George Ellery Hale Invents the Spectroheliograph (1892)

George Ellery Hale revolutionized solar observations with the invention of the spectroheliograph, an instrument that made it possible to photograph the sun's prominences in full daylight.

George Ellery Hale with a 40-inch spectroheliograph mounted on the Yerkes Observatory. Image credit: The University of Chicago.

George Ellery Hale (1868-1938) was an American astronomer interested in the evolution of the sun and stars. In the seven years after graduating from the Massachusetts Institute of Technology in 1890, he revolutionized solar observations with the invention of the spectroheliograph, an instrument that made it possible to photograph the sun's prominences in full daylight. The spectroheliograph used a clever combination of two new technologies available to astronomers in the late-1800s: photography and spectroscopy.

Normally, the sun produces so much light that faint details on its surface are lost in the blinding glare. A spectroheliograph, however, divides the light into its individual wavelengths and captures only a small amount on film. First the light from the sun enters a telescope. It is then passed through a narrow slit, which acts as a prism to spread the fire hose of light into a spectrum of individual wavelengths. An instrument called a spectroscope can be tuned to only collect a narrow band of these wavelengths – ultimately a much smaller portion of light than outpoured by the sun. The spectroscope directs this portion of light, just a narrow band of wavelengths onto photographic film.

This is useful when studying the sun because the sun is filled with heated gases that each produces light at all wavelengths, which is why its surface is so blinding to see. But, each gas produces light at only a specific wavelength. Astronomers can use a spectroheliograph to tune-in to certain gases such as hydrogen and calcium, and photograph only the solar details hot enough to emit the light from these elements. The image is now much less bright, so photographs can be taken even during the day. Moreover, different spectral lines represent gases at various temperatures from 2,000 to over 1 million C, so astronomers can map the sun’s surface and atmosphere in a wide range of different temperatures.

Within a few decades, Hale’s spectroheliograph had revolutionized the way that astronomers study the sun. Instruments such as the spectroheliograph or those based upon its principles, can now be found in most major observatories around the world – as well as on star telescopes in space.

Optical image (top) and a hydrogen light spectroheliogram (bottom) showing details on the solar surface on July 30, 1906. Image credit: Mount Wilson Observatory.
A 1998 hydrogen-alpha spectroheliogram showing enhanced detail of the sun. The dark features are prominences. The white spots are sunspots or ‘active regions'.

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