At the top of the page appears the most popular name by which the star is known. Of course, most stars are really really obscure, and so choosing the "most popular" name for such an object is essentially a coin-toss between several catalog numbers.
Proper Names: Names for the star that aren't catalog numbers. These come from folklore, Arabic, Bayer, Flamsteed, or the variable star naming system.
Catalog names: The I.D. number(s) for this star in various star catalogs.
Age: The amount of time since the star became a main-sequence star. Where reasonably accurate age data are available, this will be given in millions of years. However, for non-sunlike stars (such as most red dwarfs) the only age data usually available comes from the third (z) component of the star's velocity, and may only be enough to distinguish the star as "Galactic plane population" (newborn to 10 000 million years), "Thick galactic disk population" (3000 to 10 000 million years), "Intermediate galactic population" (around 10 000 million years), or "Galactic halo population" (over 10 000 million years).
Heavy element abundance: The amount of all elements except hydrogen and helium, relative to the amount of hydrogen and helium. About 1% of the elements in our own sun are heavy elements.  Star systems are listed with their (sometimes estimated) abundances relative to the sun's.
Galactic plane population stars (formerly called Population I stars) are young stars that formed after the Milky Way had accumulated a rich deposit of heavy elements (such as carbon). Any planets orbiting these stars would probably have enough heavy elements to support organic life, although other preconditions for life might not be met (such as the star having been in the form it is now for a long enough period to let life form and evolve).
Galactic thick-disk, intermediate, and halo population stars (formerly called Population II stars) were around when the galaxy was forming. As such, they contain little or almost no heavy elements. Since the planets of these stars are also likely to be carbon-poor, the chance for life around these stars is slim at best. The lack of heavy elements would also preclude any large solid planets (like Earth or Pluto) in such a star system; any planets would be gaseous, like Jupiter or Neptune, and comprised almost entirely of hydrogen and helium.
Arity: The number of actual stars that this "star" represents. Some points of light in the night sky are "binary" stars — that is, two stars orbiting about their common center of mass which are so close together they look like one star from a distance. Most stars are singular, while a few others are "trinary" (two stars orbiting each other with a third star orbiting the pair from a much greater distance). In very rare instances, a star may be "quarternary" (two binary stars orbiting each other), "quinary" (a trinary star system where two of the three "stars" are actually close-orbiting binaries) or even "hexary" (a trinary star system where all three "stars" are close-orbiting binaries).
Multiple stars listed under the same name are usually referred to as "A," "B," and "C" in order of relative brightness: the two stars that compose the binary star Sirius are called "Sirius A" and "Sirius B", for instance. In some rare cases, such as Castor, the A, B, etc., components of a star system may themselves be close binary star pairs, in which case they are referred to as A1 and A2, then B1 and B2 — or Aa and Ab, then Ba and Bb, depending on what the discoverer preferred.
A few stars are what are termed "spectroscopic binaries." These are stars which appear singular even to the most powerful telescopes but which, due to variances in the spectrum of light they produce, are known to be two stars very close together in a really tight, fast orbit around one another. In such cases, the spectral class and luminosity (see below) often cannot be resolved for each star individually, and is listed as the total average for both stars of the spectroscopic binary pair combined.
Points of interest: This entry is the catch-all for any specific thing that makes this star unusual, such as excessive flare activity, the fact that the star is a black hole, et cetera.
Right Ascension and Declination: "Right Ascension" and "Declination" are what Earth-based astronomers use to uniquely identify a given star. Declination is the star's angle north or south of Earth's celestial equator, ranging from -90 degrees at the south celestual pole to +90 degrees at the north celestial pole. Right ascension is the star's east-west location on Earth's celestial sphere, measured in hours, minutes, and seconds west of the "Equinox Line." (The Equinox Point is that point on the Equinox Line that faces directly toward the sun at the instant the Earth passes through the Vernal Equinox each year. By definition, it has a right ascension of 0 and a declination of 0.) Since the stars are not at rest with respect to our sun, they appear to move slowly across the sky over the centuries (a phenomenon called "proper motion"). Thus, a given right ascension and declination will only be valid for a star at a particular "epoch" (year), which in this reference is listed in parentheses.
Distance from Sol: The number of light-years from this star to our own sun. See The Scale of Things for a description of a light-year.
Standard error in distance: How inaccurate the distance measurement is. The distance to most stars is very difficult to measure precisely; a standard error of less than 1% is considered high accuracy.
Celestial (X,Y,Z) coordinates in ly: The coordinates of this star in space, relative to our own sun, in units of light-years. The first ("x") coordinate points toward the Equinox Point. The second ("y") coordinate points toward a spot in the sky at 0 declination and at a right angle to the Equinox Point. The third ("z") coordinate points toward the celestial north pole, at a declination of +90 degrees. Our sun represents the point [0, 0, 0] in this coordinate system. These coordinates are used to find the distance between any two stars, instead of just their distance from our own sun.
Note that these coordinates represent the position of the star as it was in the "epoch" given for its right ascension and declination; centuries from now, most stars will have moved enough that these coordinates will have changed noticeably. Note also that spatial coordinates will be no more accurate than the distance measurement to the star.
Galactic (X,Y,Z) coordinates in ly: Similar to Celestial coordinates, but pointing along a set of axes that's not quite so geocentric. The first ("x") coordinate points directly toward the center of our galaxy (which, in the Earth's night sky, is at a right ascension of 17h42m4s and a declination of -28°55'). The second ("y") coordinate points along the galactic plane in the direction of galactic rotation, at right angles to the "x" axis. The third ("z") coordinate points straight out of the plane of the galaxy, parallel to the galactic north pole, at right angles to both the "x" and "y" axes. As with Celestial coordinates, our sun represents the point [0, 0, 0] in this coordinate system.
Proper motion: How rapidly this star is moving across the sky as seen from the Earth. These movements are very slight; even the fastest-moving stars only appear to change their right-ascension and/or declination by about 10 seconds of arc (1/360 of one degree) per year.
Radial velocity: How rapidly this star is moving toward or away from the Earth.
Galactic (U,V,W) velocity components in km/s: How fast the entire star system is changing its first, second, and third spatial coordinates, in kilometers per second; i.e., how fast the stellar system is moving, and in what direction, relative to our own sun. Since it takes 9 455 000 million kilometers to span one light-year, a star with a velocity vector of (-1, 0, 0) would take 9 455 000 million seconds (about 300 000 years) to change its spatial coordinates from [5.5, 3.8, -2.3] to [4.5, 3.8, -2.3]. By definition, the sun's velocity vector is (0, 0, 0).
Note that the sun is in motion relative to the "local standard of rest" within our own galaxy. To get a star's galactic-relative velocity, you have to add the sun's local-standard-of-rest-relative velocity of (10.4, 14.8, 7.3) to what's listed here for that star. This is usually only important if you want to guess at how a given star is orbiting the center of the galaxy, which is not a subject for this text. Note also that the velocity vector will be no more accurate than the distance measurement to the star.
Note: It is a common misconception that most stars are binary star systems, and that our sun is an exception to that rule. While most of the stars bright enough to be seen with the naked eye are, indeed, binary or larger star systems, the vast majority of stars are singular, solitary red dwarfs. It just so happens that red dwarfs are so dim that none of them — not even Proxima Centauri, the closest star to our own sun — are visible without the aid of a telescope. However, as more data are gathered about these common, dim, and largely uninteresting objects, we may yet discover that more red dwarfs are actually binary pairs of red dwarfs than we had previosuly supposed. We shall have to wait and see.
Combined visual luminosity: The brightness of this pair of orbiting bodies added together, relative to the sun. NOTE: Calculating this value requires knowing the distance to the star system, so it will be at least as inaccurate as the distance measurement.
Period: How long it takes for the stars to orbit each other. It takes half this much time to go from Periastron to Apastron or vice-versa.
Periastron Gap: The closest-approach distance between the orbiting bodies in A.U.s. (See The Scale of Things for a description of an A.U..) "Periastron" comes from the Greek "peri" (meaning near) and "astro" (meaning star); binary stars orbit each other in ellipses with their common center of mass at one elliptical "focus," and seldom will such orbits be circular.
Bear in mind that any planets in a multiple star system must be closer to the star they are orbiting than 25% of the Periastron distance between that star and its nearest neighbor; otherwise, their orbits wouldn't be stable and they'd fly off into deep space. However, a planet may have a stable orbit much farther out if it orbits the common center of mass of a pair of stars. See "Orbital Mechanics" [not included yet] for more information on how stars orbit one another.
Apastron Gap: The farthest distance between the orbiting bodies. "Apastron" comes from the Greek "apo" (meaning far) and "astro" (meaning star).
Observed separation: Sometimes the exact shape of a binary star's orbit is not known, either because the orbital period is too long to form a complete picture or because no one has bothered to gather sufficient data. In this case, the periaston and apastron distances cannot be calculated, and the last-observed distance between the two stars is listed instead.
Year in which periastron occurs: Some multiple star systems have orbital periods lasting hundreds or even thousands of years. When data are available, and when the orbital periods aren't too short, the year C.E. (which means the same as the year A.D.) in which the last or next periastron occurs will be listed.
From the "Spectral class" field on down to the "Angular size in sky in CZ" field (as described below), binary, trinary, and more complex star systems have each component (A, B, C, etc.) listed separately.
Spectral class: The color of the star. Hot stars are bluer, cool stars are redder. In descending order from hottest (most blue) to coolest (most red), the spectral classes are: O B A F G K M. Like many naming schemes in astronomy, these letter names are archaic and are kept for historical reference to old, outdated star guides. They seem to have stayed with us only to give astronomers an excuse to think up clever mnemonic devices to remember them by, like "Oh Be A Fine Girl, Kiss Me" or "Only Big And Ferocious Gorillas Kill Morons". Furthermore, each spectral class letter can be subdivided into ten smaller steps or gradations, i.e. class A breaks down into A0 through A9. A0 would be the hottest class A star (only slightly cooler than B9), while A5 would be half way in between A0 and F0. (Dim class M dwarfs sometimes even have finer gradations, e.g. M3.5.) The spectral class corresponds to the actual surface temperature and color as follows:
|O5||40 000 K||blue|
|B0||27 000 K||blue-white|
|B5||16 000 K|
|A0||10 000 K||white|
|A5||8 200 K|
|F0||7 200 K||yellow-white|
|F5||6 700 K|
|G0||6 000 K||yellow|
|G2||5 800 K||(our sun)|
|G5||5 500 K|
|K0||5 100 K||orange|
|K5||4 300 K|
|M0||3 700 K||red|
|M5||3 000 K|
Mitchell N. Charity has a webpage that graphically demonstrates "What color are the stars?". This page is best viewed in 16-bit or better Color resolution.
An "e" at the end of a spectral classification indicates emission lines or bands in the spectrum of a star; usually, only absorption lines and/or bands are present. Such stars are surrounded by hot, rarefied gas, where the emission is occurring. For red (class M) dwarfs, this often indicates a reasonably high heavy-element abundance (see above).
A "p" at the end of a spectral classification indicates peculiarities in the spectrum, which may indicate a lack of heavier elements or that the star is in a short-lived stage of its evolution.
An "n" at the end of a spectral classification indicates that the spectral lines are diffuse, rather than sharp.
White Dwarf stars use a completely different spectral classification system from the one given above.  A white dwarf's spectral class always starts with a capital "D", which stands for Degenerate. The letter after the D is either an A, B, C, O, Z, or Q, which indicates whether certain elements are present in the star's spectral lines at all. After this second letter comes a number from 0 to 9, which indicates its surface temperature — 0 being the hottest at about 50 000 Kelvins, 9 being the coolest at around 10 000 Kelvins. It is this number, rather than the D or the second letter, which is your best indicator as to the white dwarf's color.
Luminosity Class: How the brightness of this star compares with its spectral class, after the system introduced in 1937 by W. W. Morgan and P. C. Keenan at Yerkes Observatory, as follows:
Luminosity Class Ia largest supergiants Luminosity Class Ib supergiants Luminosity Class II bright giants Luminosity Class III giants Luminosity Class IV sub-giants Luminosity Class V dwarfs (main-sequence stars) Luminosity Class VI sub-dwarfs Luminosity Class wd white dwarfs"Dwarf" stars (luminosity class V) are also known as main-sequence stars because 90% of all stars fall into this category. (That astronomers have essentially labelled average-sized objects as "dwarfs" may be indicative of the insecurity of male astronomers as a whole, but that's not a subject for this text.) Note also that the plural of "dwarf" is "dwarfs," not "dwarves"; Tolkien's slant on the plural spelling didn't enter the language until the middle of the century.
Giant-sized (class Ia-IV) stars are usually larger than normal because they're going through a short-lived evolutionary period at the end of their normal main-sequence lifetimes. As such, they don't tend to last very long, and if life ever did manage to arise on a planet orbiting one of these giants, the next stage in the star's evolution would snuff that life out before multicellular creatures or even oxygen-respiration would have a chance to evolve. Note: Subgiant stars (class IV) have a variable light output (they grow and shrink). It is possible when observing a star that is variable for some other reason (such as being an eclipsing binary) to misinterpret its variability as evidence that the star is a subgiant.
Apparent visual magnitude: The brightness of this star to an observer on Earth. For historical reasons dating back to ancient Hellenic Greece, the magnitude scale decreases with brightness; a star of magnitude +6.0 would be 100 times dimmer than a star of magnitude +1.0. Sirius, the brightest star in Earth's sky (other than our sun), has an apparent visual magnitude of -1.43.
Absolute visual magnitude: The apparent visual magnitude this star would have if it were at a distance of exactly 10 parsecs (32.64 light-years). The sun has an absolute visual magnitude of +4.85. Note that calculating this value requires knowing the distance to the star system, so it will be at least as inaccurate as the distance measurement.
Visual luminosity: The brightness of this star relative to the sun. NOTE: Calculating this value requires knowing the distance to the star system, so it will be at least as inaccurate as the distance measurement.
Variable type: If the star is a variable star, this field displays what kind of a variable it is. Stars can vary in brightness for many different reasons.
Period of variability: The time it takes this variable star to get dimmer, then brighter, until it's back to the same brightness it had at the start of the last cycle.
Color indices: The relative brightness of this star between certain frequency filters, or "colors", of light. A star's magnitude isn't just measured in the visual (yellow-centered) portion of the spectrum; it can also be measured in the blue-centered end of the visible spectrum, or in the red-centered end, or in the near ultraviolet, or in the near infra-red. There are standardized frequency-range filters that are used to measure a star's brightness in some portion of the spectrum. These filters are given one-letter names: U is ultra-violet, B is blue, V is visual (yellow), R is red, and I is near infra-red. The difference between a star's magnitude in two adjacent filters — U minus B, B minus V, R minus I — can then be used to determine the star's color, and thereby its "color temperature." The lower the magnitude difference, the brighter the star is in the higher of the two frequency-range filters, because a higher magnitude means a dimmer light source. For example, our sun has a B-V index of +0.65, while the much hotter star Sirius has a B-V index of +0.00, and the very cool star Proxima Centauri has a B-V of +1.83. For cool class M stars, R-I gives the best precision as to what color and temperature the star is; for hot class A, B, or O stars, U-B is a better color temperature indicator.
Mass: The "weight" of the star, given in solar masses. (One solar mass is 2 x 1030 kilograms, or two million million million million million kilograms.)
Handy-dandy mass chart grams MEarth MJ MSun Earth 5.976 x 1027 1 0.003144 3.00437 x 10-6 Jupiter 1.8997 x 1030 317.893 1 0.0009551 Our Sun 1.9891 x 1033 332 943 1047 1
Diameter: The size of the star, given in solar diameters. (One solar diameter is 1 391 980 kilometers.) Many of the stars in this reference don't have their diameters measured by direct observation, but instead have an inferred diameter based on their apparent brightness, their distance, and their color (surface temperature). This means the diameter listed will be at least as inaccurate as the distance measurement. The diameter will also be incorrect if the star is actually a close-orbiting binary that can't be resolved in a telescope. In the chart below, remember that the radius is half the diameter.
Handy-dandy radius chart centimeters REarth RJ RSun Earth 6.378 x 108 1 0.08873 0.009164 Jupiter 7.188 x 109 11.27 1 0.1033 Our Sun 6.9599 x 1010 109.1 9.683 1
Comfort Zone: This is the distance, in Astronomical Units, at which we're pretty sure a planet may orbit this star and support life on its surface. (One Astronomical Unit is the mean distance from the Earth to the sun.) Much closer and the life forms would fry, or the air and water would evaporate; much further and life forms would freeze. Note that, since humankind has only ever stumbled across one planet where we know for sure that life exists, it's hard to generalize about the range of conditions under which life as we know it could arise; thus, the comfort "zone" is listed as a single distance, being the distance a planet would have to be from the star to receive precisely as much light as the Earth does from the sun. Note also that even if an Earth-like planet were present at the comfort zone distance, other factors could make such a planet uninhabitable (like if the star occasionally emits large, lethal flares).
The comfort zone distance in A.U.s is computed by simply taking the square root of the star's luminosity in solar units, since the light received by a planet falls off with the square of its distance from the star. In the Internet Stellar Database, I've cheated a little bit by using the visual luminosity (the luminosity passing through a filter that approximates our own eyes' bias) as the basis for the comfort zone. This is reasonable for yellowish stars like our sun, but doesn't work as well for very cool or very hot stars. Cool red stars tend to emit more of their energy in the infrared portion of the spectrum, while hot blue stars tend to emit more energy at frequencies higher than the visual "peak" frequency. I should really be using the bolometric luminosity (the energy emitted at all frequencies), but bolometric data are pretty hard to find for most stars.
Orbital period in CZ: If a planet, whose mass was small compared with the star, were in a circular orbit about this star in its comfort zone, this entry shows how long the "year" on that planet would be. (This figure is related both to the distrance the planet is from the star and the mass of the star itself. More massive stars demand faster orbits, but greater distances both take longer to traverse and require that the orbiting object be moving more slowly.) The exact relationship for a planet orbiting at any distance is P2 = A3/M, where M is the mass of the star in solar masses, P is the orbital period in Earth years, and A is the semimajor axis (radius) of the orbit in Astronomical Units. Conveniently, for the Earth's orbit around the sun, all three of these values — P, A, and M — are equal to 1.
Tidal Index in CZ: If a planet were in a circular orbit about this star in its comfort zone, this entry shows how strong the star's tidal forces would be on said planet, relative to the strength of the Sun's tidal forces on Earth. A very high Tidal Index means that any such planet would quickly become locked in synchronous rotation around the star, so that the same side of the planet faces the star at all times (in the same way that the moon is locked in synchrorous rotation around the Earth).
Angular size in sky in CZ: If a planet were in a circular orbit about this star in its comfort zone, this entry shows how big the star would appear to an observer on the surface of said planet. The Earth's sun subtends an angle of about 0.5 degrees.
Detected companions: If any actual planets or brown dwarfs have been detected around this star, directly or indirectly, the number so discovered will be mentioned here. The "Points of Interest" field (above) will likely have a more in-depth description as to what is known about these unseen objects.