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Norman Sperling
Norman Sperling Buy Norman's Book! "What Your Astronomy Textbook Won't Tell You" Click here for info
Nebulous Categories
What is a Galaxy “Spiral Arm”?
Apples, Oranges, Rocks, and Clouds
FFNs, LBBs, and LBMs
Putting "Worlds in Collision" in its Place
Types of Meteorites
Inside Gas Balls
Planetary Magnetic Fields
Unidentified Spectral Lines
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Classifying phenomena into categories is a standard procedure by which science seeks to understand nature’s variety. Classifying nebulæ 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 Nebulæ
The process begins when somebody notices something. Observers scarcely noticed nebulæ 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 nebulæ 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 nebulæ 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 nebulæ 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 nebulæ, clusters, and galaxies.
With better telescopes and techniques, the Herschels and others listed thousands more nebulæ. 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 nebulæ 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 nebulæ could be resolved into stars, if telescopes improved enough. In the mid-1800s, spectroscopy proved that some “nebulæ” actually had the spectra of groups of stars, and could eventually be “resolved”. However, textbooks kept calling “nebulæ” 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 “nebulæ” were neither clusters, nor clouds of gas, but whole galaxies akin to our Milky Way, far beyond it. So galaxies were removed from “nebulæ” 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 nebulæ are made of gas, extremely different from collections of stars. The gas ones were labeled “nebulæ properly so called” because not only did they look cloudy – the original reason they were called nebulæ – 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 nebulæ”, 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 nebulæ into nebulous stars before 1880, nebulous stars into planetaries around 1910, and ring nebulæ into planetaries in the 1920s.
Making Light
Spectroscopy played a huge role in sorting out these classifications. So astronomers re-categorized nebulæ according to how they make their light:
- Reflection nebulæ simply act as projection screens, reflecting the light of nearby stars. The brightest stars are blue giants, so reflection nebulæ often look blue. The wispy, dusty nebulosity surrounding the Pleiades star cluster demonstrates this type.
- Emission nebulæ are heated up by nearby hot stars till they emit different wavelengths of their own. Nebulæ are largely hydrogen, and the resulting hydrogen-pink glow is the most abundant color in the universe. We see this hydrogen-pink from nebulæ and galaxies all across the universe. The Lagoon, Orion, and Swan Nebulæ are famous and beautiful examples.
- Absorption or dark nebulæ absorb light, and are seen only in silhouette. (A student once called these “omission nebulæ”.) 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 nebulæ 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 Nebulæ. Color photos show emission (pink), and reflection (blue), and absorption (black) areas, all intertwined.
- In reality, all nebulæ absorb some of the light that hits them.
- All nebulæ emit some light (often in the radio and infrared parts of the spectrum).
- And all nebulæ 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 nebulæ are in all 3 categories at all times. Not so useful!
Re-Classify Nebulæ as “Pre-Stellar” and “Post-Stellar”
Meanwhile, stellar astronomers confirmed the old suspicion that stars condense from the nebulæ that show no symmetry. Medium-mass stars die by puffing out symmetrical planetary nebulæ, while high-mass stars die in supernova explosions that leave nebular remnants, too. So some nebulæ turn into stars, and some stars turn into nebulæ. Gasses to gasses, dust to dust.
Nebulæ make sense when taught as part of stellar evolution. First, learn “pre-stellar” nebulæ and how they make stars. Then, learn stellar evolution. Finally, learn “post-stellar” nebulæ such as planetary nebulæ and supernova remnants.
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The Milky Way, and many other disc-shaped galaxies, are said to have “spiral arms”. The term comes from early drawings and photographs, which show an overall spiral impression in the bright parts.
Humans tend to “connect the dots”. When you carefully inspect photos of real galaxies, hardly any have arms so smooth you can actually trace them all the way around.
The very few that do seem to result from recent galactic encounters. M 51 is currently encountering NGC 5195, and M 81 has just passed M 82.
Most other so-called “spirals” only show overall impressions of a spiral-like theme, but notice:
- The “arms” look very patchy. It’s easy to notice the bright spots, but make a point of noticing the faint spots and places where no arm appears at all. Only by connecting the bright patches do people perceive the continuous spiral arms. The actual “arms” are almost always very discontinuous.
- Color photos reveal the bright patches to be blue, meaning they are OB associations. Of course, wherever O and B stars form, every other kind of main sequence star forms too, but the blue giant O and B stars outshine all the others, making the area look blue. O and B stars shine brilliantly but gobble up their fuel much faster than lower-mass types. No blue giant seems to live longer than a few million years. So, what the spiral-arc patches marked by blue giants show is where star-birth happened very recently, and often where it happens right now. Blue giants never live long enough to drift very far away.
- Dimmer stars keep shining long after waves of star-birth sweep over their nests. They fill the disc, including the spaces between the “arms”, with lots of stars. While the small patches that trace the “arms” are brighter than the big places between them, lots of light also comes from between arms, especially compared to places way beyond the galaxy. The disc’s A, F, G, K, and M stars put out quite a lot of light.
- Color photos reveal arcs of hydrogen-pink nebulæ, paralleling the blue tracery of OB associations. Arcs of dark, thick nebulosity parallel those, too. Radio-telescope traces of molecular clouds also reveal segments of spiral-like arcs.
However, each of those (blue, pink, black, etc.) lies in a different location! The disc is full of segments of spiral-like arcs, but no single one of them constitutes a physical arm, because they all lie in different places. Arms are not physical structures. Arms are illusions. The physical structure is a disc. On the disc, there are many segments of various sorts, mostly following overall spiral-like arcs.
Some galaxies have spiral segments that appear to be wound much looser or tighter than others. Sometimes the looser-wound segments are nicknamed “spurs”. But in some galaxies, like NGC 7217 and NGC 1398, there are large zones where the spiral segments are wound much differently than elsewhere in those same galaxies.
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All planetary data tables list planets’ diameters. However, they don’t measure the same characteristic for each planet.
Everybody agrees that both Venus and Jupiter are planets. Everybody agrees that both have rock and metal inside, surrounded by gas, which includes layers of opaque clouds.
For Venus, “diameter” refers to the top of the rock; the gas is called the “atmosphere”. That’s because Venus is so similar to its better-known sister planet, Earth, where we Earthlings walk on the rock surface, and to Mars and Mercury. The diameter that books quote for Venus doesn’t include its gasses.
For Jupiter, “diameter” refers to the top of the clouds; the rock and metal are called the “core”. When we observe Jupiter through a telescope, we see the cloudtops; Saturn, Uranus and Neptune are measured the same way. If the diameters that books quote for these giant planets used Venus/Earth standards and neglected their gasses, only measuring the rock and metal deep inside, the numbers would be much smaller. Those numbers would also be much less accurate, because we don’t actually observe those surfaces. The layers are computed by theoreticians.
Everyone may agree on the diameters of Venus and Jupiter, but that doesn’t mean they’re talking about the same things. You can’t always compare such planets using the data tables, including derived properties such as a planet’s average density.
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FFNs
When novices start to use their first telescope, they look at the sky’s major showpieces, such as the Messier nebulæ, clusters and galaxies. They’re big and bright enough to show up in binoculars, and a beginner’s telescope shows detail in many of them. In the background lurk many more faint objects.
Experienced skywatchers buy bigger and better telescopes, seeing ever-richer detail in more and more nebulæ, clusters and galaxies. But always, in the background, there are even more objects, too small and faint to make out. Some irreverent amateur astronomers in San Jose call those background objects “Faint Fuzzy Nothings” – FFNs.
FFNs continue in the background as seen by big, professional telescopes, too. Look at a picture of a galaxy in your textbook. In the background you can often notice dim smudges. Each of those is a galaxy, too, but so much farther away that you can’t make out as much detail. A 3-meter-wide telescope shows magnificent detail in objects that amateurs can barely glimpse – and in the background lurk uncountable thousands of more FFNs. A 6-meter telescope shows detail in those, and in the background, even more FFNs. A 10-meter telescope reveals detail in those objects . . . and in the background, there are ever more FFNs. No telescope has ever been made that didn’t find more FFNs in the background.
LBBs
One day when I was visiting my brother, a bird-watcher, I noticed his log of sightings. Almost every entry included “LBB”. He told me that LBB stands for “little brown bird”. They are so common, so small, and so similar, that they’re not worth examining to see which common species each one belongs to. They flock all over, they’re usually there, and they’re not the big or pretty or rare birds that bird-watchers prize.
LBMs
The university’s mycological society hosted a meeting about LBMs. Mycologists study fungi, and I didn’t have to attend to figure out that “LBM” stands for “little brown mushroom”. LBMs are so common, so small, and so similar, that they’re not worth examining to see which common species each one belongs to. They’re not the big or pretty or rare mushrooms that fungus-hunters prize.
This happens a lot in science. Beginners learn all the kinds of phenomena in the field, and quickly concentrate on certain ones, all but ignoring certain others. Sometimes practicality forces the distinction: some are available, others are too difficult to study. Often, though, it’s about what’s fashionable to study.
Technology advances at such a furious pace these days that it may be worth looking anew at common background items, using the latest devices. Most people don’t pay attention to them. You just might recognize something interesting that no one noticed before.
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In 1950, Immanuel Velikovsky published his famous book, Worlds in Collision. He attributed various biblical events to astronomical phenomena, describing Jupiter, Venus and comets in ways contrary to what science has learned, and contrary to what was already understood in 1950.
Astronomers immediately denounced the book as unscientific; some urged boycotting its publisher. There was a big ruckus, and many scientists who were unaccustomed to persuasion in public affairs came away bruised by the experience.
Scientists who have read Velikovsky’s book tell me it is absolutely not science.
Even now some people still think scientists suppressed the visionary. They have their own journal, write their own supporting literature, and keep the issue alive, at least in their own minds.
Velikovsky’s book is still on library shelves. I certainly don’t object to that. Libraries should offer a wider range of books than I personally endorse. But I most certainly do object to which shelf it is on. Both in public and in academic libraries, I find it classified as astronomy, which it is not and never was.
The problem is practical, not just theoretical. Students who base term projects of library books can waste precious weeks developing a paper which I will reject or fail if they swallow Velikovsky’s book as truth.
I have asked some public and college librarians about shelving Velikovsky’s book in a more appropriate classification. They insist that they are not empowered to do so. Ultimately, book classifications come from the Library of Congress. And while the Library of Congress has occasionally shown wisdom in classifying newer books, an insider once advised me privately that they would not reclassify Velikovsky’s because of political reasons. Is there really political support for Velikovsky’s pseudoscience to be called science? It’s time to set this record straight.
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Confronted with a variety of minerals that fell from the sky, scientists classified them by what they’re made of. All of them are made of stone, or iron, or some of both. So, for hundreds of years, the major classifications have been:
This classification is true – there are still no exceptions – but misses the main points learned since the mid-1900s.
Most meteorites are (as long suspected) fragments of asteroids. (A very few are chips of Mars and the Moon. No other source has been confirmed.) There are roughly as many different categories of asteroids as there are of meteorites, but (as of 2002) general agreement on only a few types of meteorites as samples of identified asteroid types.
Though exact identifications still elude science, the overall lessons become clear when analyzing meteorite contents. The minerals in most stony meteorites have never melted. They include little rock spheres, a few millimeters across, called “chondrules”. These were flash-melted in the solar nebula, before planets condensed. They cooled off very quickly and were cold-pressed into rocks with other minerals. The meteorites containing chondrules are “chondrites”. They have always stayed under 1100°C. Chondrites also contain carbon or iron. Chondrites are partly classified by how hot they got. Rare, primitive ones never reached room temperature. Higher and higher temperatures drove off ices, then drove off tars, then almost melted the rock.
The other meteorites have melted and refrozen. When they melted, they differentiated.
Only the term itself, “differentiation”, is difficult. The concept is blatantly familiar. In a fluid, dense stuff sinks and light stuff rises. Anything you must “shake well”, such as salad dressing, differentiates into layers.
While the planetary object is fluid, the dense stuff – iron and other metals – sink to form a core. The lightest stuff – like gases and water – rises so much that most of it escapes, unless the planet is so massive that its gravity can even hold onto such light stuff. Medium-density stuff, like rocks, form layers in between. Granite and basalt form a crust, while denser olivine forms a mantle.
Millions of years later, cooled off, the object collides with another, and fragments splatter around the solar system, some of which eventually land on Earth as meteorites.
These meteorites tell about solar system processes! The chondrites tell about cold, old, primitive objects that have hardly changed since the solar system began 4.54 billion years ago. All the others tell about bigger objects that got so hot they differentiated. Iron meteorites tell about the cores they used to be parts of. Stony-irons seem mostly to tell about core-mantle boundaries, though a few may be surface melts from collisions. Olivine stony meteorites tell about mantles. Basaltic meteorites tell about crusts.
So the more important categorization of meteorites, and the asteroids they came from, is whether they differentiated or not, and, if so, what layer they were in.
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One probe, from the Galileo mission of 1995, dipped slightly into the top of Jupiter’s massive atmosphere, and found it was hotter and more turbulent than computer models previously suggested.
Those are all the on-site measurements we have for the insides of any gas object in the universe. We observe (but don’t entirely understand) neutrinos that come directly from inside the Sun. Everything else understood about the insides of stars, brown dwarves and gas giants – every object in the universe that’s much bigger than the Earth, since most of the universe is made of hydrogen and helium – comes from observing surface vibrations and other behavior, and from computer modeling.
Astronomers were clanking out mathematical models of stellar interiors on mechanical calculators as early as the 1940s. As computing equipment improves, more and more factors can be included. And as observations sharpen, reality imposes narrower and narrower limits.
Computer models are still all we have. Some models predict certain characteristics gratifyingly well, such as the lithium abundances actually observed in the smallest dwarf stars.
Other models are probably rather farther from reality. That is certainly not the fault of the astronomical theoreticians who crank them out. These are among the sharpest, cleverest humans ever. The problem is that equations for the effects of rotation, mixing, and magnetic fields demand more computing power than early 2000s computers can deliver. So they leave out those factors, or roughly approximate them.
For example, gas turbulence is maddeningly complex. You’ve seen smoke (tiny solid particles that ride gas currents) rise, break into puffs, curl, billow, and dissipate. Describing mathematically how each particle moves is just too complicated.
Another example is everyday weather – the behavior of the gases we live in. Very sharp scientists have been studying weather for hundreds of years. Yet, in the 1960s, weather predictions for most places weren’t accurate much beyond 6 hours, and in 2000, weather predictions for most places weren’t accurate much beyond 20 hours.
One blatant result is that most computer models show smooth, spherical boundaries between layers. Yet all those objects are hot, and hot objects show internal and surface motion and mixing. Just because computers can’t yet account for those factors doesn’t mean they aren’t there. Such boundaries are probably fuzzy or lumpy or jagged, rather than smooth and sharp. Any time you see cutaways of internal structures, if the layers are smooth, sharp, and spherical, recognize that as an artifact from the computing, not actually characteristics of those objects.
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Some planets have magnetic fields. In 1919 Joseph Larmor developed the idea of self-exciting dynamos inside the Earth and the Sun to account for their magnetic fields. The “Dynamo Theory” suggested that stirring in the hot interiors of planets causes electric currents that generate the fields, somewhat as a dynamo works. The planet’s rotation rate also contributes.
The Dynamo Theory predicted that because Venus’s mass and core heat are so similar to Earth’s, it should have a magnetic field similar to Earth’s. Spacecraft have since shown that Venus has no detectable magnetic field. The prediction is wrong.
The Dynamo Theory predicted that Mars, showing volcanoes – surface evidence of a hot interior – should have a magnetic field, though probably weak. Mars spins about as fast as Earth does. Mars has no detectable magnetic field. The prediction is wrong.
The Dynamo Theory predicted that Mercury, being so small, should not have a magnetic field. Mercury spins very slowly. Mercury does have a magnetic field. The prediction is wrong.
The magnetic fields of the Gas Giant planets are not in proportion to their rotation rates or core temperatures, as far as is known.
And the Dynamo Theory predicts that the Sun would have a strong global magnetic field, when it has none; instead, there are small patchy magnetic fields scattered around the Sun, often associated with sunspot groups.
The Dynamo Theory fails to predict observed conditions. Textbooks should omit it and call planetary magnetic fields “not yet explained”.
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Physicists and chemists began investigating spectral lines in the early 1800s. Astronomers saw how they reveal the chemicals and conditions of remote objects, and immersed themselves in spectroscopy starting in the 1860s. Ever since, sharp scientists in these and other fields have industriously cataloged the spectral lines of just about all known substances under just about all conditions that laboratories can produce.
At first, some lines appeared in almost everything tested. Scientists would wash and rinse a probe, stick a chemical on it, and heat it. Each substance had its own set of lines, but in addition, they all shared certain lines. Later, they discovered that these were from the element sodium. The water they rinsed the probe with was slightly salty, the salt (sodium chloride) stuck to it when the rinse water evaporated, and the sodium lines showed up along with the intended chemical’s.
Using quantum mechanics, physicists have also figured out the spectral lines due to conditions that no lab on Earth can reproduce.
Nevertheless, after 2 centuries of intense research, there are still lines showing up in astronomical spectra that no one can explain.
They can’t be from undiscovered elements, a frequent situation in the 1800s, when many holes remained in the Periodic Table of elements. Lines of an element were seen in the Sun’s light, and, using the Sun’s Greek name “helios”, they called it “helium”. Years later they discovered helium on Earth. Other unidentified lines from nebulæ were attributed to “nebulium”, and from the Sun’s corona were called “coronium”, but those turned out to be more familiar substances under very unfamiliar conditions.
No more holes remain to be plugged in the Periodic Table, so the still-unidentified lines don’t belong to unknown elements. They might be from unfamiliar compounds, or familiar elements in extreme situations.
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