(S-2)   Our View of the Sun

    Most of those planets are as big as Jupiter or bigger, because obviously the heaviest planets shift the center of gravity by the greatest amount (the effect of an Earth-size planet is still too small to be measured). Many of them were detected by Paul Butler (Carnegie Inst.), Geoffrey Marcy (U. Cal Berkeley) and Steve Vogt (U. Cal Santa Cruz), who worked out a sensitive method for such observations.

    The observations do not tell much about how the orbit is oriented in space, but for one recently discovered planet we now know a bit more. It orbits a star catalogued as HD 209458, some 153 light years from Earth, and happens to pass right between us and it. The researchers were hoping to find such a planet, and sure enough, Greg Henry of Tennesse State Univ, using an Arizona telescope, detected on November 7, 1999 a temporary drop in the light intensity by 1.7%. It is a big planet with about 2/3 the mass of Jupiter, and its orbital period is about 3.5 days. Because it orbits very close, and its size is expanded by the heat of its nearby star, it is considerably larger than Jupiter, able to block a measurable amount of starlight.

2.     A total solar eclipse was also used in 1919 by the British astronomer Arthur Eddington to check out the general theory of relativity of Albert Einstein. Einstein predicted that the Sun's gravity would bend starlight passing close to the Sun. If that happened, stars whose position in the sky during a total eclipse was close to that of the Sun should have been very slightly shifted from their usual positions. The effect was very small, but Eddington confirmed its existence.

3. (This is related to the preceding point; illustrate by a sketch on the board)

   

       Today we know a handful of cases in which a very distant galaxy is obscured by a nearer one. You might think that the distant galaxy would be invisible, hiding (so to speak) behind the nearer one. However, the gravity of the nearer galaxy can bend light from the distant one, light which otherwise would have missed Earth, so that it does reach us. If the positioning is right, we see multiple images of the obscured galaxy. That phenomenon is known as gravitational lensing.

Chromosphere, corona--does the Sun have any other visible layers?

-- The photosphere, the layer from which sunlight reaches us. It is below the chromosphere.

Teacher's explanation of the heat outflow of the Sun:

    The heat generated at the core moves outwards, towards the Sun's surface, by processes somewhat like the ones by which heat works its way through the atmosphere, from the surface of the Earth to space (section S-1).

    No one of course can observe what happens deep inside the Sun, but a theory has been developed about the interior of stars, suggesting heat in the deepest layers travels by radiation. Atoms radiate light (or "electromagnetic radiation" related to light, like X-rays) and neighboring atoms absorb it. However, since the deep layers are compressed with atoms packed close to each other, the process is more like the conduction of heat. As heat moves outwards, the temperature keeps dropping, because net flow of heat can only take place from hot material to colder one.

    Closer to the Sun's surface, the theory predicts, heat is carried by convection, as in the Earth's atmosphere (See Sweather1.htm)--by hot gas gas rising, giving up heat and returning somewhat cooler.. All these layers are still fairly dense, and any light emitted is quickly absorbed again. The photosphere is the final layer. Not enough material remains above it, allowing any light emitted there to spread out into space. It is a relatively thin layer, of the order of 100-200 kilometers

    Because most sunlight comes from the photosphere, the much fainter chromosphere and corona are only seen during a total eclipse, when the light of the photosphere is completely blocked by the disk of the Moon.

    However, astronomers can also view the Sun through special filters which reject all of the Sun's light except one or another narrow range of color ("spectral line," discussed in a later lesson). In some such colors, the chromosphere or the inner corona shine brightly enough to be seen even without a total eclipse.

• About 5000 kilometers.

• It is less than 1% of the Sun's radius, which is about 700,000 kilometers.

• It is more than 100 times bigger.

• About 50,000° and 1,000,000°.

• Because the only credible source of the heat is the Sun itself. However, heat always flows from a hot temperature to a lower one--never the other way around. The photosphere below these layers is cooler: how then can it heat the chromosphere and corona?

•     The light from the corona is characteristic of atoms which have lost quite a few electrons (more about that, in a later lesson). At the density of the corona, this requires collisions between fast atoms, and the speed of the atoms of a gas is directly related to its temperatures.

•     The energy must arrive at the corona in a form different from heat--perhaps as sound-like waves, or as a population of fast ions produced by such waves.

• At one time it was proposed that a steady flow of small meteorites was hitting the Sun. However, nothing like them was ever observed passing near Earth.

• A fast flow of ions, streaming out from the Sun in all directions.

• Its energy comes from the heat of the corona. Its top layers are so hot that they manage to escape the Sun's gravity.

      [If the corona were like the Earth's atmosphere, its temperature would gradually drop with height. However, the corona is hot enough to be a plasma, a mixture of free-floating positive ions and negative electrons. Plasmas conduct heat well, and this does not allow the high layers to stay cool.]

•     Its velocity is about 400 km/sec, density at the Earth's orbit about 6 ions/cm³ , both these varying widely (at a distance of r astronomical units, the average density becomes 6/r²). Its composition resembles that of the Sun--mostly ionized hydrogen (protons), some helium and a small amount of heavier elements.

A calculation on the board

(This calculation tests ability to use scientific notation when working with very large and very small numbers. The teacher might call up a student to derive it on the board, with the class copying).

The area of a sphere with radius r = 1 AU around the Sun is

    Imagine behind every square centimeter a column of solar wind 400 kilometers long, waiting to cross it during the next second! (Teacher could illustrate this with a sketch on the board).

The entire surface of the sphere is therefore crossed each second by the solar wind particles contained in a volume

And with a density of 6 ions/cc, the number of ions is

The total mass lost by the Sun each second is

    The Sun therefore loses about a million tons each second. It sounds like a lot but really isn't--one cubic kilometer of the ocean contains about 1000 times more, and the Sun is not much diminished even if this loss continues for many billions of years.

(end of calculation)

•     It gets lost in interstellar space. Actually, a boundary is expected, the "heliopause" marking the outer limit of the solar wind domination, from where on the interstellar gas determines the ambient conditions. The space probe "Voyager 2" is going to reach it, but whether it will still be transmitting remains to be seen.

    [As of 2004, it has not yet reached the heliopause, and not even the "termination shock" where the solar wind abruptly slows down, before the heliopause is reached.]