Though the solar interior burns hydrogen at a steady state and seems very stable--it has been very close to the same for 4.5 billion years and is expected to remain the same for another 5 billion years--the surface and outer layers show that the Sun is not stable at all. Examined with modern techniques, especially space instruments, the solar atmosphere is seen to seethe with constant motion and violent activity. Solar distrubances of diverse kinds occur on time scales ranging from years down to thousandths of a second, involving regions ranging in size from the entire solar atmosphere to the smallest detail visible in our most powerful telescopes. We can now see radiation emitted from many different parts of the Sun. Each solar region has its own temperature, density, and other characteristic physical conditions and emits its own kind of light. We have measured temperatures on the Sun ranging all the way from a mere 4200 degrees C to over 50 million degrees C in the hottest solar flares. The upper left photograph is an ultraviolet image taken by a telescope aboard Skylab. It shows a huge erupting prominence, one of the largest seen in a decade, as it lifted off the Sun on December 19, 1973. The ultraviolet radiation was produced by electrified helium atoms at a temperature of about 70,000 degrees C. The upper right photograph shows several sunspots. Sunspots are appropriately named--they appear as spots on the disk of the Sun. A sunspot will have a very dark central region known as the umbra, and it is often surrounded by a less dark halo known as the penumbra. The umbra is dark because it is cooler (around 3500 degrees C/6300 degrees F) than the surrounding sunscape (around 5500 degrees C/10,000 degrees F). Spots change in appearance over a period of several days. In addition, they also move across the Sun as the Sun spins on its axis. Because the Sun is fluid it does not spin as a rigid body. Therefore, a spot near the equator will take about 25 days to complete one rotation while a spot near a pole, if there were ever one there, will take over a month to make the trip. Many observations over several years has revealed an 11 year cycle of sunspots. Over that period the numbers of spots goes from a maximum to a minimum and back. The Sun presents us with a bewildering variety of surface features, atmospheric structures, and active phenomena. Sun spots come and go. The entire Sun shakes and oscillates in several different ways at the same time. Great eruptions called prominences ahng high above the Sun╒s surface for weeks, suspended by magnetic force, and then sometimes shoot abruptly into space from the corona. The explosions called solar flares, seen in the lower left photograph, emit vast amounts of radiation and atomic particles in short periods of time, often with little or no warning. The lower right image shows many types of solar features. Each of these are at different temperatures and emit characteristic radiation of different forms, ranging from radio and infrared waves to visible light, ultraviolet, X-rays, and gamma rays. The drawing shows the typical temperatures, while the legend at the left indicates the corresponding type of radiation emitted. Infrared and visible light are suitable for study of the coolest layers, while much information on the hot corona can only be gleaned from X-rays. Energetic atomic particles from solar flare explosions produce gamma rays.
