Local Stars

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    A page that will help you do your research regarding our neighborhood in space in case you want to make a sci-fi work.

    Let's start with the stars within 50 light-years of The Solar System. Many of these stars are red dwarves; only the red dvarves closest to Sol (less than 12 ly) are listed here. Except Gliese 581, which is important.

    Proxima Centauri (4.22 light years away)

    Located in Centaurus, this is the closest star to us. A member of the triple system of Alpha Centauri, but far enough from the two larger members (about 0.21 ly) to be considered a separate star system for space travellers. Despite being the closest star to the Solar system, it (like all red dwarfs) is too dim to be seen with the naked eye from Earth. From a planet orbiting Alpha Centauri A or B, it would appear as an unremarkable fifth-magnitude star.

    A red dwarf with a flare tendency, it's a nice place to visit, but you wouldn't want to live there.

    • Appears in Elite 2. It's got the only base in the Alpha Centauri system and it's a stupid distance (almost 10,000 AU) from where you jump into.
    • In the obscure hex-map wargame Cerberus, humanity tries to colonize a world in the Proxima Centauri system -- which is already inhabited by aliens from Tau Ceti.
    • In the Polish novel The Magellanic Cloud by Stanislaw Lem, humans visit the Alpha Centauri system in a slower-than-light starship and discover signs of organic life on a planet orbiting Proxima Centauri. They suspect that life may have been transplanted there from yet another planet.

    Alpha Centauri (aka Rigel Kentaurus or Toliman, 4.35 ly away)

    Third brightest in the sky (although you can only see it from Miami, Brownsville or Hawaii if you're in the USA), this pair of yellow stars ("A", a very Sun-like G2V and "B", a somewhat dimmer K0V) is part of the Alpha-Proxima trio.

    B orbits A (or vice versa) in an elliptical orbit that would roughly range between Saturn and Uranus in our solar system. With enough room for close-in planetary systems around each star, you'd have a decent chance of getting an Earth-like planet there but NASA haven't found any yet -- though Alpha Centauri A and B are #1 and 2 on the list for their planned Terrestrial Planet Finder mission.

    It's not entirely surprising that no planets have been detected around either of these stars yet. The two means we currently have of detecting extrasolar planets -- doppler wobble, and eclipses -- both work best for orbits viewed edge-on. As seen from the Earth, the plane in which A and B orbit each other is inclined such that we're looking almost straight down onto it, the very opposite of edge-on. Since any planets would have formed out of the same condensing cloud of gas and dust that the stars formed out of, they'd almost certainly be orbiting in the same plane. Furthermore, we've pretty much confirmed that neither star of this pair has a debris disk orbiting around it like Vega does, which is actually a good sign that planets might exist (since forming planets would have swept up any orbiting debris near their orbit).

    From Alpha Centauri, the Sun would appear as the brightest star in Cassiopeia. (At around magnitude 0.86, it would be the brightest in that constellation by far.)

    Even though the Alpha Centauri duo may be the nearest interesting stars to us, the distance to be covered to reach them is still immense. If Neptune were a 3 km bike ride away from the Sun, reaching Alpha Centauri would be a >27000 km grand marathon in which you bike over half way around the Earth. You may think it's a long way down the road to the chemist, but that's just peanuts to space...

    • Setting of, naturally, Sid Meier's Alpha Centauri.
    • In the movie Avatar, Pandora is a moon of the gas giant Polyphemus, which orbits Alpha Centauri A. However, if there really was a giant the size of Polyphemus there, its gravitational tug on Alpha Centauri A should have been detected by now. At best, there may be terrestrial worlds we cannot yet see, but not giants.
    • In The Hitchhiker's Guide to the Galaxy, there was a time in the ancient past when men were real men, women were real women, and small furry creatures from Alpha Centauri were real small furry creatures from Alpha Centauri.
    • In Star Trek: The Original Series, the inventor of the space warp is known as "Zephram Cochrane of Alpha Centauri."
      • The Franz Joseph Star Fleet Technical Manual, one of the few canonical Trek supplements published in the 1970s, shows a flag for Alpha Centauri that features a centaur.
    • In the Starfire universe, Alpha Centauri is the only star system connected to Sol via a warp point, but is itself a major junction with no less than seven other warp points leading to other star systems. (It also has one "closed" warp point, i.e. a warp point that's only detectable from the other end -- a fact which the Arachnids use to devastating effect in the Fourth Interstellar War.)
    • In The Pentagon War, the 3rd planet orbiting Alpha Centauri A is home to a spacefaring species called (originally enough) Centaurians. The 2nd planet orbitind Alpha Centauri B is also inhabited, but it's a Waterworld, and life there hasn't gotten past the trilobite stage.
    • In Larry Niven's Known Space, the human colony planet Wunderland orbits Alpha Centauri A at a distance of 1.32 A.U.. It eventually gets taken over by the Kzinti.
    • In Lost in Space, the Robinson family's original destination was Alpha Centauri.
    • In the Transformers universe, Cybertron originally orbited Alpha Centauri, but was thrown out of the star system and wandered through interstellar space for some millions of years. It's not clear whether it orbited A or B close in, or whether it orbited the A-B pair from a long distance away.
    • In Charles Pellegrino's The Killing Star, the species that nearly wipes out humanity sends their attack from Alpha Centauri. We end up calling them "Alphans."
    • In the Polish novel The Magellanic Cloud by Stanislaw Lem, humans visit Alpha Centauri in a slower-than-light starship and discover a planet there with an advanced civilization on it.

    Barnard's Star (just under 6 ly)

    Small red dwarf, known for its rapid motion across the sky (relatively speaking - it takes about 180 years for it to move the width of the Moon in Earth's sky). In 1998, it was observed to emit a bright flare, making it a flare star and giving it the variable star designation V2500 Ophiuchi.

    Its rapid motion across the sky has given this star many names, including "Barnard's Runaway Star", "Greyhound of the Skies", and "Velox Barnardi". The fact that it's the closest star to the Sun in the constellation Ophiuchus has also given it the name "Proxima Ophiuchi."

    Thought for a while to have planets, still a possibility, which has led to Barnard's Star becoming a popular first destination for interstellar travel. However, Barnard's Star belongs to the older "Population II", which have very low concentrations of heavy elements. If there are planets around Barnard's Star, they're probably balls of hydrogen and helium, not rock and metal.

    In The Seventies, the British Interplanetary Society designed a fusion-powered interstellar rocket called Daedalus. (This rocket could not be built with the technology of the time, and still can't, but still stands on the drawing board in case efficient controlled nuclear fusion ever becomes a reality.) Barnard's Star was chosen as the destination, because the trip could be made in 50 years, so young folk at the time Daedalus was launched would have a good chance to still be alive when it reached its destination.

    Wolf 359 (7.7 ly)

    A red dwarf with flare tendencies in Leo, it is not visible to the naked eye. At 0.09 solar masses, it's pretty close to the lower limit for actual stars.

    • In the Star Trek: The Next Generation episode "The Best of Both Worlds," served as the location for the Battle of Wolf 359, in which most of the Federation Star Fleet's forces were destroyed in a futile attempt to stop a Borg cube.
    • Wolf 359 appears by its Variable Star designation ("CN Leonis") in The Pentagon War. (There, CN Leonis II is a world colonized by aliens from Alpha Centauri, who declared independence from their homeworld.)
    • There was an episode of The Outer Limits titled "Wolf 359".
    • The Moon Dagger, the third level in Terminal Velocity, orbits Wolf 359.
    • Wolf 359 is also the destination of the first Node jumps conducted by humans in Sword of the Stars, seeing as that's the nearest star to have a node connection to Sol.

    Lalande 21185 (8.3 ly)

    Another flaring red dwarf, this one in Ursa Major. It's flares are very mild compared with other flare stars, and as such it has not received an official variable star designation. Lalande 21185 may or may not have planets -- probably not.

    • In Gregory Benford's Across the Sea of Suns, Lalande 21185 was home to the synchronously-rotating planet Isis, on which lived aliens that were biological radio telescopes.

    Sirius aka the Dog Star (8.6 ly)

    A binary system, with a white main star (A0V) and a white dwarf secondary. The primary star is the brightest star in the night sky and the Canis Major of Canis Major, so to speak. As such, it played a big role in ancient calendars. Urban Legend attaches this to the "dog days" of late August/September - the Greeks thought that when Sirius was close to the sun in summer, it added its light and heat to the body.

    Sirius B's orbit is highly elliptical, coming closer to Sirius A at periastron than the Saturn-Sun distance, then winding up farther away at apapstron than the Neptune-Sun distance. It takes 50 years to complete one orbit.

    Neither star probably has planets, and if either of them does, habitable ones are unlikely. Any planet far enough away from Sirius A not to be toasted would pass close enough to Sirius B to get thrown out of the system. Any planet orbiting Sirius B would have been burned to a crisp when Sirius B went through its Red Giant stage in eons past.

    • In Doctor Who, Sirius becomes an independent dominion of the First Human Empire and is the location of Androzani Major and Minor.
    • V has the visitors come from Sirius.
    • Home to the Sirius Cybernetics Corporation in The Hitchhiker's Guide to the Galaxy.
    • Dogsbody stars (no pun intended) Sirius.
    • In Isaac Asimov's Foundation series, Earth's sun is in the "Sirius sector", since Sirius is the brightest star in our neck of the woods.
    • In The Pentagon War, Sirius A IV is a colony world that declared independence from Sol, and now calls itself "America".
    • In Voices of a Distant Star Sirius A has a habitable planet, Agharta. The deciding battle takes place nearby.


    Luyten 726-8 (8.58 ly)

    A pair of red dwarf flare stars. Their Variable Star designations are "UV Ceti" and "BL Ceti". They orbit each other once every 26-and-a-half years, ranging in separation from a little over the Mars-Sun distance to nearly the Saturn-Sun distance.

    UV Ceti, the smaller of the two stars, is the most violent flare star known. Its name, in fact, is used by astronomers as the prototype for the flare star class:

    Astronomer #1: "Hey, Charlie! What kind of variable star is V1216 Sagitarii?"
    Astronomer #2: "Um ... it's a UV Ceti type variable."
    Astronomer #1: "Oh, another flare star! Thanks, Charlie!"

    From either of these stars, the Sun would appear as a 1st magnitude star near Arcturus. Sirius would appear in the constellation of Sextans, Procyon would be in Leo, and Alpha Centauri would be next to Spica in Virgo.

    • Was the site of the first Phased Antimatter Bomb test in The Pentagon War.
    • In Larry Niven's Known Space novel A Gift from Earth, a gas envelope around this pair of stars was used by unmanned Ramscoops to brake and/or collect fuel en route between Sol and Tau Ceti.

    WISE 1541-2250 (~9.3 ly)

    Currently (January 2012) the brown dwarf closest to the Sun, with a temperature of about 350 K (less than cooking water). As the search has started only recently, even closer brown dwarfs might exist.

    Ross 154 (9.68 ly)

    Yet another red dwarf that flares. Most of the time, though, you'll need a big telescope to see it. Sometimes called by its lesser-known variable star designation, V1216 Sagitarii.

    • Frontier: Elite II has Ross 154 as a starting point.

    Ross 248 aka HH Andromedae (10.33 ly)

    It's red. It flares.

    Epsilon Eridani (10.5 ly)

    An orange (K1V) star and among the closest systems likely to have a habitable planet. (The only closer one is Alpha Centauri). There may be a brown dwarf or a very dim red dwarf orbiting it widely. May have planets – a lot of astronomical attention in that field looks over here, as do searches for intelligent alien life of the "point a radio telescope at it and see if we hear anything" variety. NASA has marked them as #9 for their planned Terrestrial Planet Finder mission.

    While there are probably planets in the habitable zone here, finding life "as we know it" is improbable, as the entire system is far too young.[1]

    • Babylon 5 can be found here.
    • Part of the Solarian League in the Honor Harrington novels, and the site of a horrific slaughter via Colony Drop that prompted the creation of the Eridani Edict.
    • Yellowstone and its orbiting Glitter Band, both important colonies in Alastair Reynolds' Revelation Space universe, are here.
    • In the Halo universe, the planet Reach is located here.

    Ross 128 (10.9 ly)

    Yet another red dwarf flare star. (Variable star designation: FL Virginis.)

    • In Gregory Benford's Across the Sea of Suns, Ross 128 was home to a gas giant planet, around which orbited a moon called Pocks that housed amphibious aliens.
    • In Free Space, The first point of contact between the Terrans and the Shivans takes place in this system.

    Procyon aka Alpha Canis Minoris (11.41 ly)

    A white (F6IV) star with a white dwarf companion, its name comes from the Greek prokyon, meaning "before the dog" as it "precedes" Sirius when "travelling across the sky". It's a subgiant star that's starting to run out of gas.

    Given the A star's intrinsic brightness, its comfort zone would be at least 2.7 A.U. away based on visible light output alone; the actual comfort zone distance is probably closer to 3 A.U.. Unfortunately, at closest approach, A and B are only 9.5 A.U. apart, meaning any planet orbiting more than 2.4 A.U. away from Procyon A would be thrown out of the system by the other star. Procyon B, on the other hand, is a white dwarf, and any planets it could have are all fried to a crisp when it was a red giant and then frozen. Thus, if Procyon A or Procyon B has any planets, they're probably not going to be habitable. (Planets orbiting Procyon A might have been habitable back when the star was in the middle of its main sequence lifetime, when it would have been dimmer.)

    • In Larry Niven's Known Space, Procyon A is home to the planet We Made It.
    • In the Justice League comics, Manhunters (the robots built by the Guardians, not the Martian Manhunter) come from the planet Orinda in the Procyon system.
    • The Andorians in Star Trek hail from a moon of a gas giant named Procyon VIII.
    • Treasure Planet mentions the Procyon Armada. There was a video game based on the movie named Treasure Planet: Battle at Procyon which prominently features the Procyon Empire -- but, oddly, there's no actual battle at Procyon.

    61 Cygni aka Bessel's Star, or Piazzi's Flying Star (11.4 ly)

    A pair of orange K stars, visible but not noticeable. These are among the coolest main sequence stars that are still visible to the naked eye, being barely heavier and hotter than red dwarfs. Moving rapidly, relatively speaking. No planets or brown dwarfs so far detected. 11.4 light years away, and the first star other than the Sun to have its distance estimated (they got 10.4 light years - not bad for a first try!). Appears a fair bit in fiction.

    • The only unsurveyed bit of the Federation near Earth in Blakes Seven- the locals are hostile and released a virus on base one time.
    • And speaking of The Federation, 61 Cygni is also home to the Tellarites in Star Trek.
    • In Isaac Asimov's Foundation series, they've forgotten which world humans originally came from. 61 Cygni is suggested as a possible site for the origin of humanity.
    • Planet Crythania, the second level of Terminal Velocity, orbits 61 Cygni.

    Groombridge 34 (11.7 ly)

    A binary pair of red dwarfs, both of which are flare stars. (They carry the Variable Star designations "GX Andromedae" and "GQ Andromedae".) Their orbit is very wide -- at closest approach, they're over 100 A.U. apart, and it takes nearly three millennia for them to go 'round each other once.

    • Was the site of at least one battle in Space: Above and Beyond. It wasn't portrayed as a binary star system in that series, though.

    Epsilon Indi (11.8 ly)

    An orange star, quite bright for its class (K2). Notable in that it has a binary pair of magenta-ish T2 brown dwarfs as companions. The binary pair orbits Epsilon Indi with a wide separation (over 1000 AU) and itself is fairly close (2,5 AU). Like Epsilon Eridani, this is a very young system, less than a quarter the Sun's age.


    Tau Ceti (11.9 ly)

    The closest solitary Sun-like star to us, so appears a lot in fiction, and is #3 on the list for NASA's planned Terrestrial Planet Finder mission. Can be seen in the northern sky as a third-magnitude star – it's clearly visible but you'll probably only notice it specifically if you're looking for it. It's only about half as luminous as the Sun, despite being in the same spectral class - Tau Ceti is a G9V, Sol is a G2V.

    40 Eridani, aka Omicron2 Eridani or Keid (16.5 ly)

    Triple system with a main orange (K1V), a white dwarf (DA) and red dwarf (M5V). #10 on the list for NASA's planned Terrestrial Planet Finder mission.

    Altair aka Alpha Aquilae (16.8 ly)

    A class A7 blue-white main sequence star, the brightest in Aquila, twelfth brightest in the night sky. Name is an abbreviation of an Arabic phrase meaning "the flying eagle". It spins so fast (once every nine hours) it's noticeably egg-shaped. Altair is unusually bright for its temperature, suggesting that it may be a subgiant star, about to stop hydrogen fusion and begin expanding into a red giant. Despite this, it is much younger than the Sun, perhaps 900 million years old at most.

    Eta Cassiopeiae (19.42 ly)

    A yellow G class, very similar to our Sun. Has an orange K7V companion, Eta Cassiopeiae B, which is a likely candidate for Earthlike planets, too. The distance between the two stars is 77 au, which is more than enough for both stars to have full planet systems with terrestrials and gas giants. Larger-than-Jupiter gas giants and brown dwarfs, though, are highly unlikely, since they would be already detected. #4 on the list for NASA's planned Terrestrial Planet Finder mission.

    82 Eridani (19.77 ly)

    A class G8V star in Eridanus, visible but inconspicuous to the naked eye. It's not widely featured in fiction, however, it is rated as being fairly likely to support habitable planets.

    • In Poul Anderson's Orbit Unlimited, 82 Eridani was home to the (barely) habitable world Rustum. It was colonized by refugees for whom Earth had grown too authoritarian.

    Delta Pavonis (19.92 ly)

    A class G subgiant star, the fourth-brightest in the constellation Pavo (the Peacock). It's at the end of its lifespan as a main-sequence star, but is in many other ways similar to the Sun; it might be useful to think of it as the Sun's semi-identical older brother. It about to stop fusing hydrogen and is in the process of becoming a red giant, so any habitable planet nearby is about to get fried.

    • Caladan in Dune is stated to orbit Delta Pavonis.
    • The mystery planet Resurgam and the neutron star Hades are in the Delta Pavonis system in Alastair Reynolds' Revelation Space.
    • In the Transformers universe, Delta Pavonis IV is home to humanoid cats who, thanks to being hypnotized by the Quintessons, attack a neigboring planet of humanoid dogs.

    Gliese 581 (20.3 ly)

    A Class M3V red dwarf star in Libra. Although it's technically a flare star (variable star designation: HO Librae), it's a very mild one; a flare event increases the star's brightness by only about a hundredth of a magnitude.

    It's the closest star known to have a solar system -- at least six planets, ranging from 1.7 Earth masses (Gliese 581 e, the smallest known exoplanet) to 15.6-30.4 Earth masses (Gliese 581 b). Gliese 581 d and g are the most interesting -- at 5.6 and 3.1 Earth masses respectively, they are probably rocky planets with 581 g orbiting entirely within Gliese 581's habitable zone (581 d is the local equivalent of Mars: close but not close enough). Gliese 581 g is believed to be tidally locked with its star. Two messages have been sent in Gliese 581's general direction; the first will arrive in 2029, with any response arriving no sooner than 2049 (assuming its hypothetical inhabitants don't have Faster-Than-Light Travel).

    Beta Hydri (24.33 ly)

    Another yellow subgiant (G2IV), similar to Delta Pavonis. A large, four times the mass of Jupiter, gas giant is suspected in a roughly 8 AU orbit. In our skies looks like the brightest star near the South Pole. It's #5 on NASA's Terrestrial Planet Finder mission.

    • In Stellvia of the Universe, a star "Hydrus Beta" goes supernova. It is described very similar to Beta Hydri.
      • Do note that in reality, Beta Hydri is not nearly massive enough to go supernova - supernova progenitor stars are generally class B or O when on the main sequence, while this one was G or F.

    Vega (25.3 ly)

    Also known as Alpha Lyrae. A white A0V type similar to Sirius, spinning fast enough to be bulged at the center. Possesses a debris disk, like a large asteroid belt, but no actual planets known of so far. Was the pole star as seen from Earth 14000 years ago and will be the pole star again in about 11000 years.

    • Was the location of the alien radio transmitter and part of the wormhole network in Contact, both book and film.

    Fomalhaut (25.7 ly)

    Also known as Alpha Piscis Austrini. A class A3 main-sequence star similar to Sirius or Vega, with a pronounced dust disk. Its name means "Mouth of the Whale/Fish" in Arabic.[2] It has at least one planet, which was the first to be detected by direct imaging, and the first discovered around an A-class star.

    Pi3 Orionis (26.3 ly)

    A yellow-white (F6) star in Orion, three times as bright as the Sun. Some kind of substellar companion is detected, which could be a large gas giant planet (or several) or a brown dwarf, with an approximate semimajor axis of 5.2 AU. This star is largely forgotten by science fiction, but scores a #7 on the NASA Terrestrial Planet Finder list.

    Chara aka Beta Canum Venaticorum (27.7 ly)

    A solar analog yellow dwarf (G0V or F9.5V depending on who you ask), and the second brightest star in the dim constellation Canes Venatici (the hunting dogs). It is a very promising candidate for the presence of Earth-like planets. It's been mentioned as one of the best stars to search for signs of life, as well.

    Somewhat like the Sun, its nearby stellar neighborhood contains several other stars which may be potential abodes for life. Most prominent among these is its near neighbor Beta Comae Berenices, which is about 5-8 light-years away. (As the two stars are similar in distance from the Sun, they appear a good distance apart in Earth's sky, though still in the same general area.)

    Beta Comae Berenices (29.9 ly)

    A yellow dwarf star in the constellation Coma Berenices, which, despite its "beta" designation, is the brightest star in that constellation. It's an F9 or G0 star, just slightly younger than the Sun, rated as having pretty good odds of having an Earth-like or Mars-like planet in the habitable zone. It's a little bit hotter and whiter than the Sun. There don't seem to be any stellar-mass companions or closely orbiting gas giants, which bodes well for the presence of Earth-like planets.

    While it hasn't made much appearance in widely published fiction, it's not infrequently featured in web original works recently.

    Beta Virginis (35.6 ly)

    A sunlike star (class F9 V, metal-rich) with no orbiting companion star. Also known as "Zavijava" and "Alaraph." Sadly, no planets have (yet) been detected orbiting it.

    Arcturus aka Alpha Boötis (36.5 ly)

    A bright late K-class red giant which is the fourth brightest star in the sky behind Sirius, Canopus and Alpha Centauri. It's the third brightest single star, however. (Alpha Centauri's two components would be individually quite a bit dimmer than Arcturus, but they're close enough together to be perceived as a single star.) It's hot for a red giant, but quite a bit cooler than the Sun. As a red giant, Arcturus has probably destroyed or ejected any habitable planets it once had, but again the fact that it's a Population II low-metal[3] star means they likely weren't chock full of the building blocks of life anyway.

    • Many mentions in The Hitchhiker's Guide to the Galaxy. In particular, Arcturan Mega-Gin is a major ingredient in a Pan Galactic Gargle Blaster. Presumably it's carried aboard Arcturan Mega-Freighters. Also home to a variety of deadly megafauna, including the Arcturan Mega-Donkey, Mega-Leech, Mega-Voidwhale, Mega-Gnat, Mega-Elephant, Mega-Puppy and Mega-Camel.

    Zeta Reticuli (39.8 ly)

    A wide binary star in the constellation Reticulum (the Net - it's supposed to be the targeting recticule for a telescope). Reputedly the home solar system of The Greys, thanks in part to the account of Betty and Barney Hill. As the system consists of two Sun-like main sequence stars (G2V/G5V) orbiting each other very widely (almost 4000 AU), it is possible that the system harbors habitable planets - potentially around both stars. A spacefaring civilization seems unlikely, albeit not impossible – current means of detecting interstellar radio signals could very likely not detect Earth at a distance of 10 ly, much less the nearly 40 ly to Zeta Reticuli. Oddly, both stars are somewhat dimmer than expected for their mass and temperature, possibly due to their fairly low metallicity.

    • In the first Alien movie, the moon where they encounter the xenomorph orbits a gas giant that orbits Zeta2 Reticuli.
    • The homeworld of the Chigs in Space: Above and Beyond orbits one of these two stars.

    55 Cancri, a.k.a. Rho1 Cancri (41 ly)

    A binary dwarf star in Cancer; the primary star, 55 Cancri A, is a class G8V yellow dwarf, and notable for having the second-biggest known solar system (after ours, of course), with 5 planets. 55 Cancri f is the most interesting of these, as it orbits entirely within 55 Cancri A's habitable zone (in fact, it is the first planet discovered to do so). 55 Cancri f is itself a gas giant roughly half the mass of Saturn, but if it is anything like our gas giants, it will have a veritable swarm of moons, some of which may be conducive to life, and so 55 Cancri A has the #63 slot for NASA's planned Terrestrial Planet Finder mission. A radio message has been beamed to this star's vicinity; it will arrive in 2044.

    When referring to the other planets, be careful to use lowercase letters. 55 Cancri b was the first planet discovered to be orbiting 55 Cancri A, and is about a tenth of an A.U. away from it. 55 Cancri B (with a capital B), on the other hand, is the second star in the binary system, a class M4 red dwarf orbiting 1000 A.U.s away from 55 Cancri A.

    Upsilon Andromedae (44 ly)

    A wide binary system consisting of an F8V main sequence star and a cool, dim red dwarf. Since it's been confirmed to have at least three planets, the system has turned up from time to time in fiction. One of these planets is known to be in the habitable zone: it's a gas giant, but could have Earth-like moons. The companion star orbits far enough out to potentially have its own planetary system, but none has been detected so far.

    18 Scorpii (45.1 ly)

    A Class G2Va star in Scorpio. We don't know if it has any planets, but it is a nearly-perfect solar twin, and the closest of all such known stars.


    Some related info

    Magnitude- Apparent and Absolute

    Magnitude is the brightness of a star in the form of a number. Created by Hipparchus or Ptolemy, a star of Magnitude 1 is about 2.512 times (the 5th root of 100) brighter than one of Magnitude 2. It used to be that 1 was the highest, and 6 was the lowest, but they found that this didn't quite work and had to go into negative numbers, hence the Crab Supernova having -6.

    Most famous stars are between 0 and 2. The human eye can see to about 6, with 7.72 being the current world record. Vega is approximately magnitude 0. Sirius, the brightest star in the night sky is at -1.46. -3.9 is the faintest you can see in daylight, the Moon is -12.6 when it's full and the Sun is -26.73. This is the apparent magnitude of course.

    To judge the true relative brightness of stars, the magnitude that they would be at if 10 parsecs (32.6 light years) away is determined and called absolute magnitude. For example, Alpha Centauri has an absolute magnitude of 4.38, but an apparent one of -0.27, as it is very close to Sol, relatively speaking (4.3 light years). Far-off (700-900 light years) Rigel has an apparent magnitude of 0.18, but at the 10 parsec distance it shines at absolute magnitude -6.7.

    (There is also "planetary absolute magnitude", which is the brightness a planet would show at full illumination from 1 astronomical unit - that is, if the planet was moved to the Earth's orbit and viewed from the center of the Sun.)

    Distance determination

    Nearby stars have it determined by something called parallax. It's basically trigonometry that uses the relative movements of objects against a "fixed" background of far-off stars and galaxies- the closer the near star is, the more it will seem to move (you can see this on a train or in a car). Because of the distances involved, the change in angle is very small, less than a second of arc (or 1/3600 of a degree). This is the source of the unit of distance known as a "parsec" – it's the distance [3.26 light years] at which an object viewed from two points one AU apart would show a parallax of one second of arc. Most stellar distances are calculated by taking two observations six months apart (with a baseline of 2 AU), but plans exist to send out probes to provide baselines of tens or even hundreds of AU.

    But after a certain point current instruments simply can't measure the infinitesimal parallax of truly far away bodies. That's where we turn to an indirect means called the standard candle -- we look for an object whose intrinsic brightness (i.e. absolute magnitude) is known, then use how dim it appears (i.e. its apparent magnitude) to calculate how far away it must be. Not many objects have predictable brightnesses, but one of the few that do is a type of giant star known as a Cepheid variable. These stars pulse regularly at a rate proportional to their intrinsic luminosity, and some of them are also close enough to have measurable parallax. Therefore, far-off Cepheids (and objects such as the nebulae or galaxies they reside near or within) can have their distance judged simply by noting their pulse rate.


    Names

    Stars have a variety of naming methods:

    • The Arabs named a large batch of them back in the day (Betelgeuse, Altair, Deneb and Rigel, among others).
    • Others have been named more recently - Alpha Pavonis was named "Peacock" by the Royal Air Force because it didn't have one yet and was a navigation star for pilots operating in the Southern Hemisphere.
    • A Greek letter (Bayer designation) or number (Flamsteed designation) and genitive constellation name, such as "Alpha Centauri" or "40 Eridani." Since both of these naming schemes pre-date the astronomical use of the telescope, they are only used for stars visible with the naked eye. Some of these objects turned out to be 2 different stars that lay along the same line of sight; their modern names carry an additional superscript to tell the two objects apart, e.g. Zeta1 Reticuli and Zeta2 Reticuli. Some of them weren't stars at all; for example, "34 Tauri" turned out to be the planet Uranus - as a sidenote, this error left that designation available for the star system at the heart of a certain 'Verse...
    • Variable stars that don't already have Bayer or Flamsteed designations have their own wonky naming scheme. They're named in order of discovery, starting with "R" -- R Ceti was the first variable star discovered in Cetus, S Ceti was the second variable star discovered in Cetus, etc.. When they get to Z, they go back and use RR through RZ, then SS through SZ, then TT through TZ, etc.. When they get to ZZ, they go back and use AA through AZ, then BB through BZ, etc., all the way to QQ through QZ. Finally, if still more variable stars are discovered in the constellation, they give up and name the next ones V335, V336, V337, etc..
    • Catalog numbers - usually a prefix followed by an ID number or coordinate reference. Some catalogs include the Henry Draper (HD), Smithsonian Astrophysical Observatory (SAO) and "Bonner Durchmusterung" (BD). Early catalogs contained fewer stars, and are only referred to if the star in question first appeared there, e.g. "Wolf 359."

    A lot of stars are named in multiple ways, but go primarily by one designation or another. For example, while most people have heard of Rigel or Alpha Centauri, the same folks probably wouldn't recognize Beta Orionis or Toliman, which are alternative names for the same stars (respectively). As another, the star called both Rho1 Cancri and 55 Cancri is nearly always known by the latter name.

    In Bayer designations, stars are generally named in descending order of brightness. In other words, Alpha Centauri is brighter than Beta Centauri, which is brighter than Gamma Centauri and so on. There are, of course, exceptions – Beta Orionis (Rigel) is usually brighter than Alpha Orionis (Betelgeuse) – but then, no system is perfect. Flamsteed designations seem to be arbitrary were originally supposed to have the number increase from west to east, but precession has changed the relative position of some stars relative to celestial longitude.

    Types of stars

    Most stars are found along something called the Main Sequence, characterized by their balance between inward gravity and outward pressure generated by hydrogen fusion. Other stars exist that are off of it and fusing other elements, or else are dead or dying. These types have varying letters (spectral classifications) applied to them, with numerical sub-groups and a corresponding informal color (you can see the color in a good telescope).

    The higher up the scale, the bigger (and brighter) the star, but the faster the rate at which the hydrogen in them is used up and so the shorter their lifespan.

    For a number of reasons, very large stars - called giants, supergiants and hypergiants - like to live at the extreme ends of the spectral scales. Giant stars really do not like to be classes F or G, seeming to stay there for a very short time while heating up or cooling down to either extreme. There are a few known, but are decidedly in the minority. In general, most of the visible giants are either class-M or rather cool class-K (Betelgeuse, Arcturus) red giants, or class-B (Eta Carinae, Rigel, Deneb) blue giants.

    • Main Sequence Classifications, in order from hottest to coolest:
      • O – Blue-violet stars. The hottest and most massive main sequence stars, with most of their energy output in the ultraviolet regions of the spectrum. Pretty rare, but also conspicuous. Delta Orionis and Zeta Puppis (Naos) are examples.
      • B – Blue-white stars, e.g. Rigel, or all the bright stars in the Pleiades.
      • A – White stars. Sirius A and Vega are examples.
      • F – Yellow-white stars. Upsilon Andromedae and Procyon are of this type. Canopus is a rare class F giant.
      • G – Yellow stars. The most famous is a G2V type known in Latin as Sol, and in English as The Sun. Alpha Centauri A, Tau Ceti and Zeta Reticuli are this type as well.
      • K – Orange stars. Alpha Centauri B, Epsilon Eridani.
      • M – Red dwarf stars. (No, not that one.) Have very long life-spans i.e. a trillion years. Proxima Centauri and Barnard's Star are of this type. Luminosity class is always V or VI, as more massive types are actually red giants, and entirely different kind of creature.

    If you want to memorize the above sequence, use a handy mnemonic like "Oh Be A Fine Girl, Kiss Me" or "Oh Big And Ferocious Gorilla, Kill Mikey."

    The term "dwarf star" refers to main sequence stars of spectral classes F, G, K and M. Classes A, B and O usually aren't called dwarfs, because the terms "white dwarf" and "blue dwarf" refer to something else.

    Each spectral classification letter is subdivided into 9 numbers, running from 0 (hottest) to 9 (coolest). An F9 star is only ever-so-slightly hotter and whiter than a G0 star, while a G9 star is considerably cooler and oranger than either.

    • Non-Main Sequence Classifications:
      • W – Wolf-Rayet stars, former O-types which have long expended the hydrogen in their core and are using more exotic heavy element fusion, shedding mass rapidly and likely to go supernova sooner or later. They undergo Type-Ib/c core-collapse supernovae, generating spectacular gamma ray bursts. They have an onion like structure with helium on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding.
      • K or M - Red giant stars - classified as having a similar spectrum to M-type or cooler K-type red dwarfs but with luminosity classes I through IV. These are actually former B- through K-types that have expended their hydrogen and are fusing helium now. Their surfaces are cooler than they were in their hydrogen-fusion days, causing them to shift down the color spectrum to look like red dwarfs even though they may have originally been larger and hotter. They may either shed their outer layers and become white dwarfs upon expending their helium, or else undergo a Type-II core-collapse supernova. The main difference with Wolf-Rayets, apart from being less massive, is that they still have hydrogen in their outer layers when they explode, and their explosions are less energetic. For eg., Arcturus, Antares, Betelgeuse. Betelgeuse, in particular, is presently on course for a supernova in the near future (read: next 10000 years) that would be visible from Earth.
      • D – White dwarfs. (No, not that one.) Small, dead stars that aren't undergoing fusion at all, but are held up against gravity by electron degeneracy pressure, created by quantum effects regarding electrons. Their average densities are on the order of a ton to the cubic centimeter, and the surface gravity is usually around 100,000 G's. They are generally made of a dense core of carbon and oxygen and lack the gravity to fuse these elements further. Sirius B is one of these, there's another orbiting 40 Eridani. This is the final fate of most small to medium stars, including the Sun. Binary white dwarfs can do a bit more though - they may siphon off hydrogen from their companion stars, heating it on their surfaces until it undergoes flash fusion in an event called a nova. If the white dwarf accretes enough material to exceed 1.44 solar masses, electron degeneracy will no longer be able to support the star's weight, and the whole star will collapse in a spectacular explosion called a Type-Ia supernova.
      • L or T – Brown dwarfs. (No, not Gary... wait, never mind.) Collections of gas that never got big enough to start proper fusion reactions. They usually have some sub-par fusion like deuterium-deuterium and leftover heat of formation to give off a dull magenta glow, but cannot fuse ordinary hydrogen (protium, i.e, raw protons). Sort of intermediate between huge gas giant planets and true stars. The exact amount of mass needed to sustain fusion and not just peter out like this is unclear. A very few actual hydrogen-fusing red dwarfs may be cool enough to fall under the early L-class, as well.
      • Neutron stars. The dead remains of heavier stars, known for being extraordinarily dense (a billion times denser than a white dwarf) and occasionally spinning at extraordinary speeds while blasting out radio noise and generating intense magnetic fields (a.k.a., pulsars and magnetars). They are formed from core-collapse supernovae and are composed of pure neutronium, as their name suggests, with whatever charged particles that may be left strewn over their surfaces. Like white dwarfs, they are stabilized by quantum effects, except with neutrons instead of electrons. Also like white dwarfs, neutron stars in binary systems can accrete material from their companion, and eventually ignite it -- except that while a white drawrf that does so can erupt as a nova every few years, a neutron star that does so will erupt as an X-ray burster every few minutes. A neutron star will be the final fate of most of the larger red/blue giant stars, like Spica, Betelgeuse or Rigel.
      • Black Holes. The ultimate product of core-collapse supernovae, these were once stars whose core gravity was so strong that no force in the known universe could oppose it, so they contracted all the way down into gravitational singularities. This is the final fate of extremely large stars, like Eta Carinae or probably Deneb.
      • B VI - Blue subdwarfs. They're spectral class B, and are hence blue-white, but are much smaller and less luminous than the main sequence B-types. They form either when two white dwarfs merge and core gravity needed to restart reactions, or when a companion star strips a relatively small red giant of its outer layers. As a result, they have no hydrogen, and are fusing helium, or carbon/oxygen. They are longer-lived than the usual kind of blue stars, but still no more than some millions of years. More commonly found in globular clusters than in the main galaxy, since star densities there are higher and make the interactions needed to create blue subdwarfs more common.
      • Black dwarfs[4]. White dwarfs that have finally radiated away all their residual heat. As best we can tell, there aren't any of these yet, since the universe is still too young.
      • Blue dwarfs. There aren't any of these yet, either. A blue dwarf is what you get when a small red dwarf is out of fuel. It turns hotter for a brief time, becoming a blue dwarf, then shrinks and becomes inert, turning into a small white dwarf. Such stars never had the mass to even enter the red giant phase, and so instead have this as their intermediate stage before death. Such stars however are also extremely long-lived, and none of them have died yet.

    There's two other elements to a spectral class designation. The first is a number which roughly indicates where, within a given class, that star falls. Lower numbers are hotter. For example, Alpha Centauri B is a K0 star, meaning that it's very hot for a class K, and is in fact on the borderline of being a class G. Upsilon Andromedae is an F8, meaning that it's quite cool for an F.

    The second addition is a Roman numeral representing the star's luminosity class. This has to do with the brightness of the star, but as a rule of thumb, it can be thought of as a size and mass designation. In general, the top three classes started out as unusually massive stars. The numerals are as follows:

    • 0. Hypergiants like Eta Carinae or the Pistol Star.
    • I. Supergiants like Betelgeuse or Rigel.
    • II. Bright giants like Dabih (which is a rare class G giant).
    • III. Giants like Arcturus.
    • IV. Subgiants like Procyon. These tend to be main sequence stars that have run out of hydrogen to fuse and are expanding toward their red giant stage.
    • V. Main sequence stars like the Sun or Sirius. These are sometimes called "dwarf stars", though in fact, those of spectral class G or hotter outshine 80-90% of all stars in the galaxy. (Because most of them are dim red dwarfs).
    • VI. Subdwarf stars. There aren't very many of these known - this class is mostly used to designate odd, metal-poor stars. Their low metallicity lets radiation escape easier; consequentially there's less support for the star's outer layers and they tend to shrink and become hotter. This results in a net 1-2 magnitude drop from the brightness a "normal" dwarf star would demonstrate; hence the term "subdwarf".

    Two's Company, Three's Better

    Stars are pretty frequently[5] found in pairs, triples or more - Sol is rare for being a single star system. These stars can be very close together (like the twin suns of Tatooine in Star Wars), or quite far apart (like Alpha Centauri), but either way it can lead to a very cool Alien Sky.

    Close binaries are less likely to have useful planets than single stars. But distant binaries/multiples can have a LOT! -- a full system for each of the component stars.

    According to one paper published in the late 1970s, a planet in a multiple star system cannot be in a stable orbit unless its orbital radius is either (A) less than one-quarter (1/4) the closest-approach distance between the two stars, or (B) greater than twice the farthest-separation distance between the two stars (so as to orbit both stars as a pair). In Alpha Centauri's case, the A and B stars get within 11.5 A.U. of each other every 80 years, so no planets should exist more than 2.9 A.U. away from either star. Fortunately for writers, this still allows either or both to have a habitable planet.

    Extrasolar Planets

    Or exoplanets, for short.

    Giant stars are very unlikely to have useful planets: red giants gobble their close-by planets up when they grow, retaining only their outermost worlds, while blue giants, for one thing, blow their protoplanetary disks away and, for another thing, are too short-lived to form planets anyway. Some pulsars have been found to have planets, but these were formed probably after the original star went supernova, from the remaining gas and dust that the explosion left behind. Needless to say, they'd be pretty inhospitable to any known forms of life. Class A stars can host planets (Fomalhaut has one, for example), but almost certainly don't last long enough for life to evolve there naturally. Hotter class F stars (F0 through about F4-5) may have similar problems - they tend to enter their subgiant phases after about five billion years, which is roughly the present age of the Sun. Cooler F, G, and hotter K stars are the most hospitable to Earth-like life to evolve. Red dwarfs (M and cooler K stars) are too dim, that means their habitable zone isn't wide enough to probably have a planet there, with the exception of, perhaps, the possible rare small tide-locked world (see below). Many red dwarfs are also prone to violent solar flares. But there are a hell of a lot of these M-types in the universe, more than any other type, and some percent of them aren't flaring, and some percent of these may have a planet in the life-zone. So quite a number of red dwarfs could be hospitable.

    Another wrinkle with red dwarf planets is that a red dwarf's habitable zone is so close to the star that any planets there stand a good chance of being tidally locked; that is, always keeping the same side toward the star, as the Moon does to Earth. Without a very dense atmosphere[6] (to avert Convection, Schmonvection), the light side might become searing hot while the dark side becomes cold enough to freeze the air, resulting in a planet with one baked side and the other covered in nitrogen ice. Only if the planet has a dense atmosphere and sufficient water and carbon to become an "eyeball Earth" would it be habitable - the name given because they would be ice-covered except for a liquid ocean in the sub-solar region. Large, bright red dwarfs probably have less of a problem with this since their habitable zones are wider.

    By now, a lot of exoplanets have been discovered, but keep in mind that our current methods can generally only discover really big planets (2-3 Earth masses to gas giants) that are very close to their stars, or else planets that happen to be orbiting in a plane where we could see them (partially) eclipse their suns. With our current technology, if we were observing the Sun from Alpha Centauri, we might be able to detect Jupiter and maybe Neptune, but terrestrial planets like Mars, Venus or Earth are right out. That means, if a star's description says "no planets found", don't despair: there may be quite a lot we haven't been able to find yet. And we have no way to detect things like dwarf planets and moons, still - they're hard enough to spot reliably in our own system.

    In general, however, don't expect to find habitable planets around anything other than F, G, K or M main sequence stars or possibly subgiants. Unless your lifeforms are based on Artificial Intelligence and can live anywhere, have otherwise come from elsewhere and Terraformed an inhospitable planet or moon, or have evolved with some exotic form of biology, any exceptions to that rule would be extremely strange - and without a sufficiently good Hand Wave, it will look like you Did Not Do the Research.


    A couple of useful external links

    1. ↑ Our sun is is 4.57 billion years old. Earth came around about 30 million years later, and the simplest forms of life 740 million after that (or so sayeth the other wiki). Epsilon Eridani itself is at most 1 billion years old, probably younger, and its planets would be younger still. Life there is at best in the "single-celled organism" phase--if not the "parents grunting and groaning in the next apartment over" bit.
    2. ↑ The word "ḥawt" means "large fish-shaped sea creature". It strictly means "whale" in Modern Standard Arabic, but archaic Arabic and some modern dialects--particularly Moroccan--extend it to medium-to-large sea fish as well.
    3. ↑ To an astronomer, anything above helium on the periodic table is a "metal".
    4. ↑ Now I know what you're thinkin' - that's racist. But actually it's not, it's just cosmic vocab.
    5. ↑ While the majority of star systems visible to the naked eye, or visible in any given field when you look through a telescope, are binary or larger systems, this is because the more intrinsically bright a star is the more likely it is to have companions. A comprehensive survey of all stars within a given volume of intra-galactic space would reveal that most of them are in fact solitary red dwarfs.
    6. ↑ At least one-tenth as thick as Earth's atmosphere
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