# Extra catalogue stars (`data/stars_extra.csv`) — sources and licence

`data/stars_extra.csv` adds the 3,952 exoplanet host stars that the HYG
catalogue (`data/stars.csv`) does not contain, at any distance — faint M
dwarfs such as TRAPPIST-1, Teegarden's Star, L 98-59, LHS 3844, K2-18 and
most TESS hosts within 100 pc (the first 312 rows, the 2026-09-27 build), then
the Kepler, K2, TESS, WASP, HAT, imaging and microlensing hosts beyond it, out
to the Galactic bulge (27,700 ly) — so their planets have a star on the map.
1,281 of them lie within 1,000 ly (the Gaia field's reach), 2,671 beyond. It has the same columns as
`data/stars.csv`; ids start at 10,000,001 so they can never collide with HYG
ids (all below 120,000). `StarCatalog` loads it after `data/stars.csv`, so
these stars take catalogue indices 30,036 and up. The game runs on HYG alone
when the file is missing.

Generated by `tools/build_extra_stars.py` (Python 3 standard library; run it,
then `tools/build_exoplanets.py` and `tools/fetch_gaia.py --build`, to rebuild
the planets and the Gaia cross-match against it). Built 2026-09-27, extended
to every distance 2026-09-29 (new rows appended, earlier ones unchanged). Rows for
these stars in `data/star_aliases.csv` (HD, HIP, TIC, Gaia DR3 designations)
come from the same tool.

## Sources

* **NASA Exoplanet Archive**, Planetary Systems Composite Parameters table
  (`pscomppars`, DOI [10.26133/NEA13](https://doi.org/10.26133/NEA13)): which
  stars host confirmed planets, their names, HD / HIP / TIC / Gaia DR3
  identifiers and spectral types (`st_spectype`, empty when the archive has
  none), queried 2026-09-27.

  *This research has made use of the NASA Exoplanet Archive, which is operated
  by the California Institute of Technology, under contract with the National
  Aeronautics and Space Administration under the Exoplanet Exploration
  Program.*

* **Gaia Data Release 3** (`gaiadr3.gaia_source`), ESA Gaia Archive: position,
  parallax (distance = 1 / parallax), proper motion (to move the epoch-2016.0
  positions back to J2000, the epoch of the HYG positions), G and BP−RP.

  *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.* Credit **ESA/Gaia/DPAC**.
  Reference: Gaia Collaboration, Vallenari A. et al. 2023, A&A 674, A1.

* **Photometric transformations** (Gaia G, BP−RP → Johnson V, B−V):
  * G − V for −0.5 ≤ BP−RP ≤ 2.75: Riello M. et al. 2021, A&A 649, A3,
    Table C.2 (G − V = −0.02704 + 0.01424 c − 0.2156 c² + 0.01426 c³,
    σ = 0.03 mag).
  * G − V for redder stars, and B − V throughout: the dwarf colour sequence of
    Pecaut M. J. & Mamajek E. E. 2013, ApJS 208, 9 (E. Mamajek, "A Modern Mean
    Dwarf Stellar Color and Effective Temperature Sequence", version
    2022.04.16, https://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt),
    linearly interpolated in BP−RP (or in T_eff when BP−RP is missing).
  * Check: TRAPPIST-1 comes out at V = 18.73 (measured 18.80, Costa et al.
    2006), Teegarden's Star at 15.02 (measured 15.13).
* **Constellations**: Roman N. G. 1987, PASP 99, 695, "Identification of a
  Constellation From a Position" (CDS catalogue VI/42), after IAU 1976
  precession of the J2000 position to B1875.
* Hosts within 400 pc the archive gives no Gaia id get one from a 10″ cone
  search in Gaia DR3 (TOI-6000 and others; TOI-2267 B shares TOI-2267 A's
  source, so its planet is drawn at that point). Those with only a blended
  Gaia source (K2-288 B, 2MASS J01033563-5515561 A, Kepler-632, Kepler-636)
  use the archive's position, distance and V.
* **Distances beyond 100 pc**: the Gaia parallax when it is measured to 20 %
  (parallax_over_error ≥ 5), else the archive's `sy_dist` (mostly the
  Bailer-Jones et al. 2021, AJ 161, 147 photogeometric distances).
* **Colours beyond 100 pc**: interstellar reddening makes BP−RP too red, so
  B−V comes from the archive's T_eff along the Pecaut & Mamajek sequence
  (BP−RP when there is no T_eff).
* **No optical magnitude** (most microlensing hosts, 259 stars): V is
  M_V + 5 log₁₀(d / 10 pc) with M_V from the host's T_eff, else its mass,
  along the Pecaut & Mamajek dwarf sequence — no extinction, so an upper
  limit on the brightness.
* Left out (no planet system for them): 18 hosts with no distance at all
  (no archive `sy_dist`, no Gaia parallax to 20 %) and 15 with no magnitude of
  any kind (three nearby T / Y brown dwarfs and twelve microlensing
  brown-dwarf lenses); `tools/build_extra_stars.py` lists them.

## Licence

The file combines Gaia DR3 values, which are distributed under
**Creative Commons Attribution-NonCommercial 3.0 IGO (CC BY-NC 3.0 IGO)**
(https://creativecommons.org/licenses/by-nc/3.0/igo/; see
`data/GAIA_LICENSE.md` for the ESA terms and what the NonCommercial term means
for Expanse), with facts from the NASA Exoplanet Archive (which asks for the
acknowledgement above). `data/stars_extra.csv` is therefore distributed under
**CC BY-NC 3.0 IGO**, credit ESA/Gaia/DPAC and the NASA Exoplanet Archive.
Expanse is a free, MIT-licensed game, which is fine; commercial distribution
needs ESA's permission — or drop this file (the map falls back to HYG alone,
and the planets of these stars lose their system).

## What was changed

Selected stars and columns; distances from parallax; V and B−V computed from
Gaia photometry with the transformations above; positions converted to
heliocentric equatorial parsecs (HYG's x, y, z convention) at epoch J2000;
constellation computed. See `tools/build_extra_stars.py` for the exact
queries and formulas.