129
See also: Galileo’s telescope 56–63 ■ The surface of the sun 103 ■
The sun’s vibrations 213 ■ Maunder (Directory) 337A
merican George Hale was
just 14 when his wealthy
father bought him his first
telescope, and 20 when his father
built him an observatory on the
family property. Two years later,
while at MIT, Hale developed a new
design for a spectroheliograph—
a device for viewing the surface
of the sun one wavelength of light
at a time. He used this device to
study the spectral lines of sunspots.
Some years later, Hale organized
the building of some of the largest
telescopes in the world at the time,including the 60-inch (150-cm)
Hale Telescope, built at California’s
Mount Wilson Observatory in 1908,
paid for by a bequest from his
father. Working at Mount Wilson
that same year, Hale was able
to take clear images of sunspots
in a deep red wavelength emitted
by hydrogen. The speckled images
reminded Hale of the way iron
filings mapped the force field
around a magnet. This led him to
look for signs of the Zeeman effect
in light coming from the sunspots.
The Zeeman effect is a split
in spectral lines caused by the
presence of a magnetic field, first
observed by the Dutch physicist
Pieter Zeeman in 1896. The
spectral lines in light coming from
sunspots had indeed been split,
which suggested to Hale that
sunspots were swirling magnetic
storms on the surface of the sun. ■THE RISE OF ASTROPHYSICS
SUNSPOTS
ARE MAGNETIC
THE PROPERTIES OF SUNSPOTS
IN CONTEXT
KEY ASTRONOMER
George Ellery Hale
(1868–1938)
BEFORE
800 bce The appearance of dark
spots on the sun is recorded in
the Chinese Book of Changes.
1600 English physicist William
Gilbert discovers that Earth
has a magnetic field.
1613 Galileo demonstrates
that sunspots are features
on the surface of the sun.
1838 Samuel Heinrich
Schwabe notes a cycle in
the numbers of sunspots
seen each year.
1904 British astronomers
Edward and Annie Maunder
publish evidence of an 11-year
sunspot cycle.
AFTER
1960 US physicist Robert
Leighton introduces the field
of helioseismology, a study of
the motion of the solar surface.
The variations in the strength
of the sun’s magnetic field are shown
in this magnetogram, produced
using the Zeeman effect. The marks
correspond to the locations of sunspots.