88
ON THE
CONSTRUCTION
OF THE HEAVENS
THE MILKY WAY
IN CONTEXT
KEY ASTRONOMER
William Herschel (1738–1822)BEFORE
1725 English astronomer
John Flamsteed’s catalog of
3,000 stars is issued, followed
by his star atlas in 1729.1750 Thomas Wright suggests
that the solar system is part
of a disk of stars.1784 Charles Messier
produces his final catalog
of nebulae.AFTER
1833 John Herschel continues
his father’s work, publishing a
systematic mapping of the sky
including observations made
from the southern hemisphere.1845 Lord Rosse observes
that some nebulae have a
spiral structure.1864 William Huggins uses
emission spectra to determine
that some nebulae are masses
of stars.O
ne of the most spectacular
features in the sky visible
to the naked eye is the
dense band of light called the Milky
Way. This light from billions of stars
is not seen by many people today
because of light pollution, but was a
common sight before street lighting.
In the 1780s, British astronomer
William Herschel attempted to
determine the shape of the Milky
Way and the sun’s position within
it by observing the stars. In this
endeavor, Herschel built upon
the work of his compatriot Thomas
Wright, who, in 1750, had argued
that the stars appeared as a band
of light because they were notrandomly scattered but formed
a vast ring around Earth, held
together by gravity.
The Milky Way appeared to circle
Earth and so Herschel concluded
that the galaxy was disklike. He
observed the numbers of stars of
different magnitudes (brightness)
and discovered that these were
equally distributed within the band
of the Milky Way in all directions.
This led him to assume that theFrom Earth, the Milky Way appears
as a band of light whose individual
stars cannot be seen with the naked
eye. The band is the galaxy’s disk-
shaped structure viewed from within.