Star catalogue

The star catalogue of Pax Abyssi is a table of 119,626 real stars, each one a measured object with a position, a brightness and, for almost all of them, a spectral type. It began as the public HYG database, a merger of the Hipparcos, Yale Bright Star and Gliese catalogues compiled by David Nash, and has been corrected row by row against newer measurements, above all the parallaxes of ESA's Gaia mission. ^{[1] [2]} Every star in the game's night sky is a row in this table, and every correction sits in a column of its own beside the value it replaces, so the original measurement is never lost.

Image pendingFile:Catalogue_sky_sim.pngA black sky scattered with stars of different brightness, a few of them faintly orange or blue-white.

Where the stars come from

HYG is named for its three sources. ^[1]

• Hipparcos, the ESA satellite that measured 118,218 stars between 1989 and 1993 and gave the first large set of accurate parallaxes, about a thousandth of an arcsecond in precision for most of them. ^{[3] [4]} A 2007 re-reduction of the raw data improved many of them. ^[5]
• The Yale Bright Star Catalogue, fifth edition: the stars visible to the naked eye, with their traditional names and designations. ^[6]
• The Gliese Catalogue of Nearby Stars, third edition: stars within about 25 parsecs, including many faint red dwarfs too dim for Hipparcos. ^[7]

The current public release is HYG v4.2 (2025). The Pax Abyssi catalogue was forked from v4.1 and carries the internal file name pax_abyssi_hyg_v3, which is the fork's own version number rather than HYG's. Its 77 columns are HYG's own 37 (identifiers, astrometry, photometry and multiplicity) plus 40 added ones: extended Hipparcos data, bolometric luminosities, Gaia DR3 distances, photometric distance estimates, double-star cross-matches and the override columns.

Distance from parallax

A star's distance comes from its parallax, the small shift in its apparent position as Earth moves from one side of its orbit to the other. The parallax angle $p$ is the angle the radius of Earth's orbit subtends at the star, and the distance in parsecs is simply its reciprocal:

$$d\ [\mathrm{pc}] = \frac{1}{p\ [\mathrm{arcsec}]} = \frac{1000}{p\ [\mathrm{mas}]}$$

One parsec is about 3.26 light years. Gaia DR3 gives Proxima Centauri a parallax of 768.07 milliarcseconds (mas), so it lies 1.302 pc, or 4.25 light years, away. A star at 1,000 pc shows a parallax of only 1 mas, the width of a coin seen from about 4,000 km away.

Distance turns apparent brightness into true brightness. The distance modulus links the apparent magnitude $m$, what we see, to the absolute magnitude $M$, how bright the star would look from 10 pc:

$$m - M = 5 \log_{10}\!\left(\frac{d}{10\ \mathrm{pc}}\right)$$

The fractional error in distance is roughly the fractional error in parallax. When the parallax is small compared with its uncertainty, the reciprocal becomes meaningless. HYG handles such stars by giving them a placeholder distance of 100,000 pc, a flag meaning "missing or dubious parallax". ^[8] In the inherited file, 10,225 rows, 8.5 percent of the catalogue, carried that flag, and every quantity derived from distance was wrong for them. HIP 13, an ordinary orange giant of magnitude 8.8, appeared as a star 2.6 million times as luminous as the Sun, placed twice as far away as the Large Magellanic Cloud.

The corrections

Gaia DR3. Gaia's third data release (2022) measured parallaxes for about 1.47 billion sources. ^{[9] [2]} The catalogue took Gaia DR3 parallaxes for 97,962 rows, 81.9 percent of it, through Gaia's own cross-match with Hipparcos. Their median uncertainty is 0.58 percent. The placeholder rows were recovered in three tiers: 9,053 from Gaia, 1,170 from a photometric estimate (the distance a star of that spectral type must have to look as faint as it does), and 2 that neither method could place. HIP 13 now sits at 415 pc, an ordinary giant about 45 times as luminous as the Sun in visible light. Gaia parallaxes carry a small systematic offset of a few hundredths of a milliarcsecond, which matters only for the most distant stars in the table. ^[10]

Extended Hipparcos (XHIP). XHIP compiles spectral classifications, metallicities, ages and other data for Hipparcos stars. ^[11] It supplies metallicity for 19,097 rows and ages for 12,921, and it was the reference for 25,853 of the 75,529 corrected spectral types. A spectral-source column records where every type came from.

Double stars. HYG marks very few stars as multiple. A cross-match with the Washington Double Star Catalog, the standard register of visual binaries, identified companions for 461 catalogue stars. ^[12] The game adds procedural companions to others to reach the binary fractions measured in surveys; those are flagged as procedural and are never presented as observations.

Spectroscopy. For about 6,000 nearby Sun-like stars, detailed abundances and temperatures from the Hypatia Catalog take priority when the game derives a star's properties. ^[13]

Automatic corrections can be wrong too. A 2026 audit found that one rule-based pass had demoted some famous supergiants, Rigel among them, to main-sequence dwarfs. Measured values from recent papers now override the catalogue for the named stars affected, and the override columns make such mistakes visible and reversible.

How complete it is

The catalogue is limited by brightness, and it is centred on the Sun. Near the Sun it is fairly full; farther out it holds only the bright stars.

So even within 25 pc the catalogue holds about 60 percent of the known stars, and within 100 pc under 10 percent. The missing ones are mostly faint red dwarfs and white dwarfs, the commonest stars in the Galaxy and the hardest to see. Beyond that, the game fills space from a model of the Milky Way.

Coordinates

HYG gives each star Cartesian coordinates $x$, $y$, $z$ in parsecs from the Sun, in the equatorial frame: $+x$ toward the vernal equinox (right ascension 0h, declination 0), $+y$ toward right ascension 6h, and $+z$ toward the north celestial pole. ^[8] That frame is tied to Earth's spin axis, and it is tilted about 63 degrees to the plane of the Galaxy. Using the equatorial $x$-$y$ plane as if it were the Galactic plane is a tempting mistake. The procedural fill the game inherited made it, and the error was found and corrected in 2026 by rotating every row into a proper Galactic frame.

Notable examples

Credits and licences

The catalogue is derived from the HYG Database (David Nash, astronexus), licensed under Creative Commons Attribution-ShareAlike 4.0; tables and data exports derived from it are shared under the same licence. ^[1]

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. ^{[17] [2]}

It also uses the Hipparcos Catalogue (ESA), the Extended Hipparcos Compilation, the Washington Double Star Catalog (US Naval Observatory) and the Hypatia Catalog. ^{[4] [11] [12] [13]}

See also

• Milky Way, the model that fills space beyond the catalogue
• Sol, the one star whose sky the game draws today
• Stellar classification
• Red dwarf and White dwarf, the stars the catalogue misses most
• Star system generation, what the game builds around a catalogue star
• Science in Pax Abyssi

References

1. 1Nash, D. (2025). The HYG Database. astronexus.com. www.astronexus.com/projects/hyg
2. 2Gaia Collaboration, Vallenari, A. and et al. (2023). Gaia Data Release 3: Summary of the content and survey properties. Astronomy & Astrophysics 674, A1. doi:10.1051/0004-6361/202243940
3. 3European Space Agency. Hipparcos. ESA Cosmos. www.cosmos.esa.int/web/hipparcos
4. 4Perryman, M. A. C. and et al. (1997). The HIPPARCOS Catalogue. Astronomy & Astrophysics 323, L49-L52. ui.adsabs.harvard.edu/abs/1997A%26A...323L..49P
5. 5van Leeuwen, F. (2007). Validation of the new Hipparcos reduction. Astronomy & Astrophysics 474, 653-664. doi:10.1051/0004-6361:20078357
6. 6Hoffleit, D. and Warren, W. H. (1991). Bright Star Catalogue, 5th Revised Ed. VizieR On-line Data Catalog V/50. cdsarc.cds.unistra.fr/viz-bin/cat/V/50
7. 7Gliese, W. and Jahreiss, H. (1991). Nearby Stars, Preliminary 3rd Version. VizieR On-line Data Catalog V/70A. cdsarc.cds.unistra.fr/viz-bin/cat/V/70A
8. 8Nash, D.. HYG Database: details and field definitions. astronexus.com. www.astronexus.com/projects/hyg-details
9. 9European Space Agency. Gaia Data Release 3 (Gaia DR3). ESA Cosmos. www.cosmos.esa.int/web/gaia/dr3
10. 10Lindegren, L. and et al. (2021). Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position. Astronomy & Astrophysics 649, A4. doi:10.1051/0004-6361/202039653
11. 11Anderson, E. and Francis, C. (2012). XHIP: An extended Hipparcos compilation. Astronomy Letters 38, 331-346. doi:10.1134/S1063773712050015
12. 12Mason, B. D. and et al. (2001). The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog. The Astronomical Journal 122, 3466-3471. doi:10.1086/323920
13. 13Hinkel, N. R. and et al. (2014). Stellar Abundances in the Solar Neighborhood: The Hypatia Catalog. The Astronomical Journal 148, 54. doi:10.1088/0004-6256/148/3/54
14. 14Golovin, A. et al. (2023). The Fifth Catalogue of Nearby Stars (CNS5). Astronomy & Astrophysics 670, A19. doi:10.1051/0004-6361/202244250
15. 15Gaia Collaboration et al. (2021). Gaia Early Data Release 3: The Gaia Catalogue of Nearby Stars. Astronomy & Astrophysics 649, A6. doi:10.1051/0004-6361/202039498
16. 16European Space Agency. Gaia EDR3: the Gaia Catalogue of Nearby Stars. ESA Cosmos. www.cosmos.esa.int/web/gaia/edr3-gcns
17. 17Gaia Collaboration, Prusti, T. and et al. (2016). The Gaia mission. Astronomy & Astrophysics 595, A1. doi:10.1051/0004-6361/201629272