The Shape of Nebulae, Then and Now
by Duane Dunkerson
The Epicureans thought the Moon, the stars, and the rest
in the sky were as they appeared to be. The Milky Way was
a diffuse band, a plume or a fog in the sky. Patches of
light here and there such as in the constellations Orion
and Andromeda and Hercules had an appearance not to be improved
Simon Marius of Galileo's time observed the patch in Andromeda
with the new discovery called a telescope and said the patch
was like a distant candle seen through a thin obstruction.
Galileo himself focused the Milky Way into stars, that band
became a band of starlight. All nebulosities, nebulae, that
he found in his telescope became stars. He assumed any remaining
blurs in the sky were due to the optical imperfections of
his telescope. Nebulosity was banished from the sky.
Not so. Improved telescopes found still more patches and
they blurred, obstinately so. Most spectacular of these
recalcitrant objects was in Orion. A pale blue mist persisted
around a group of four stars. Huygens thought some stars
shone through a fog that made the surrounding space placid
and very dark with a hole containing a "more lucid
region". By 1700, ten nebulae were known and regarded
The curiosities as nebulosities sometimes moved. The comet
hunters had found their prey. Those that persisted as blurs,
as luminous indistinct patches, were put on a list by Messier
and Méchain in 1784. They were later to become known
as M objects or Messier objects. M and M had found over
100. Some were in the zone of the Milky Way.
Thomas Wright in 1750 had accepted Galileo's contention
that the Milky Way was largely unresolved stars. The Milky
Way, according to Wright, is a band because it is a flattened
layer of stars in our area. Kant of Königsberg took
up the subject of the nebulous stars from Wright and working
from a sense of uniformity in the Universe, removed the
"nebulous" from the "nebulous stars".
That is, the nebulosities were a mass of stars, unresolved.This
was in opposition to Jean-Jacques Dortous de Mairan who
thought the exhalations of monster stars put forth what
we see as the nebulosities. Obviously uniformity was lacking
if monstrosities exhaled. But Kant thought his notions applied
only to pure nebulosities. If they were nebulous stars,
of stars and nebulosity together, then there was more to
it than a question of unresolved resolution.
A monster here on Earth, a forty-foot telescope, the "ultimate
object" of William Herschel, was put into use to examine
the list of M and M. The telescope's 48 inch aperture in
"proper situations" found stars in the nebulae.
But then Herschel also found 2,000 nebulae in seven years.
Some of them didn't have a stellar aspect. He called them
"planetary" because they resembled in general
appearance the look of a small, indistinct planet. So some
nebulae were nebulae and made of gas about a central illuminating
star. Other astronomers listened politely to what he concluded
in 1811 - that some nebulae were a little brighter in the
middle or much brighter in the middle. He argued from uniform
appearance to continuity of structure. Astronomers of his
day had no telescopes to equal his. They busied themselves
with tracing Newtonian motion and measuring distance to
Laplace traced planetary orbits into the future and into
the past. Six editions of his The System of the World would
be published. Not until the 4th edition is Herschel mentioned.
As discovered by Herschel, there are stars in nebulosities,
and Laplace wanted to make use of such an observation to
support his nebular hypothesis for the origins of the planets.
This nebulosity, a cloud of gas, contracted and produced
the Sun. Rings of the condensation of the cloud eventuated
into the planets. Later, in 1900, Moulton and Chamberlain
wanted to substitute a planetesimal hypothesis that provided
for small particles that aggregated into planets.
Before Moulton and Chamberlain more concern for nebulosities
arose again from another Herschel. This time it was John,
son of William Herschel. John Herschel thought space to
be filled with dissipated matter that did not glow. Though
it could not glow, it carried particles that could glow.
So then unresolved nebulae could contain glowing particles.
The younger Herschel observed for a time from South Africa
and made drawings of the clouds of Magellan. These Clouds
were two nebulosities seen from Earth's Southern Hemisphere.
He found stars and nebulae within the Clouds. He had also
made lists of genuine nebulae that he had seen in the Northern
The Third Earl of Rosse and an associate, the Rev. Thomas
Robinson, used the Earl's giant telescope at Birr Castle
in Northern Ireland to observe many of the nebulae on J.
Herschel's lists. The telescope, having a mirror of 72 inches
and a 50 foot focal length, allowed Rosse and Robinson to
resolve those nebulae into individual stars. They felt that
other nebulae not examined by them to be composed of stars
and that all nebulae, if sufficient resolving power existed,
would be shown to be stars.
In 1848 Rosse found a spiral pattern in one of the objects
of the lists done by Messier and Méchain. Called
M51, it displayed swirls tightly wound on a central area.
He found spiral patterns in other nebulosities too. He and
his colleagues carefully drew what they had seen. Their
drawings were soon to be superceded by photography.
The new process, photography, was to be combined with the
spectroscope. William Huggins had begun to examine the dark
lines in the spectrum of the Sun as had been seen by Kirchhoff.
Light from stars seen through the spectroscope showed multiple
lines in their spectrum. These were multiple lines like
those seen from the Sun. But then Huggins spectroscopically
examined a bright planetary nebula in Draco. He saw only
a single narrow band of color. Here now was a way to know
if nebulae were starlike or not. He checked 60 other nebulae
and found one third to not have starry spectra.
By 1900 it was known that nebulae were not evenly positioned
in the sky. Starry nebulae were near the Milky Way. Other
nebulae, some seen as spirals, having no stars, were not
near the Milky Way. Also by 1900 astronomers became preoccupied
with concerns about scale and form. Were the nebulae with
us, that is relatively close to us in space and was the
Milky Way's form unique in the Universe?
Easton in 1900 depicted the Milky Way as a spiral. He also
contended that "the great majority of the small spiral
nebulae" were a part of the Milky Way. There were definitely
star-like or not nebulae in the Milky Way and so they were
close to us. Then there were the spiral nebulae, star-like,
and maybe not a part of our Milky Way. The key form was
a spiral, easily discernible, in many telescopes of the
day. Our Milky Way might be a spiral but then those small
spiral nebulae were with us or apart from us? Some evidence
was lacking. In 1924 Eddington surmised that there were
three kinds of nebulae - irregular, planetary, and spiral.
For the most part the irregular and the planetary were with
us. But as for the spirals they had been called "island
universes" apart from us.
Were they like us or we like them or did we have a uniqueness?
Was it that the Universe was what we called our Milky Way
a galaxy plus nebulae or were the nebulae, the spiral nebulae
in particular, like our galaxy and then the galaxies made
up the Universe?
To look at the spiral nebulae was not enough. They weren't
going to be drastically enlarged, magnified, by newer and
larger telescopes. If they weren't of our galaxy then they
must be distant from us. Our own galaxy's limits would have
to be made known along with a determination of the distances
to spiral nebulae.
The star Delta in the constellation of Cepheus was to be
the omega for the localized spiral nebulae. The alpha was
the Small Magellanic Cloud and 2,400 variable stars contained
therein. These Cepheid variables were also scattered about
in the Milky Way. It was found in 1912 that the brightness
of a Cepheid was on the increase for longer periods of variability.
So one compared the brightness of variables know to be local
with the same kind of variable (a Cepheid having the same
period of variability) to be found in the Small Magellanic
Cloud. Hertzsprung did this and found the Small Magellanic
cloud to be at the edge of the Milky Way. Thus it could
be seen as a continuation of our Milky Way or apart from
us but a very near piece of us.
Harlow Shapley picked up on Hertzsprung's work though Shapley
was doing distances to eclipsing variables. His first task
on Mt. Wilson in California was to determine a cause for
Cepheid variation. They must mostly be pulsating stars (bigger
is brighter, lesser is dimmer) he concluded. He went on
to examine variables in globular star clusters and wanted
to show that Hertzsprung's variables that were relatively
close to us were like those in the globulars. Also, those
in the Small Magellanic Cloud were like the ones in globulars
and closer to the Sun, our backyard. They were all of a
type and varying in brightness in a periodic way. But based
on comparisons of true brightness and apparent brightness,
he found the globular clusters to be well beyond our Sun
and in a pattern around the Milky Way and not in the plane
of the Milky Way. They were spread out around the Milky
Way like the spiral nebulae.
Also at Mt. Wilson, but later, was Hubble. In 1919 he was
discovering that nebulae could be put into one of two groups
- reflection or emission. Emission nebulae were associated
with blue, bright stars. The fainter these blue stars appeared
then the more distant they were. Among these distant nebulae
were spirals. Ritchey, Curtis, Lundmark, and Duncan all
found, from Mt. Wilson, that distant spirals can contain
stars. Hubble intensely studied an irregular shaped nebula,
NGC 6822. He found variable stars, Cepheids, and five diffuse
nebulae within NGC 6822.
The variables gave him distance, a distance far far beyond
our backyard and far beyond our Milky Way. Other spiral
nebula also were shown to be far away and containing many
stars. Now there were galaxies out there. But for us? Were
we a galaxy? And the nebulae now only in our galaxy? Yes,
we are a galaxy, and a spiral as it turns out. We would
be a "spiral nebula" for others far away from
us. Distant galaxies feature nebulae, our galaxy has nebulae
but not spiral nebulae as of old. These are now beyond us.
The nebulae still persist as local features of our galaxy
or of other galaxies. Those of a class called spiral nebulae
became galaxies immensely far away. The blurs, the patches
in the night sky, remain near and far. Those closer retain
their status as curiosities, beautiful and diffuse. They
are clouds and like the clouds of earth we can be fanciful
about what we think their shapes signify. One is seen as
a horse's head, another as a continent, and another as a
coalsack. We have these in the clouds of Earth. Still other
nebula are shaped like a crab, a dumbbell, a trifid, a ring,
an eagle, a veil, a lagoon, a cocoon, a helix, a bubble
and a cave. And more - a rosette, a ghost, a state, a bug,
a tarantula. All these too are in the clouds of Earth. But
earthly are not these nebulae. Their shapes are more fixed
in fancy because they are so far away and so very large.