Astronomy Lecture
The Sun: Our Extraordinary
Ordinary Star
Filaments
Across the Sun
Bulk Properties of the Sun
• Diameter
109 Earth Diameters
• Mass
333,000 Earth Masses
• Density
1408 kg/m3
• Rotation Periods
Equatorial: 25 Days
Polar: 35 Days
• “Surface” Temp.
5800 K
• Core Temp.
15,500,000 K
Limb Darkening
Mercury
• The sun is not as bright near the limb as it is in the
center. Also it is more yellow, indicating that we are
looking through cooler layers near the limb than at the
center.
• This is because we see deeper into the photosphere
when we look straight down than when we look
obliquely.
Explanation of Limb Darkening
The Sun’s Atmosphere
• The photosphere is the visible layer of the Sun.
• The chromosphere is a mostly cooler layer that
lies just above the photosphere. This region
creates the Sun’s absorption line spectrum.
• The transition region is a thin region above the
chromosphere, where the temperature rises
rapidly from about 10,000 K to a million K.
• The corona is the Sun’s outer atmosphere. The
temperature of the corona is 1 to 2 million K.
The corona extends several times the diameter
of the Sun.
Photosphere
showing
Solar Granulation
Solar Granulation
High-resolution
photographs of the
Sun’s surface reveal a
blotchy pattern, called
granulation. Granules,
which measure about
1000 km across, are
convection cells in the
Sun’s photosphere.
Solar Granulation Video
Spicules and Supergranules in the Chromosphere
Supergranules are regions of rising and
falling gas, spanning hundreds of granules
The Solar Corona
The Active Sun
• Sunspots and the Sunspot Cycle
• Solar Magnetism and the Solar Cycle
• Other Atmospheric Phenomena
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Plages
Active Regions
Prominences
Filaments
Coronal Holes
Flares
Coronal Mass Ejections
Sunspots
• Sunspots are typically about 10,000 km in diameter –
about the size of the Earth. They have a dark central
umbra surrounded by a grayish, structured penumbra.
• They appear dark only by comparison to the brighter
surrounding photosphere. They are cooler regions of the
photosphere. The temperature of the umbra is about
4500 K and that of the penumbra is about 5000 K.
• A large group of sunspots typically lasts about 50 days.
• Galileo determined the Sun’s rotation period by timing
the movement of sunspots. The Sun rotates in 25.4
days at the equator and in 33 days in the polar region.
Motion of Sunspots Video
The Sunspot Cycle
Solar Magnetism
• Sunspots are directly linked to intense magnetic fields on
the Sun. When atoms are in magnetic fields, their
spectrum lines are split into two or more lines on each
side of the central line. This is called the Zeeman effect.
• The strong magnetic field in sunspots lowers their
temperature by interfering with the convective flow of hot
gas toward the surface.
• Sunspots usually come in pairs with the magnetic field
coming out of one member of the pair and going in at the
other member. In opposite hemispheres, sunspot pairs
are reversed in their polarity.
• Solar Cycle: The 11-year sunspot cycle is ½ the solar
cycle. In alternate sunspot cycles, the Sun’s magnetic
field reverses direction causing the polarity of the
sunspots to reverse.
Zeeman Splitting of Spectrum Lines
Magnetic Field Lines and Sunspot Pairs
Effect of Sun’s Differential Rotation on its Magnetic Field
Other Atmospheric Phenomena
• Plages – brighter (hotter) areas in chromosphere
• Filaments – dark streaks above the chromosphere.
Huge volumes of gas uplifted into the corona
• Prominences – filaments viewed from the side
• Coronal Holes – darker (cooler) areas in corona,
visible in X-rays, where gases easily can escape from
the Sun
• Flares – violent eruptive events seen in UV & X-rays
• Coronal Mass Ejections – huge, balloon-shaped
volumes of high-energy gas being ejected
Solar Prominences
Prominences and the Corona photographed during the
solar eclipse of July 11, 1991, near sunspot maximum
Active Sun in Hα
A Solar Prominence from SOHO
Prominences
X-ray picture of a Coronal Hole
UV picture of a Solar Flare
A Coronal Mass Ejection
The Sun’s Composition
The main elements are hydrogen and helium, as in
the gas giant planets and in other stars and nebulae in
the universe.
Element
by Atoms by Mass
Hydrogen, H
92%
74%
Helium, He
8%
25%
0.1%
1%
Others
This data is needed for the homework.
Main Regions of the Sun
The Sun’s Interior
• The Standard Solar Model. Is a mathematical model of
the Sun, made by combining all available observations
with theoretical insight into solar physics. This model
shows that the Sun’s interior has three major regions,
listed from outside to inside.
• Convection Zone. This zone extends downwards from
the photosphere about 200,000 km. The material is in
constant convective motion.
• Radiation Zone. Below the convection zone and
extending to the core, is the radiation zone, where
energy is transported toward the surface by radiation
rather than by convection.
• Core. The central core, about 200,000 km in radius, is
the site of the nuclear reactions that generate the Sun’s
enormous energy output.
Density & Temperature Profiles of the Sun’s Interior
The Sun’s Source of Energy
• Solar Constant is 1400 W/m2.
• Luminosity of the Sun is 1400 W/m2  4π  (1 AU)2 =
41026 W.
• The Conversion of Mass to Energy: Mass and energy
are related through Einstein’s equation E = mc2.
• Solar Energy from Nuclear Fusion, the combining of light
nuclei into heavier ones. The sum of the masses of the
light nuclei is a little greater than the mass of the heavier
nucleus that is formed.
• The Sun gets its energy from the Proton-Proton Chain,
fusing 4 hydrogen atoms into 1 helium atom.
The Proton-Proton Chain
Hydrogen to Helium Animation
How Energy Gets from the Sun’s
Core to Its Surface part 1
• Radiative Transfer.
– In the central regions of the Sun, the temperature is
so hot that all the electrons are stripped from the
nuclei. Thus there are no bound electrons to move
from one state to another, absorbing radiation.
– This region is relatively transparent to radiation,
allowing energy to flow out freely.
– Radiation diffuses slowly outward in a haphazard
zigzag pattern, taking about 170,000 years on the
average to go from the core to the bottom of the
convective zone.
How Energy Gets from the Sun’s
Core to Its Surface part 2
• Convective Transfer. As the temperature drops outside
the inner core, atoms can retain some electrons. This
causes the gas to become more and more opaque to
radiation. The energy must still get out, but since the
radiation is blocked, convection begins and carries the
energy away to the surface. This takes about 10 days to
reach the photosphere.
• Convection Cells. In the deep solar interior they are
thought to be large, perhaps 30,000 km across. At
higher levels the convection cells are smaller, about
1000 km across just below the photosphere.
Solar Neutrinos
• Neutrinos are “ghostly” particles with no charge
and having an immeasurably small mass.
• They go through matter like it isn’t there.
Therefore they are very difficult to detect.
• Solar neutrinos are going from the Sun through
the Earth and through your body right now –
roughly 100 billion of them per square
centimeter each second.
Helioseismology
Sound waves
resonating within the
solar interior cause the
photosphere to move in
and out, rhythmically
distorting the shape of
the Sun. This heaving
motion can be
described as the
superposition of millions
of oscillations, like the
one shown here.
Rising Gasses
Descending Gasses
Sound waves inside the
Sun, like seismic waves
in the Earth, are
refracted back out. The
Sun’s surface reflects
waves back in. How
deep a wave penetrates,
and how far around the
Sun it goes before it hits
the surface depends on
its wavelength.
Summary of Key Ideas
Know and Understand:
• The size of the Sun compared with Earth
and its “surface” temperature
• The regions of the Sun’s atmosphere
• The nature of sunspots and the sunspot
cycle
• Where and how the Sun gets its energy
• The regions of the Sun’s Interior