Introduction to the Sun
The Sun has been worshipped, despised, and feared by humans throughout history. Ideas about the Sun are as strange nowadays as they have ever been, but there are some statements about our parent star that we can make with confidence.
Size And Distance
The Sun is the largest nuclear reactor from which humanity has ever derived energy, and until we venture into other parts of the galaxy, this will remain the case. The Sun has a radius of about 695,000 kilometers (432,000 miles), more than 100 times the radius of Earth. If Earth were placed at the center of the Sun (assuming the planet would not vaporize), the orbit of the Moon would fit inside the Sun with room to spare (Fig. 4-7).
Figure 4-7. Earth-moon system would easily fit inside the Sun.
The commonly accepted mean distance from Earth to the Sun is 150,000,000 kilometers (93,000,000 miles) in round numbers. But the day-to-day distance varies up to a couple of million kilometers either way. Earth’s orbit around the Sun, like the Moon’s orbit around Earth, is not a perfect circle but is an ellipse with the Sun at one focus. Earth’s closest approach to the Sun is called perihelion , and it occurs during the month of January. Earth is farthest away from the Sun— aphelion —in July. Surprisingly enough, for those of us in the northern hemisphere, the Sun is closest in the dead of the winter. It is not Earth-Sun distance that primarily affects our seasons but the tilt of Earth on its axis.
Measuring The Sun’s Distance And Size
Centuries ago, people did not know how large the Sun was, nor how far away it was. Estimates ranged from a few thousand miles (kilometers hadn’t been invented yet) to a few million miles. The distance to the Sun could not be measured by parallax relative to the background of stars because the Sun’s brilliance obliterated the stars near it. The distance to the Moon had been measured by parallax, as well as the distances to Mars and Venus at various times, but the Sun defied attempts to measure its distance until someone thought of finding it by logical deduction. What follows is an example showing the sort of thought process that was used, and can still be used, to infer the distance to the Sun. Let’s update the measurement techniques from those of our forebears and suppose that we have access to a powerful radar telescope, with which we can measure interplanetary distances by bouncing radio beams off distant planets and measuring the time it takes for the signals to come back to us.
Given a central body having a known, constant mass, such as the Sun, all its satellites obey certain physical laws with respect to their orbits. One of these principles, called Kepler’s third law , states that the square of the orbital period of any satellite is proportional to the cube of its average distance from the central mass. This is true no matter what the mass of the orbiting object; a small meteoroid obeys the rule just as does Earth, Venus, Mars, and Jupiter. We know the length of Earth’s year and the length of Venus’s year; from this we can calculate the ratio (but not the actual values) of the two planets’ mean orbital radii. Knowing this ratio is not enough, all by itself, to solve the riddle of Earth’s mean distance from the Sun, but it solves half the problem.
The next step involves measuring the distance to Venus. If we could do this when Venus is exactly in line with the Sun, then we could figure out our own distance by simple mathematics. Unfortunately, the Sun produces powerful radio waves, and our radar telescope won’t work when Venus is at inferior conjunction (between us and the Sun) because the Sun’s radio noise drowns out the echoes. However, when Venus is at its maximum elongation (its angular separation from the Sun is greatest either eastward or westward), the radar works because the Sun is out of the way. At maximum elongation, note (Fig. 4-8) that Venus, Earth, and the Sun lie at the vertices of a right triangle, with the right angle at the vertex defined by Venus. One of the oldest laws of geometry, credited to a Greek named Pythagoras , states that the square of the length of the longest side of a right triangle is equal to the sum of the squares of the other two sides. In Fig. 4-8 this means that a 2 + b 2 = x 2 , where x is the elusive thing we seek, the average distance of Earth from the Sun.
Figure 4-8. If we know the radius of Venus’ solar orbit and we can measure the distance to Venus at its maximum elongation, then we can calculate our own distance from the Sun.
Now that we know the value of a in the equation (by direct measurement) and also the ratio of b to x (by Kepler’s third law), we can calculate the values of both b and x because we have a set of two equations in two variables. Let’s not drag ourselves through a detailed mathematical derivation here. If you’ve had high school algebra, you can do the derivation for yourself. It should suffice to say that this scheme can give us a fairly good idea of Earth’s mean distance from the Sun if the measurement is repeated at several maximum elongations and the results averaged. However, even this will only give us an approximation because the orbits of Earth and Venus are not perfect circles. In recent decades, astronomers have made increasingly accurate measurements of the distance from Earth to the Sun using a variety of techniques.
Once the distance from Earth to the Sun was known, the Sun’s actual radius was determined by measuring the angular radius of its disk and employing surveyors’ triangulation in reverse (Fig. 4-9).
Figure 4-9. The Sun’s actual radius can be determined by reverse triangulation, on the basis of its apparent radius and its known distance from Earth.
How It “burns”
The Sun “shines” by constantly “burning” hydrogen, the most abundant element in the universe. You know, if you have taken chemistry classes, that hydrogen is flammable and that it burns clean and hot. (It might someday replace natural gas for heating if a method can be found to cheaply and abundantly produce it and safely distribute it.) However, the Sun “burns” hydrogen in a far more efficient and torrid fashion: by means of nuclear fusion . The enormous pressure deep in the Sun, caused by gravity, drives hydrogen atoms into one another. Hydrogen atoms combine to form helium atoms, and in the process, some of the original mass is converted directly into energy according to Einstein’s famous equation E = mc 2 (energy equals mass times the speed of light squared).
The earliest theories concerning the Sun involved ordinary combustion, the only question being what, exactly, was burning. Coal was suggested as a fuel for the Sun, but if this were the case, the Sun would have burned out long ago. Besides this, there was the little problem of how all that coal got up there into space. Another idea involved the direct combination of matter with antimatter, resulting in total annihilation. However, if this theory were true, the Sun would be far brighter and hotter than it is. The hydrogen-fusion theory accounts for what we see and is consistent with theories concerning the age of the Universe and the age of Earth.
About The Sun’s Life
How long will the Sun’s supply of hydrogen fuel last? Should we worry about the possibility that the supply will run out soon and Earth will cool off and freeze over?
Eventually, the Sun will burn out, but it will not happen for quite awhile. In fact, most scientists believe that the Sun will continue to shine for at least 1 billion more years at about the same level of brilliance as it does today. There are a lot of hydrogen atoms in that globe. It has a radius, remember, of 695,000 kilometers, or 69,500,000,000 centimeters. Scientists would write that as 6.95 × 10 10 cm. Remember the formula for calculating the volume of a sphere from your middle school geometry class: V = 4/3π r 3 , where V is the volume and r is the radius. Using your calculator, you can figure out, using 3.14 as the value of π, that there are about 1.4×10 33 , or 1.4 decillion, cubic centimeters of matter in the Sun. Written out in full, that number looks like this:
1,400,000,000,000,000,000,000,000,000,000,000
With this number in mind—or out of mind, because it’s incomprehensibly large—you might be willing to accept practically any claim as to the Sun’s longevity, except, of course, life everlasting. The Sun will perish. The symptoms of aging will begin in 1000 or 2000 million years.
As the supply of hydrogen runs out, the Sun will expand, and its surface will cool off. However, Earth will heat up because the bloated Sun will appear much larger in the sky and will send far more energy to Earth’s surface than is the case now. The climate will become intolerably hot; the polar ice caps will melt; wildfires will reduce all plant life to ashes. Sometime during this process, any remaining humans and other mammals will die off. The oceans, lakes, and rivers will boil dry. All living things, even the hardiest bacteria and viruses, will die. The atmosphere will be blown off into space. Some astronomers think that the Sun’s radius will grow until it exceeds the radius of Earth’s orbit so that the Sun will swallow Earth up and vaporize it.
Don’t let this scenario depress you. By then humanity will have colonized a couple of dozen other planets and will grieve no more about the fate of Earth than we do today about the buried houses of ancient cities. If our descendants remember us at all, it will be with fascination. Knowledge of our present society might be conveyed by legend, by stories told to children at bedtime, by tales about a place called Terra that sank beneath the surface of a stormy star after its inhabitants had fled, a place where people burned the decomposed by-products of dead plants and animals in order to propel surface transport vehicles. And the children will laugh and say that such a ridiculous place couldn’t have existed.
After the red-giant phase, the Sun will fuse helium into carbon, iron, and other elements, and will shrink as gravity once again gains dominance over the pressure of nuclear heat. However, this process cannot continue forever. A point will be reached at which no further nuclear reactions can take place, and then gravitation will assert its ultimate power. The Sun will be crushed into an orb of planetary size and, as the last of its heat dissipates, will fade away and spend the rest of cosmic time as an incredibly dense, dark ball.
Spots And Storms
If you’ve never looked at the Sun through an appropriately filtered telescope, don’t pass up the chance. Be sure that you use the proper type of filter (one that fits over the telescope’s objective lens, not in the eyepiece or the star diagonal), and use it according to the manufacturer’s instructions. Chances are good that you will see at least one dark spot on that bright disk.
Before the time of Galileo in the seventeenth century, the idea of sunspots did not cross the minds of people in Western civilizations—or if it did, no one ever voiced their thoughts aloud. The Sun was regarded as perfect, and if someone had suggested otherwise, they would have been disciplined or put to death. There is reason to believe that the ancient Chinese knew about sunspots and accepted their existence, having seen them, most likely, when the Sun was rising or setting in a hazy sky.
Sunspots enable astronomers to calculate the rotational period of the Sun because the blemishes seem to move across the solar disk, disappear over the edge, and then reappear a couple of weeks later on the opposite limb. The Sun rotates approximately once a month. Careful observation of spots has revealed that the Sun spins faster at lower latitudes than at higher latitudes. Using spectroscopy, this has been verified, and the solar “day” is in fact 25 Earth days long at the equator and 34 Earth days long at the poles. This fact has been used to prove that the Sun is not a solid body like Earth or the Moon.
Sunspots have an appearance that reminds some observers of biological cells (Fig. 4-10). The darkest part, at the center, is called the umbra ; it is surrounded by a brighter region called the penumbra . Sunspot sizes vary, but they can be, and often are, much larger in diameter than Earth. The spots tend to form in groups and are believed to be depressions in the solar surface resulting from magnetic disturbances. They are, in a sense, storms on the Sun. The overall average number of sunspots rises and falls in a cycle of roughly 11 years. The most recent sunspot maximum took place in late 2000 and early 2001.
Figure 4-10. A sunspot can have a diameter greater than that of Earth.
If sunspots are like hurricanes on the Sun, then solar flares are like nuclear explosions: sudden, bright eruptions that send high-speed, charged subatomic particles flying off into space. Solar flares are more difficult to see with an ordinary telescope than are sunspots, but astronomers constantly watch the Sun for signs of these outbursts. A large solar flare is followed, in a day or so, by destabilization of Earth’s magnetic field. As the charged particles come near our planet, they accelerate toward the north and south geomagnetic poles . This acceleration produces its own magnetic field, which interacts with that of Earth. As a consequence of this, the ionized layers of Earth’s upper atmosphere change dramatically; this can create vast halos of light around the geomagnetic poles. If you happen to live at a high latitude, especially in North America, you are familiar with this glow as the aurora borealis (northern lights). Similar effects take place in the south polar regions, but the aurora australis (southern lights) are not spectacular, except as seen from Antarctica and from the far southern ocean on rare occasions when the sky over them is not socked in with grim overcast. The ionospheric disturbances affect radio communications and broadcast, especially on the so-called shortwave bands. In the extreme, even wire, cable, and satellite communications systems are disrupted by the powerful, erratic magnetic fields.
Practice problems of this concept can be found at: The Moon and the Sun Practice Problems
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