Among the world's specialized observatories, the observatory located on Big Bear Lake in California is quite unique. First, the telescope is situated on a lake—an idea developed by Caltech astronomer Harold Zirin. In the late 1960s, Zirin decided to build a facility for studying the Sun and was attracted by the stable, clean air at high-altitude Big Bear Lake, about 100 miles east of Los Angeles. He eventually put his plan into practice by equipping the observatory with three telescopes having apertures ranging from 20 to 65 centimeters.
Second, the Big Bear Solar Observatory (BBSO) was at the time home to the world's largest solar telescope. In October of the previous year, the New Jersey Institute of Technology (NJIT) completed assembly of this unusual telescope after five years of work. Among its notable features were a 1.6-meter aperture, actuators used to adjust the primary mirror for the sharpest possible images, and an off-axis f/52 design providing an unobstructed path for the light.
Third, and finally by way of introduction, the telescope had recently produced its first official image of sunspot 1084—an image hardly comparable in detail with the image obtained by one of the cameras on the SOHO space telescope.
The BBSO image, taken on July 2, reveals an entirely different level of detail in the sunspot. According to NJIT, its resolution is about 80 kilometers. For comparison, the dark center of the spot—the umbra—is approximately the diameter of Earth. Sunspots lie at a lower level than their surroundings in the photosphere, and newer findings indicated that each sunspot has beneath it an independent vortex—somewhat like a terrestrial tornado—of magnetic-field lines.
The grain-like structures visible throughout the image are known as granules. They are bubbles of hot gas, each covering an area roughly comparable to Iran. These bubbles carry the Sun's internal energy to the surface of the photosphere, where it is radiated into space as heat and light. The phenomenon might be compared to a pot of boiling soup.
Sunspots are typically 1,700 to 2,200°C cooler than the surrounding material, yet they are still hot enough to glow. Why, then, do these regions appear dark? Because radiation is proportional to the fourth power of temperature. Thus, if the Sun's surface temperature were reduced to one-third of its present value, its surface radiation would fall by about 80 percent.
Another image, recording red hydrogen-alpha emission at 656.3 nanometers, shows a region extending to an altitude of about 1,100 kilometers in the chromosphere. This image was taken on July 1, and the same sunspot can be seen in the upper-left corner.
Observatory director Philip Goode explained that along the edges around the spots, dark hair-like strands can be seen. These are actually eruptions—or jets—of energetic plasma being thrown outward. Interestingly, when we look at these spots in visible light they appear relatively calm. The hydrogen-alpha view, however, reveals the true dynamics of these magnetic regions and the jets erupting from the edges of the spots.