Mount Wilson – the Birthplace of Solar Physics

Perched 5,710 feet above the Los Angeles Basin in the San Gabriel Mountains, Mt. Wilson Observatory is noted for the ground breaking work of Edwin Hubble during the 1920’s.  In that decade, Hubble would discover galaxies beyond the Milky Way and the expansion of the universe at the observatory’s 100-inch telescope, then the world’s largest.  Located a few hundred feet from the famous telescope lies three solar telescopes whose observations provided the groundwork for our current understanding of the Sun.  This story did not begin in the warm climes of Southern California, but in the Upper Midwest at Yerkes Observatory, 90 miles northwest of Chicago.

The first director of Yerkes Observatory was George Ellery Hale.  The observatory, established in the 1890’s, is dubbed the birthplace of modern astrophysics.  Hale was the guiding force behind the building of the observatory and wanted to move astronomy from the study of the positions of celestial bodies in the night sky to the physics behind those objects.  Hale had an intense interest in the study of the Sun and set out to build a solar telescope on the grounds at Yerkes.  The result was the Snow Solar Telescope built in 1903.  The name of the telescope is not derived from Wisconsin winters, but from Helen Snow of Chicago who anted up $10,000 ($258,000 in 2014 dollars) to build it.  However, poor optical quality necessitated a move of the Snow from Wisconsin to California.

Driving down a highway on a hot summer day, you have probably seen heat waves rising from the ground and distorting your vision.  This effect is magnified if you attempt to take a picture through a telephoto lens.  The Snow Solar Telescope design had a movable mirror (coelostat) reflect the Sun’s image to a 30-inch mirror which in turn reflected the light 60 feet to 24-inch mirror that projected the final 6-inch image of the Sun.  Heats waves from the ground interfered with the image quality as the light traveled its 60 foot path horizontally to its final destination.  Hale thought relocating the Snow to an area with thinner air would reduce the heat interference problem.

As a result, the Snow was dismantled and transported to Mt. Wilson in California in 1904.  One does not normally associate the Los Angeles basin with good optics, but the summit of Mt. Wilson lies above the atmospheric inversion layer that traps the infamous Los Angeles smog like a lid on a pot.  This, combined with the thinner air of the higher altitude, improved the image quality of the Snow.  Hale set out to study sunspots, which would provide the first significant scientific finding from Mt. Wilson.

The Sun on July 28, 1906. Earth superimposed for scale. Credit: Mt. Wilson Observatory.

The oldest known observations of sunspots dates back to 800 B.C. both from ancient Chinese and Korean astronomers.  Historical recordings of sunspot numbers dates back to the 1600’s and constitute one of the longest ongoing scientific programs of observation.  At the dawn of the 1900’s, the nature of these spots on the Sun’s surface were not known.  Among the competing theories at the time were sunspots as debris clouds from solar tornadoes, areas hotter than the surrounding surface, and one of the most colorful ideas, sunspots as holes in a shroud of the Sun that hid a solid surface underneath.  The Snow Solar Telescope would begin the process to clarify the nature of sunspots.

The Snow was equipped with a high resolution spectrograph.  With this, Hale was able to record and compare spectra lines from regions of the Sun’s surface with and without sunspots.  These spectra lines were in turn compared to spectra produced in a laboratory under different temperature regimes.  In the cooler regime, many spectra lines were strengthened, and a few were weakened.  The spectra obtained from sunspots correlated with the spectra obtained in the laboratory in the cooler regime.  Hence, sunspots were regions on the solar surface that are cooler and thus, darker than the surrounding area.  The question remained, why were these regions cooler?  To answer this would require better solar images than the Snow could provide.

As George Ellery Hale was wont to do, he built a bigger and better telescope.  Despite the thinner air at Mt. Wilson, heat interference still proved to be an issue with the Snow.  To solve this, Hale built a telescope with a vertical, rather than horizontal design.  At 60-feet, the new solar tower was completed in 1908.  In his observations of sunspots, Hale was reminded how their structures were similar to the classic iron filings magnetic field experiments.  Based on this hunch, Hale set off to detect the presence of Zeeman lines in sunspot spectra.

60-foot Solar Tower (left) next to Snow Solar Telescope (right). Credit: Gregory Pijanowski
60-foot Solar Tower (left) next to Snow Solar Telescope (right). Credit: Gregory Pijanowski

The black lines seen in spectra are absorption lines.  Different elements absorb light at different wavelengths and this is how astronomers can figure out what stars, including the Sun, are made of.  If an atom absorbs light of the same energy as the difference between two electron orbital levels, the light energy is converted to energy that moves an electron to a higher orbit.  The result is the absorbed light creates a black line on a spectra.  The presence of a magnetic field creates more potential electron orbital levels.  As a consequence, a single absorption line can split into several absorption lines as can be seen below:

Credit: Astrophysics and Space Research Group, The University of Birmingham.

Using the new 60-foot solar tower, Hale was able to detect the presence of Zeeman lines in the spectra of sunspots.  In fact, the magnetic field in sunspots are several thousands times stronger than Earth’s magnetic field.  The intense magnetic fields in these areas of the Sun push plasma convection to areas outside of sunspot regions.  As it is this convection that transports heat to the solar surface, the magnetic blockage of this convection causes sunspots to be cooler by about 2,000 Celsius than the surrounding region.

Hale published this result in 1908six years after Zeeman won the Nobel Prize for his discovery of this effect.  This was the first time a magnetic field was discovered beyond Earth.  Hale would be nominated for a Nobel as a result of this discovery, but ultimately was not awarded.  Health issues eventually forced Hale away from Mt. Wilson, but not before building what would be the largest solar observatory from 1912 to 1962.

Mt. Wilson 150-foot Solar Tower. Photo: Gregory Pijanowski
Mt. Wilson 150-foot Solar Tower. Photo: Gregory Pijanowski

The 150-foot solar tower would be Hale’s last major contribution to solar astronomy.  The 150 foot vertical focal length produces a 17-inch image of the Sun at its base.  It was with this facility that Hale was able to determine the magnetic polarity of sunspots and the 22-year solar cycle.  The 11-year solar cycle had been long known and pertains to sunspot numbers only.  It usually takes 11 years (sometimes longer, sometimes shorter) for the solar cycle to reach one maximum to the next.

Magnetic fields are dipoles.  That is, a magnetic field will have a north and south pole.  Sunspots occur in pairs with one being the north pole and the other being the south pole, albeit at times a single spot in a pair will break up into several spots with the same polarity.  Hale discovered that sunspot pairs exhibit the opposite order of polarity in each solar hemisphere.  The polarities then reverse at the end of each 11-year cycle.  Consequently, a Hale 22-year solar cycle would look like this:

     Cycle (11-years)    Northern Hemisphere   Southern Hemisphere
                   1                        N-S                    S-N
                   2                        S-N                    N-S

The most recent occurrence of polarity reversal happened on January 4, 2008.  This event heralded the arrival of the current solar cycle.

By the mid-1920’s, Hale spent most of his time at his private solar observatory located in his residence in Pasadena.  He passed away in 1938 as work was ongoing for the 200-inch Mt. Palomar Observatory.  A new generation of solar astronomers would carry on his legacy at Mt. Wilson.

In 1957, Horace Babcock would install the first magnetograph in the 150-foot tower.  Rather than just study the strong magnetic fields of sunspots, the magnetograph was sensitive enough to map the magnetic field across the entire solar surface.  Essentially, the magnetograph maps the Zeeman effect across the entire solar disk.  Astronomers would take the work at the 150-foot tower a step further in the 1960’s by using it to study the Sun’s interior with a field called helioseismology.

In 1962, Robert Leighton discovered oscillations all across the solar surface which occurred in 5 minute cycles.  The theoretical modeling of these oscillations were refined by Roger Ulrich, who also kept the 150-foot Solar Tower in operation after the Carnegie Institution pulled their financial support in 1984.  These oscillations are caused by acoustic waves trapped inside the Sun.  Measurements of these waves allowed for modelling the solar interior.  One of the findings is the amount of hydrogen converted to helium in the solar core via fusion reactions.  This finding verified current models of solar evolution.  In other words, we know the Sun will be around for another 5 billion years or so.

The mapping of the solar magnetic field and helioseismology forms a key part of NASA’s current Solar Dynamics Observatory’s mission, as explained in the video below:

During the late 1990’s, I had the opportunity to go inside all three of the Mt. Wilson solar telescopes as a student in the observatory’s CUREA program.  The Snow was used primarily and I’ll never forget cleaning off the direct current switches, seemingly straight out of Frankenstein’s laboratory.  Also had encounters with both tarantulas and rattlesnakes.  In between those adventures, got to study the Sun’s spectrum (just as Hale did 90 years earlier), imaged the Moon at night, and gaze out over the cliff into Pasadena and the Rose Bowl.  The  60-foot solar tower had AC/DC blasting in the observation room, while the 150-foot tower had a visitor’s book signed by both Albert Einstein and Stephen Hawking.

Looking down the ladder on the 150-foot Solar Tower. Credit: Gregory Pijanowski
Looking down the ladder on the 150-foot Solar Tower. Credit: Gregory Pijanowski

Since then, there has been two recessions and a major financial crash.  The result has been cutbacks in funding and a need for the solar towers to reduce staff like many businesses have.  The Snow is still used by CUREA students every summer.  The 60-foot solar tower is run by USC.  The 150-foot tower had its funding shut down and is run on a volunteer basis.  The historic magnetograph made its last observation in 2013.  It could be much worse, in 2009, a forest fire came within a few hundred yards of the observatory which was saved by the efforts of several hundred firefighters.

Below is a video on the current effort to keep the 150-foot solar tower’s record of observations unbroken:

The observatory continues to reinvent itself.  The Pavilion, closed in the 1990’s, is now home to the popular Cosmic Cafe.  The grounds, once open to the public only on weekends is now open daily.  Public viewing is now offered on both the 60 and 100-inch telescopes.  The CHARA interferometer, which had started construction when I was there, is producing scientific results.  How do the solar towers fit in?  The 150-foot tower has been an important link in the continuous observations of sunspots since the early 1600’s.  In fact, those records compose 25% of that history.  And so far, it has been able to continue to do so.  I truly hope that chain is not broken.

*Image on top of post is the 60 and 150-foot towers keeping their vigil on the Sun.  Photo:  Gregory Pijanowski

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