In this exercise you will compare the size of the Sun to the planets and other stars. You will also plot Plancks curve for the Sun and from it estimate important characteristics like its temperature, solar constant and luminosity.
You will need a calculator and graph paper
The Sun is just a nearby star By studying the Sun, we are able to learn things that apply to other, far off stars we can learn things about our own star.
Our star, the Sun, is a middle-aged star approximately 5 billion years old, Theoretical models from astrophysics predict that it will live another 5 billion years before it depletes its thermonuclear fuel of hydrogen. At this point it will evolve into a red giant engulfing the current orbits of the terrestrial planets, although the enlarged size at the Sun will cause the planets to move into more distant orbits, probably allowing Earth and Mars to avoid being engulfed.
For a star, the Sun is average in size and mass. Its diameter is 1.4 x 106 km the same as the Jovian planets. However, the density at the core is about 150 g./cm3, 20 times the density of iron. As a result, 90% of Sun’s mass is within the inner 50% of radius
Compared to the Earth, the Sun is enormous. The Earth’s diameter is a meager 13,000 km and its mass is 6x1027g. This makes the Sun’s diameter 100 times the Earth’s diameter and its mass is 300,000 times that of the Earth. The Sun is large enough to fit one million Earth’s inside it.
But compared to the red giant star Antares, the Sun is a dwarf. The super red giant Antares has a diameter of 5 1 AU which would put its outer edge between the planets of Mars and Jupiter it it were at the center of Solar System like the Sun is!
Compared to Sinus B, the Sun is a giant. Sihus B is the white dwarf companion star to Sirius, the brightest star in the night sky located in the constellation Canis Major (“Big Dog’). Sirius B has a diameter of 0.008 times That of the Sun but, with a mass of 1.1 solar masses, it has almost the same mass as the Sun.
We believe the internal structure of the Sun is like most stars of its size. We can make studies of the interior of the Sun by observing how it vibrates, much like we can learn a out t e interior c t e art y a serving waves caused by earthquakes. This is called ‘helioseismology’. The model used to describe the Sun’s structure is called the Standard Solar Model, We are also able to study the Sun by observing subatomic particles emitted by the Sun. To a first approximation, the Sun consists of a super dense core, surrounded by an interior, then an atmosphere.
The Suns core is a thermonuclear fusion furnace with a temperature of about 16,000000 K. At this temperature four hydrogen nuclei are slammed to ether in a number of steps called the proton-proton reaction to fuse into a nucleus of helium. This process consumes mass and releases large amounts of energy. This is the same process which occurs in the core of a hydrogen bomb. The amount of energy released by the Sun is 3.9 x 1033 erg/s which is the equivalent of the detonation at 100 billion 1 megaton bombs every second.
Because of the high temperatures in the core, all of the electromagnetic energy … by this intense radiation. This means there are no electrons to get excited and the matter is transparent to high energy photons emitted by the core. Occasionally, a gamma ray photon will scatter off one of the nuclei and will gradually cool down to lower and lower temperatures. For this reason, the area just outside of the core is called the ‘radiation zone’. It will take a few tens of thousands of years for a photon to cross the zone,
Above the radiation zone it the convection zone, which extends down about 200,000 … atoms to start having electrons. This causes them to become opaque to electromagnetic radiation and energy is now carried by convection, All photons generated in the core as gamma rays have been downgraded to x-rays and can now be absorbed by atoms. This … physically move upwards until it reaches the atmosphere and allows the energy to be radiated out , mostly as visible light.
e en ire process is ac ua y muc more comp ca e . ousan s o convec ion cells are located on top of each other and it takes a photon hundreds of thousands of years to cross the convection zone. The total time for a photon to go from the core to the surface of the Sun can be millions of years and transforms lethal hard radiation into visible g tin t e process.
Once the photon reaches the surface of the Sun it is in the solar atmosphere’. This is the part of the Sun where the gases are thin enough that we can see through them The atmosphere consists of three parts, the photosphere, the chromosphere, and the corona.
The part of the Sun we see is cabed the photosphere. This is where all light comes from, Photos means light in Greek We can apply our spectroscopic techniques to this … 5.9% helium. The last 1% is a mixture of aH other elements. The evidence we have leads us to believe that the composition throughout the Sun is about the same. Early spectroscopic studies identified spectral lines that did not correspond to any known … for the Sun, helios. It was later detected in our own atmosphere after scientists knew what to look for.
The lighi erTIhted iruin the phu~usphere PS SLIOHyeSL Iii L[IC region of visible light. This is not very surprising, because our eyes evolved to see visible light because that is what was most available for them We can only speculate what our vision would be like if the Sun radiated in a different wavelength and our atmosphere was different,
The Sun has a spectral type G2, whch makes it a pretty typical star, with a surface temperature of about 5800 K. Due to turbulence in the air during the day, we can study the Sun with a resolution of only about 700 km But when we study it in white light, we see a texture that is called granulation. These granules have a salt-and-pepper appearance and are about the same size as our best resoluton. So we arent able to study them in any finer detail, but they give the same appearance as a boiling liquid, leading us to believe that the are convection cells.
Just above the photosphere is a thin layer that glows pinkish, and is thus called the chromospbere (chromos means color in Greek) If we use a filter that will pass only the The uen of the red Ii ht emitted b h dr en then the chromos here will be o a ue and the only thing we will be looking at is the chromosphere. This gives us the ability to isolate and study this layer of the sun by itself. When we observe it, what we find is that there are light and dark areas in it. Curiously, the bright areas correspond to the dark, sunspot nreac in the phntn~phere The temperature in the rhrnmnsphere ic hetween Zflflfl K ~nd 15,000 K, compared to 5,800 K for the photosphere.
When we make high resolution studies of the chromosphere, what we find is that it isnt a shell at all, but a series of smaH spikes, called spic’les, that resemble blades of grass. They appear to be somewhat cylindrical in shape, about 700 kilometers across and about 7000 kilometers tall, and they seem to have lifetimes of about 5 to 15 minutes. The Sun is covered vsith hundreds of thousands of these at any given moment.
We can also study the relative velocities of these spicules and we find that there are large organized cells called supergranulation. These cells look somewhat like polygons approximately 30000 kilometers in diameter. Each of these supergranulation … of the supergranule, move across it, and then sink downwards at the edges. It is thisrelative motion that we can detect by the Doppler effect.
By taking spectra of other stars like the Sun we have been able to detect unmistakable evidence of chromospheres in other stars. So, by studying the Suns chromosphere, we can determine something about other stars. Above the chromosphere is a huge structure called the corona. The corona can be seen during eclipses as a great, brilliant area surrounding the Sun. When the Sun is eclipsed by the Moon we can see a bright halo surrounding it. It is this halo we call the corona (the ‘crown’). It is composed of the outer part of the solar atmosphere and extends throughout the solar system. This extension is called the solar wind and is made up of electromagnetic radiation and fast-moving particles, moving with typical velocities of about 500 km/s.
At the lower levels, close to the chromosphere, the corona has a temperature of 2,000,000 K. It is not known exactly how the corona is heated to this high temperature, and at first hundreds of ideas were proposed. But it is now believed to be due to bending magnetic field lines associated with sunspots - kind of like bending a coat hanger. This temperature does not equate to the same kind of temperature we are familiar with. Temperature is motion, so a high temperature means the particles are moving very fast. But there are very few particles in the corona, so energy is not transferred very efficiently. Even though we can see it, the corona is actually a good vacuum by the standards of Earth based laboratories.
The corona is irregularly shaped, with long streamers, and changes shape continuously. Its shape is maintained by the magnetic field of the Sun. Normally, it can be seen only during eclipses, but it can be also be seen from mountain tops. AJso, space based observatories can block out the solar disk and make observations of the corona. When the spectrum of the corona was taken, there were many emission lines that did not correspond to any known element. It was speculated that this was an unknown element, and it was called coronium. Later, it was found that the emission lines did not correspond to a new element, but that they corresponded to atoms that had lost many electrons, twelve or more. This could happen only if the corona was very hot.
Since the corona is so hot, it emits mainly in the x-ray region of the spectrum. The chromosphere and the photosphere are too cool to emit x-rays, therefore, if we use space-based observatories to study the Sun in the x-ray band, we will isolate the corona for study. When we study the corona in the x-ray spectrum, we find there are areas that are dark. This means that the corona is colder in these areas, too cold to emit x-rays. These cooler areas are called coronal holes. There is usually a coronal hole over the poles and less often at lower latitudes. The solar wind that reaches the Earth flows out of the coronal holes.
AbeII, George 0.; Exploration of the Universe, Fourth Edition. Saunders College
Publishing, 1982.
Chaisson, Eric, and Steve McMillan; Astronomy Today. Prentice Hall, Englewood Cliffs, NJ, 1993.
Kaler, James; Astronomy! Harper Collins College Publishers, New York, 1994.
Kippenhahn, R., and A. Weigert; Stellar Structure and Evolution. Springer-Verlag, Berlin, 1990.