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FAQ: Freely Anticipated Questions

I don't yet have any Frequently Asked Questions. Instead, I have Freely Anticipated Questions -- the sort that I expect to encounter regularly.

  1. How accurately are the distances to the Encyclopedia stars known?

    Except for Xi Ursae Majoris, all the stars in the Encyclopedia are in the Hipparcos catalogue. The typical error on Hipparcos parallaxes is 1 milliarcsecond, whereas the most distant stars in the Encyclopedia have parallaxes of 65 milliarcseconds. Thus, the relative uncertainty for the furthest stars is 1 / 65 = 1.5 percent. Most of the stars in the Encyclopedia are considerably closer, and have errors well under 1 percent.

    Xi Ursae Majoris is only in the Gliese catalog, which gives an error of 13 milliarcseconds in its parallax, corresponding to a relative uncertainty of 13.5% in its distance.

  2. How accurately are the compositions (metallicities) of the Encyclopedia stars known?

    Metallicity is somewhat difficult to measure. There are several ways of measuring it, the best (and most direct) being a detailed analysis of the spectral lines of iron and hydrogen. Estimates of the uncertainty vary, but uncertainties of 25% appear to be typical.

  3. How accurately are the ages of the Encyclopedia stars known?

    Stellar ages are the most uncertain data in the Encyclopedia. The main method for estimating ages, based on Ca H and K emission line strengths, can have an uncertainty of 100% (contrast this with the ~1% for distance and ~25% for composition described above). This is still normally sufficient to discriminate between very young, medium, and old stars. Nevertheless, data for ages are not very precise. I quote stellar ages to two significant digits, but in practice this is too much precision.

    There are other ways of estimating stellar ages. The most reliable -- probably more reliable for many stars than the Ca emission measurements -- involves stellar evolution theory and models. Astronomers have calculated how the luminosity and spectrum of a star vary as a function of various parameters, notably stellar mass, composition, and age. In principle, if the luminosity, spectrum, mass, and composition of a star are well characterized, the age can be determined from these models of stellar evolution. I have not yet found a comprehensive source of stellar ages based on stellar evolution models, but may incorporate this information into the Encyclopedia if I can.

  4. Why don't all the stars have text descriptions?

    In general, I have omitted text descriptions for stars fainter than apparent magnitude +6.0, since these generally:

    • are much harder to find than brighter stars
    • have less of a "history" than brighter stars (most lack common names, and don't form a conspicuous part of a constellation)
    • have less information known about them (particularly data such as composition or age)

    The key information for these stars -- luminosity, location, metallicity (when known), age (when known), etc. -- is always given in the information box next to the star chart. For most of these fainter stars, a text description would just repeat details that are already given, and not add much useful information.

    I have always included a text description if the star has some unusual interest, such as a companion star, or unusual physical properties. I have also always given a text description, if only a brief one, when the star is well known by something other than a catalog number. For example, any star that has a Greek letter ID (e.g., Sigma Draconis or Alpha Mensae), or one of the Flamsteed numbers (e.g. 54 Piscium) has a description, often of the constellation it is in.

  5. I'm looking at one of your references for metallicities and a lot of the numbers are negative. What does this mean? (I assume these are not antimatter stars.)

    In the previous section, I describe metallicity as a ratio of a star's iron content to the Sun's iron content. In catalogs and research papers, though, astronomers usually go one step further. They calculate the common (base 10) logarithm of the ratio. Thus a star with the same metallicity as the Sun -- a ratio of 100%, or 1.00 -- would be given as 0.00, and a star with half the metallicity (50%, or 0.5) would be given as -0.30. Although this is the usual convention in research papers, I chose to give just the ratios in the Encyclopedia because the numbers are more meaningful to non-astronomers.

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