Mount Wilson Observatory: View from the mountaintop

As the 2009 Station Fire threatens Mount Wilson Observatory, Astronomy magazine offers a look back at this facility's legacy.
By | Published: September 2, 2009 | Last updated on May 18, 2023
Mount Wilson and CHARA
CHARA’s Operations Center and Beam Synthesis Facility buildings are seen near the large 100-inch telescope dome at center. The smaller silver domes of the 1-meter CHARA telescopes are seen to the right of the 100-inch dome, in the lower left, and in the upper left (to the left of the 60-inch telescope dome).
Eric Simison / Sea West Enterprises
September 2, 2009
In September 2003, Astronomy magazine published this article about the birthplace of the discovery of the expanding universe. Here is that article, in its entirety. For updates on Mount Wilson Observatory’s status, visit Fire threatens Mount Wilson Observatory

Twilight sinks into the jagged mountains, draining color first from the valleys, then from beneath the scrub oak and Coulter pines whose cones sway, fat with sap. Buildings fade from noontime’s electric white to hoary gray to black silhouettes against the stars.

Breezes bring air faintly scented of the sea as domes open to the night. On a mountaintop above Los Angeles, distant galaxies and stars have showered light onto the optics of powerful telescopes at the Mount Wilson Observatory for almost 100 years. The observatory is the legacy of one tireless individual, George Ellery Hale, who, while trained in solar physics, found his real calling making American astronomy foremost in the world. Even today, a year from its centennial, Mount Wilson is a testament to his boundless drive and ambition.

Hale (1868-1938) came from a wealthy background and hobnobbed easily with the industrial barons of the Gilded Age. Blessed with energy, organizational ability, and a flair for public relations, Hale swept through astronomy at the turn of the 20th century, when astronomers were changing emphasis from cataloging the positions of celestial objects to understanding how stars worked. Hale realized that as astronomy became astrophysics, it would demand big telescopes with great light-collecting ability.

“Make no small plans,” he decided.

In the 1890s, Hale persuaded a streetcar tycoon to build the largest telescope in the world, the 40-inch refractor at Yerkes Observatory in southeastern Wisconsin. Characteristically, even before Yerkes was finished, Hale already was planning his next big project. Through his father’s generosity, Hale ordered a 60-inch diameter glass disk for a reflector telescope larger than any in existence. Mirrors, Hale noted, can be supported at the back of the disk, unlike refractor lenses for which the 40-inch was, then and now, the practical limit. Mirror optics thus opened the road to vastly larger telescopes.With astronomy’s future in mind, Hale designed the 60-inch telescope for astrophysical research, using photographic plates to capture images rather than an astronomer’s eye and sketching hand.

As the 60-inch was being built, Hale convinced the Carnegie Institution of Washington to fund a new observatory in which to house it, support facilities, and several additional telescopes. Wisconsin’s Yerkes Observatory suffered from cloudy skies, so he sought a clear-sky site. The solution he found was outstanding — and it’s still paying dividends for astronomers. After carefully surveying several locations, Hale selected a wild 5,700-foot-high mountain named Wilson’s Peak in the San Gabriel Mountains north of Los Angeles.

Star-test observations proved the astronomical seeing, or image steadiness, was exceptional owing to local geography and meteorology. Observations from the mountaintop are sharp because Wilson’s Peak — soon renamed Mount Wilson — is tall enough that it juts above the so-called inversion layer capping the Los Angeles basin. In an “inversion” of the normal dropoff in temperature with height, smooth, tepid marine air rides atop chillier onshore breezes. This gives the lofty observatory spectacular seeing and helps telescopes work at high efficiency.

Daytime star
Hale’s primary scientific interest was the Sun. Thus, in 1904, the Snow Solar Telescope became the first permanent instrument on Mount Wilson. The telescope uses a movable mirror to send sunlight into the horizontally mounted optics. But Hale found that daytime ground-heating degraded the resolution, so he built a 60-foot tower for the optics. At the top of the tower, the Sun-tracking mirror stands high above the trees; from there, sunlight is beamed down to instruments on the ground.

The 60-foot Solar Tower began operations in 1907. Its prime instrument, called a spectroheliograph, measures the Sun’s spectrum and studies sunspots as they spin in and out of view due to the Sun’s rotation. Hale found the spots’ spectral lines indicate strong magnetic fields — up to 10,000 times that of Earth’s. Ever the perfectionist, Hale sought even sharper resolution and built the 150-foot Solar Tower in 1912.

Astronomers at the observatory also studied the rise and fall in numbers of sunspots, which vary over about an 11-year period. In 1925, Hale and Seth Nicholson discovered a cycle twice as long when they looked at the spots’ magnetism. For example, in one 11-year cycle, the leading spot in a pair displays positive polarity in the Northern Hemisphere (negative in the Southern). In the next 11-year cycle, the pattern of polarity in the leading spot reverses. Thus, it takes a total of 22 years for leading sunspots to return to the same polarity.

60-inch telescope at Mount Wilson Observatory
On December 13, 1908, astronomers first gazed at the heavens with the 60-inch telescope at Mount Wilson Observatory in California. It was the most technologically advanced viewing instrument of its age. Credit: MWO
Mount Wilson Observatory
Expanding the universe
Yet solar studies were to be only part of Mount Wilson’s repertoire. The 60-inch saw “first light” in 1908, and Walter Adams used it to study the spectra of stars, measuring velocities, chemical compositions, and variations. In 1906, Hale ordered the mirror for a 100-inch telescope, nearly tripling the light-grasp of the 60-inch. After technical delays, first light reached the 100-inch in November 1917.

The two big telescopes — each, in succession, the world’s largest — made fundamental discoveries. Harlow Shapley employed the 60-inch to extend the boundaries of the Milky Way, then regarded as the entire universe. He plumbed the distances to globular clusters, showing they lie tens of thousands of light-years away. And their locations in the sky revealed that the Sun and solar system do not take center stage in the Milky Way.

Yet the greatest work done at the observatory was discovering the nature of galaxies and the expanding universe. Through the 1920s and ’30s, Edwin Hubble and his colleague Milton Humason showed that “spiral nebulae” were distant galaxies, each as vast as the Milky Way. Their work also showed that the more distant a galaxy appeared, the faster it seemed to recede. Thus the motions of galaxies trace the expansion of the universe from a beginning, now estimated at 13.7 billion years ago. This result fit predictions from Albert Einstein’s theory of general relativity, and it established the field of observational cosmology. As astronomer Allan Sandage wrote in his book, The Hubble Atlas of Galaxies, “What are galaxies? No one knew before 1900. Very few people knew in 1920. All astronomers knew after 1924.”

Postwar changes
As Los Angeles grew after World War II, the skies brightened and Mount Wilson’s fortunes declined. Hale saw it coming. In 1928 he began planning a 200-inch telescope on Palomar Mountain, in the dark-sky country between Los Angeles and San Diego. Delayed by the Depression and the war, first light for the 200-inch, named for Hale, came in 1948. Sadly, Hale died 10 years before it was finished.

With the arrival of the 200-inch, cosmology research shifted to Palomar and, later, to newer big telescopes at remote sites in Chile, Hawaii, and Australia. In the late 1960s, Mount Wilson Observatory also branched out with a new Southern Hemisphere station in Las Campanas, Chile.

Despite bright night skies, superb seeing has kept solar astrophysics thriving on Mount Wilson. Continually upgraded instruments at both towers provide state-of-the art digital data on the Sun, extending a decades-long record of solar change. The 60-foot Solar Tower, now managed by Ed Rhodes of the University of Southern California, continues to be a vital instrument. It’s where Robert Leighton of the California Institute of Technology found 5-minute oscillations on the solar surface, which launched the field of helioseismology, the study of the Sun’s interior through its surface vibrations. (Helioseismology was developed by Roger Ulrich of UCLA, who leads the 150-foot Solar Tower program.)

At the 150-foot Solar Tower, Ulrich and his colleagues are continuing to study the Sun’s magnetic field. The magnetograph, installed in 1953 by Horace Babcock, has been in constant use. There is now a 50-year record of solar surface magnetism, including complex flows and fluctuations. This lengthy and detailed observational record helps refine the solar dynamo model, which still cannot give good predictions on how the Sun’s magnetic field changes over long periods.

New directions
In the 1980s, tightened finances forced the administrators of the observatory to shift its emphasis from the Mount Wilson site to Las Campanas, where dark skies and large telescopes still permit research on faint stars and galaxies. In response, astronomers from several institutions formed an independent Mount Wilson Institute to keep the observatory’s telescopes operating.

The 100-inch telescope closed in 1984, but in 1994 the Mount Wilson Institute re-opened it with new computer-controlled pointing and tracking and adaptive optics. In this last technique, the surface of a flexible mirror dimples up to several hundred times per second to counteract blurring in the atmosphere. The adaptive optics system, built by Chris Shelton of the Mount Wilson Institute, yields images that are as sharp as the telescope’s maximum theoretical performance — the views look as if the telescope were in space. Ancillary instruments, among them a high-resolution near-infrared camera built by Laird Thompson, hitchhike on the adaptive optics.

To remove a limitation in the adaptive optics that forces the system to look only at relatively bright stars, Thompson built a different instrument that shines a laser spot on the sky to detect the air’s distortions near the object of interest, for example, the core of a galaxy. This system, which has just become operational, can take high-resolution extragalactic images.

Even sharper focus
Near the 100-inch, a new metal building hums as computers and lasers merge starlight from six 40-inch telescopes. The pictures they create are sharp enough to detect a nickel 10,000 miles away. When the six scopes join forces, they form the world’s largest optical interferometer, or linked array. Together they have the resolution of a giant telescope with a diameter as large as their distance apart — a fifth of a mile. The interferometer is called the CHARA Array, for its parent institution — Georgia State University’s Center for High Angular Resolution Astronomy. Another instrument on Mount Wilson, the University of California at Berkeley’s Infrared Spatial Interferometer (ISI), views the sky at longer wavelengths.

These two interferometers take astrophysics into the 21st century. Interferometry is not new on the mountain; Albert Michelson measured star diameters in the 1920s with a 20-foot interferometer atop the 100-inch (see Astronomy, July 2003). But advances in technology now allow much larger baselines and higher resolution. CHARA can measure star diameters, luminosities, distances, and masses. It can search for evidence of extrasolar planets and image starspots on some stars. Similarly, ISI, led by Charles Townes, can penetrate nebulae obscured at visible wavelengths. This lets astronomers peer into the dusty environments that cocoon very young or very old stars.

Another project involves students. Gil Clark directs the Telescopes in Education project, which has worked with students in kindergarten through 12th grade in several hundred schools around the world for nearly a decade. The project uses 24-inch and 14-inch telescopes at the observatory, plus 14-inch telescopes in Chile and Australia. All are equipped with CCD cameras. Students control the telescopes remotely from their classrooms. They’ve searched for supernovae, assisted in planning for NASA missions, followed variable stars, and measured asteroid rotations and distances.

Hunting other suns
One project that would have greatly pleased Hale examines Sun-like and other cool stars for evidence of “starspots.” Olin Wilson began the HK Project in 1966 to see if stars showed decade-long sunspot cycles like the Sun.

The project’s name comes from the spectral lines of ionized calcium the project monitors: H and K. These lines are bright when magnetic fields on a star’s surface cause spots and other Sun-like features. By measuring changes in the two lines over many years, Wilson found starspot cycles like the Sun’s in some of his program stars.

Wilson retired in 1978, but the program keeps expanding — to date, more than half a million measurements have been made of more than 3,000 cool stars. Project records extend through several starspot cycles, allowing scientists to study long-term changes.

Hale’s legacy
As Mount Wilson enters its second century, studies of the Sun and other stars continue to benefit from the site’s incredibly steady seeing. Advanced telescope technologies are bringing renewed activity to the mountaintop.

And George Ellery Hale’s vision of an astrophysics laboratory is still expanding, just like the universe it discovered.