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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 upon.

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 as curiosities.

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 nearby stars.

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 Hemisphere.

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.

 

   

 

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Copyright © 2004
by Duane Dunkerson

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