Entering a New Era in Solar Research
By Mike Simmons
“It cannot too often be repeated that the Sun is the only star whose phenomena can be studied in detail.” With this statement, George Ellery Hale explained the importance of establishing a solar astrophysical observatory, a dream which became a reality with the founding of the Mount Wilson Solar Observatory in December 1904. Hale and his staff found new ways of dealing with the many questions they raised, and within a few years Mount Wilson was the world’s leader in astrophysical research.
At the beginning of the twentieth century the study of the physical nature of celestial objects was in its infancy, with most research still directed toward the description of the appearance, position, and motion of the stars and planets. In 1817, Joseph Fraunhofer, the great telescope maker, had described his observation of the Sun’s spectrum with a modified apparatus. He placed a card at the eyepiece of the telescope with only a narrow slit allowing light through a prism. Within the horizontal band of colors from red to blue were 600 vertical lines where very little light appeared. Fraunhofer named the more prominent lines with letters (some of which are retained today such as sodium D and calcium H and K), but he did not know that his discovery would prove to be the tool needed for studying the physical nature of the Sun and stars. In 1859, Kirchhoff showed that such lines were associated with certain chemicals when those chemicals are vaporized in a flame. Ångstrom applied these findings to the Sun a year later and the composition of the stars suddenly became an area for study rather than speculation. Still, the “new astronomy” was not quickly embraced; the traditional observers saw little use for it. The famous observer, S.W. Burnham once remarked after visiting University of Chicago physicist Henry Gale, “Gale showed me the soda lines once at the Ryerson Laboratory but I didn’t think much of them.”
While still a student, Hale invented the spectroheliograph, a modified spectrograph which could photograph the Sun in the light of a single element. Later, as directory of Yerkes Observatory, on Lake Geneva, Wisconsin, he had the use of the world’s largest telescope for this instrument as well as for the spectrograph – but still he was frustrated. Only instruments of very limited size could be attached to the moving end of this telescope, but larger spectrographs were needed in order to spread the spectrum out and separate the lines sufficiently for detailed study. This and other problems led to the establishment of the Mount Wilson Solar Observatory.
The Snow telescope, a coelostat donated by Helen Snow of Chicago to the Yerkes Observatory, was brought to Mount Wilson in 1904. The coelostat is a telescope in which the flat mirrors reflect the light of the Sun into a stationary lens or mirror which forms the images. This type of telescope had until then only been used for temporary tasks, such as eclipse expeditions; but Hale felt that this design could be further exploited. Transported to Mount Wilson by mules and horses in 60 separate trips from Pasadena, the Snow telescope became the world’s first permanently mounted solar telescope. Many of the pieces were carried on the backs of the animals, but for the heavier loads a special carriage was designed. Its front wheels were steered by a man riding on the carriage, while, in order to negotiate the sharp turns, the rear wheels were guided by a tiller operated by a man walking behind the contraption. The narrowness of the trail limited the width of the bed to 24 inches. With this device and two horses, 1,000-pound loads could be transported to the mountain. The sight of this entourage on the trail – men, animals and machine – always figures prominently in the recollections of those on the mountain at this time.
The mode of transportation figured in the design of the telescope’s housing, for no structural element of the Snow building (with the exception of one ten-foot steel rod) is more than eight feet long. Similarly, the doors of the original “Monastery”, the astronomer’s living quarters (which burned in 1909), were made very narrow to keep them from dragging on the ground while being hauled up to the mountaintop on the animals’ backs. Nevertheless, several arrived with rounded corners.
This new type of telescope was not without problems, as Hale’s tests with a temporary coelostat had shown. His experience led to some novel features in the Snow building’s design, especially in attempting to avoid the distortion caused by the reflected beam of light passing close to the sun-heated ground. The coelostat mirrors were placed 35 feet above the ground atop a 29-foot high stone pier on a slope overlooking the San Gabriel Valley almost a mile below. The light then traveled through a wooden-framed building covered with canvas (later replaced with metal) to the primary mirror, 24 inches in diameter with a 60-foot focal length.
Even the sunlight on the tracking mirrors presented a problem since the silver-coated plate glass mirrors distorted as they heated. Low-expansion fused quartz was tried as a material for new mirrors, but it was to be many decades before quartz mirrors would be successfully produced. The mirrors were covered and air blown on them with electric fans between uses, but the problems persisted. At one point heat was applied to the back sides of the mirrors to keep the temperature uniformly high throughout the glass, but this method also proved successful. Finally, air was blown on the mirrors continuously, even during the exposure, and this, along with the other improvements in technique, produced the best quality plates. Two years passed before these were fully solved.
With the Snow telescope operational, the spectrum of sunspots could be studied in detail for the first time. Hale and his colleagues had many questions about these irregular dark areas, questions asked since Galileo’s time. But , as with many pioneering efforts in science, more new questions than answers developed with the first photographic plates. Some spectral lines were strengthened (showing more light missing from the spectrum) compared to the rest of the Sun, while others were weakened, and still others were widened. Bands of lines not seen before appeared in the spot spectrum. Now the enormous value of Hale’s two-pronged attack, using both the telescope and the laboratory, would be proven.
Working in the laboratories on Mount Wilson and in Pasadena, Henry Gale and Arthur King were able to recreate the lines seen in the sunspot spectra. By manipulating the temperature and pressure of gases in a special electric furnace and around a metal electrode, and by evaporating metal rods with a high-voltage spark, they could photograph and identify the spectral lines as light was shined through the gas. They found that the line changes seen in sunspots are caused by decreased temperature, proving that these dark areas are cooler than the rest of the solar surface. The identifications of bands of lines as due to molecules which could not exist at the temperatures existing outside of spots confirmed this result.
The importance of this early work cannot be overstated. The explanation of many of the features of the solar spectrum would later be the basis of ionization theory and the analysis of spectra according to atomic energy levels. This and other information would eventually lead to understanding the relationship between the luminosity and spectral classes of other stars, and then to the method of spectroscopic parallax by which the distances of stars are determined by their spectra. This is one of the primary tools at the disposal of the modern stellar astronomer. Walter Adams later said, “It is probably not too much to say that this investigation of sunspot and laboratory spectra … has been one of the most fruitful in its results of any in the field of astrophysics and spectroscopy.”
The study of the Sun on a previously unknown scale now became routine. Every day that the weather was suitable for use of the Snow telescope, photographs were taken of the Sun. The spectroheliograph made daily records of the solar image in the light of calcium (the H and K lines) and hydrogen (H delta). The spectra of spots and flocculi (now called “plages”) were taken regularly.
The rotation of the Sun had been measured since spots were first seen on the surface, but many questions remained. Since the Sun is gaseous, does it rotate as a solid sphere, or do different parts rotate at different rates? Is the rotation rate variable, as some had suggested? Walter Adams began work on these problems in 1906. By watching the motion of the flocculi he could monitor the solar rotation at different latitudes almost continuously. The positions were measured by projection of the photograph on a special globe, the heliomicrometer, devised by Hale. Adams also measured the doppler shift by photographing the spectra of the Sun’s east and west limbs together on one plate. The spectral lines were shifted in opposite directions as a result of the gases’ motion towards us and away from us at the Sun’s opposite edges.
Adams’ work confirmed that the Sun’s rotation is constant, but that is does not rotate as a solid sphere. The Sun rotates fastest at the equator and slowest near the poles. By looking at the flocculi in the light of the various atoms, he could see the rotation vary as he looked deeper into the Sun.
Turning to the Stars
Though the Snow solar telescope was meant for solar research, its large spectrograph – much larger than had ever been applied to the study of stars – provided a new opportunity for stellar spectroscopy as well. The silvered mirrors of the coelostat were pointed to some of the brighter stars beginning in 1905, and the first large-scale spectra of other stars were obtained. The red supergiants were of interest, but with the photographic plates most sensitive to the weak blue light of those stars, a single plate required 14 hours exposure taken over three nights. When the mirrors were tarnished, 24 hours were needed on five nights. However, the results were worth the trouble. This solar telescope revealed, for the first time, that the surface of these brilliant red stars has a spectrum similar to that of sunspots; they are cooler than the Sun.
By September of 1908, over 5,000 plates had been taken with the spectroheliograph of the Snow telescope. In March 1908, new red-sensitive plates became available for the first time, and spots were recorded in the light of the Balmer alpha line of hydrogen. More detail was seen immediately, and a whirlpool-like vortex structure was seen in many spots. These results prompted still more questions at what was an opportune time: the world’s first tower telescope was just nearing completion on Mount Wilson.
Hale had conceived the idea of a telescope with a “high tower and no tube” in 1904. This would place the mirrors and lens high above the heated ground and avoid many of the problems encountered with the Snow telescope. It would enable the astronomers to use a large spectrograph in a pit beneath the tower where the temperature would be almost constant. The 60-foot vertical coelostat, with an enormous subterranean 30-foot spectrograph, was completed early in 1908 and became the world’s greatest tool for solar research.
With the 60-foot tower complete, the stage was set for what would prove to be Hale’s greatest astronomical discovery. The much greater resolution provided by this telescope and spectrograph allowed a more careful examination of the spectra of sunspots. Hale had hypothesized that the vortex structure observed in spots was due to a powerful magnetic field, arranging charged particles like iron filings around a magnet. With the new telescope he began to look for spectral lines that were split into two or more lines indicating the presence of a magnetic field – the Zeeman effect. On June 25, 1908, five years to the day after his first visit to Mount Wilson, Hale photographed the Zeeman effect in a sunspot, this proving that the magnetic force was not unique on Earth. Many types of line splitting were soon observed, with one line of chromium resolved into 21 separate components. In 1941, Robert Richardson would show at Mount Wilson that the vortices Hale found were not caused by the magnetic fields after all, but rather by the Coriolis effect resulting from the Sun’s rotation – but this would not diminish the importance of Hale’s discovery.
Hale soon discovered that sunspots usually occurred in pairs of opposite polarity, like the “north” and “south” poles of a magnet. This was interesting in itself, but something startling occurred when the first spots of the new solar cycle appeared in 1912. While spot pairs of the old cycle had magnetic polarities oriented in one particular fashion, the polarities in the spots of the new cycle were reversed. Not for another 11 years, when the next cycle began and the spots’ polarities again reversed, would Hale be confident of the significance of this unexpected finding: the solar cycle was actually a 22-year magnetic cycle, not just an 11-year spot cycle.
With the success of the tower telescope, Hale immediately began working on a 150-foot tower. The structure of this tower is actually double, an inner tower supporting the coelostat, and an outer tower (which completely surrounds every part of the first tower) holding the dome over the instrument. With the outer tower as a shield, observations could be made in winds as great as 30 miles per hour. The structure was completed in 1910, in time for the meeting of the International Union for Solar Cooperation (later the International Astronomical Union) on Mount Wilson, but the planned lens was a failure and difficulties were encountered in the manufacture of the huge 75-foot spectrograph to be installed beneath the tower. Two more years would be required to complete the telescope. In 1912, with details of the 17-inch solar image being transformed into a 70-foot spectrum with thousands of lines, Hale began work on the greatest challenge of his life: the search for the general magnetic field of the Sun.
The possibility of a general solar magnetic field had been discussed since Frank Bigelow first suggested it in 1889, based on the appearance of the corona during a solar eclipse. The difficulty in observing this effect was enormous; even with the great dispersion of the new telescope, a weak field would cause a displacement of lines on the spectrographic plate of only 0.0001 inches. Several observers measured each plate, making thousands of measurements of 26 different lines. The results indicated the probable presence of a magnetic field; but the Sun’s general magnetism was not conclusively shown until 1952 when Harold and Horace Babcock invented the solar magnetograph at the Hale Solar Laboratory in Pasadena.
When the Observatory was first established, Hale invited the Smithsonian Institution to send an expedition to Mount Wilson to measure the amount of radiation received from the Sun in the clear, dry mountain air, and to look for variations in the solar radiation. Charles Abbot headed this expedition and subsequently spent many years on the mountain, building his own laboratories and towers. Abbot was one of the “pioneer” astronomers of the day, and he enjoyed the primitive life of those early years: the challenges and long hours, and the gatherings around the huge fireplace of the Monastery in the evening. This was to change with the construction of the 60-inch telescope in 1908. With the advent of the night work, the evening gatherings in the Monastery lost their charm, and Abbot complained that “the 60-inch telescope has spoiled the mountain.”
The 60-inch telescope would be the fourth telescope on Mount Wilson to carry the title, “world’s largest”, but it would not be the last. With the completion of the 100-inch telescope in 1917, the word “Solar” was dropped from the name of the Mount Wilson Solar Observatory. However the solar work in those early days was of such great importance that it became the foundation for almost all solar and stellar research to follow. Bray and Loughhead, in their modern book, Sunspots, state: “So important, indeed, were the contributions made by Hale, and his chief collaborators, W.S. Adams, F. Ellerman, C.E. St. John, H.D. Babcock, S.B. Nicholson, and E. Pettit, that the period between 1905 and 1930 may justly be referred to as the Mount Wilson era in solar physics. Under Hale, the art of solar observing reached a peak which has seldom been surpassed.”
Additional reading, Eight decades of solar research at Mount Wilson By Howard, Robert, Solar Physics (ISSN 0038-0938), vol. 100, Oct. 1985, p. 171-187.