Lecture VII: The Sun
In a kind of desperation physicists began looking at a new "non-mecahnaical" description of atoms and light.
From this came Quantum Mechanics (QM) - the most powerful theory ever devised by humans.
The Structure of the
Sun
Six Main Regions Of The Sun - Six main regions of the Sun, not drawn to scale, with physical dimensions labeled.
Theoretical Model Of Solar Interior - Theoretically modeled profiles of density (b) and temperature (c) for the interior of the Sun, presented for perspective in (a). All three parts describe a cross-sectional cut through the center of the Sun.
Computer Model Of Seismic Vibrations On Sun - The Sun has been found to vibrate in a very complex way. By observing the motion of the solar surface, scientists can determine the wavelength and the frequencies of the individual waves and deduce information about the solar interior not obtainable by other means. The alternating patches represent gas moving down (red) and up (blue).
Physical Transport Of Energy From Sun's Interior - Physical transport of energy in the Sun's convection zone. We can visualize the upper interior as a boiling, seething sea of gas. Each convective loop is about 1000 km across. The convective cells are arranged in tiers with cells of progressively smaller size as the surface is neared. (This is a highly simplified diagram; there are many different cell sizes, and they are not so neatly arranged.)
The Granulated Photosphere - Skylab photograph of the granulated solar photosphere. Typical solar granules are comparable in size to the Earth's continents. The bright portions of the image are regions where hot material is upwelling from below. The dark regions correspond to cooler gas that is sinking back down into the interior.
Formation Of Solar Absorption Lines - Formation of solar absorption lines. Photons with energies well away from any atomic transition can escape from relatively deep in the photosphere, but those with energies close to a transition are more likely to be reabsorbed before escaping, so the ones we see on Earth tend to come from higher, cooler levels in the solar atmosphere. The inset shows a close-up tracing of two of the thousands of solar absorption lines, those produced by calcium at about 395 nm.
The Solar Spectrum - A detailed spectrum of our Sun in a portion of the visible domain shows thousands of spectral lines, which indicate the presence of some 67 different elements in various stages of excitation and ionization in the lower solar atmosphere.
Photo Of A Total Solar Eclipse - This photograph of a total solar eclipse shows the solar chromosphere, a few thousand kilometers above the Sun's surface.
Solar Spicules - Solar spicules, short-lived narrow jets of gas that typically last mere minutes, can be seen sprouting up from the solar chromosphere in this Ha image of the Sun. The spicules are the thin, dark, spikelike regions. They appear dark against the face of the Sun because they are cooler than the solar photosphere.
The Solar Corona - When both the photosphere and the chromosphere are obscured by the Moon during a solar eclipse, the faint corona becomes visible. This photograph shows clearly the emission of radiation from the solar corona.
Gas Temperature Changes In The Lower Solar Atmosphere - The change of gas temperature in the lower solar atmosphere is dramatic. The minimum temperature marks the outer edge of the chromosphere. Beyond that, the temperature rises sharply in the transition zone, finally leveling off at over 1,000,000 K in the corona.
The Sun Seen In X-Ray Light - Images of X-ray emission from the Sun observed by the Skylab space station. These frames were taken at one-day intervals. Note the dark, boot-shaped coronal hole traveling from left to right, where the X-ray observations outline in dramatic detail the abnormally thin regions through which the high-speed solar wind streams forth.
Sunspot Pair - This photograph of the entire Sun, taken during a period of maximum solar activity, shows several groups of sunspots. The largest spots in this image are over 20,000 km across-twice the diameter of the Earth. Typical sunspots are only about half this size
Sunspot The Size Of Earth - (a) An enlarged photograph of the largest pair of sunspots in Figure 16.15. Each spot consists of a cool, dark inner region called the umbra, surrounded by a warmer, brighter region called the penumbra. The spots appear dark because they are slightly cooler than the surrounding photosphere. (b) A high-resolution, true-color image of a single sunspot shows details of its structure as well as much surface granularity surrounding it. The spot is about the size of the Earth.
Magnetic Field Between Susnspot Pair - Sunspot pairs are linked by magnetic field lines. The Sun's magnetic field emerges from the surface through one member of the pair and reenters the Sun through the other.
Distribution Of Susnspot Pairs - The leading members of all sunspot pairs in the solar northern hemisphere have the same polarity-if the magnetic field lines are directed into the Sun in one leading spot, they are inwardly directed in all leading spots in that hemisphere. The same is true in the southern hemisphere, except that the polarities are always opposite to those in the north.
Differential Rotation On The Sun Twisting Magnetic Fields - This diagram illustrates how the Sun's differential rotation wraps and distorts the solar magnetic field. Occasionally, the field lines burst out of the surface and loop through the lower atmosphere, thereby creating a sunspot pair. The underlying pattern of the solar field lines explains the observed pattern of sunspot polarities. (If the loop happens to occur on the limb of the Sun and is seen against the blackness of space, we see a phenomenon called a prominence, described in a later section.)
Sunspot Cycle In 20th Century - This graph presents the annual number of sunspots throughout the twentieth century, showing the 5-year average of the annual data to make long-term trends more evident. The (roughly) 11-year solar cycle is clearly visible. At the time of solar minimum, hardly any sunspots are seen. About 4 years later, at solar maximum, as many as 100-200 spots are observed per year.
Sunspot Location At Latitude Vs Activity - Sunspots cluster at high latitudes when solar activity is at a minimum. They appear at lower latitudes as the number of sunspots peaks. Finally, they are prominent near the Sun's equator as solar minimum is again approached. The most recent solar maximum occurred in 1990.
Sunspot Activity Over 400 Years - This graph plots the number of sunspots occurring each year. Note the approximate 11-year "periodicity" and the absence of spots during the late seventeenth century.
The
Active Sun
Loop-Like Solar Prominence - The looplike structure of this prominence clearly reveals the magnetic field lines connecting the two members of a sunspot pair.
Unusually Large Solar Prominence - This image of a particularly large solar prominence was observed by ultraviolet detectors aboard the Skylab space station in 1979. (See also Figures 16.18 and 16.19.)
Much more violent than a prominence, a solar flare is an explosion on the Sun's surface that sweeps across an active region in a matter of minutes, accelerating solar material to high speeds and blasting it into space. Visible here through a red Ha filter.
July 1995 Photo Of Solar Corona - Photograph of the solar corona during the July, 1991 eclipse, at the peak of the sunspot cycle. At these times, the corona is much less regular and much more extended than at sunspot minimum (compare Figure 16.12). Astronomers believe that coronal heating is caused by surface activity on the Sun. The changing shape and size of the corona are the direct result of variations in prominence and flare activity over the course of the solar cycle.
Overview Of Energetic Events On Sun's Surface - This summary piece of art illustrates many of the salient features of our Sun-from prominences in its upper atmosphere, to spicules and flares in its lower atmosphere, to granules and spots on the surface.
Powering
The Sun
Thermonuclear Fusion
Proton Collision - Two protons collide violently, initiating the chain of nuclear fusion that powers the Sun.
Proton-Proton Chain Collision - Diagram of the entire proton-proton chain. A total of six protons (and two electrons) are converted into two protons, one helium-4 nucleus, and two neutrinos. The two leftover protons are available as fuel for new proton-proton reactions, so the net effect is that four protons are fused to form one helium-4 nucleus. Energy, in the form of gamma rays, is produced in each reaction.