Say which band each star was measured in, whose catalogue it is, and how sure its distance is

The readout named Hipparcos, Yale and Gliese while 378 775 of the 455 608 stars are described by
Gaia DR3, printed one "Magnitude" for G and V alike, and gave every distance to the parsec. The
band cannot be read off the star's source, which records whose position it has: 62 002 stars Gaia
places keep HYG's V and B-V, and the 575 hosts renamed after their planets are Gaia's, in G.

stars-meta.bin gains two byte columns, 14 to 16 bytes a star (6 378 512 to 7 289 728 bytes; gzip
-9 2 804 049 to 3 110 991). One holds the magnitude's band (V 76 555 stars, G 378 744, none 309,
whose magnitude is a stand-in), which colour the colour index is (B-V 75 203, BP-RP 376 703), and
whether the distance is Gaia's parallax. The other holds the distance's relative error as its
square root in 255ths: a step is 0.08 % of distance at 1 %, 0.35 % at 20 %, and 100 % is the top.
encode, decode, BYTES_PER_STAR_META and the build.ts round trip cover both; no workflow reads the
format.

Where the errors come from:
- Gaia rows keep the parallax_error their query already fetched: median 0.3 %, 90th percentile
  1.2 %, at most 20 %, the query's own cut.
- A HYG star at Gaia's distance takes the cross-match's parallax_over_error, and a star merged
  into a Gaia entry keeps that entry's error with its position.
- The 3 067 Hipparcos stars that keep their Hipparcos distance, Rigel, Deneb and Alnilam among
  them, take e_plx from van Leeuwen's 2007 reduction: a new cached query of
  public.hipparcos_newreduction on the ESA archive, whose 117 955 rows HYG's distances invert.
- The archive's stars take sy_disterr1/2 from pscomppars, in a query and cache file of their own
  so the composite rows already cached were not refetched.
- 439 distances have no published error: 357 Gliese rows and 82 archive hosts.

Of the errors, 392 786 are 1 % or less and are not printed; 61 156 print as "117 ± 12 pc" to the
distance's own digits; 1 196 between 20 and 100 %, and 30 past it, print as the range the
parallax gives, since a symmetric error in parallax is a lopsided one in distance.

The star card (measured on the dev server) now reads, for example:
- Rigel: 265 ± 23 pc, V 0.18, B-V -0.03, source HYG.
- Deneb: 433 ± 60 pc.
- Alnilam: "476 pc to 833 pc" (Hipparcos 1.65 ± 0.45 mas).
- Gaia DR3 5612323414549657984: 111 ± 2 pc, G 4.63, BP-RP -0.15, source Gaia DR3.
- Proxima Centauri: 1.30 pc, V 11.01, source "HYG, Gaia DR3 distance".
- TRAPPIST-1: G 15.62, BP-RP 4.90, source Gaia DR3.
- Kepler-186: V 15.14, source NASA Exoplanet Archive.
The neighbourhood's subtitle reads "Gaia DR3 378,775 · HYG 73,556 · NASA Exoplanet Archive
3,277", counted by the catalogue describing each star.

build.ts validateStars now fails a catalogue with more than 1 000 stars without a band (309
today) or without a distance error (439). Dropping G from the Gaia rows gave 379 040 without a
band, and dropping their parallax_error gave 441 216 without an error; both runs failed.

Decoding the catalogue in Node took a median 29 ms before and 24 ms after (nine runs each, within
noise). In the app, five cold boots gave a 654-786 ms long task after the data landed and the
HUD at 1.83-2.07 s.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tools/etl/sources/gaia.ts

@@ -133,9 +133,9 @@ const MIN_ROW_SHARE = 0.95;
 export class GaiaAnswerError extends Error {}
 
 /**
- * Gaia publishes no spectral classifications, but `bp_rp` is a colour index on the same footing
- * as HYG's `ci` — so the app's existing colour and spectral-class handling works unchanged, and
- * the spectral type is left as unknown rather than invented from the colour.
+ * Gaia publishes no spectral classifications. Its `bp_rp` is a colour index, though not HYG's
+ * B−V — `colorSystem` says which — and the spectral type is left as unknown rather than
+ * invented from it.
  */
 const UNKNOWN_SPECTRAL_TYPE = 'Unknown';
 
@@ -209,10 +209,13 @@ function rowsToStars(rows: readonly Record<string, string>[], idBase: number): S
       return;
     }
 
+    const parallaxErrorMas = parseOptionalNumber(row['parallax_error']);
     const pmRaMasYr = parseOptionalNumber(row['pmra']);
     const pmDecMasYr = parseOptionalNumber(row['pmdec']);
     const j2000 = propagateProperMotion(raDeg, decDeg, pmRaMasYr ?? 0, pmDecMasYr ?? 0, CATALOGUE_EPOCH - GAIA_DR3_EPOCH);
     const { x, y, z } = raDegDecDistanceToXyz(j2000.raDeg, j2000.decDeg, distancePc);
+    const magnitudeG = parseOptionalNumber(row['phot_g_mean_mag']);
+    const colorIndex = parseOptionalNumber(row['bp_rp']);
     stars.push({
       id: idBase + index,
       name: `Gaia DR3 ${row['source_id']}`,
@@ -222,9 +225,13 @@ function rowsToStars(rows: readonly Record<string, string>[], idBase: number): S
       // Only the nearby query has sources without a G, 45 of them, and they are given its limit.
       // That errs bright: 26 of the 31 that 2MASS measured are at J 12.6–15.7, fainter still in G
       // for stars this red, and 5 at J 7–8.
-      magnitude: parseOptionalNumber(row['phot_g_mean_mag']) ?? MAGNITUDE_LIMIT,
+      magnitude: magnitudeG ?? MAGNITUDE_LIMIT,
+      ...(magnitudeG === undefined ? {} : { magnitudeBand: 'G' as const }),
       spectralType: UNKNOWN_SPECTRAL_TYPE,
-      colorIndex: parseOptionalNumber(row['bp_rp']) ?? null,
+      colorIndex: colorIndex ?? null,
+      ...(colorIndex === undefined ? {} : { colorSystem: 'BP-RP' as const }),
+      ...(parallaxErrorMas === undefined ? {} : { distanceError: parallaxErrorMas / parallaxMas }),
+      distanceFromGaia: true,
       source: 'gaia',
       pmRaMasYr,
       pmDecMasYr
@@ -243,7 +250,8 @@ function rowsToStars(rows: readonly Record<string, string>[], idBase: number): S
 const MIN_USABLE_HIP_DISTANCES = 90_000;
 
 /**
- * Gaia's distance for every Hipparcos star it has a usable parallax for, keyed by HIP number.
+ * Gaia's distance for every Hipparcos star it has a usable parallax for, and that distance's
+ * relative error, keyed by HIP number.
  *
  * Taken from the archive's own cross-match (`hipparcos2_best_neighbour`) rather than from
  * matching positions here, since Gaia's team made that identification star by star with the
@@ -255,7 +263,7 @@ const MIN_USABLE_HIP_DISTANCES = 90_000;
  * distance, the 6 833 that Gaia puts past 250 pc move back inside, and the published map would
  * flip between the two with the archive's availability.
  */
-export async function fetchGaiaDistancesByHip(): Promise<Map<number, number>> {
+export async function fetchGaiaDistancesByHip(): Promise<Map<number, { distancePc: number; relativeError: number }>> {
   const query = [
     'select top 200000 b.original_ext_source_id as hip, g.parallax, g.parallax_over_error',
     'from gaiadr3.hipparcos2_best_neighbour b join gaiadr3.gaia_source g on g.source_id = b.source_id'
@@ -264,13 +272,13 @@ export async function fetchGaiaDistancesByHip(): Promise<Map<number, number>> {
   console.log('Fetching Gaia DR3 distances for Hipparcos stars (archive cross-match)...');
   const rows = parseCsvObjects(await fetchTextCached(url, `gaia-dr3-hip-${createHash('sha1').update(url).digest('hex').slice(0, 8)}.csv`));
 
-  const distances = new Map<number, number>();
+  const distances = new Map<number, { distancePc: number; relativeError: number }>();
   for (const row of rows) {
     const hip = parseOptionalNumber(row['hip']);
     const parallaxMas = parseOptionalNumber(row['parallax']);
     const overError = parseOptionalNumber(row['parallax_over_error']);
     if (hip !== undefined && parallaxMas !== undefined && parallaxMas > 0 && overError !== undefined && overError > 1 / MAX_PARALLAX_ERROR_RATIO) {
-      distances.set(hip, 1000 / parallaxMas);
+      distances.set(hip, { distancePc: 1000 / parallaxMas, relativeError: 1 / overError });
     }
   }
   if (distances.size < MIN_USABLE_HIP_DISTANCES) {
@@ -282,3 +290,33 @@ export async function fetchGaiaDistancesByHip(): Promise<Map<number, number>> {
   console.log(`  ${distances.size} Hipparcos stars have a Gaia distance (of ${rows.length} cross-matched).`);
   return distances;
 }
+
+/** The new Hipparcos reduction holds 117 955 stars, and every one has a parallax error. */
+const MIN_HIPPARCOS_ERRORS = 110_000;
+
+/**
+ * The relative error of every Hipparcos parallax, keyed by HIP number, for the stars that keep
+ * their Hipparcos distance: 3 067 of them, Rigel and Deneb among them, where Gaia saturates.
+ *
+ * From van Leeuwen's 2007 reduction, which the ESA archive hosts beside Gaia and HYG's distances
+ * are the inverse of. HYG publishes the distance and not its error.
+ */
+export async function fetchHipparcosParallaxErrors(): Promise<Map<number, number>> {
+  const url = `${GAIA_TAP_URL}?REQUEST=doQuery&LANG=ADQL&FORMAT=csv&QUERY=${encodeURIComponent('select top 200000 hip, plx, e_plx from public.hipparcos_newreduction order by hip')}`;
+  console.log('Fetching Hipparcos parallax errors (new reduction)...');
+  const rows = parseCsvObjects(await fetchTextCached(url, `hipparcos-errors-${createHash('sha1').update(url).digest('hex').slice(0, 8)}.csv`));
+
+  const errors = new Map<number, number>();
+  for (const row of rows) {
+    const hip = parseOptionalNumber(row['hip']);
+    const parallaxMas = parseOptionalNumber(row['plx']);
+    const errorMas = parseOptionalNumber(row['e_plx']);
+    if (hip !== undefined && parallaxMas !== undefined && parallaxMas > 0 && errorMas !== undefined) {
+      errors.set(hip, errorMas / parallaxMas);
+    }
+  }
+  if (errors.size < MIN_HIPPARCOS_ERRORS) {
+    throw new Error(`the Hipparcos reduction gave ${errors.size} parallax errors (of ${rows.length} rows), not the ~117 955 it holds; delete tools/etl/.cache/hipparcos-errors-*.csv once the archive answers properly`);
+  }
+  return errors;
+}