Lesson Plan #37               http://www.phy6.org/stargaze/Lsun2vue.htm

(S-2)   Our View of the Sun

    An introduction to solar observations--by eye or telescope (with cautions) and during eclipse. Also, the distance of the Sun, its layers, the corona and the solar wind.

Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern

This lesson plan supplements: (S-2) Our View of the Sun: on disk Sun2view.htm, on the web http://www.phy6.org/stargaze/Sun2view.htm

"From Stargazers to Starships" home page: ....stargaze/Sintro.htm
Lesson plan home page and index:             ....stargaze/Lintro.htm

Goals: The student will learn

  • Safety rules for observing the Sun

  • How the distance of the Sun, about 150,000,000 km, must be measured indirectly, using Kepler's laws.

  • About the visible layers of the Sun--photosphere, chromosphere, corona.

  • About the high temperature of the corona, the evidence for it and what makes it so puzzling.

  • About the solar wind and its connection to the Sun's corona.

Terms: astronomical unit, plasma, photosphere, chromosphere, corona, solar wind. (Sunspots are discussed in the next section)

Start the lesson by discussing a solar eclipse:

Who here has seen an eclipse of the Sun? Total? Partial? Actual observations, or over TV?
    How does one observe an eclipse? What should be avoided? What may one do?

        --As long as any part of the Sun remains uncovered, do not look directly. Instead, project the Sun's image onto a flat surface from a pinhole or telescope, or view the Sun through a completely blackened B&W film. The Sun should appear comfortably dim.

    Looking directly can be harmful--and you will be too dazzled to see any details. Looking through binoculars or through a telescope is very dangerous--as if you focused a magnifying glass onto your eye! On the other hand, once the Sun is completely covered (in a total eclipse only), it is safe to look and to photograph.)

What can one see during a total eclipse that is not visible otherwise?

  • The chromosphere, a reddish ring around the Sun.
  • --The corona with streamers extending above active sunspots,
  • "Plumes" above the poles and arches above sunspot groups, both these suggesting magnetic fields.

  Students who observe a total eclipse should be cautioned, however, not to expect to see all the fine details one finds in eclipse photographs. Pictures that appear in journals often use time exposures and superpositions to make the outer fringes of the corona appear brighter than they actually are. For instance, a picture shown on the web of the total eclipse of 11 August 1999 was actually obtained by blending more than 20 pictures.

Additional points the teacher may raise:

  1.     Total solar eclipses were once used to look for an additional planet orbiting very close to the Sun. Because of the glare of the Sun, such a planet would be invisible at any other time. Astronomers had even proposed a name for is--Vulcan. No such planet was ever seen, and astronomers today agree that none exists.

        In the early 1800s, the German amateur Heinrich Schwabe, using a telescope (with a projected or filtered image) watched the Sun day after day for years, trying to see Vulcan passing in front of the disk of the Sun, when it should be visible as a dark spot (click here for a picture of the planet Mercury in front of the Sun). To distinguish such spots from sunspots, Schwabe carefully noted down all the sunspots he saw. He never found a planet, but after about 17 years, in 1843, he discovered the 11-year cyclical rise and fall in the number of sunspots, which had eluded professional astronomers.

A recent discovery (14 Nov'99): As discussed in connection with Kepler's first law, planets orbit not around their central star but around the common center of gravity of their planetary system (see here). The central star also orbits that point, and this causes its position in the sky to wobble slightly. Astronomers have used such wobbles--or more accurately, the changes of speed associated with them, which slightly shift spectral colors--to detect the existence of planets around more than 20 stars.

    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.

  1.     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.

  2. (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.

(end of additional material)

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 of the Sun is generated deep inside, in the Sun's core, by protons combining to form helium nuclei (such "nuclear fusion" will be studied in a later lesson). This process require very high temperatures, and matter would not stay together at these temperatures, were it not for the pressure produced by the enormous weight of the other layers 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.

How thick is the chromosphere?
  • About 5000 kilometers.

How does this thickness compare to the radius of the Sun?
  • It is less than 1% of the Sun's radius, which is about 700,000 kilometers.

How does the radius of the Sun compare to that of Earth?
  • It is more than 100 times bigger.

What are the approximate temperatures of the chromosphere and Corona, in degrees centigrade?
  • About 50,000° and 1,000,000°.

Why is that so puzzling?

  • 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?

How can we be sure that the corona is so hot?

  •     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.

So, what can be the explanation of the heat of the corona?

  •     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.

Any older explanations?

  • 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.

What is the solar wind?

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

What produces the solar wind?

  • 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.]

How fast does the solar wind move, what is its density at the Earth's orbit, and what is it made up of?

  •     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

    A proton's mass is 1.67 10-27 kg. Assuming the solar wind consists entirely of protons, how much mass does the Sun lose each second? Assume the mean Sun-Earth distance ("astronomical unit") is 150,000,000 km.

(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).

1 AU = 150,000,000 km = 1.5 108 km = 1.5 1011 meter = 1.5 1013 cm

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

4πr2 = (12.56) x (2.25 1026) cm2 = 2.826 1027 cm2

    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

(2.826 1027 cm2) x (4 107 cm) = 1.13 1035cm3

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

(1.13 1035) x 6 = 6.78 1035

The total mass lost by the Sun each second is

(6.78 1035) x (1.67 10-27 kg) = 1.13 109 kg = 1.13 106 ton

    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)

What ultimately happens to the solar wind?

  •     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.]

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated: 11-15-2004