. The Sun sustains all living creatures and plants on Earth. Sunlight is absorbed by green plants where it strikes chlorophyll, giving it the energy to break water molecules apart and energize photosynthesis. Plants thereby use the Sun’s energy to live and grow, giving off oxygen as a byproduct, and animals are nourished by eating these plants. Here the eyes of sunflowers turn in unison to follow their life-sustaining star. (Courtesy of Charles E. Rodgers.)
. All forms of radiation consist of electric and magnetic fields that oscillate at right angles to each other and to the direction of travel. They move through empty space at the velocity of light. The separation between adjacent wave crests is called the wavelength of the radiation and is usually designated by the lower case Greek letter lambda or l.
. This picture was made by using crystals to liberate the spectral colors in visible sunlight, refracting them directly onto a photographic plate. It was obtained in the rarefied atmosphere atop Hawaii’s Mauna Kea volcano, where many of the world’s best optical telescopes are located. (Courtesy of Eric J. Pittman, Victoria, British Colombia.)
. When the intensity of thermal radiation is spread out as a function of wavelength, the resultant spectrum is most intense in a band of wavelengths that depends on the temperature. This occurs at visible wavelengths when the temperature is about 6,000 degrees Kelvin. Such a hot gas emits radiation at longer wavelengths, but at a lower intensity. In contrast, non-thermal radiation is more intense at longer radio wavelengths. High-speed electrons emit non-thermal radiation in the presence of a magnetic field.
. The Sun’s spectral flux at the Earth plotted as a function of the wavelength in nanometers, given here in units of watts per square meter per nanometer, or W m-2 nm-1. At each wavelength, the amount of solar energy received at the Earth’s surface is less than the amount received outside the Earth’s atmosphere. The attenuation is much greater at certain wavelengths than at others owing to absorption in the Earth’s atmosphere by molecules of oxygen, O2, water, H2O, and carbon dioxide, CO2. There is a general reduction at shorter wavelengths, less than 320 nm, due to absorption by ozone, O3, molecules, and to scattering by molecules, aerosols and small particles in the atmosphere. At any given location on the Earth’s surface, the solar insolation, or power per square meter, can be obtained by integrating the spectral flux over all wavelengths.
. Radiation from the Sun and other cosmic objects is emitted at wavelengths from less than 10-12 meters too greater than 104 meters. The visible spectrum is a very small portion of the entire range of wavelengths. The lighter the shading, the greater the transparency of the Earth’s atmosphere. Solar radiation only penetrates to the Earth’s surface at visible and radio wavelengths, respectively represented by the narrow and broad white areas. Electromagnetic radiation at short X-ray and ultraviolet wavelengths, represented by the dark areas, is absorbed in our air, so the Sun is now observed in these spectral regions from above the atmosphere in Earth-orbiting satellites.
. The spectral plot of thermal radiation intensity as a function of wavelength depends on the temperature of the gas emitting the radiation. At higher temperatures the wavelength of peak emission shifts to shorter wavelengths, and the thermal radiation intensity becomes greater at all wavelengths. At a temperature of 6,000 degrees Kelvin, or 6,000 oK, the thermal radiation peaks in the visible, or V, band of wavelengths. A hot gas with a temperature of 100,000 oK emits most of its thermal radiation at ultraviolet, or UV, wavelengths, while the emission peaks in X-rays when the temperature is 1 to 10 million oK.
. If our eyes could see X-ray radiation, then the Sun would look something like this. Million-degree gas is constrained within ubiquitous magnetic loops, giving rise to bright X-ray emission from active regions. Relatively faint magnetic loops connect active regions to distant areas on the Sun, or emerge within quiet regions away from active ones. The extended corona rings the Sun, and dark coronal holes are found at its poles (top and bottom). This image was taken on 8 May 1992 with the Soft X-ray Telescope (SXT) onboard the Yohkoh mission; it has been corrected for instrumental effects and processed to enhance solar features. (Yohkoh is a project of international cooperation between the Japanese Institute of Space and Astronautical Science (ISAS) and NASA. The SXT was prepared by the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo with the support of NASA and ISAS.)
. A spectrograph has spread out the visible portion of the Sun’s radiation into its spectral components, displaying radiation intensity as a function of wavelength. When we pass from short wavelengths to longer ones (left to right and top to bottom), the spectrum ranges from violet through blue, green, yellow, orange and red. Dark gaps in the spectrum, called Fraunhofer absorption lines, represent absorption by atoms or ions in the Sun. The wavelengths of these absorption lines can be used to identify the elements in the Sun, and the relative darkness of the lines establishes the relative abundance of these elements. (Courtesy of the National Solar Observatory/Sacramento Peak, NOAO).
. The Crab Nebula (M1 or NGC 1952) is an expanding cloud of interstellar gas that was ejected by the supernova explosion of a massive star, about 9 solar masses, in 1054 A.D.. The filamentary gases are expanding at a velocity of 1.5 million meters per second. The red-yellow wisps of gas shine primarily in the light of hydrogen, while the blue-white light is the non-thermal radiation of high-speed electrons spiraling in magnetic fields. The south-westernmost (bottom-right) of the two central stars is the remnant neutron star of the supernova explosion, and a radio pulsar with a period of 0.33 seconds. This supernova remnant has angular dimensions of 4.5 x 7 arc minutes; it is located at a distance of 1,930 ± 110 parsecs and has a diameter of 2.9 parsecs. This famous supernova remnant is also a source of intense emission at radio and X-ray wavelengths. (Courtesy of Rudolph Schild, Harvard-Smithsonian Center for Astrophysics.)
. The most massive stars end their lives in violent explosions called supernovae. These tenuous red wisps mark the expanding debris of the supernova that exploded in the constellation Vela, perhaps 12,000 years ago. Its gases are still expanding outward as a result of the explosion. The Vela supernova remnant now subtends an angle of about 255 arc minutes; it is located at a distance of about 500 parsecs and is now about 30 parsecs across. (Royal Observatory Edinburgh © 1979. Photo prepared by David F. Malin.)
. The abundance of the elements in the Sun, plotted as a function of their weight, on a logarithmic scale that spans twelve orders of magnitude, or one million million. Hydrogen, the lightest and most abundant element in the Sun, was formed 10 to 20 billion years ago in the immediate aftermath of the big-bang explosion that led to the expanding Universe. Most of the helium now in the Sun was also created then. All the elements heavier than helium were synthesized in the interiors of stars that no longer shine, and then wafted or blasted into interstellar space where the Sun subsequently originated. Carbon, nitrogen, oxygen and iron, were created over long time intervals during successive nuclear burning stages in former stars. Because any nuclear reaction involving the iron group must absorb energy rather than release it, these elements cannot serve as fuel in further chains of nuclear reactions inside stars. Elements heavier than iron, such as gold and uranium, were produced by neutron capture reactions during the supernova explosions of massive stars that lived and died before the Sun was born. The exponential decline of abundance with increasing mass can be explained by the rarity of stars that have evolved to later stages of life.
. Unformed planets circle a nascent star, our Sun, before its nuclear fires burst forth. According to the nebular hypothesis, the Sun and planets were formed at the same time during the collapse of an interstellar cloud of gas and dust that is called the solar nebula. (Courtesy of Helmut K. Wimmer, Hayden Planetarium, American Museum of Natural History.)
. The Sun can be observed with both refractors and reflectors. The earliest telescopes were refractors (left). The curved surfaces of the lens bend the incoming parallel solar light rays and bring them to a focus at the center of the focal plane, where an image of the Sun is created. The reflecting telescope (right) uses a large, parabolic primary mirror to collect and focus light. The prime focus can be used for direct images by removing anything that intervenes in the light path. Large equipment, such as a spectrograph, requires a more mechanically stable platform, so the Cassegrain focus is often used. For massive equipment, the light might be further reflected to the Coudé focus.
. A moveable heliostat, perched atop this telescope, follows the Sun and directs its light downward through the long fixed shaft of the telescope. A figured mirror at the bottom reflects and focuses the sunlight toward the observation room. The shaft’s axis is parallel to the rotation axis of the Earth, and about three-fifths of it is underground. It is kept cool by pumping cold water through tubes in the exterior skin, thereby reducing turbulence in the air inside and keeping the Sun’s image steady.
. The McMath Solar Telescope waits for winter skies to clear after a storm (left). In another view, scattered sunlight colors the telescope a stunning red, while stars make streaks across the evening sky (right). [Courtesy of Gary Ladd (left) and William C. Livingston, NOAO (right).]
. A small section of the image at the focal plane of a telescope is selected with a narrow entry slit, S1, and light passes to a diffraction grating, producing a spectrum. A second slit, S2, at the focal plane selects a specific wavelength from the spectrum. If the plate containing the two slits is moved horizontally, then the entrance slit passes adjacent strips of the image. The light leaving the moving exit slit then builds up an image of the Sun at a specific wavelength.
. Sunlight enters from the left, and is focused by an objective lens, L1, on an occulting disk, blocking the intense glare of the photosphere. Light from the corona, which is outside the photosphere, bypasses this occulting disk and is focused by a second lens, L2, forming an image of the corona. Other optical devices are placed along the light path to divert and remove excess light.
. When an incoming radio wave approaches the Earth at an angle, the crests of the radio wave will arrive at two separated telescopes at slightly different times. This delay in arrival time is the distance X divided by the velocity of light. If X is an exact multiple of the wavelength, then the waves detected at the two telescopes will be in phase and add up when combined. If not, they will be out of phase and interfere. The angular resolution of the interferometer is equal to the wavelength divided by the effective baseline. When the object being observed is directly overhead, the effective baseline is equal to the distance between the two telescopes.
. The Very Large Array, abbreviated VLA, operates at radio wavelengths. In this photograph, it is strikingly portrayed against the colors of a double rainbow. The VLA is a collection of radio telescopes interconnected electronically to provide a total of 351 pairs of telescopes. The combined signals obtain two hundred thousand pieces of information every hour, so giant computers are also required to carry out radio investigations of the Sun. (Courtesy of Douglas Johnson, Batelle Observatory, Washington.)
. Each component of the Very Large Array is a radio telescope measuring 25 meters in diameter and weighing 235 tons. Twenty seven of these telescopes are placed along the arms of an Y-shaped array on a desert near Socorro, New Mexico. Each arm of the Y is 20 thousand meters long. The telescopes can be rolled along tracks to change their configuration and create a radio zoom lens. When the telescopes are crowded in towards the center of the Y, they provide a wide-angle view, and when they are pushed to the outer ends of each arm, the telescopes zero in for a closer look. Their output, when combined in a computer, creates a radio telescope with a diameter as large as 34 thousand meters and an angular resolution that can be smaller than 1 second of arc.
. An artist’s impression of the SOlar and Heliospheric Observatory, or SOHO for short. It may be the greatest solar observatory in history. The 1.33 ton, one billion dollar spacecraft was launched from Cape Canaveral Air Station, Florida, on 2 December 1995, and was still in full operation at the turn of the century. SOHO is a joint project of the European Space Agency (ESA) and the United States (US) National Aeronautics and Space Administration (NASA). The spacecraft contains 12 instruments that study the Sun 24 hours a day, every day for years, from the Sun’s deep interior, out through its expanding atmosphere and solar wind, to the Earth and beyond.
. An artist’s conception of the Solar-Terrestrial Relations Observatory, or STEREO, in which two spacecraft will provide the images for a stereo reconstruction of Coronal Mass Ejections, or CMEs. One spacecraft will lead Earth in its orbit and one will be lagging. When simultaneous telescopic images form the two spacecraft are combined with data from observatories on the ground or in low Earth orbit, the buildup of magnetic energy, the lift off , and the trajectory of CMEs can be tracked in three dimensions. (Courtesy of Johns Hopkins, Applied Physics Laboratory).