The Science of Battlestar Galactica

by Di Justo, Patrick

  Most of the stars Galactica visits—in fact, most of the stars in the Galaxy itself—are main sequence stars. After the hot volatility of the T Tauri phase, stars settle down to a more sedentary life, done with the giant flare-ups of adolescence. This lack of excitement during the main sequence inevitably makes science writers talk about “middle-aged stars.” As that term suggests, main sequence stars are normal, middle-of-the-road stars. They exist because of the nuclear fusion of hydrogen into helium deep inside their cores. There’s nothing too outrageous about their behavior.bu

  All stars have to put on a balancing act between the inward pull of their own gravitation, which aims to collapse all the material in the star into the smallest volume possible, and the outward push of the nuclear reactions going on in their core, which aims to blow the star apart. Main sequence stars have found that equilibrium point: the outward nuclear pressure perfectly balances their inward gravitational pull. Larger stars counterbalance their increased gravitational pressure with more active nuclear fusion cores; smaller stars under less gravitational pressure have less active cores. When the data are plotted on a graph, there is a nearly straight-line relationship between the star’s mass and the star’s luminosity. On the main sequence, larger stars are brighter stars.

  This link between mass and luminosity also has consequences for a star’s longevity as well. A star that’s ten times the mass of the Sun isn’t ten times as luminous—it is more than three thousand times as luminous,1 and burns its fuel three thousand times faster than our Sun. Instead of living the 10 billion years our Sun is expected to live, the more massive star will burn itself out in only about 30 million years. It seems odd that a star with more nuclear fuel should have a shorter lifespan, but as with humans, packing on the pounds can take years off a star’s life. Some of the more massive stars live for, perhaps, a million years. The smallest stars may live for trillions.

  Astronomers classify stars according to both their total energy output and their temperature (or color). More massive stars are both brighter and hotter. Stellar surface temperature and color are, in some respects, the same measure. As an object heats up, it appears to change color. Anybody who has been around a blacksmith or a welder has seen this. The blacksmith places his cold metal into the fire and, as it heats up, it begins to glow red. Cool stars similarly appear red. As the metal gets hotter it glows orange, then yellow. The color corresponds to increasingly higher energies on the EM spectrum as it heats until it glows “white-hot” (white-hot metal actually has a bluish tinge, and very hot stars are similarly blue). In fact, the progression of colors is the similar to the order of colors on the rainbow: red, orange, yellow, and blue.bv So the color red is associated with cooler stars; blue is the color of very hot stars.

  From large-and-hot to small-and-cooler, stars are mainly classified by the letters O, B, A, F, G, K, and M.bw Each spectral type is divided into subclasses from 0 to 9, indicating where the star falls in its classification continuum: a G5 star, for instance, is halfway between a G0 and an F0 star.

  It should make sense that smaller stars are far more common than larger stars, simply because they can be formed from smaller parent nebulae. M and K red dwarfs like nearby Proxima Centauri, Barnard’s Star, or Wolf 359bx comprise 93 percent of known main sequence stars. This implies that although Sol is often referred to as “average,” it is actually larger than 95 percent of known main sequence stars. Large blue stars like Rigel and Sirius are comparatively rare and represent roughly only one in 3,000,000 known stars. The seven different types of stars have vastly different life spans, and vastly different fates.

  A Type M star, less than one half the mass of the Sun, will end its life as a degenerate starby called a white dwarf. Over the hundreds of billions of years in its life span on the main sequence, the star will continue to undergo nuclear reactions in its core, converting its hydrogen fuel into helium until it runs out of hydrogen. Since the star is so small, it probably will not have the gravitational pressure needed to spark the fusion of helium. The star will leave the main sequence and will continue to contract until it is about the size of Earth. It will still glow from residual gravitational friction, but by itself it will never experience nuclear fusion again. Eventually, even its gravitational friction will cease. The white dwarf will cool even further and will eventually fade into darkness, becoming a black dwarf. At least, that’s what we expect—the process of turning a white dwarf into a black dwarf takes so long that the universe probably isn’t old enough for black dwarfs to exist yet.

  A star larger than one half solar mass but smaller than about three solar masses—a star ranging from a smaller K to approximately a midrange type A—will also end its life as a white dwarf, but it will take an entirely different path to get there. When the hydrogen fuel in the star’s core is all used up, the core will start to collapse gravitationally. This collapse has a strange side effect: it creates enough heat to initiate fusion in the layer just above the core, where there is still plenty of hydrogen. This layer of fusion provides energy to the rest of the star—enough energy to overcome the star’s inward gravitational pressure. The outer layers of the star expand and cool until a new energy-gravity balance is reached. The star has now left the main sequence and become a red giant.bz

  After a few tens of millions of years, the hydrogen fusion layer burns itself out, causing the core of the star to collapse even further. In medium stars like our Sun, this can result in the helium flash, a momentary fusion of the star’s helium core into carbon. (Yes, this is the same thing Lieutenant Gaeta yelled out just before the Algae Planet’s star exploded. We’ll explain that in a moment.) The flash lasts only a few seconds, but the energy pulse is eventually felt throughout the star. Over the course of hundreds of millions of years, the star will pulsate, shrinking and expanding as various layers of the star undergo nuclear fusion, then burn themselves out. Finally, the last burp of energy will blow off approximately half of the star’s remaining mass, creating a thin shell of expanding gas—a planetary nebula. The remaining core of the star is yet another white dwarf.

  A blue giant Type B or an even larger Type O star will end up as a supernova. After only a few dozen million years on the main sequence, a blue giant star will have used up the hydrogen fuel in its core. Much as the midrange stars did, the lack of core hydrogen will create a layer of hydrogen fusion just above the core. But these stars are so massive that the helium flash is actually not a flash, but a continued process of helium fusion. And once the helium fuel is used up, the temperatures inside the star are so great that the core begins to fuse carbon. Over time, the fusion process proceeds in layers throughout the star: an outer shell of hydrogen fusion, a shell of helium fusion below that, carbon below that, then oxygen fusion, neon fusion, magnesium fusion, all the way up the periodic table until chromium atoms in the core fuse with lighter elements to form iron.

  This is where the star begins to die, because iron fusion does not release any energy. The core quickly runs through its chromium-to-iron conversion, and without any more fusion energy, it collapses in the blink of an eye. The rapid collapse of the star’s core releases an enormous amount of gravitational potential energy in the form of heat, and the rest of the star explodes into superheated plasma moving at nearly the speed of light. The star has become a supernova.

  There’s an interesting side effect to this explosion: it is so hot that it can briefly cause iron atoms (and all the other atoms remaining in the star) to fuse into every naturally occurring element in the periodic table. Every element not already created in the star’s shell—everything from the copper in our pennies to the gold in our bank vaults and the uranium in our nuclear reactors—is created during the supernova explosion and spewed out into space.

  What about the core of the star? Does it also become a white dwarf?

  No. The core of the star collapses on itself to the point where protons and electrons merge together to become neutrons. If the core is between about 1.4 and 2.1 solar
masses, the collapse stops there. The neutrons compact together until the core becomes essentially one giant neutron,ca approximately 10 kilometers in diameter! A neutron star is born.

  WHAT ABOUT ALL THOSE OTHER STARS?

  Soon after the discovery of Dead Earth, Admiral Adama barked out an order to search for all nearby type K, G, and F stars. These are the types of stars most like our own Sun, a type G2. We would expect Kobol’s sun to be similar, as well as those of the Twelve Colonies. These stars are relatively cool, and their initial nebula contained quite a bit of metals.cb As a result, these stars have a relatively long life span, and are very likely to have solid planets. If you’re looking for a place to settle humanoid life, you’ll first look for a K, G, or F star.

  Why would he eliminate the O stars? The A stars? All those M stars? It’s because we would expect none of those classes of stars to have habitable planets. A good argument can be made that if life, especially intelligent life, is found in the cosmos, it will be around a star very similar in size, mass, and color to Sol. Sol-like stars are the kind for which Adama asked Gaeta to search.

  Sam Anders on Dead Earth. ›

  Naturally, the closer you are to a star, the hotter you are. The farther away, the cooler. There is a region around every star where a planet could have water in its liquid form—the planet is not too hot, it’s not too cold, it’s “just right.” Not surprisingly, this region is called the star’s habitable zone or Goldilocks Zone. For the solar system, the Goldilocks Zone extends from near the orbit of Venus to near the orbit of Mars. For small cool stars, like M and small K class stars, this zone is narrower and closer to the parent star. For larger hot stars—like O, B, and A stars—the Goldilocks Zone is not only farther away, but it is wider than that of Sol’s.

  Then why did Adama exclude these?

  Although they have larger Goldilocks Zones, there are other factors that come into play when looking for a habitable planet. Earth II is 4.6 billion years old and according to the fossil record, it has had life for nearly that long. The oldest fossilized algae are 3.8 billion years old. Therefore it took 800 million years for life to appear on Earth. Stars in the large F to O ranges live a billion to a few tens of millions of years. That’s simply not enough time for life to develop.

  Most planets would need preexisting life to be habitable by Colonials. Kobol’s (and Caprica’s and Earth’s) atmosphere was initially delivered to the surface by comets, so that atmosphere had a cometary composition: water, methane, carbon dioxide, and ammonia (among other compounds). That combination of gases would be deadly to humans, but the first life forms on Earth, cyanobacteria (or blue-green algae), literally ate it up. That algae inhaled the carbon dioxide-rich atmosphere and exhaled a gas that was, for it, a deadly metabolic toxin: oxygen. Oxygen is a highly reactive gas, and is extremely rare in nature in an unreacted form (that is, not bound to other atoms in a compound). As an oxygen-breathing species, humans would need to find a planet whose initial atmosphere had been “processed” by preexisting life. On a related note, if humans ever discover a planet with a high concentration of oxygen, life is a near-certainty.

  So large stars simply do not live long enough to harbor human- or Cylon-habitable planets. What about the end of the stellar spectrum? What about the bulk of the stars in the Galaxy, the cooler M and K stars? These stars live extremely long lifetimes, so certainly they’re better candidates for life. The key word is “better.” They are still not “good” candidates. The Goldilocks Zones for cooler small stars are very close to the star, and also very narrow. This presents three problems. First, the mere fact that the habitable zone is small dramatically decreases the likelihood that a planet will form in the zone. Since the habitable zone of a small star is necessarily close, the gravitational pull of the star would tend to cause a planet at that distance to become tidally locked. This means that the same face of the planet would always face the star, like our moon—which is tidally locked to Earth—always presents the same face. That means one side of the planet would roast while the other side froze. Finally, small stars tend to fire off more stellar flares. A stellar flare is a violent eruption in a star’s atmosphere ejecting a stream of hot, highly energetic, charged subatomic particles into space. If a young planet were in the early stages of creating life-forming compounds, stellar flares could sterilize the planet’s surface—especially since a nearby planet gets a bigger dose of radiation from a stellar flare than a planet farther away (like Earth), where the flare has time to dissipate.

  Stars in the middle of the Main Sequence on the H-R Diagram,cc the F, G, and K stars, survive on the main sequence long enough for life to form, they have moderate-sized not-too-close Goldilocks Zones, and they aren’t as likely to spew stellar flares as smaller stars. So in the search for a habitable planet, Adama knew exactly where to look. ‹

  If the core is between about 2.1 and 5 solar masses, the collapse might continue until the neutrons break down into their constituent up and down quarks. The conversion releases an enormous amount of gamma radiation, and results in a star that is essentially one giant elemental particle, only a few kilometers in diameter.cd

  If the core is more than about 5 solar masses, the collapse will continue even past the quark stage, resulting in a singularity, or a black hole.

  CHAPTER 17

  The Many Different Types of Planets

  In Battlestar Galactica, the Fleet has passed through a number of star systems. The planets the Rag Tag Fleet has visited are a reasonable sample of the kinds of worlds we can expect to find as we explore the universe.

  Planets

  Name: The Twelve Colonies

  Type: Terrestrial

  The original Battlestar Galactica made reference to the fact that all of the Twelve Colonies orbit the same star: in the pilot episode, references are made to Cylon attacks of both the inner and outer planets. It strains credibility to expect there to be twelve habitable planets in the Goldilocks Zone of a single star, certainly not a G-type star like Sol or larger F-type stars like nearby Procyon. Hot O- and A-type stars have very wide habitable zones, but as we saw in the sidebar “What about Those Other Stars?” in chapter 16, these types of stars live very short lifetimes, sometimes barely long enough to allow planets to form. No, we’re constrained to base the Twelve Colonies around F-, G-, and K-type stars. Even if we say two habitable planets are in mutual orbit, and that planets span the entire habitable zone, it’s still difficult to think that the Twelve Colonies orbit a common star.

  The beaches of Canceron are burning.

  The plains of Leonis are burning.

  The jungles of Scorpia are burning.

  The pastures of Tauron are burning.

  —Cylon Hybrid, Battlestar Galactica, “The Plan”

  How, then, can twelve human-habitable planets be packed into as close a space as possible to become the Twelve Colonies? The series bible for the reimagined Battlestar Galactica made a statement, with the same implication as in original series, with one major difference: the thirteen tribes traveled far away from Kobol, and eventually twelve of them settled in a star system with twelve planets capable of supporting life.

  Although a lone star with twelve habitable planets in orbit may be a stretch scientifically, the term “star system” implies a small number of stars that are gravitationally bound. We can ask ourselves the broader question “What type of star system, as opposed to a single star, might be home to the Twelve Colonies?”

  Half of the points of light in Earth’s night sky are multiple star systems. While the vast majority of multiple star systems are binary stars—two stars in mutual orbit around their common center of mass—the nearest star to Sol, Alpha Centauri, is a trinary: Alpha Centauri A, Alpha Centauri B, and Proxima Centauri. Polaris, the North Star, is also a trinary system.

  Alternately, perhaps the Twelve Colonies orbit a few different, albeit close, stars—stars that are a fraction of a light-year apart, such as the stars in an open cluster. Open clusters are physica
lly related groups of stars held together by mutual gravitational attraction, typically with only a few hundred to a few thousand stars.

  An open cluster is not, strictly speaking, a “star system,” though. The Twelve Colonies are more likely to be orbiting stars in a multiple star system. Also, based upon what we’ve seen in the series and excerpts from the series bible, the colonies are probably closer than they would be were the planets orbiting stars in an open cluster.

  Lee Adama and Anastasia Dualla.

  Anastasia Dualla.

  The Twelve Colonies existed separately for most of their history, fiercely independent worlds with different cultures and societies. While they were clearly all linked together by heritage, they still found ways to war with each other, and presumably different alliances among the twelve rose and fell over the centuries according to the ebb and flow of history.

  The Cylons were originally simple robots that grew increasingly complex with more and more powerful artificial intelligence. They eventually were used for dangerous work such as mining operations, and then they were used as soldiers in the armies of the Twelve Colonies.