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It wasn’t until the early 1920s that the objects we know
today as Spiral Galaxies—like the Whirlpool—were proven to
be what they are: enormous and very distant masses of
billions of stars, like the Milky Way. Before that,
astronomers believed these objects to be “nebulae”—great
clouds of gas in space. The Whirlpool Galaxy, along with
its smaller companion galaxy, is about 23 million light
years away and 65 million light years across.
A Brief Observational History of
M-51, The Whirlpool Galaxy
 When it was discovered in 1773, the Whirlpool Nebula was
described by its discoverer, Charles Messier, as “a very
faint nebula, without stars, which is difficult to see.” In
1784 Pierre Méchain, reported “It is double, each has a
bright center…the two atmospheres touch each other, the one
is even fainter than the other.” In 1830, John Hershel made
this drawing (right) of the Whirlpool, describing it as having “a
nucleus and double or divided ring.”
 In 1845, Lord Rosse
discovered the Whirlpool’s spiral structure, making it the
first object to be classified as a “Spiral Nebula.” At the
time, the nature of Spiral Nebulae was not known, and most
believed that they were relatively nearby clouds of
material. Above:
Drawing of M-51 by Lord Ross, 1945.
However, in 1755, Immanuel Kant
conjectured that our Milky Way is disk-shaped, and that from
our point of view inside this disk of stars it would appear
as it does: a band of light across the sky. At the same
time, he suggested that some of the faint nebulae seen
through telescopes might be other, “island universes” like
the Milky Way, composed of huge numbers of stars.
 It
wasn’t until the early 1920s that Spiral and Elliptical
“Nebulae” were proven to be composed of huge numbers of
stars, and not just clouds of gas.
Right: Photograph of
M-51, Mt. Wilson
Observatory, May 15, 1926.
Edwin Hubble, using a new
telescope, was able to capture images of individual stars in
several Spiral Nebulae. At the same time, Hubble was able to
make use of a special type of stars—called Cepheid
variables—to estimate the great distances to some these
objects, showing that they were not part of the Milky Way,
but “galaxies” in their own right.
 The Whirlpool Galaxy, 23 million
light years away and 65 thousand light years across, is the
dominant member of a whole cluster of galaxies.
Left: Hubble Space
Telescope image of M-51. In
comparison, our Milky Way galaxy is believed to be about
100,000 light years across. The smaller companion galaxy
that Pierre Méchain discovered may be responsible for the
highly pronounced spiral structure of the Whirlpool,
sculpted over time by gravitational interaction. The Hubble
Space Telescope has given us the sharpest, most detailed
portrait of the Whirlpool to date, showing clearly the lanes
of dark dust, arms of stars, and bright pink regions of
hydrogen gas where new, massive stars are busy being born.
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