There are three secrets to real estate: Location, location, location.
Whether you're looking to start an interstellar colony, found a galactic empire, or merely find a great location for your next tale of adventure in outer space, it pays to know what the neighborhood is like. That's the plan for this column and the next: to take a look around the neighborhood (the solar neighborhood, that is!) and see what's out there waiting for us. In this column, we'll start with the nearby stars—what and where they are, which ones have had starring roles in science fiction, and what recent discoveries have added to science fact. Next time, we'll zoom in to the rapidly expanding catalog of known planets around nearby stars, and their surprisingly bizarre properties—just the latest example of astrophysical truth proving once again to be stranger than fiction.
"Excuse me?" said Arthur. "Are you trying to tell me that we just stuck out our thumbs and some green bug-eyed monster stuck his head out and said, Hi fellas, hop right in. I can take you as far as the Basingstoke roundabout?"
"Well," said Ford, "the Thumb's an electronic sub-etha signalling device, the roundabout's at Barnard's Star six light years away, but otherwise, that's more or less right."
—Douglas Adams, "The Hitchhiker's Guide to the Galaxy"
Our sun is, as you may know, located in a fairly ordinary spiral galaxy, the Milky Way. Slightly more surprising is the fact that the Milky Way's properties are less well understood than those of many more distant galaxies: from our location within the galaxy, we can see tremendous detail close by, but the overall structure is harder to discern. We know that the Milky Way is a spiral galaxy, about a hundred thousand light years across and containing some 200-400 billion stars, but astronomers still debate fundamental properties such as the number of spiral arms, the presence or absence of a central bar, and even the overall size of the galaxy. At present, NASA's Spitzer Space Telescope has been conducting an ambitious survey of a huge portion of the galactic plane, hoping to answer some of these questions with its power to peer through obscuring dust at infrared wavelengths. An even more ambitious mission planned for next decade, the European Space Agency's GAIA, will map the three-dimensional positions and measure the stellar properties for over one billion stars, providing an unparalleled view of our galaxy and its structure.
But we do not need that comprehensive a map of the whole galaxy to describe the sun's surroundings. The sun is located about a quarter of the way out from the galactic center, about 25,000 light years away. Earlier estimates put that distance between 30 and 35 thousand light years, but better measurements have consistently revised it downward. (This downward trend does not mean the Milky Way is shrinking! In fact, our increasing ability to detect faint stars in galactic outskirts has suggested that the galaxy may be much larger than previously thought, so that 25,000 light years may in fact be much less than a quarter of the way out.) The sun is also off-center in the vertical direction, currently resting about 50 light years above the midplane of the roughly thousand-light-year thick galactic disk. Its orbit carries it wobbling up and down through the midplane many times over the course of its quarter-billion-year cycle around the galactic center.
A common misperception of spiral galaxies is that the regions between spiral arms are really devoid of stars. For instance, in Larry Niven's short story "At the Core," as Beowulf Schaeffer speeds toward the galactic core he avoids colliding with stars by flying down the gap between two spiral arms. In reality, no such trick is possible (not even if you did have a Quantum II Hyperdrive at your disposal!). The bright spirals seen in many galaxies trace only massive hot blue stars, which burn brightly for a scant few million years before exploding as supernovae. More average stars, which spend hundreds of millions or billions of years as middle-aged members of the stellar main sequence, have time to orbit away from their birthplaces, and fill the galactic disk uniformly. In fact, the sun itself is currently located between two of the major spiral arms: the Sagittarius arm lies several thousand light years inward toward the core, while the similarly distant Perseus arm stretches past on the opposite side of the sky. Our home between these two major arms is on the outskirts of a minor "armlet," the Orion Spur, which may be a side-branch of the Perseus arm. A nearby part of that spur forms the Orion molecular cloud complex, the closest major site of star formation some 1600 light years away. Some lower-mass stars form in closer regions, in particular the Taurus molecular cloud, but the Orion nebula is the closest place where high-mass stars are born, such that it would appear bright and blue, truly spiral-arm-like, from a vantage point outside the galaxy. And even the Taurus stellar nursery is 450 light years away—there are no newborn suns nearby. (However, there are a scattered handful of moderately young systems, 10-200 million years old, which have drifted within a hundred light years or so of our position. These systems are top targets for astronomers seeking to understand the processes through which planets are assembled from circumstellar disks around young stars.)
Thus if we look only within our immediate "galactic backyard," we find only middle-aged stars like the sun itself, ranging from one to ten billions years old, plus a scattered handful of more elderly star systems. Developing the definitive catalog of all nearby stars—"nearby" meaning within 25 parsecs (80 light years) or so—is the goal of several ongoing research projects, such as NStars and RECONS. Identifying nearby stars is no easy matter: Because of the tremendous variation in stellar luminosities, the brightest stars in the night sky are often quite distant, massive, and bright, while literally thousands of stars in our own immediate neighborhood hide dimly in the dark. In fact, the closest star, the faint red dwarf Proxima Centauri, is not only invisible to the naked eye, it's fainter than more than a million other stars! Makes finding a needle in a haystack look easy, no? The key to successfully identifying nearby stars is their motion: the sun and its neighbors move with roughly the same velocities as they slowly orbit the galactic center, but small differences from star to star cause their apparent positions to shift over time. The closer a star is, the larger its apparent motion will be. After a star has been identified as a nearby candidate on account of high motion, its distance can then be confirmed through painstaking parallax measurements. By measuring these shifts in stellar positions from year to year for hundreds of thousands of stars, astronomers have carefully sifted some fast-moving needles from the haystack, resulting in an ever-growing catalog of the nearest stars.
"Starfleet reports it has engaged the Borg at Wolf 359, sir."
—Lt. Cdr. Data, ST:TNG, "The Best of Both Worlds"
Wolf 359 is a real place. While Star Trek's Alpha through Delta Quadrant system bears no relation to any present-day astrocartography, many of the individual stars mentioned in the series are real, including the site of the disastrous battle against the Borg. Wolf 359 is a red dwarf sun, spectral type M6 in the standard MK classification. In fact, it is the fifth closest star to the sun, merely 7.8 light years distant. Only Barnard's Star and the three members of Alpha Centauri are closer. Barnard's star, another M dwarf, is named for its discoverer, famed 19th-century astronomer Edward Barnard. He spotted it in 1916 as it zoomed across the sky at the unprecedented rate of ten arcseconds per year, giving it the fastest apparent motion of any star in the entire sky, a title it retains to this day. Proxima Centauri, although closer at only 4.3 parsecs, moves with a velocity more similar to the sun's, and so shows less than half as much annual motion across the sky. The prize for second-fastest apparent motion goes instead to Kapteyn's Star, 13 light years distant but moving in a bizarre high-velocity retrograde orbit. Kapteyn's Star is an interloper from the galactic halo, perhaps even originally from a different galaxy, whose highly elliptical path carries it from the galaxy's outer rim to deep near the core (similar to the home star of the Spiders in Vernor Vinge's A Deepness in the Sky).
These stars are all red dwarfs, stellar class M, with masses about a tenth of the sun's, sizes barely larger than Jupiter, and total light output only one thousandth of solar. Beware astronomers' casualness with language: these stars are "red" only in comparison with hotter stars like the sun. With surface temperatures of 3000-3500 Kelvin, to the eye they would appear similar in color to a typical incandescent light bulb. For a suitably located planet or space habitat, a red dwarf sun could provide very pleasing lighting indeed! (But unrealistically dim and very very red lighting makes for better television, I suppose.)
These red dwarfs, and others like them, make up the bulk of the population of the galaxy. Even Proxima Centauri, the closest of all, is far too faint to be seen with the naked eye, but they are all around us, and far outnumber the more massive stars that shine so brightly: of the 348 stars currently known within 10 parsecs (32 light years), 239 are red dwarfs. It may be tempting for any putative Galactic Empire to concentrate on only solar-type stars (defined loosely as spectral types K, G, and F, say) but ignoring the red dwarfs means giving up on more than two-thirds of the galaxy. Their faintness is such that the present census of red dwarfs is certainly incomplete, even for this closest region of the galaxy. There is a continual stream of newly identified nearby red dwarfs, which has inflated the neighborhood address book by 16 percent in the last few years. Astronomers have recently become able to locate even dimmer objects, the "failed stars" called brown dwarfs, and some think that the ultimate census of these objects may in turn outnumber even the red dwarfs. Because of their dimness, these red and brown dwarfs alike have only "phone number"-style catalog names, like Luyten 789-6 and BD +36 2147.
There are, however, a few handfuls of solar-type stars in the neighborhood, and not surprisingly these have some of the most famous names in science fiction. It is an amazing cosmic coincidence that the nearest star system, Alpha Centauri, contains a star nearly the sun's twin. Alpha Centauri A, 10 percent more massive and about 60 percent brighter than the sun, is only 4.3 light years distant. The next closest star this similar to the sun, Eta Cassiopeiae, is more than 60 light years away. Because it is our nearest neighbor, Alpha Centauri has played host to more stories than perhaps any other star system: from Philip K. Dick's Alphane moon to Larry Niven's Wunderland to Douglas Adam's local Vogon planning office to countless "first spaceflight" tales. At one point, it was even named as the setting of the Transformers' home world, Cybertron. And what a setting it is! Alpha Centauri A is slightly hotter and brighter than the sun, while Alpha Centauri B is cooler and half as luminous. Elliptical orbits carry them as close as 11 AU and as far apart as 35 AU, every 80 years. Because of this, any hypothetical planet around either of these two stars would have to orbit no more than a few AU from its parent star. Imaging searches thus far have failed to turn up any such worlds, despite controversial orbital evidence that Alpha Centauri B might host a massive brown dwarf companion, but present surveys lack the sensitivity to see any possible low-mass planets. From a vantage point on such a world, the second sun would appear a thousand times brighter than the Earth's moon does to us. Proxima Centauri is so dim that it would be only a faint 4.5th magnitude star, completely unremarkable in appearance.
Beyond Alpha Centauri's two suns, the next closest sunlike star is Tau Ceti, 12 light years away. As the closest sunlike star not in a binary system, Tau Ceti's light has illuminated fictional worlds including Asimov's Aurora, Niven's Plateau, Simmons's Tau Ceti Center, Turtledove's Home, Le Guin's Anarres and Ursas, Cherryh's Downbelow Station, and many more. That's one busy star system! Alas, the real Tau Ceti may not be quite such a happening place: it is an old system, 10 billion years or so. Because it was born so long ago, at a time when the universe was poorer in the heavy elements that form planets, Tau Ceti is relatively less likely to contain massive planets, and indeed present radial velocity searches have as yet detected nothing. But all is not lost: Tau Ceti has turned out to be a surprisingly dirty place, with 10 times the amount of dust as our own solar system, indicating a large population of comets and asteroids. The presence of so many small bodies suggests there may be rocky planets as well, after all, but that any such worlds would be battered and crater-heavy.
Slightly closer than Tau Ceti, but also slightly less sunlike, is Epsilon Eridani. "Eps Eri" has also played host to many beloved fictional places, from Asimov's Baleyworld to Babylon 5 (despite some major astrophysical inconsistencies on J. Michael Straczynski's part; perhaps we can blame the Shadows). It even served for a time as Vulcan's sun, though Roddenberry ultimately relocated Vulcan to the nearby 40 Eridani instead. Despite its role as hot Vulcan's temporary home, in reality Epsilon Eri is a somewhat cooler and dimmer star than the sun. Its claim to fame is that it is one of the younger nearby stars, being only about half a billion years old (this youth is what led Roddenberry to relocate Vulcan—not even Spock could evolve that quickly!). Eps Eri is also one of the closest star systems to have a known planet. After many years of controversy, recent Hubble measurements seem to confirm the presence of a 1.5 Jupiter-mass world in a 7 year orbit. Attempts to detect additional planets are ongoing. And if you thought Tau Ceti was dusty, look out for Eps Eri, which has a thousand times more dust than the solar system! The dust is concentrated in a ring some 35 AU in radius, comparable to the size of the Edgeworth-Kuiper comet belt around the sun. The combination of at least one confirmed planet and a whopping huge comet belt suggests that cataclysmic collisions may be common, similar to the Late Heavy Bombardment period early in the solar system's history.
These are just the closest sunlike stars, of course. Within 32 light years (10 parsecs), there are 19 G-type stars such as Alpha Centauri A and Tau Ceti, plus twice that many cooler K stars like Alpha Centauri B and Epsilon Eridani. Compare that to 239 red dwarfs! Even rarer than these sunlike stars are the few more massive stars. Within 32 light years, there are 8 stars of spectral type F, and only 4 of the hot type-A stars. These latter are of course some of the brightest stars in the sky: Sirus, Altair, Fomalhaut, and Vega. Each is unique: Sirius has a tiny but blazingly hot white-dwarf companion. Fomalhaut has a narrow comet belt, lopsidedly offset by the gravitational influence of an unseen planet. Vega spins so rapidly, turning once every 12 hours, that its shape is distorted far out of spherical and its equator is 2500 Kelvin cooler than its poles! (This is unfortunate for astronomers, who long ago chose Vega as the brightness standard against which all other stars were measured, only to find out recently that it's a rapid-rotating oddball.) Rounding out the solar neighborhood is an exotic handful of white dwarfs, 22 within 32 light years, including the companions to Sirius and Procyon. In contrast, the closest neutron star is about 100 light years away, and the closest black hole is 1600 light years distant. The closest that we know of now, anyway—black holes are tricky devils, and space can easily hide surprises, even in our own galactic backyard.
Next time, Settings for Space Opera, Part II: A Perplexing Plethora of Planets!
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