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Nebulous Categories

Posted by Norman Sperling   12/22/2004 00:00:AM

Classifying phenomena into categories is a standard procedure by which science seeks to understand nature’s variety. Classifying nebulae has been a recurring problem throughout the 1800s and 1900s, with more than 25 categories going in and out of use. New categories enter one-by-one as they are recognized. But adopting a new paradigm (usually pushed by technology) may outmode whole sets of them all at once.

Finding Nebulae

The process begins when somebody notices something. Observers scarcely noticed nebulae until the mid 1700s. That’s because the telescopes used till then had extremely long focal ratios, as much as 300 times their diameter, f/300. They had high magnification, little light-gathering power, and tiny fields of view. Due to imperfect materials and imperfect manufacturing, they also scattered more than a little light. Most nebulae are so faint and large that the human eye can’t notice them when their light is spread out so far that it is diluted that much. Scattered light from stars and planets masked nebulosity. And the fields of view were so tiny that most telescopes only showed a small piece of a nebula at a time, and observers didn’t notice that there was a whole nebula there.

In the 1750s, John Dollond began selling his affordable achromatic telescopes. Their focal ratios dropped to f/15 or f/20, making a number of nebulae obvious, and hundreds more detectable as fuzz-blotches. That’s why comet-hunting became popular at the same time: comets have similar sizes and surface-brightnesses. Charles Messier, the French observer, could hunt comets so well because he had a telescope with a much faster focal ratio than earlier astronomers. It concentrated light more, and showed a wider field of view, so comets stood out.

This is also why Messier could record nebulae that no previous astronomer noticed: the telescopes of the late-1700s concentrated nebular light, just as they did with comets. Messier and his successors catalogued over 100 un-moving patches of light, to warn other astronomers not to waste time on them because they weren’t comets. Today, hardly anyone remembers the 13 comets Messier discovered. But he is honored for his catalog of things to avoid – the hundred handsomest nebulae, clusters, and galaxies.

With better telescopes and techniques, the Herschels and others listed thousands more nebulae. They drew pictures of them, and tried to classify the shapes. Many showed no symmetry. Many were oval. In the 1840s a bigger and better telescope, Lord Rosse’s Leviathan of Parsonstown, revealed spiral structure, so “spiral” became a category. The newly-found nebulae were too faint to take spectra of, so nobody knew what they were made of.

Clouds of Stars

By the late 1700s, improving telescopes resolved some objects (that looked like fuzz-patches to Messier) into clusters of stars. A major question for the next century was whether all nebulae could be resolved into stars, if telescopes improved enough. In the mid-1800s, spectroscopy proved that some “nebulae” actually had the spectra of groups of stars, and could eventually be “resolved”. However, textbooks kept calling “nebulae” those which new telescopes resolved into stars clear into the 1880s, decades after forefront researchers relegated them.

Then, in the 1920s, Edwin Hubble proved that spirals, most ellipticals, and a few non-symmetrical “nebulae” were neither clusters, nor clouds of gas, but whole galaxies akin to our Milky Way, far beyond it. So galaxies were removed from “nebulae” and established as a separate type of deep-sky object, though textbooks didn’t give them a separate category until 1940.

Clouds of Gas

On the other hand, many other nebulae are made of gas, extremely different from collections of stars. The gas ones were labeled “nebulae properly so called” because not only did they look cloudy – the original reason they were called nebulae – they were thus known to be physically true clouds, a physical description.

The ones that looked round and greenish, reminiscent of Uranus, William Herschel (discoverer of Uranus) called “planetary nebulae”, a horrible name we still haven’t lived down. That is the only category-name to survive the whole 1800s and 1900s. However, three other categories have been merged into it: stellar nebulae into nebulous stars before 1880, nebulous stars into planetaries around 1910, and ring nebulae into planetaries in the 1920s.

Making Light

Spectroscopy played a huge role in sorting out these classifications. So astronomers re-categorized nebulae according to how they make their light:
Reflection nebulae simply act as projection screens, reflecting the light of nearby stars. The brightest stars are blue giants, so reflection nebulae often look blue. The wispy, dusty nebulosity surrounding the Pleiades star cluster demonstrates this type.

Emission nebulae are heated up by nearby hot stars till they emit different wavelengths of their own. Nebulae are largely hydrogen, and the resulting hydrogen-pink glow is the most abundant color in the universe. We see this hydrogen-pink from nebulae and galaxies all across the universe. The Lagoon, Orion, and Swan Nebulae are famous and beautiful examples.
Absorption or dark nebulae absorb light, and are seen only in silhouette. (A student once called these “omission nebulae”.) The Horsehead is a famous example; Barnard 86 is often pictured in textbooks, too.

This mid-1900s paradigm still dominated textbooks at the end of the 1900s, calling nebulae emission OR reflection OR absorption, as if a whole nebula is one and only one of those. By then, however, many objects were understood to show different parts in 2, or all 3, of these categories, such as the Trifid and Orion Nebulae. Color photos show emission (pink), and reflection (blue), and absorption (black) areas, all intertwined.

In reality, all nebulae absorb some of the light that hits them.

All nebulae emit some light (often in the radio and infrared parts of the spectrum). And all nebulae reflect some light (though if there’s no bright star nearby it won’t show up, but that’s like blaming a projection screen for not having a projector shining on it at the moment).

So considering the physical manners in which dust and gas handle light, all nebulae are in all 3 categories at all times. Not so useful!

Re-Classify Nebulae as “Pre-Stellar” and “Post-Stellar”

Meanwhile, stellar astronomers confirmed the old suspicion that stars condense from the nebulae that show no symmetry. Medium-mass stars die by puffing out symmetrical planetary nebulae, while high-mass stars die in supernova explosions that leave nebular remnants, too. So some nebulae turn into stars, and some stars turn into nebulae. Gasses to gasses, dust to dust.

Nebulae make sense when taught as part of stellar evolution. First, learn “pre-stellar” nebulae and how they make stars. Then, learn stellar evolution. Finally, learn “post-stellar” nebulae such as planetary nebulae and supernova remnants.

The Pillars With the Fringe On Top

In the Horsehead, Rosette, Eagle, and other nebulae, black patches show bright fringes. Though clear on pictures taken since the 1920s, no one paid much attention to the bright fringes until the Hubble Space Telescope imaged them in the mid-1990s. The spectacular view of 3 dark pillars in the Eagle Nebula made newspaper front pages all around the world, not because editors understood the picture, but because it is gloriously beautiful.

The pillar tops, and somewhat the sides, show streaky billows of gas coming off the dark pillars. These resemble comet tails. That’s because the same process is happening. Both in comet nuclei and in the dark nebular clumps, the dense, black matter is cold, filled with ices, tars, and rock bits. Both in comets and in these nebulae, nearby hot stars heat up the dark stuff until it vaporizes. Both in comets and the bright fringes, the vapors jet away in wispy streaks. Those wispy fringes resemble comet tails because they result from the same materials under the same conditions, responding in the same way.