The Astronomy Nexus

There are a number of other important stellar properties given in the Encyclopedia.

  1. Identifiers. The Encyclopedia lists the catalog number of the star in three widely used catalogs:
    • the Hipparcos catalog of stellar positions, distances, and motions
    • the Yale Bright Star Catalog, a widely used catalog of relatively bright stars
    • the Catalogue of Nearby Stars compiled by the German astronomers W. Gliese and H. Jahreiss.
  2. Apparent Magnitude. Apparent magnitude indicates how bright a star looks from Earth. It is based on a logarithmic scale, in which a difference of 5 magnitudes corresponds to a brightness ratio of 100. Furthermore, brighter stars are given lower magnitudes -- this seems a little weird initially, but think of "first rate" versus "third class" and it will make more sense.

    For historical reasons, astronomers defined the scale so the brightest stars visible to the naked eye are roughly magnitude 0, and the dimmest roughly magnitude 6. Here are the magnitudes of some representative celestial objects:

    • Pluto (faintest planet): +14
    • Dimmest naked eye stars (dark skies): +6 to +7
    • Dimmest naked eye stars (city skies): +4 to +5
    • Polaris (North Star): +2.0
    • Sirius (brightest star): -1.4
    • Sun: -26.7

    Most of the stars in the Encyclopedia are fairly faint. With the exception of Alpha Centauri, none is brighter than magnitude +3, and the majority are fainter than +4.5. None of the stars require a large telescope to see. However, from a city, where the city lights interfere with the night sky, those fainter than about +4.5 will often require binoculars (ordinary birding binocs are fine) to see clearly. From a very dark sky you will be able to see most of the Encyclopedia stars without binoculars, though the fainter ones (magnitude +6 and fainter) will be much clearer in binoculars or a small telescope.

  3. Luminosity. The luminosity is just the ratio of the star's light output to that of the Sun. A star with a luminosity of 1 is a near-exact twin of the Sun in energy output.
  4. Absolute Magnitude: Since stars are at different distances from Earth, stars with the same apparent
    magnitude can differ greatly in total energy output. The absolute magnitude is the apparent magnitude a star would have if its distance from the Sun were 10 parsecs (32.6 light years).

    The absolute magnitude is really just another way of describing the star's luminosity. The Sun's absolute magnitude is +4.85; stars similar to the Sun generally have absolute magnitudes in the range +3.5 to +7.

  5. Metallicity. Stars are primarily made of the two lightest elements, hydrogen and helium. Normally, elements heavier than helium -- called, somewhat perversely, "metals" by astronomers -- make up less than 2% of the outer atmosphere of a star (the part we can see). Astronomers define a quantity known as the metallicity to describe how rich in metals a star is. Since planets form from the same materials as their parent stars, and you can't make an Earthlike planet out of just hydrogen and helium, metal-poor stars are considered less likely to harbor Earthlike worlds than metal-rich ones.

    Astronomers usually measure metallicity by comparing the ratio of iron (Fe) to hydrogen (H) in a star. Although other elements can be (and are) measured, iron is the most commonly measured, and is widely used as a proxy for total metal content. Astronomers then divide the iron/hydrogen ratio by that of the Sun, whose chemical composition is well known, so a metallicity of 50% indicates an iron abundance (relative to hydrogen) that is only one-half that of the Sun.

  6. Spectral Type. The star's spectral type is a shorthand code describing the key features of a star, primarily its temperature and luminosity class. The "temperature" code is a letter in the series O,B,A,F,G,K,M, where O is hottest and M is coolest. The temperature scale is further subdivided by a number from 0 to 9, with 0 being hottest and 9 coolest: a star with a spectral type of G5 is about halfway between one of G0 and one of K0. The stars in the Encyclopedia have temperature codes between F5 and K5 (the Sun's is G2). Stars outside this range are too different from the Sun to be included in the Encyclopedia.

    The "luminosity class" code is a Roman numeral from I ("supergiants": extremely luminous) to VII (white dwarfs: extremely dim). Nearly all of the stars in the Encyclopedia, as well as the Sun, have luminosity class V: ordinary main-sequence stars. Only two (so far) have a different class: IV-V, representing a star that is just beginning to evolve away from the main sequence and become a subgiant (an intermediate stage between main sequence and red giant).

  7. Age. Astronomers have found that the spectra of stars change as they get older. Specifically, some emissions from calcium (known as the Ca II H and K lines) are much stronger in young stars than in older stars. Thus, by measuring the strengths of these emission lines, an estimate of the star's age can be obtained.

    The Ca II H and K emissions of a typical star vary somewhat over the short term. The Sun, for example, produces measurably more of these emissions during solar maximum (high sunspot number and flare activity) than at minimum. Because of this, the age of a star has fairly high uncertainty (up to about a factor of 2). In the Encyclopedia, I give the ages to the nearest tenth of a billion years; in reality the uncertainty will generally be considerably larger than this, possibly a billion years or more. Nevertheless, the age figures are accurate enough to group stars into broad age groups, particularly into groups that are much younger than the Sun (less than ~2 billion years), similar to the Sun (~2 - ~6 billion years), and much older than the Sun (more than ~6 billion years). SETI enthusiasts and "hard" science fiction fans ought to be quite interested in older stars...

  8. Companions. Many stars have one or more other stars orbiting them, and quite a few have planet-sized bodies orbiting them. The Encyclopedia notes which companions are known and what their orbits are like. By long standing convention, the brightest star in a multiple star is generally labeled "A" (e.g., Alpha Centauri A), and the successively dimmer stars are labeled "B", "C", etc. Planets are normally labeled "A", "B" etc. in order of increasing orbit size instead. Even though they use similar lettering schemes, it's usually pretty clear from context which type of companion is involved.

    If a companion object moves quickly enough, astronomers can measure its position over time and determine its orbital elements -- the physical parameters that describe the orbit. The most important orbital elements are (a) the period, which is the time it takes the companion to orbit once; (b) the eccentricity, or how elongated the orbit is (larger numbers mean narrower ellipses), and (c) the semi-major axis, which is one-half the length of the long axis of the ellipse. The semi-major axis is usually expressed in arcseconds (i.e., apparent angle as seen from Earth), but if the distance to the star is known, astronomers can express the semi-major axis in units like kilometers or astronomical units. Since the distances to all the stars in the Encyclopedia are very well known, I have calculated the semi-major axes of all known orbits in astronomical units.

    In the case of extrasolar planets, I give another figure: the estimated mass of the planet. Because of limitations of the techniques used to find these planets, their precise mass M cannot be determined; what astronomers actually measure is the quantity M sin i, which is the mass times the sine of the orbital inclination, as seen from Earth. Since the orbital inclination is unknown, sin i can be anywhere from 0 to 1, so M sin i is a lower limit on the planet's mass. The planet's mass could be many times the value of M sin i, but statistically speaking, most of the time it will be less than twice that value.

    Stars orbiting very slowly or very far away do not have well characterized orbits. For these stars, I note that the period of the orbit is unknown, and give the apparent separation, which is simply the distance between the two stars in angular units (i.e., arcseconds). I also give the separation in astronomical units, assuming the line between the stars is perpendicular to our line of sight to them. Since the stars could really be in any orientation, without further information, this separation is a lower limit.

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