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(S-2) Our View of the Sun |
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 |
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Goals: The student will learn
Terms: astronomical unit, plasma, photosphere, chromosphere, corona, solar wind. (Sunspots are discussed in the next section)
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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?
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?
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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.
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.
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 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.
[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.]
(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)
[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
Additional points the teacher may raise:
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.
(end of additional material)
-- 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.
How thick is the chromosphere?
How does this thickness compare to the radius of the Sun?
How does the radius of the Sun compare to that of Earth?
What are the approximate temperatures of the chromosphere and Corona, in degrees centigrade?
Why is that so puzzling?
How can we be sure that the corona is so hot?
So, what can be the explanation of the heat of the corona?
Any older explanations?
What is the solar wind?
What produces the solar wind?
How fast does the solar wind move, what is its density at the Earth's orbit, and what is it made up of?
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.
What ultimately happens to the solar wind?
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 11-15-2004