The Fictional Element Rant

As an astrophysicist, I enjoy a good science-fiction story as much as anyone. That same fact, though, also makes me pretty good at spotting where the authors are playing fast and loose with physics (for anything but the hardest of science-fiction). Many things I can accept as necessary for a story to work (superluminal travel, for instance), but there's one thing that can break my suspension of disbelief faster than almost anything else if handled poorly: fictional elements.

To see why, let's take a look at the periodic table, which summarizes what we know about the elements. To begin with we need to have a definition of elements themselves. Basically, all atoms have an integer number of protons in their nucleus, known as their atomic number. Atoms with the same number are the same element, and have the same attributes.

An element's atomic number uniquely identifies it.

An element's atomic number is so central to its identity, that it's the first thing I want to know about a new fictional element. And thence the problem: its atomic number can't be one, because that's hydrogen. It can't be two, because that's helium. It can't be three, because that's lithium, and so on all the way up to one-hundred and eighteen, the highest the period table goes at the moment (while elements with numbers 113, 115, 117 and 118 are not named yet, the International Union of Pure and Applied Chemistry is expected to approve names for them before the year is out).

And thus we see the problem that a fictional element (in a moderately hard science fiction setting) must overcome: all the elements up to a hundred and eighteen have been discovered already. Having a new element without further explanation feels to me like someone trying to tell me that they've discovered a new integer between 1 and 118; my immediate response is always going to be, “But we already know all of those integers.”

Now, while this is a problem, there are two approaches that I can see to justifying it:

1. The easy way: give it an atomic number higher than 118. The problem with this is that everything we know about nuclear physics says that anything with such a number should decay so fast it'd be nearly impossible to detect. But this is science fiction we're talking about here. It's pretty trivial to throw in a section discussing why what we thought we knew about physics was in fact wrong, allowing stable high-atomic-number elements to exists. At the very least, lampshade it by having someone remark how they have no idea how such an element is stable, but it'll give the physicists and chemists something to work on! I've seen this done in a free game set in the Half-Life universe released recently called Transmissions: Element 120. They went with the route of explicitly disclaiming knowledge of how element 120 (the putative source of the game's gimmick) was so stable, but it showed to me that they had at least acknowledged that a problem existed and had thought about it, and it didn't break my immersion the way simply announcing some new element would have. (They even used what would be the correct chemical symbol for element 120, Ununbium: Uub).
2. The slightly odd way: have the fictional element replace a real one. So this universe is like ours, except that, say, ruthenium is replaced by fictitium. I've never seen this used, and it would really only sort of paper over the problem, but it could be interesting if the replacement element is judiciously chosen. Elements that are uniformly highly radioactive and decay quickly might be good choices to replace; no one's really going to notice if astatine didn't exist, whereas history would be very different if iron didn't exist.

Of course, it's painfully obvious that most of the fictional elements that most badly break my immersion were only added to their respective settings to drive the plot (usually through opposing factions warring over it) and had no thought whatsoever given to them. To those needing such a plot driver, might I suggest a third path? Use an existing element, and give it a new property that explains why it's in such high demand. It's much easier to accept that an obscure element has suddenly become much more sought after due to a recent discovery than a totally new, never-really-explained element. Plenty of elements that were simply intellectual curiosities to their 19th-century discoverers are now critical components of modern items and infrastructure due to discoveries in the intervening time that could scarcely be imagined at the time they were discovered. (Various rare-earth metals being important in making high-end electronics, for one example.)

Note that this whole rant really only applies to science fictional settings of at least a moderately hard nature where it can be assumed that the periodic table exists. Mithril in The Lord of the Rings doesn't bother me because it's a fantasy setting. (You could also rationalize it as maybe being some real element that was unknown to any but the dwarves at the time. A light, strong, silvery metal could apply to aluminum, which is normally found in compounds but can very rarely occur in pure form as nuggets, explaining why it's only found in the mines of Moria. Just a fun thought exercise.)

Similarly, I don't mind if all the elements in a series are fictional. Star Wars goes this route, with the Expanded Universe (before Disney disavowed it) having probably dozens of fictional elements and minerals. That's interesting, and it gives you a lot of latitude in coming up with interesting elements. It's mixing real and fictitious elements (without justifying the latter) that really starts the smoke coming out of my ears as my brain tries to reconcile the fictional element with reality (for settings where it's assumed things are like reality unless specifically noted).

To put it simply, in science fiction stories, don't mix your real and fictitious elements without comment or justification, please. One or the other, not both.

Well, I don't know how interesting any of that was, but it's been buzzing about in my head for a while and I wanted to get it out. Sorry if I've now retroactively spoiled anyone's favorite story. A hui hou!

One Ring Nebula to Rule Them All

Today I have something besides another globular cluster picture for your perusal. It's a picture of a nebula fairly famous in astronomical circles that I've seen prob-ably hundreds of times in the telescope (it's a popular target during the summer) but have never actually imaged before.

Perhaps it's appropriate that I have this picture less than a week after The Hobbit came out, as this object, Messier 57, is popularly known as the Ring Nebula. It's a small planetary nebula (small on the sky, not physically) found in the constellation Lyra, the Lyre, best seen during the summer and autumn. When I say small, it's only about 1.5 by 1 arc-minutes in diameter; compare that with Messier 55 from my last post, at 19 arc-minutes across. I've therefore cropped out the central region for easier viewing.

Messier 57, the Ring Nebula, in Lyra, at 100% resolution from the camera.

The Ring Nebula is about 2,300 light-years from Earth, and is currently about two and a half light-years across. Measurements of its expansion rate suggest that it has been expanding for about \(1,610\pm240\) years.

The processes forming the Ring Nebula have to do with the life cycles of stars. When stars about the mass of the Sun exhaust the hydrogen in their cores, they go through a complex process of fusing the helium produced by hydrogen fusion into heavier elements, then those into heavier elements, up the periodic table till they get stuck at carbon, having insufficient mass to fuse it to anything higher. During this time, due to other concurrent processes, their atmospheres swell up to become hundreds of times larger than before. As the star runs out of fusible material in its interior it gradually loses its grip on its outer atmosphere which puffs off into space, and which would have been observed starting sometime between A.D. 250 and A.D. 670.

This escaped atmosphere is what we're actually seeing when we look at the Ring Nebula. The core of the progenitor star has contracted down to a small white dwarf of mostly carbon about the size of Earth, but containing about the mass of the Sun. It currently has a temperature of about 125,000 K (~225,000 \(^\circ\)F) and lights up the surrounding atmosphere like a beacon as it blows away. The white dwarf at the center of the Ring Nebula is too faint to be seen in this picture, but is estimated to weigh about 20% more than the Sun currently does.

One of the reasons that I haven't had a picture of this famous (and not un-photo-genic) nebula up before, is because I'd already taken a picture of it...sorta. Sometime during the summer of 2010, I think, I tried imaging it using the narrow-band filters on the imager. Unfortunately, the night I chose had some very thin, high clouds, and I quickly learned that just because a star is bright enough over the entire visible light spectrum to serve as a guide star, does not mean it will be bright enough when you are only looking at the minuscule fraction of its light that comes through a narrow-band filter. Basically, it lost tracking during the exposure, the resulting picture was ruined, and I just never got around to imaging it again, there being plenty of other objects in the summer and autumn sky to keep me busy. This September I finally got around to imaging it and I'm glad I did, for completeness' sake if nothing else.

Supernova Remnants, and the Pulsars that Light Them

Last week I showed you a picture of what happens to small stars (those less than ten times more massive than our Sun) when they run out of fusible hydrogen in their core. Today, let's take a look at what happens to a big star.

Upon running out of hydrogen to fuse, large stars initially proceed very similarly to their less weighty brethren. They begin to fuse the helium produced by hydrogen fusion into heavier elements such as carbon, oxygen, and neon, but unlike smaller stars they have enough mass and gravitational force in their cores to continue. Neon gets fused into heavier elements such as silicon, magnesium, and calcium, slowly working up the periodic table until the star starts making iron in its core. Iron is an interesting element because its nucleus has the highest binding energy per nucleon of any element. (Though certain isotopes of nickel have virtually the same binding-energy that iron does.) What this means, practically, is that you cannot get any energy from iron by either fusion or fission, either by fusing it into more massive elements or by splitting it apart.

Once a star arrives at iron in its core, it's done. It can't squeeze any more energy out of its core, so all that is left supporting the star against its own great weight is electron degeneracy pressure in a slowly-growing iron-nickel core. Electron degeneracy pressure is a quantum mechanical effect with no real analog in classical mechanics. Simply put, electrons exert pressure because two fermions (a class of particle of which electrons, protons, and neutrons are a member) cannot be in the same place and the same quantum mechanical state at the same time. Squeezing them together causes all the low-energy states to be taken, so the electrons vigorously resist any further compression, which would require large amounts of energy to raise electrons into high-energy states. However, there is a limit to how much pressure electrons can exert; if the mass of the core exceeds the Chandrasekhar limit of about 1.38 solar masses, electron degeneracy pressure catastrophically fails and the core collapses in on itself.

At this point, if the star is less than about 20 solar masses there is only one mechanism that can save the star from collapsing into a black hole: neutron degeneracy pressure. Similar to electron degeneracy pressure but involving neutrons, the star's core collapses into a neutron star, a sphere of neutrons packed as tightly as an atomic nucleus and about the size of a city.

As the core collapses into a neutron star, the star's outer layers fall inward tremendously fast, at speeds up to 23% of the speed of light. The neutron star at the center, however, is already packed as tightly as it possibly can be, so the infalling material rebounds off the core in a colossal shockwave to produce what we see as a supernova.

(There is a lot more going on at the same time, of course – supernovae are incredibly fascinating events where relativity and quantum mechanics are both in play, and I'm giving you merely the barest overview of all the processes happening.)

If the star in question started off heavier than about 20 solar masses, even neutron degeneracy pressure will be unable to support the core and nothing in the universe can prevent it from continuing to collapse into a black hole. However, I'm going to focus on neutron stars in this post because the picture I have for you today contains one. The story behind this particular supernova is ancient and varied, so settle in...

The story starts about a thousand years ago, in Anno Domini 1054, when a new star appeared in the constellation Taurus. As was customary, Chinese and Japanese astronomers noted the appearance of a "guest star" and recorded its location. It was also apparently observed by as least one person in the Arabic world. This star was apparently bright enough to be seen in the daytime for a period of several weeks, after which it slowly faded over a period of about two years and finally disappeared, whereupon it dropped out of history.

In 1731 a mysterious nebula (one of the first discovered telescopically) was discovered just off the tip of one of the horns of Taurus by one John Bevis. In 1758, while searching for the return of Halley's comet, Charles Messier stumbled upon this nebula and initially mistook it for his quarry. After watching it for a few weeks he realized that it wasn't moving, and came up with the brilliant idea to publish a catalog of objects that looked like comets but weren't, so that other amateur comet hunters wouldn't be fooled as he had. Thus, this nebula became the first object on what would become his now-famous list of not-comets: Messier 1.

In 1844, almost a hundred years later, the nebula was sketched for the first time by William Parsons, 3rd Earl of Rosse, whose love of astronomy and independently wealthy nature led him to build the largest telescope in the world at that time ("the Leviathan of Parsontown", 6 feet in diameter). His sketch reminded him of a crab, and so he gave our nebula the whimsical name the Crab Nebula. It remained a popular object of observation with astronomers, both amateur and professional.

In 1921 the American astronomer Carl Lampland noted changes in the Crab Nebula which implied a small size for it. In the same year, another astronomer demonstrated that the nebula was expanding. Several astronomers noticed its proximity to the "guest star" of 1054, but nothing was made of it until 1928 when the venerable Edwin Hubble definitively proposed that the nebula be associated with the star. However, it wasn't until later, when the theory behind supernovae had been worked out, that Nicholas Mayall showed that the Crab Nebula was nothing less than the remains of the supernova that exploded into the sky nearly 900 years earlier.

Although the association of the nebula with a supernova was now clear, it wasn't until the 1960's that neutron stars were first predicted by Franco Pacini. A few short years later, in 1968, a neutron star was detected in the center of the Crab Nebula, which made it both the first neutron star ever known and a shining confirmation of Pacini's hypothesis. It also explained why the nebula was so much brighter than a 900-year-old supernova remnant was expected to be.

The neutron star that lurks at the center of the Crab Nebula – known as the Crab Pulsar – is a fascinating beast by terrestrial standards. It is about 25 kilometers (about 15.5 miles) across, and makes a complete rotation every 33.08471603 milliseconds – 30 times a second! As the neutron star spins, it sends out a constant stream of electromagnetic radiation all across the electromagnetic spectrum (including visible light) from both poles of its extremely powerful magnetic field. The axis of its magnetic field is not the same as its rotational axis (much like the Earth, though a bigger offset), and as it spins around it sends off a powerful beam that appears to "blink" on and off as seen from Earth, much like the beam from a lighthouse. The neutron star is thus known as a pulsar, a portmanteau of pulsating star.

This electromagnetic energy being given off comes from the rotational energy of the Crab Pulsar, which is slowly slowing down by 38 nanoseconds per day. The energy being given off along with the star's powerful magnetic field (thousands of times more powerful than the Earth's) being spun through the Crab Nebula 30 times a second causes it to light up. Electrons are accelerated to nearly half the speed of light and spiral along the magnetic field lines of the pulsar, giving off synchrotron radiation as they do, which create a blueish glow visible in the center of the Crab Nebula in long exposures.

Now, after all this background, I suppose I should show you the picture you no doubt read this post for. I hope you can now better appreciate just how amazing this object is, even if my picture cannot do it justice. Here it is, the Crab Nebula:

The Crab Nebula, Messier 1, in Taurus.

While the Crab Pulsar itself is quite invisible in a small telescope such as the imaging telescope, you can easily see the remnant of that titanic explosion 958 years ago. Within that shell of gas exist fantastic filaments and mysterious structures, brought about by electrons powerfully accelerated to relativistic speeds by the pulsar's magnetic field. Despite being around 6,000 light-years away it is persistently the strongest source of X-rays and gamma rays from outside the solar system. The central neutron star itself is a sphere of ultra-dense matter more massive than the Sun compressed into an area smaller than New York city and spinning over 30 times a second. This matter (commonly called "neutronium") is so dense that a single thimble-full would weigh over 100 million tons. I could go on and on at length about how fascinating this single member of the group of objects know as pulsars are, but you get the idea.

I hope this post has given you a sense of the wonder and excitement I get when I study astronomy and physics. Learning more about the incredibly varied denizens of our universe never fails to amaze and astound me, and I enjoy nothing more than bringing that feeling to others. If this post made you stop and think at all, then I will feel I have succeeded. A hui hou!