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About the Star Data

  1. What objects from the Hipparcos catalog did you include?

    This depends on the section of 3D Universe you're looking at.

    The most comprehensive star database in 3D Universe serves the Distant Worlds Star Mapper. The Mapper contains all Hipparcos stars meeting the following criteria:

    1. Stars brighter than apparent magnitude +9.00, regardless of distance
    2. Stars closer than 50 parsecs (about 165 light years), regardless of magnitude

    The high magnitude limit is needed to create accurate star maps far from Earth -- with a lower (brighter) limit, many stars would be missing from a naked eye view at places far from Earth. With the chosen limit, Distant Worlds Star Mapper can create an accurate simulation of a dark naked-eye sky (i.e., apparent magnitude limit +6.5) at more than 100 light years from Earth, and a simulation of a yypical "suburban" sky (magnitude limit +4.0) at more than 300 light years.

    The magnitude and distance limits chosen encompass over 80,000 stars in Hipparcos -- approximately 3/4 of the catalog. The remainder of the catalog has poorer quality data, since it contains mostly dimmer stars (i.e., harder to measure accurately) at high distances (i.e., also harder to measure accurately). Since the Hipparcos catalog has few stars below about magnitude +10, I filled in many of the gaps with data from the next-best source of distance data, the Gliese catalog (version 3). This combined catalog contains essentially all known stars out to about 80 light years, as well as all fairly bright stars out to many hundreds of light years. Finally, I used the 5th edition of the Yale Bright Star Catalog to add some common name information (such as Bayer [Greek letter] designations and Flamsteed numbers), and also to add the "HR" number, another common catalog number for bright stars.

  2. How'd you cross-reference the Hipparcos Catalog to the Gliese and Yale catalogs?

    Henry Draper numbers, whenever possible -- it's the only identification common to all three catalogs. For some of the dimmer stars in Hipparcos, which lack HD numbers, I was normally able to match them by comparing position and apparent magnitude to those in Gliese. These cross-references are less robust, but generally involve only very dim stars that don't show up at the magnitude limits I use in 3D Universe.

    There are some known problems with this fairly simple method (in particular, it's not robust against errors in HD numbers tabulated in one or more of the catalogs), but it beats the other alternatives: (a) trying to match all 80,000+ entries by eye, and (b) rewriting SIMBAD or one of the other astronomy catalog servers out there.

  3. It doesn't look like you go down to magnitude +9.0 on your other charts.

    For most other parts of the site, I used smaller catalogs, since the charts mostly represent naked-eye views from, or near, Earth. There was thus no need to include any stars below typical naked-eye limits (i.e., magnitude 6-7 from dark skies). Depending on the details of the page, I used different subsets of the Hipparcos data, but for all pages besides Distant Worlds, the Hipparcos dataset had a magnitude cutoff of +7.5, as opposed to +9.0 (plus fainter stars from Gliese).

    Even this magnitude-7.5 Hipparcos subset is overkill for most of the charts. For the Basic Stereo Views, I used stars brighter than magnitude +5.50. For the Advanced Stereo Views, up to the full limit (+7.5) is possible, though most people probably choose lower limits. For the animations, I used stars brighter than +6.00, as seen from Earth. These subsets contained from 1500 to 25000 stars, depending on magnitude limit and location.

  4. How did you choose which areas of the sky to display in the Basic Charts section of the stereo charts?

    The chart centers and areas of sky covered are based on the 26 charts in Wil Tirion's Sky Atlas 2000. The 3D Universe charts have a much brighter magnitude limit (5.5 in 3D Universe vs. 8.0 in the Tirion atlas), and slightly different map projections and orientations. Otherwise the charts are pretty comparable. Thus you can use Sky Atlas 2000 quite conveniently to look up star names and labels.

  5. In the Animations page, you have some proper motion animations that show stars approaching or receding from Earth. I thought Hipparcos didn't measure radial velocities.

    You're right -- it didn't. I took the radial velocities for my stars from the third edition of the Gliese Catalog of Nearby Stars whenever it was available and could be assigned unambiguously to a Hipparcos entry. For the rest of the stars, I took the value in the second edition of the Yale Bright Star Catalogue if it was available. Between the two catalogs, most stars visible in the 3D Universe animations have a reasonably well-known radial velocity.

  6. I'm interested in stars like the Sun (for settings in science fiction novels, or armchair SETI speculation, or whatever). What stars in the Hipparcos data set (and in 3D Universe) are Sunlike?

    In Hipparcos, hundreds if not thousands. My "master subset" of Hipparcos data for 3D Universe has about 800 stars whose luminosity ranges from 0.4 to 2.5 times that of the Sun; the full Hipparcos catalog certainly has many more. Of these, about a hundred or so are reasonably easy naked-eye objects from a dark sky site (from the center of a city, only about ten, unfortunately).

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