This web page is somewhat of a white paper discussing the astronomical possibilities of life-bearing planets within the 'immediate' stellar neighborhood; that is, within roughly 50 LY (light years) of Sol. The general audience I envision is fans of science fiction, most particularly hard science fiction, though beginning writers (like me) may find this page useful. For those with a background in astronomy, this may be a refresher course.
Primarily, these pages grew out of my own research into a story I wrote about a human colony around another star. I thought I would just pick some well-known star the appropriate distance from Sol from a star chart (the story required a distance of 40 LY, but it wasn't too picky about local conditions other than local plants and fish). It turns out that named stars tend not to be good places for life, or colonies. Named stars have names because they are quite visible in our sky, but in terms of a scientific possibilities for life-supporting planets they turn out to be too young (Vega), too old (Betelgeuse), too big (Capella), too small (Barnard's), too far (Rigel), too hot (Sirius), too cold (Wolf 359), or too many (Castor). My research into local stellar environments, coupled with my lifelong interest in astronomy, made me decide to write an article about what it takes for a star to be capable of hosting life supporting planets, consolidating the research of many that have gone before me (see Bibliography).
For the purposes of life, I am assuming Earth type life, that is organic (carbon based) requiring a lot of water and oxygen. We may speculate about exotic life such as silicon based, but it is harder to predict what the best environment would be as we have yet to run across any examples of it among perhaps 20-100 million species of life on Earth. Earth life has proven to be extraordinarily resilient, from life in 600 degree volcanic water to bacteria accidently sent to the moon on a Surveyor probe and proved alive years later after their return by Apollo astronauts. Because of this, I feel it is not too far-fetched to assume that carbon/water life will take hold given the proper environment. Be aware that this is an extremely debatable subject. However, in science fiction we try to exploit the possible and aren't necessarily trying to win Nobel prizes in exobiology.
The scientific plausability of all of this is further debatable. Many astronomers (or biologists for that matter) refuse to go too far out on a limb to suggest which stars look promising because this science is so far in its infancy, you might as well call it pre-fetal. However, there are quite a lot of theories to suggest which types of stars would be incapable of hosting life, or at least would make it extremely unlikely. Therefore, it is up to the author to decide how far to push current theories to support a story around a particular star. I try to remember the classic editor's rejection of far-fetched science: 'Suspension of disbelief does not mean hanging it by the neck until dead'.
There are three main areas of astronomy that can help narrow down the possibilities of finding a likely star. The first and biggest help, though the most complicated, is the 'Spectral Class' of the star and it's associated trivia. The second is the star's age and lifespan, and the third is the multiple star characteristics of the system.
The Hertzsprung Russell Diagram
The Hertzsprung Russell Diagram is probably familiar to anyone who has taken an astonomy class, or has ever picked up a generic book about astronomy. Here's a version from Jim Smith's Free Virtual Galaxy Project, I'm sure you've seen a similar one:
Here's a quick review. The brighter a star is (usually from an arbitrary distance of 32.6 LY -- 10 parsecs), the higher it is on the chart. Dimmer is near the bottom. The horizontal aspect of the chart is temperature -- hotter stars to the left, cooler stars to the right. Your Astronomy 101 class probably pointed out where the sun lies (and almost all H-R diagrams have a big yellow dot in the middle of the main sequence saying 'We Are Here', just like this diagram does). You were probably taught the spectral classes of stars, O B A F G K M, and probably several mnemonics to help you remember them, from 'Oh Be A Fine Girl Kiss Me' to the classic rebuttal 'Only Boys Accepting Feminism Get Kissed Meaningfully'. Spectral classes are further subdivided into numbers, 0 is warmer than 9, and Sol turns out to be a G2 star. O stars are usually hot/bright, and M stars are usually cool/dim, with exceptions such as white dwarfs like Sirius B (hot, dim) and red supergiants like Betelguese (cool, bright). You were probably also taught, usually with a strange smugness, that our star is a yellow dwarf star with no distinguishing characteristics to make it seem special in any particular way.
It's all coming back, right? There are a few things on this chart that make things interesting for planet hunting, and almost all of it revolves (so to speak) around the mass of the star. Mass is not explicitly plotted on the H-R diagram, but it generally corresponds with the temperature of the star -- all the stars on the main sequence to the left of Sol tend to have more mass and stars to the right tend to have less mass. This is less evident off the main sequence, or at least it doesn't translate directly. For instance, white dwarfs tend to be at the lower left in the OBA areas, but generally have masses near that of Sol, while some red giants (not supergiants) may also be near Sol's mass, though they fall to the right and above.
Because mass doesn't disappear, stars don't generally drift through the main sequence. After the first few percent of a star's age (less than 5 percent), the star generally stablizes and stays at the same general spot on the H-R diagram for up to 90 percent of it's life. Even at the end of Sol's life, when it expands off the main sequence to a red giant, it may still technically be a G (or perhaps low-K) class star based on its temperature, luminosity, and mass. Some variable stars shift slightly, however, and there is a theory that the ice ages may have been caused by small 'wiggles' in solar luminosity (though after seeing the effects of the last El Nino cycle, I tend to agree with the theory that the ice ages were caused by the rising Himalayas shifting the jet stream).
For purposes of likely life sustaining planetary systems, we probably don't want to look too far below the K class stars. At that point, we're looking at a typical mass of half that of Sol's (with 0.2 times the luminosity of Sol), and the M class stars run around a quarter of Sol's mass (with 0.005 times Sol's luminosity). When stars head towards the basement of K class, any warm planets would have to snuggle closer to its host star to stay where water has a good chance at remaining a liquid. This runs the risk of tidally locking the planet so that one face stays towards its star -- which can cause undesirable weather and temperature problems. Additionally, many stars flare, which causes a greater risk of deadly radiation regularly sterilizing close planets.
On the other end of the scale, we probably don't want to look above F class stars. O through A stars have high levels of ultraviolet radiation and tend to have high stellar winds which could strip away an atmosphere. OBA stars tend to rotate rapidly as well, and there are theories that rapidly rotating stars don't generate many planets (or perhaps it's the other way around and OBA stars rapidly rotate because they don't have many planets to slow the stellar rotation).
This implies likely candidates in the F, G, and K classes, with G perhaps as optimum merely through being the center of the range. As it turns out, there are many more stars of low mass than stars with higher mass. For every 100 stars, statistically there are 67 M class stars, 13 K class, 7 G class, 3 F class, and maybe 1 A class (and probably none of the rare O and B class stars). Out of 400 billion stars in the galaxy, this gives us roughly 90 billion FGK candidates, less than twenty-five percent, with seven percent being G type (26 billion).
Spectral classes can be further subdivided by a roman numeral based on size (related to luminosity) called the Yerkes scheme:
Ia Luminous Supergiants Ib Less Luminous Supergiants II Luminous Giants III Normal Giants IV Subgiants V Dwarfs VI Subdwarfs
Generally class V stars are called dwarfs because they are much smaller than classes Iab through IV. However, the term 'dwarf' is statistically a misnomer -- 90 % of stars fall in the main sequence V class and are class F and cooler. Due to problems of tidal locks and flares, we want the V class stars (though we can, perhaps, get away with a small IV class member under ideal conditions). The star Canopus (in the constellation Argo) provides a good example for checking the Yerkes class; Canopus is a 'F0II' star. F0 is perhaps a little toasty for life, but still yellow-white and on the borderline of our acceptable FGK selection. The II, however, labels it as a luminous giant and in fact its diameter is 65 times that of Sol, with a violent corona emitting strong X-rays and radio waves.
There is one more set of miscellaneous data usually prefixed or appended to spectral class:
D Degenerate. A white dwarf, as in Sirius B, type DA2 d usually a red dwarf, as in Proxima Centauri, type dM5e. Yerkes
class V is more modern.
sd subdwarf (Yerkes VI these days)
e strong emissions (indicate a large, thick hot gas cloud or corona surrounding the star) m abnormally abundant heavy elements (usually metals, sometimes non-hydrogen/helium) n nebulous absorption lines (usually due to high stellar rotation) neb nebula spectrum mixed with the star p other peculiar feature (usually abundance of an unexpected element) var variable star wl weak lines (usually old, metal-poor stars)
If one looks at stellar potentials, such as Canopus, Sol may seem like an uninteresting, dim, small, yellow dwarf. However, since spectral class is linked to mass, Sol actually has more mass than the average star. As there are more K and M stars than any other, Sol's G2 status actually makes it brighter than most stars. Given the actual statistics, Sol is actually brighter, bigger, warmer, and rarer than most of the stars in the galaxy.
If we use Earth evolution as a guide, it took roughly 3 billion years for evolution to get out of the single-cell stage once life evolved. It took that long to 'terraform' the atmosphere into one that provided free oxygen, and thus give the potential for more active, multicellular life. It took 540 million years from the 'Cambrian Explosion' of multicelled evolution to Homo sapiens. There were a lot of false starts in this due to great extinctions, some of them extra-terrestrial catastrophies (such as the possible Yucatan impact at the end of the Cretaceous), some were terrestrial in origin (the trilobite extinction may have been caused by planetary weather changes due to mass vulcanism in what is now Siberia). Many other events, from sunspot cycles to continental drift, have affected the evolution of our modestly intelligent species. Perhaps these events were necessary to weed out life that may have restricted or competed with evolved intelligence. Perhaps if the Yucatan impact had not occurred, an intelligent, warm-blooded dinosaur would have evolved 50 million years earlier than intelligent mammals.
In any case, based on Earth, we can estimate roughly 500 million years of multi-cellular evolution to get to humans, plus 3 billion years of single-celled life, plus 500 million to 1 billion years for the Earth to absorb solar system debris and to cool down from formation. We can speculate on variables: Perhaps a star formed from a high oxygen/water nebula could cut out a billion years or two of single-celled terraforming. Perhaps evolution is quicker and more competitive on a planet that's surface is one-third water instead of three-quarters. Unfortunately for the biologist, these guesses are purely theoretical. Fortunately for the science fiction writer, unproven theories don't always hinder story development. From planet formation to intelligent life, call it maybe 3 billion years on the low end, maybe 6 billion on the high end. Non-intelligent prolific life may occur in a shorter period of time, but remember that Earth life was mostly slime, bacteria, and algae until the Cambrian.
Mass affects the density of the star, which affects the fusion rate of the star, which in turn affects the potential age of the star. An A0 star like Vega will probably only live for a billion years. An M4 star like Barnard's may live for 50 billion. A giant O star's life is miniscule; Alnitak (the first star on the left of Orion's belt) is around 6 million years old and has already run out of hydrogen. During the lifetime of Alnitak, humanity has evolved from just before Australopithecus afarensis (Lucy) to Homo sapiens. Alnitak probably wasn't more than a lump in a nebula in the dinosaur era.
The lifetime of a star again limits us to our FGK spectral class for potential evolved life, though it's not out of the question for a non-FGK star to host a potential colony. Science fiction fans of Larry Niven may note that Jinx is a habitable colony around Sirius A. Sirius A is an A1V class star, twice the mass of Sol, doomed to a short lifetime of maybe a billion and a half years, and probably pumping out enough UV radiation to fry any life on a nearby planet. However, Niven's stories establish that Jinx is actually a moon around a gas giant primary. Although it is not mentioned in his stories, the primary may be far enough away from Sirius A so that the UV radiation is low. Gas giants tend to be heat providers; Jinx may be in a water-temperate zone outside of the normal liquid-water range of Sirius A. Furthermore, the local species on Jinx, mainly Bandersnatch, have been established as being genetically engineered lifeforms planted there by another race (Bandersnatch are even immune to genetic radiation damage due to large chromosomes). I don't know how much of this was known when Niven wrote his first Jinx story, but his scenario is possible theoretically.
Which brings us to the question of a star's actual age. How old is Sirius A? If its 100 million years old, then planets probably haven't finished forming (if they're there at all) and the star wouldn't have even started fusing yet when the Slavers were moving Bandersnatch around. On the other hand, if Sirius A is fairly mature, say 1.2 billion years old, things look more possible.
Unfortunately for the astronomer (and also for us, in this case), we really don't have a good way of figuring out the actual age of an individual star (there are exceptions, especially in the case of Sirius). We know that Sol is about 5 billion years old because that's how old the rocks are on Earth and the Moon (and, lately, Mars). There are theories currently being tested: One is that sunspot cycles decline as a star ages. Another theory is trying to determine how much hydrogen has fused from the interior of a star; like Carbon-14 or Uranium testing, this may tell us how long a star has been on the main sequence by calculating the ratio of helium to hydrogen. However, there is no reliable way currently being used to age a star, with the exception of very young stars (like the Pliedes members, which are still in their birth nebulae) or very old stars (like Betelgeuse, which is currently fusing heavy elements, having run out of Hydrogen and Helium).
Multiple Star Systems
The final stellar obstacle to work around is the large presence of multiple stars. In the closest 100 stars to Sol, 23 are members of binary or multiple star systems. Orbital dynamics of planets in these multiple star systems may be tricky for many reasons.
Close multiples may just not have any room for planets. Take Castor, for instance, one of the 'Twin' stars of Gemini and appearing to the naked eye like a bright, hot class A star. When examined closely, Castor looks like three stars -- a pair with a 400 year orbit, surrounded by a faint companion 1000 AU from the inner pair. Spectroscopic analysis actually shows that Castor A is a two star binary orbiting each other in an ellipse about the size of Mercury's orbit, revolving once every 9.2 days. Castor B is also a binary, with closer stars making an orbit every 2.9 days. The faint companion star is also a binary, two M stars that are so close they nearly touch and orbit each other every 20 hours. Castor turns out to be a six star system, one in which planets would probably not last long enough to even finish forming, let alone develop life.
A planet probably cannot hold a orbit around two stars at once; that is an ellipse or circle with two stars within the orbit. The stars orbiting each other just cause too many orbital pertubations for a stable orbit, unless the orbit is tremendously huge and the two center stars extremely close. Figure eight orbits, or other complex round-robin orbits, also probably wouldn't last more than a few thousand years in a realistic environment (a figure eight might be temporarily stable with two stars of identical mass, far apart, and slowly revolving. However, the odds of such a stellar system are bad enough without throwing in a renegade planet).
Far binaries tend to be better from an orbital dynamic point of view. Take Alpha Centauri, our nearest neighbor. This system is actually three stars. Proxima Centauri (Alpha Centauri C) is a dM5e red dwarf over 13 thousand AU from Alpha Centauri A and B, making an orbit once every few hundred thousand years. Due to this distance, Proxima would most likely not interfere with the orbital dynamics of any close-in planets. Alpha Centauri A and B turn out not to be that close, either. They orbit each other with a period of around 80 years in a wide ellipse, coming as close as 11 AU (just past the orbit of Saturn), then receeding to 35 AU (just past Neptune). It turns out that orbits of planets under 3 AU per star may be stable. Since Alpha Centauri A is a G2V (virtually a twin of Sol), and Alpha Centauri B is a cooler K0V, this makes it possible for a stable planet in the liquid-water zone around each star! Imagine how quickly space travel might develop in a system with two habitable planets no farther apart than half our own solar system.
Unfortunately, things may not be so rosy in such a system. In our system, the Kuiper belt of asteroids and dust starts just past Pluto (or, perhaps Pluto and Charon are the biggest Kuiper objects, in which case the belt starts just past Neptune). There is also the Oort cloud of comets starting at around 10 thousand AU from Sol, extending perhaps all the way out to 100 thousand AU. If the Centauri system were similar to ours, Alpha Centauri A and B would start drifting near their Kuiper belt every 80 years and Proxima would be sweeping through the suburbs of the Centauri Oort cloud. The number of comets and asteroids buzzing through that system must be enormous, and may regularly impact any planets (would this make life more likely or less?). Or, perhaps the system may have been swept clean pretty early in its lifetime, though there seems to be evidence that Proxima Centauri has a super-Jovian companion planet due to orbital wobbles.
What the Neighborhood Looks Like
Using this data, which nearest stars might support life, if not actually generate it? Let's hit the closest 25 stars, in order, heading away from Sol.
Personally, I lean against the Centauri system for intelligent life, though I like it for multicellular if it's old enough. It would be a great place to get our feet wet with a colony or two. I would guess that the Centauri system might have too many asteroid and comet impacts to generate any lasting civilization (though this is just my gut instinct and probably wouldn't stop me from writing a story about it). The views, however, would be spectacular -- two suns and maybe yearly or monthly comets.
The next two stars, Barnard's (M5V) and Wolf 359 (the M6 system of Star Trek Federation vs. Borg fame) are M type reds, which I tend not to like for life of any kind.
Gliese 411, also known as UV Ceti, (M2Ve, probably a double) seems a close borderline to K, but it has this unfortunate habit of flaring more than several times it's brightness in a matter of seconds.
Next is Gliese 65, which are two identical 'dM5.5e' red dwarfs that would only warm the heart of an astrophysicist.
Sirius is next, probably too young for life and it's too hot anyway. If life had made a start, I would guess that the entire system had been sterilized when Sirius B collapsed.
Two more red dwarfs follow, Gliese 729 (dM4.5e) and Ross 248 (dM6e). A whole lotta nothing with either of them.
Epsilon Eridani (K2V, about 10.7 LY out) is one of those interesting borderline stars. It's a cool star, and one that flames a lot, so things don't generally look good for the gonads. However, there is evidence in the star's wobble that it has at least one large planet around it, and infrared tests have found a dust ring. Epsilon Eridani may be a young star just in the beginning stages of forming planets. If the flares calm down, this may be a spot to watch (in a billion years or so). However, if I had a choice on whether or not to send a probe here, I'd roll the dice and send one out.
Next in line are Ross 128 (dM4.5), Gliese 866 (a possible double M5e), and GX Andromeda (a double, M2V and M6Ve). All cool reds, probably fairly sterile.
Epsilon Indus is next, a K5Ve (possibly a mildly warmer K4Ve) about 11.3 LY out. I couldn't find much information either way about this star. It's a little chilly, but it might be older and a better candidate than Epsilon Eridani.
61 Cygni is a double K class (K5Ve and K7Ve) also 11.3 LY away. They are far apart enough (a mean of 84 AU with a 653 year orbit) that either might have several planets. 61 Cygni A might be a flaring variable, and that's the warmer one of the pair. 61 Cygni B might be inviting, however, and they're close enough that it might be close enough to try a two-for-one.
After 61 Cygni is Gliese 71, a double red dwarf (dM4 and dM5) of no obvious interest.
We may hit the jackpot with Tau Ceti, a G8Vp star 11.4 LY away. This star is slightly cooler than Sol, but still a G type star. Unlike Alpha Centauri A, Tau Ceti isn't a member of a multiple. If Tau Ceti is old enough, this is probably the best place in the nearest 50 stars to look for life or find a nice, comfortable planet to settle on.
Procyon is the last out of the 25 nearest stars and it appears to be a cooler cousin of Sirius. At F5IV-V, it's warm and big, but in our range of likely stars. Procyon, like Sirius, has a white dwarf companion star with an orbit of almost 41 years. The mean distance of the stars is 14 AU, however, which is fairly close -- any planets might have to be fairly close to Procyon A, which may make them too hot for life.
One of the nice things about the web is that it is easier to create 'living' documents that may change as time goes on. In this section, I'll add links to new theories about planet formation or life that might fit in with the theme of this document.
Here Come the Suns -- (added 19 Mar 00) Scientific American article. We've found almost two dozen planets orbiting other stars by now, and the similarities between the stars they orbit are interesting: The stars tend to be metal rich (metals or heavy elements, to an astronomer, is any element other than H or He). Note Tau Ceti above -- as a G8Vp, the 'p' stands for a peculiarly large amount of an element (Calcium, if I remember my research). Does this make Tau Ceti an even more likely candidate for planets? A corollary -- metal rich stars tend to be near the core of the galaxy. A metal rich star has to be formed from the supernova remnants of an earlier, dead, star. The optimum distance might mean out a ways from the galactic core (high metals, but also a lot of bombarding junk and frequent supernovae), yet not too far out (where there aren't many older stellar graveyards). Like the 'water zone' of the solar system, a zone where life is likely due to the probability of liquid water, the galaxy may have a 'planet zone' of stars where the amount of metals in the nebulae stars are formed from reaches optimum. This 'planet zone' may be fairly narrow, and the Sun may be right in the center of it.
Stellar Database -- (added 19 Mar 00) Stellar database. Alpha Centauri has 150% the metal content of Sol, which may be a good sign for planet formation. It's age may also be 4.2 billion years, which would be good for multicellular life. In Earth evolution, 4.2 billion years might range from the Cambrian Explosion to around the time of the first land animals (bugs, mostly, like millipedes), but before trees and even ferns -- plant life on land is waxy algae. Neat database -- has a lot of information for close stars, and annotations as to where the information comes from.
Chromospheric Age Dependence -- (added 19 Mar 00) Astrophysical Journal article. Some serious Hard Science: How does one measure the age of a star? This is a Journal publication of a procedure tested by Don C. Barry of USC. It is a dense academic article, you've been warned. The theory is that stars slow down their rotations as they age, and eject more mass into their solar wind. Errors in age can sometimes be up to 20% for mid-life stars, but it might give a good ballpark estimate. The paper calculates the ages of the 115 F and G Dwarf stars in known clusters, though the catalog it uses to name the stars seems to escape me (all stars are refered to by a number that isn't the Gleise number or any other catalog I recognize -- it may be the Woolley catalog). Interesting tidbit: In the observed stars, there was a burst of star formation ending around 4 billion years ago, and another burst beginning 400 million years ago. Oddly, there aren't many stars in this group that are between 0.5 billion and 4 billion years old.
3-D Starmaps An utterly cool set of pages containing star databases and pictures of the local stellar neighborhood. Some maps are in 3-D if you have those funky red-blue 3-D glasses. You can get the unedited Gliese catalog here, which is a database of the closest several thousand stars and their spectral types.
Bibliography (for the main paper)
Stars Gives a history of one star per week, usually mentioning size, class, and other interesting trivia.
Stars of the Milky Way A page about spectral types of stars.
The Zeta Reticuli Incident Okay, on the surface, this starts as a discussion about Betty and Barney Hill (you remember, they claim they got abducted by grey aliens back in the 60's) and explores the possibility that the abducting Greys come from Zeta Reticuli based on a star chart Betty Hill dragged out of her mind during hypnosis. The page quickly degenerates into the reprint of the Astronomy Magazine flame war between Carl Sagan and Steven Soter vs. a host of others who tried to be more 'open minded' about the incident. (IMHO, both sides are fairly rigid blinder-wearing extremists). In the flotsam, however, are interesting theories about what it takes for stars to contain planets, and the possibilities of planets in multiple star systems. It also demonstrates how much in its infancy this 'science' is.
The Classification of Stellar Spectra A page that discusses the lesser-known aspects of spectral types, especially stars that don't quite fit the H-R diagram.
Hertzsprung-Russell Diagram University of Oregon Atronomy department class notes about H-R and spectral types.
Astronomy103:H-R Diagram Ditto, but this is George Mason University's.
Planets around Binary Stars A NASA 'Ask the Scientist' question and answer about planets around multiple stars.
Centaurus A discussion of the constellation Centaurus and stars within. Interesting info about Alpha Centauri.
Spectroscopic Binary Catalogue If you want to get down-and-dirty in the details about the known binary/multiple stars (such as orbital period, semi-major axes, etc...), then this is the database for you.
Hipparcos FAQ Hipparcos was a probe the Europeans sent out into the middle of the solar system to look at stars. Like the Gliese catalogue, it came back with a database of thousands of stars with probable distances and spectral classes. Unlike the Gliese catalogue, it limited itself to stars above a certain magnitude. This means it possibly missed very close, but dim stars.
Hipparcos Closest Stars Here's a list of the closest stars in the Hiparcos Catalogue. You can see it missed some I used in the essay above -- that's because I used the Gliese catalogue, which includes dim stars.
Alpha Centauri More Alpha Centauri data, supplied by SciFi writer Robert J. Sawyer.
Nearby Stars Another list of the closest stars. I think this one is Gliese based.
Free Virtual Galaxy Project More H-R discussion, and I stole the H-R GIF from this site.
Scientific American's Ask the Expert A page about how to tell how old a star is.
Sky & Telescope's Spectral Classes Further discussion about H-R and spectral classes.
Vulcan's Sun Okay, this is indeed a discussion about which star Spock is from, but it's from Sky & Telescope and it discusses the possibility of planets around multiple star systems.
Nearby Stars This is actually part of a group of pages about Biblical interpretation, but it mentions an interesting puzzle about Sirius being described 2000 years ago as a red star.
White Dwarfs A discussion about the end-life of Main-sequence stars, from Ohio State University
The Dogon and Sirius From the Skeptic's Dictionary, a discussion about the Dogon tribe of Africa and Sirius.
Extra-Solar Planets The Darwin Mission's list of planets discovered around nearby stars (usually either dust rings, like Vega, or super-jovians like Bernards).
(Update 20 March 2000 -- fixed some things pointed out to me concerning dwarf and subdwarf designations.)
(Update 19 March 2000 -- fixed some broken links, a mis-label of Canopus, changed some wording that bugged me, added theory section)
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