* The solar ultraviolet spectrum has been mapped and studied, first with sounding rockets in 1946 and later with the Orbiting Solar Observatories (OSO satellites).
* With radio telescopes, it was found that radio bursts of many kinds are emitted by the Sun.
* Ground-based telescopes discovered "supergranulation," the existence on the Sun of convection cells with typical sizes of 30,000 kilometers (19,000 miles).
* Sounding rocket instruments discovered that solar soft (lower energy) X-rays are produced primarily by active regions.
* OSO instruments revealed that bursts of hard (higher energy) X-rays accompany solar flares.
* Sounding rocket observations unexpectedly detected neutral hydrogen emission from the solar corona, allowing astronomers to measure the temperature of the coronal hydrogen and to infer the speed of the solar wind moving out through the corona.
* Ground-based telescopes show that the five-minute oscillations of the Sun are composed of superposed oscillation modes.
* An OSO instrument discovered gamma ray emission lines, indicating that nuclear reactions sometimes occur in solar flares.
* Observations from Skylab revealed that:
* The corona consists largely of arcades of magnetic-field arches.
* The basic structure of a solar flare is a magnetic arch or loop.
* Coronal holes do not exhibit the differential rotation which characterizes the solar photosphere, but rotate as though they were solid objects.
* The top of a flare loop is hottest, and its foot points are relatively cool.
* There are numerous small regions of emerging magnetic flux, which appear as bright points on the Sun in soft X-rays, with lifetimes of three to six hours.
* Observations from OSO-7 and Skylab showed that large "solar bubbles" or coronal transients pass outward through the corona after flares and prominences erupt.
* OSO-8 measurements showed that, contrary to earlier expectations, acoustic waves do not carry sufficient energy to heat the corona. Perhaps the dissipation of magnetic energy is responsible for the high temperature of the corona.
* Sounding rocket observations showed that jets, perhaps related to the spicules seen with ground-based telescopes, are ejected from the solar surface and reach speeds of 300 kilometers per second (190 miles per second).
* Combining the results of observations from the Solar Maximun Mission (SMM) satellite and ground-based radio telescopes, it was found that hard X-rays are emitted at the foot points of flare loops, while microwave radio bursts are emitted at the tops of the loops.
* An instrument on SMM discovered that the total light of the Sun varies from week to week by amounts of plus or minus 0.05 percent and that there are some larger variations as well.
* SMM observations revealed how solar flares occur after hot plasma fills preexisting magnetic loops, which then explode.
* Ground-based measurements found that there is an inverse correlation between solar activity and cosmic ray intensity.
* Interplanetary spacecraft, including the Mariners, detected and measured the solar wind.
* An abundance of such atoms as carbon, nitrogen, oxygen, and iron have been measured in the solar wind.
* Enhanced amounts of the isotope helium-3 were discovered in the matter ejected from solar flares and in high-speed streams of the solar wind.
* Waves and discontinuities were discovered in the solar wind.
* The interplanetary magnetic field was found to possess sector structure that rotates with the Sun.
* Skylab observations identified coronal holes as the sources of high-speed wind streams and showed that coronal holes are the cause of recurrent geomagnetic storms on Earth.
* Correlation of the interplanetary magnetic field with the magnetic field at the solar surface was accomplished.
* A warped-disk model for the magnetic neutral sheet in interplanetary space has been developed.
* Pioneer 10 has established that the heliosphere extends out beyond the orbit of Uranus.
* The International Sun-Earth Explorer(ISEE) mission discovered electron bursts that originate in the outer corona and traced the paths of the bursts outward through the corona, along the spiral magnetic field.
* The Van Allen radiation belts of the Earth were discovered by Explorer 1.
* The magnetopause, or boundary between the solar wind and the Earth's magnetosphere, was located at a distance of about 20 Earth radii toward the Sun.
* The Earth's "bow shock," a collisionless shock wave, was found to be slightly closer to the Sun than the magnetopause and separated from the latter by a region called the magnetosheath.
* The Earth's magnetic tail (magnetotail) was found to extend far into space, beyond the distance of the Moon.
* It was determined that significant numbers of charged particles enter the magnetosphere directly from the solar wind.
* Evidence was found for a magnetic field reconnection process in the magnetotail, which accelerates charged particles to high velocities.
* Displays of the aurora on Earth were found to originate from disturbances in the magnetotail.
* Low-frequency waves were detected in the magnetosphere.
* It was found that electric currents move along the magnetic field lines above the Earth's polar regions. These currents are maintained by electric fields aligned along the magnetic field, which once were thought to be impossible.
* Rocket-borne ion mass spectrometers showed that molecular ions predominate among the charged particles in the lower layers of the Earth's ionosphere, at altitudes of 90 to 200 kilometers (about 55 to 125 miles).
* Rocket observations showed that the charged particles in the upper layers of the ionosphere, at altitudes of 200 to 800 kilometers (125 to 500 miles), are mainly ions of oxygen.
* Metallic ions derived from meteorites were discovered in the E region of the ionosphere.
* Measurements showed that the temperatures of the ionospheric electrons are higher than those of the ions; the ion temperatures are higher than those of the neutral gas.
* Observations by Explorer satellites showed a division between the polar ionosphere and the ionosphere over lower latitudes of the Earth.
* Plasma instabilities were artificially excited for study in the ionosphere by means of radio-wave injection.
* Extensive satellite measurements solved the long-standing problem of why there are more electrons in the winter ionosphere than in the summer ionosphere at the low-to-medium activity phases of the sunspot cycle. (More electrons would be expected in the summer, since the Sun is then more nearly overhead.) Systematic measurements were taken by the atmosphere explorers. This allowed variations associated with satellite height, latitude, time of day, and season to be statistically separated. Three factors were found to contribute to the winter anomaly. First, there is an increase in neutral atomic oxygen due to atmospheric circulation and dynamics; this results in an increase in electron production due to more ionization by the Sun. Second, the temperature of the neutral nitrogen is lower in winter, so that the existing ionization is removed more slowly by a chemical recombination process. Third, there is a more rapid production of ionization in winter due to increased quenching of an intermediate excited state of atomic oxygen.
* Convective bubbles that cause the equatorial "spread-F" effect were detected. These involve electron density variations by factors of 100 within scale sizes of approximately 10 meters (33 feet). The bubbles move at speeds of hundreds of meters per second.
* A "polar wind" of ions that are convected rapidly upward from the ionosphere above polar regions was discovered. The ions either travel out along magnetic field lines to interplanetary space or are transported into the magnetotail.
(The following is from "The Solar System" NASA/ASEP, 1989, p. 1.)
* Apollo, the Sun god, brings life-giving heat and light.
* The egg of creation is the symbol for the Sun.
* The Sun formed from a huge cloud of gas 5 billion years ago.
* The Sun (an average star) is the star nearest the Earth.
* In ancient times, people thought the Sun was a perfect sphere or celestial fire created by the gods.
* The Sun's energy is nuclear reaction.
*The Sun shapes our life on Earth (the weather and the climate).
*The Sun consists mainly of hydrogen and helium.
* The diameter of the Sun is 864,000 miles. One million Earths could fit inside the Sun.
* The Sun contains 99 percent of our solar system's total mass.
* The Sun spins on its axis from left to right.
* The Earth receives only one-billionth of the total energy output of the Sun.
* Energy we receive from the Sun was formed 50 million years ago.
* The Sun consists of three layers: the photosphere, the chromosphere, and the corona.
* "Sun-spots" are giant magnetic fields that cool areas of the Sun.
* The temperature of the Sun's core is 15 million degrees Kelvin.
* The Sun will burn for another 5 billion years.
(NASA, Jet Propulsion Laboratory, "Our Solar System at a Glance," NASA Information Summaries, PMS 010-A (JPL), June 1991.)
A discussion of the objects in the solar system must start with the Sun. The Sun dwarfs the other bodies, representing approximately 99.86 percent of all the mass in the solar system; all of the planets, moons, asteroids, comets, dust and gas add up to only about 0.14 percent. This 0.14 percent represents the material left over from the Sun's formation. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths.
As a star, the Sun generates energy by the process of fusion. The temperature at the Sun's core is 15 million degrees Celsius (27 million degrees Fahrenheit), and the pressure there is 340 billion times Earth's air pressure at sea level. The Sun's surface temperature of 5,500 degrees Celsius (10,000 degrees Fahrenheit) seems almost chilly compared to its core temperature! At the solar core, hydrogen can fuse into helium, producing energy. The Sun also produces a strong magnetic field and streams of charged particles, the field and streams extending far beyond the planets.
The Sun appears to have been active for 4.6 billion years and has enough fuel for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow Earth. After a billion years as a "red giant," it will suddenly collapse into a "white dwarf"čthe final end product of a star like ours. It may take a trillion years to cool off completely.
Many spacecraft have explored the Sun's environment, but none have gotten any closer to its surface than approximately two-thirds of the distance from Earth to the Sun. Pioneers 5-11, the Pioneer Venus Orbiter, Voyagers 1 and 2 and other spacecraft have all sampled the solar environment. The Ulysses spacecraft, launched on October 6, 1990, is a joint solar mission of NASA and the European Space Agency. After using Jupiter's gravity to change its trajectory, Ulysses will fly over the Sun's polar regions during 1994 and 1995 and will perform a wide range of studies using nine on-board scientific instruments.
We are fortunate that the Sun is exactly the way it is. If it were different in almost any way, life would almost certainly never have developed on Earth.