Download The Corona

The
Interior of
The Sun
Jonathan Correa Magdalena
Contents:
 General Features of the Sun
 Layers:
o
o
o
o
o
o
The
The
The
The
The
The
Core
Radiative Zone
Convection Zone
Photosphere
Chromosphere
Corona
 Spacecrafts
 References
General Features of the Sun
A star is a huge mass of hot, glowing gas. The
strong gravitational pull of the Sun holds Earth
and the others planets in the solar system in
orbit. The Sun’s light and heat influence all of
the objects in the solar system and allow life to
exist on Earth.
The Sun’s radius is 695,508 km, 109 times as large
as Earth’s radius. The Sun has a lower density
(about 1,409 g/cm3). Which is a quarter of the
average density of Earth. It has about 333,000
times the Earth’s mass.
It generates 3,83x1026 joules of energy every
second. All of this energy is generated in the
Core by Nuclear Fusion.
The Sun is a second-generation star, meaning that
some of its material came from former stars. The
first stars were composed only of the hydrogen and
helium produced in the early universe. Although
hydrogen (94%) and helium (6%) also are the main
ingredients of the Sun, it contains heavier
elements (0.13%), such as carbon, nitrogen and
oxygen as well. Spectroscopy shows us all these
values.
The Sun is about 4,6 billion years old and we can
calculate that it will keep shining for about
another 7 billion years.
The average distance from Earth is 150 million km.
Layers
The solar interior is separated into four regions
by the different processes that occur there:
The Core
The core extends from the center to about one
quarter of the radius of the Sun. Is in this
central region where nuclear reactions consume
hydrogen to form helium.
These reactions need
special conditions of density (about 150 g/cm3,
which is more than 13 times the of lead) and
temperature (about 15x106 C).
The hydrogen nuclei must collide with enough
energy to give a reasonable probability of
overcoming the repulsive electrical force between
thee two positively charged particles.
Every second, fusion reactions convert about 700
million tons of hydrogen into helium. Every fusion
reaction also produces a neutral particles called
“neutrinos”. The Sun creates a huge amount of
these particles. In one second 70 billion of
neutrinos pass through every square centimeter of
Earth that is facing the Sun.
The fusion take place through three different
steps. Firstly two protons collide to produce
deuterium, a positron and a neutrino. In the
second step one proton collides with the deuterium
to produce a helium-3 nucleus and a gamma ray. And
finally, in the third step, two helium-3 collide
to produce helium-4 nucleus and two protons.
The Radiative Zone
This layer extends outward from the edge of the
Core to the Convection Zone (between 25-70 percent
of the Solar radius). Particles in the radiative
zone repeatedly absorb, radiate, and deflect
photons. The matter in this zone stays in the same
place while the energy moves through it. Because
of this, a photon requires about 170,000 years to
go from the Sun’s Core to the bottom of the
Convective Zone. The density drops from 20 g/cm3
down to 0,2 g/cm3 from the bottom to the top fo
the radiative zone. The temperature falls from 7
to about 2 million C over the same distance.
The Convection Zone
This zone extends from of about 200,000 km to the
visible surface. At the base, the temperature is 2
million C which is enough for the heavier ions
(such as carbon, nitrogen, etc) to hold onto some
of their electrons. These materials at the bottom
heat up with blocked radiation and become less
dense than surrounding material. Then moves up
through the convection zone carrying energy toward
the atmosphere of the Sun. When reaches this point
(where the density is much less than at the
bottom) the energy can radiate into space. The
material at the top of the convection zone becomes
then colder and denser so falls down to the bottom
to pick up more energy. This process takes about
ten days. At the surface of the Sun the
temperature is about 5,700 K and the density is
only 2x10-7 g/cm3.
The Photosphere
This layer is the visible surface of the Sun. This
is not a solid layer but has about 100 km thick
(about 0,05% of the radius). The Photosphere is
opaque because it contains hydrogen ions which
block, absorb, and emit light. The photosphere has
different features as well:
o
Granulation and Supergranulation
Under close inspection with a telescope, the
photosphere breaks into a million tiny bright
points, called granules, with a strongly textured
and varying pattern.
The granules are small (about 1,000 km across).
They are the top of the convection cells where hot
fluids rise up. They last only for 20 minutes. The
flow within the granules can reach 7 km/s
Supergranules are large versions of granules
(about 35,000 km across). These features also
covert the entire Sun. An individual supergranule
last for a day or two. Here, the flow speed is
about 0,5 km/s.
o
Sunspots
Large dark spots are often visible in the
photosphere. The biggest sunspots exceed Earth in
size. The position and the number of this spots
changes in a 11-year cycle.
This spots are places in the Sun’s surface where
the magnetic field is so strong. Appear dark
because
they
are
much
cooler
than
their
surroundings.
The
concentrated
magnetism
in
sunspots keeps them cold. They are ten times
brighter than the full Moon for example.
This spots act as a valve choking off the heat,
light
and
energy
flowing
outward
from
the
interior. The temperature of is about 3230 C
(approximate the half of the surroundings).
The Chromosphere
The Chromosphere is a thin layer (about 2,0003,000 km) thick, just above the Photosphere. Here
the temperature rises from 5510 C to 9700 C near
the Corona. The hydrogen emits a deep red color.
Calcium ions also produce distinctive radiation
here (UV).
It is an irregular layer. Here we can find other
interesting feature:
o
Prominences
Prominences are dense clouds of material suspended
above the surface of the Sun by loops of magnetic
field. They can remain in a quiet or quiescent
state for days or a week, depending of how quick
changes the magnetic field.
The Corona
The Corona is a very hot layer of the solar
atmosphere. At 3,000 km from the solar surface the
temperature rises to about 106 K. This was a
problem
for
astrophysics
in
the
past.
The
Chromosphere and Photosphere are closer to the Sun
Core but the Corona is several hundred times
hotter than these layers. Now we know that the
heat at the Corona is an effect of the solar
magnetic field and not of the radiation from the
Core. In fact, low frequency magnetic waves
dissipate their energy in this layer when they are
damped.
At these high temperatures both hydrogen and
helium are completely stripped of their electrons.
Carbon, nitrogen and oxygen are stripped as well.
This produced a really strange emission spectrum.
This layer emits most of its radiation at very
short UV an X-ray wavelengths. Much of this
radiation hits Earth’s atmosphere and is absorbed
by atoms and molecules. So, scientists use
instruments in space to study the Corona.
The Corona is visible during total eclipses of the
Sun as a pearly white crown surrounding our star.
The most important features of the Corona are:
o
Coronal Loops
These features are found around sunspots and in
active regions. Usually they are associated with
the closed magnetic field lines. Some of them last
days or weeks but most change quickly.
o
Coronal Holes
Coronal holes are regions where the corona is
dark. Usually associated with "open" magnetic
field lines
poles.
o
and
are
often
found
at
the
Sun's
Polar Plumes
Polar plumes are long thin streamers that project
outward from the poles. Like the feature before,
are associated with the "open" magnetic field
lines at the Sun's poles.
Spacecrafts
Right now there are a couple of missions studying
the Sun. Here we can find some of them:
o
SOHO
SOHO was designed to study the internal structure
of the Sun, its extensive outer atmosphere and the
origin of the solar wind, the stream of highly
ionized
gas
that
blows
continuously
outward
through the Solar System.
o
Ulysses
This mission was designed to sample the solar wind
and the heliosphere at latitudes unexplored by any
other spacecraft. The Ulysses spacecraft got the
Sun's south pole in the fall of 1994 and its north
pole in the summer of 1995. Instruments onboard
Ulysses measured particles, magnetic fields, and
electro-magnetic radiation from radio wavelengths
to gamma-rays.
o
Trace
TRACE explores the magnetic field in the solar
atmosphere by studying: the 3-dimensional field
structure, its temporal evolution in response to
photospheric flows, the time-dependent coronal
fine structure, the coronal and transition region
thermal topology, etc.
References

“Lecture Notes on Space Physics” Kjell Rönnmark

“Solar Physics – NASA”
http://science.msfc.nasa.gov/ssl/pad/solar/interior.htm

“Instituto de Astrofísica de Canarias”
http://www.iac.es/

“Living Reviews”
http://solarphysics.livingreviews.org/

“Institute for Solar Physics”
http://www.solarphysics.kva.se/

“Sun Science”
http://cse.ssl.berkeley.edu/hessi_epo/html/sunmain.htm