104
A SPIRAL FORM
OF ARRANGEMENT
WAS DETECTED
EXAMINING NEBULAE
IN CONTEXT
KEY ASTRONOMER
Lord Rosse (1800 –1867)BEFORE
1784 Charles Messier
publishes a catalog
of the visible nebulae.1785 William Herschel
publishes catalogs of
nebulae and speculates that
many are similar in shape
and size to the Milky Way.1833 John Herschel expands
his father’s catalogs by
surveying objects from the
southern hemisphere.1864 William Huggins
discovers that some nebulae
are clouds of luminous gas,
not aggregations of stars.AFTER
1917 Vesto Slipher concludes
that spiral galaxies are “island
universes” and that the Milky
Way is one such galaxy seen
by us from within.I
n the 1840s, a British aristocrat
named William Parsons, Lord
Rosse, decided to commit
some of his considerable wealth
to building the world’s largest
reflecting telescope. Rosse was
curious to reexamine some of the
nebulae listed by John Herschel in
the early 19th century, in particular
those nebulae that did not appear
to be clusters of stars.
To re-observe these nebulae,
Rosse needed to build a larger and
better telescope than that used by
Herschel. He experimented for many
years with methods for casting a
36-inch (0.9-m) mirror. Mirrors at
the time were made from a metal
called speculum, an alloy of copper
and tin—a brittle material that was
prone to cracking as it cooled.Despite this difficulty, by 1845
Rosse had succeeded in casting
a mirror that was 72 in (1.8 m)
in diameter. He mounted it in
his telescope at Birr Castle, near
Parsonstown in Ireland, where it
became known as the Leviathan
of Parsonstown. This telescope
remained the world’s largest
reflecting type until the 100-in
(2.5-m) reflector was built at Mount
Wilson in California in 1917.
Central Ireland proved a far
from ideal place to build a telescope,
as overcast or windy conditions
often prevented viewing. The
telescope itself had limited mobility,
meaning that only a small area
of the sky could be examined.
Nonetheless, when the weather
was clear, Rosse was able to useLarger telescopes
reveal a spiral form
of arrangement.Telescopes show
some nebulae to be
clusters of stars.To the naked eye, nebulae are fuzzy patches
of light that could comprise gas or stars.