Download The Interior of the Sun

Umeå University
Department of Physics
21st October 2010
THE INTERIOR OF THE SUN
Space Physics – Project on
by
Ursula Schlager
Supervisor: Prof. Kjell Rönnmark
CONTENT
1.
Introduction ……………………………………………………….
3
2.
The Structure of the interior of the Sun: ………………………..
3
2.1 The Core ……………………………………………………..
3
2.2 The Radiation Zone …………………………………………..
4
2.3 The Interface Layer (Tachocline) …………………………....
5
2.4 The Convection Zone ………………………………………...
5
2.4.1
Granules ……………………………………………
6
2.4.2
Supergranules ………………………………………
7
3.
Helioseismology …………………………………………………...
7
4.
References …………………………………………………………
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2
1. Introduction
The Sun is our nearest star. It’s just a normal star but for us, it is very important. Without
the Sun the Human race and all living creatures on earth wouldn’t exist. It gives us light
and warmth and it holds our Solar System together.
In the following abstract The structure of the interior of the Sun I will describe the
interior of the Sun without talking about the atmosphere. It’s like you would talk about
the interior of the earth: You wouldn’t pay attention to its atmosphere.
But like in the case of the earth we cannot see inside the Sun. So there is still the
question: How do we know what we know about the Sun? One answer to this question is
Helioseismology which will be described in the same titled abstract.
2. The structure of the interior of the Sun
2.1
The Core
The core is the central region of the Sun. It extends from the centre to one quarter of the
radius of the Sun which is 696,128km. So the core has a radius of approximately 174·10³
km. It includes only 2% of the Sun’s volume but nevertheless it contains half of its mass.
The Core is composed of approximately 72% hydrogen, 26% helium and 2% other
elements. The temperature in this place is about 15·106 K and its density 150g/cm³.
Because of this high temperature and density, all atoms are broken down into their
constituent parts which means in the core of the Sun exist only electrons, protons and
neutrons. Furthermore nuclear reactions take place which create helium from hydrogen.
3
These nuclear reactions generate also the energy which later will leave the Sun’s surface
as visible light.
About 85% of the nuclear reactions in the core follow the following scheme:
1
1
2
1
(1)
( 2)
(3)
3
2
p +11p
→
d +11p
→
2
1
d + e+ +ν
3
2
4
2
He + 23He →
Each of this reaction is
exothermic and the total
thermonuclear energy which is
released is 26.2MeV per
4
2 He nucleus formed.
He + γ
He + 211 p
The remaining 15% of nuclear reactions take up at step (2):
2
1
d + 11p →
3
2
He + γ
He + 24 He →
7
4
Be + γ
( 2)
3
2
e − + 47 Be
1
1
7
3
p + Li
7
3
→
→
4
2
1
1
Li + ν
4
2
He + He
p + 47 Be
8
5
B
8
4 Be *
.
Branch II
[15%, released energy: 25.2MeV]
8
5
→
→
→
8
4
B+γ
Be * + e + + ν
4
4
2 He + 2 He
Branch III
[0.02%, released energy: 19.1MeV]
Whereas steps (1) to (3) make up Branch I, the ones above are the Branches II and III of
the so called Proton-Proton Chain. Notice that a pre-existing 24 He acts as a catalyst (in
branches II and III).
During these reactions neutrinos ν are produced. These elementary particles leave the
Sun very quickly because they have no charge and thus they don’t interact so much with
the medium that surrounds it.
When you move outside the centre, the density and temperature decreases. As a result the
nuclear burning is almost completely shut off beyond the core. At the edge of the core the
temperature is only half as high as in the centre and the density has decreased to 20g/cm³.
2.2
The Radiation Zone
This Zone expands outward from the edge of the core to around 70% of the distance to
the surface of the Sun. It includes 32% of the Sun’s volume and 48% of its mass. The
density is 20g/cm³ at the bottom and it decreases to 0.2 g/cm³ on the top of the layer
which is less than the density of water. The temperature also decreases with increasing
radius from 7 million Kelvin to 2 million Kelvin.
Because of the temperature being a little cooler than in the core, it is a region with highly
ionized gas. As a result, every photon coming from the core and carrying energy is
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absorbed. Thus energy can be stored for a while and emitted as new radiation later. The
new created photon bounces again into another particle due to the high density and is
absorbed and emitted later and so on. This is it what gives this layer its name Radiation
Zone.
Due to the jumping from particle to particle the light needs about a million years from the
bottom to the top of this layer. If there weren’t any obstacles, light would need only two
second for the same distance.
2.3
The Interface Layer (Tachocline)
The next region is the so called Interface layer. It is a very thin layer which is around
200,000km beneath the surface of the Sun. It combines the turbulent outer region, the
Convection Zone, with the orderly interior, the Radiation Zone. So fluid motions that are
found at the bottom of the Convection Zone disappear from the top of the Interface Layer
to its bottom where the Radiation Zone begins which is calmer. Consequently, the speed
of gas within this layer changes abruptly and there is even a difference between how fast
this fluid motion changes when you look at the equator in comparison to looking at the
poles: At the equator the outer gas moves around the Sun’s axis of rotation faster than the
inner gas whereas at mid-latitudes and near the poles the outer gas rotates slower.
The changes in fluid flow velocities across the layer give this layer its alternative name
Tachocline. They can stretch a magnetic field and they can make the magnetic field lines
of force stronger. That is why it is currently thought that the Sun’s magnetic field is
generated by a magnetic dynamo within this layer.
In addition, it was discovered that the contrast in speed between the outer and inner layer
of the Tachocline can change by 20% in six month. That means when the lower gas
speeds up then the upper gas slows down and vice versa.
Also very interesting is that there appear to be sudden changes in the chemical
composition across this layer.
With more details being discovered during the recent years like the ones mentioned
above, the Interface Layer became more and more interesting for scientists and is still a
topic of current research.
2.4
The Convection Zone
This layer is the top layer of the solar interior, so it extends from the Interface Layer up
to the visible surface of the Sun. The Convection Zone brings the missing 66% to the
Sun’s volume and the 2% to its mass.
The temperature at the base is “only” 2 million Kelvin, so it is cool enough for heavier
ions like carbon, nitrogen, oxygen, calcium and ion to hold some of their electrons. This
makes the material more opaque or non-transparent. Now it is more difficult for a photon
to get outward with the help of radiation. The photon will be absorbed by an atom which
doesn’t release it so readily again due to the cool temperature and the density which is
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still very high. This traps heat what consequently makes the fluid unstable and
convection starts. So, with the beginning of the Convection Zone, the transport of energy
by radiation slows down significantly and it is replaced by transport with the help of
convection.
This kind of energy transport works much faster than the one by radiation. It takes only a
little more than a week to transfer the energy from the bottom to the top of the
Convection Zone.
At the visible surface the temperature has dropped to 5,700 K which makes it possible
that even neutral hydrogen exists. The density also dropped to 0.2·10-6 g/cm³ which
corresponds to approximately 1/10000 of the density of the air at sea level.
The convective motions are visible at the surface of the Sun in form of Granules and
Supergranules.
Just to have a better overview about the temperature and density in different layers, here
some diagrams:
2.4.1 Granules
Granules are the top of convection cells where hot gas
rises up from the interior (bright areas), spreads out
across the surface, cools down and sinks inward again
(dark lines). They are about 400-1000km in diameter and
move vertically a distance of the order of 200km.
Granules cover the entire surface of the Sun except those
areas covered by Sunspots. An individual Granule can
last 15 to 20 minutes and it is continually evolving. That
means old ones are pushed aside by new Granules
emerging from the interior.
The flow within the Granules can even reach supersonic
speed of more than 7km/s. In this way it can produce sonic “booms” and other noise that
causes waves on the surface of the Sun.
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2.4.2 Supergranules
Granules are grouped in Supergranules which expand about
35,000km across and extend 5000 to 8000km down into the
Sun. Like the Granules, they cover the entire surface of the Sun
and they are continually evolving. An individual Supergranule
can last up to a day or two.
Supergranules are best seen when measuring the “Doppler
shift”, like you see in the picture. Red means that the material
where the light comes from is moving away from us, whereas
light from material moving towards us is shifted to the blue.
3. Helioseismology
In the 1960’s it was discovered that sound waves are propagating in the Sun by Robert B.
Leighton. About ten years later it was explained by Ulrich (1970) and Leibache and Stein
in 1971. This has lead to the development of a new technique called helioseismology.
The Sun is a ball of hot gas so its interior transmits sound waves very well which can be
seen by the doppler shifting of light emitted at the Sun's surface. Helioseismology uses
these sound waves to probe the interior of the Sun. It is the same like geologists use
seismic waves from earthquakes to probe the inside of the earth.
Some of these waves travel right through the centre of the Sun others are bent back
towards the surface and stay in shallow depths. The lifetime of such waves can be as
short as one or two days or as long as some months. Their frequencies depend on the
thermodynamic, compositional and dynamic state of the material where the wave goes
through. Consequently, with the help of these waves it is possible to construct extremely
narrow probes of the temperature, chemical composition and motions throughout the
interior of the Sun. In this way Helioseismology is a method to measure the internal
structure and dynamics of the Sun directly.
Some things which have been found out with the help of Helioseismology:
The theory and application of Helioseismology has largely confirmed the main elements
of the Standard Solar Model and it has ruled out solar models with low abundance of
heavy elements or such in which Helium produced by fusion in the core is mixed with
Hydrogen from outside the core.
This method has fixed the bottom of the Convection Zone at a depth of about 200,000km
and it has been found that the temperature at the bottom of this zone is higher than
predicted by the Standard Solar Model. The Core on the other hand is cooler than
expected, so the core and its nuclear reactor are maybe more complicated then thought.
With the use of this theory it was also discovered that Superganules go 5000 to 8000 km
deep. Before, theorists expected a depth of 15000 to 20000 km.
Without Helioseismology some of the most important processes in astrophysics would
remain only conclusions from theory and because our Sun is just a normal star, it is
possible to generalize our results from the Sun to other stars. So Helioseismology offers a
springboard to study the interiors of other stars as well.
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4. References
1
“The Physics of Stars”
Wiley, 2nd edition, by A.C.Phillips
2
http://solarscience.msfc.nasa.gov/interior.shtml
3
http://www.nasa.gov/worldbook/sun_worldbook.html
4
http://sohowww.nascom.nasa.gov/newsroom/oldesapr/042000pr/
5
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=30851
6
http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter1.html
7
http://solar.physics.montana.edu/YPOP/Spotlight/SunInfo/Structure.html
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