ch23)
Black Holes (Ch24)
ASTR 113 – 003
Lecture 02
Spring 2006
Feb. 01, 2006
Our Star, the Sun
Chapter Eighteen
Basic Facts
• Radius: 700,000 Km
• Distance to Earth: 1 AU = 1.5 X 108 km
• Light travel time: 8 minutes
• Angular size: 30 arcmin
• Effective Surface Temperature: 5800 K
Guiding Questions
What is the source of the Sun’s energy?
What is the internal structure of the Sun?
How can astronomers measure the properties of the
Sun’s interior?
4. How can we be sure that thermonuclear reactions are
happening in the Sun’s core?
5. Does the Sun have a solid surface?
6. Since the Sun is so bright, how is it possible to see its
dim outer atmosphere?
7. Where does the solar wind come from?
8. What are sunspots? Why do they appear dark?
9. What is the connection between sunspots and the
Sun’s magnetic field?
10. What causes eruptions in the Sun’s atmosphere?
1.
2.
3.
The Sun’s energy is generated by thermonuclear
reactions in its core
• Sun’s total energy output: 1026 watts
• Not chemical energy (only last 10,000 years)
• Not gravitational contraction (only last 25 million
years)
• Energy from nuclear reaction
– Corresponds to a reduction of mass according
Einstein’s mass-energy equation
E = mc2
The Sun’s energy is generated by thermonuclear
reactions in its core
• Thermonuclear fusion occurs at very high
temperatures
• Hydrogen fusion occurs only at temperatures in
excess of about 107 K
• In the Sun, hydrogen fusion occurs in the dense, hot
core
Proton-Proton Chain Reaction
•The Sun’s energy is produced by hydrogen fusion, a
sequence of thermonuclear reactions in which four
hydrogen nuclei combine to produce a single helium
nucleus; called proton-proton chain reaction
Proton-Proton Chain Reaction: Step 1
Proton-Proton Chain Reaction: Step 2
Proton-Proton Chain Reaction: Step 3
Proton-Proton Chain Reaction
4 H  He + energy + neutrinos
Mass of 4 H > Mass of 1 He
•In every second, 600 million tons of hydrogen
converts into helium to power the Sun
•At this rate, the Sun can continue the hydrogen
burning for more than 6 billion years.
Theoretical Model of the Sun
•
Using physical equations to calculate the
distribution of temperature, density, and pressure
along the radius of the Sun from the core to the
surface.
•
Based on physical conditions of
1. Hydrostatic equilibrium: no expansion, no
contraction
2. Thermal equilibrium: no temperature change
with time
3. Energy transportation
1. Conduction
2. Convection
3. Radiative diffusion
Hydrostatic Equilibrium
Theoretical Model of the Sun
Theoretical Model of the Sun
•From the center to the surface, luminosity increases,
and mass increases
Theoretical Model of the Sun
•From the center to the surface, temperature
decreases, and density decreases
Sun’s Internal Structure: three layers
1. Energy Core: Hydrogen
fusion takes place, extending
from the Sun’s center to
about 0.25 solar radius
2. Radiative Zone: extending to
about 0.71 solar radius
– In this zone, energy
travels outward through
radiative diffusion
3. Convective Zone: an opaque
zone at relatively low
temperature and pressure
– energy travels outward
primarily through
convection
Astronomers probe the solar interior using
the Sun’s own vibrations
• Helioseismology is
the study of how the
Sun vibrates
• These vibrations have
been used to infer
pressures, densities,
chemical
compositions, and
rotation rates within
the Sun
Neutrinos reveal information about the Sun’s
core—and have surprises of their own
• Neutrinos emitted in
thermonuclear reactions
in the Sun’s core have
been detected, but in
smaller numbers than
expected
• Recent neutrino
experiments explain why
this is so
• Nuclear reaction is
indeed occurring in the
Sun
The Sun’s Atmosphere
•
The Sun atmosphere has three main layers:
1. Photosphere (400 Km thick, innermost)
2. Chromosphere (2000 Km thick, above photosphere)
3. Corona (millions of Km, extended)
•
Everything below the solar atmosphere is called the solar
interior
The photosphere is the lowest of three main layers
in the Sun’s atmosphere
• The photosphere
(sphere of light) is the
visible surface of the
Sun
• It is only 400 km thick
because of its
opaqueness; Photons
emitted below 400 km
can not escape.
• Temperature
decreases upward
Photosphere: Limb Darkening Effect
• At the limb, one can not see as deeply as at the center
• The gas at higher altitude is less hot (or lower
temperature), and thus emit less energy
• At the limb, appear dimmer
Convection in the photosphere produces granules
• Granulation is the direct evidence of convection
• Each granule is about 1000 km
•Granules from, disappear and reform in cycles lasting a
few minutes
Super-granules in photosphere
• Very large convection cell
• Each super-granule is about 35,000 km
• Moves slowly, lasting about one day
•Hard to observe directly; better seen in Doppler images
The chromosphere (sphere of color)
• Above the photosphere is a
layer of less dense but higher
temperature gases called the
chromosphere
• Chromosphere is best seen in
spectral emission lines, e.g.,
Hα line at 656.3 nm (Hydrogen
level 3-2 transition)
• Spicules, jets of rising gas,
extend upward from the
photosphere into the
chromosphere along the
boundaries of supergranules
The Corona:
the
outermost
The corona ejects
mass into
space
to formlayer
the solar wind
of the Sun’s atmosphere
• Corona is made of very
high-temperature
gases at extremely low
density
• It extends to several
million Km
• Because of hot
temperature, it expands
into the outer space
forming solar wind
Corona is directly seen in EUV light, which is
sensitive to 1 million Kelvin plasma emission
The Sun’s Atmosphere
The Sun’s Atmosphere
The Sun’s Magnetism
•The outer corona is much hotter than the
inner chromosphere and photosphere
•The corona must be heated by a source other
than the conduction or radiative diffusion from
the underlying atmosphere, because the
energy transfer of conduction and radiative
diffusion is always from high temperature to
low temperature
•The corona heating is related to the
ubiquitous presence of magnetic field in the
Sun’s atmosphere.
Sunspots are low-temperature regions in
the photosphere
• Sunspots are shaped dark regions in the photosphere
•mIt appears dark because it is cooler (radiate less energy)
• Sunspot Umbra: core
• Sunspot Penumbra: the brighter border
Tracking the Sun’s Rotation with Sunspots
11-year sunspot cycle, or solar cycle
• The average number of sunspots increases and decreases
in a regular cycle of approximately 11 years, with reversed
magnetic polarities from one 11-year cycle to the next
• The last solar maximum is in 2000
• We are now in the declining phase of solar cycle 23rd
• The next solar minimum is in 2007
Sunspot is the area of concentrated
magnetic field
• This is proved by direct measurement of magnetic field
using Zeeman effect
• Zeeman effect: a spectral line splits in magnetic field
Magnetic Field Measurement
• Zeeman effect
•The splitting of a spectral line because of the presence of
magnetic field.
Magnetogram: image of magnetic field
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•
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•
Magnetic field has two polarities: positive (north) and negative (south).
Sunspot always appears in groups
A sunspot group resembles a giant bar of magnet.
In a sunspot group, the sunspots in front are called “preceding
members”, while the sunspots following behind are called “following
members”
• In one solar hemisphere, the preceding members all have the same
magnetic polarity; the following members have the opposite polarity
• The polarity is opposite in the other hemisphere
22-year of solar magnetic cycle
• The Sun’s polarity pattern completely reverse every 11 years
• In one 11-year sunspot cycle, the hemisphere that has
preceding positive polarity will have preceding negative polarity
in the next 11-year cycle, and vice versa
• The north and south magnetic poles of the Sun itself also
reverses every 11 years
• The same magnetic pattern repeats after two sunspot cycle, or
22 years.
Butterfly Diagram of Sunspot
Creation of Solar Magnetic Field: Magnetic
Dynamo
• An electric dynamo (or generator in power plant) is a
device that converts the energy of motion (kinetic
energy) to that of the electric current
•Similarly, a magnetic dynamo is to convert the energy of
motion into magnetic field
•The solar magnetic dynamo is caused by the Sun’s
differential rotation
Rotation of the Sun
Differential Rotation of the Sun’s Interior
• The Sun’s rotation varies with latitude and depth
• A short rotation period at the equator (25 days), and longer
periods near the poles (35 days)
Solar Cycles are caused by the Sun’s differential
rotation and convection: Babcock’s Magnetic
Dynamo Model
•Solar magnetic field is generated inside of the Sun by the
mechanism called solar magnetic dynamo
Active Sun: Coronal Loops
• Coronal loops are
tracing magnetic
field lines in the
corona
• Coronal loops are
rooted in the
photosphere, one
end with positive
polarity, and the
other end negative
polarity
Active Sun: Chromospheric Features
•Filament: the dark streaks on the disk, cooler and denser
chromospheric material suspended in the corona; it is
supported by magnetic field in the corona
•Prominence: the same as filament when viewed above the
limb; it appears bright because it is against the dark
background of space.
•Plage: bright areas associated
with sunspots.
Active Sun: Solar Flares
• Flare: sudden and intense increase of electromagnetic
radiation from the Sun in almost all wavelengths from radio
wavelength to X-rays and Gamma rays.
Active Sun: Coronal Mass Ejections
• A large scale eruption of coronal material and magnetic field.
It is ejected into space at high speed. It impacts the Earth a
few days later if at the right direction.
Space Weather Effect of Solar Activities
Space Weather Effect of Solar Activities
Key Words
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22-year solar cycle
chromosphere
CNO cycle
conduction
convection
convective zone
corona
coronal hole
coronal mass ejection
differential rotation
filament
granulation
granule
helioseismology
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hydrogen fusion
hydrostatic equilibrium
limb darkening
luminosity (of the Sun)
magnetic-dynamo model
magnetogram
magnetic reconnection
negative hydrogen ion
neutrino
neutrino oscillation
photosphere
plage
plasma
positron
prominence