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>

src/app/shared/models/star-catalog.ts

@@ -26,13 +26,38 @@ export const STAR_POSITION_COMPONENTS = 3;
 export const BYTES_PER_STAR_POSITION = STAR_POSITION_COMPONENTS * Float32Array.BYTES_PER_ELEMENT;
 
 /**
- * Columns in `stars-meta.bin`, in order: catalogue id, apparent magnitude, colour index, and an
- * index into the spectral-type dictionary. Stored column by column rather than record by record
- * so each one is a single typed-array view over the buffer, with no per-record stride or
- * alignment padding.
+ * Columns in `stars-meta.bin`, in order: catalogue id, apparent magnitude, colour index, an
+ * index into the spectral-type dictionary, what those were measured in (see {@link PHOTOMETRY}),
+ * and the distance's relative error (see {@link DISTANCE_ERROR_STEPS}). Stored column by column
+ * rather than record by record so each one is a single typed-array view over the buffer, with no
+ * per-record stride or alignment padding.
  */
 export const BYTES_PER_STAR_META =
-  Int32Array.BYTES_PER_ELEMENT + Float32Array.BYTES_PER_ELEMENT + Float32Array.BYTES_PER_ELEMENT + Uint16Array.BYTES_PER_ELEMENT;
+  Int32Array.BYTES_PER_ELEMENT +
+  Float32Array.BYTES_PER_ELEMENT +
+  Float32Array.BYTES_PER_ELEMENT +
+  Uint16Array.BYTES_PER_ELEMENT +
+  Uint8Array.BYTES_PER_ELEMENT +
+  Uint8Array.BYTES_PER_ELEMENT;
+
+/**
+ * Bits of the photometry column. The band takes two: none (a stand-in magnitude), V or G. None
+ * of it follows from the source, which records where the *position* came from: 62 002 stars
+ * Gaia places keep HYG's V and B−V, and the archive's stars in G have a B−V from their
+ * temperature. The last bit says whose parallax the distance is, which for a HYG star Gaia did
+ * not place can still be Gaia's.
+ */
+const PHOTOMETRY = { bandV: 1, bandG: 2, bandMask: 3, colorBpRp: 4, distanceFromGaia: 8 } as const;
+
+/**
+ * The distance error column holds the square root of the relative error, in 255ths, and 0 where
+ * none was published. The errors span three orders of magnitude — Gaia's are a median 0.3 % and
+ * at most 20 %, the cut its queries make, while a Hipparcos parallax the map keeps for a bright
+ * star can be as large as itself — and the square root keeps a step small at each end: 0.08 % of
+ * distance at 1 %, 0.35 % at 20 %, under 1 % at 100 %. Anything past 100 % is stored as that,
+ * where it no longer bounds the distance from above.
+ */
+const DISTANCE_ERROR_STEPS = 255;
 
 /** `stars-index.json`: everything that is a string, plus the count the columns are sized by. */
 export interface StarCatalogIndex {
@@ -81,6 +106,8 @@ interface StarMetaColumns {
   magnitudes: Float32Array;
   colorIndices: Float32Array;
   spectralTypeIndices: Uint16Array;
+  photometry: Uint8Array;
+  distanceErrors: Uint8Array;
 }
 
 /** Lays typed-array views over the meta buffer at the offsets the format defines. */
@@ -93,8 +120,12 @@ function metaColumns(buffer: ArrayBuffer, count: number): StarMetaColumns {
   const colorIndices = new Float32Array(buffer, offset, count);
   offset += count * Float32Array.BYTES_PER_ELEMENT;
   const spectralTypeIndices = new Uint16Array(buffer, offset, count);
+  offset += count * Uint16Array.BYTES_PER_ELEMENT;
+  const photometry = new Uint8Array(buffer, offset, count);
+  offset += count * Uint8Array.BYTES_PER_ELEMENT;
+  const distanceErrors = new Uint8Array(buffer, offset, count);
 
-  return { ids, magnitudes, colorIndices, spectralTypeIndices };
+  return { ids, magnitudes, colorIndices, spectralTypeIndices, photometry, distanceErrors };
 }
 
 /**
@@ -152,6 +183,13 @@ export function encodeStarCatalog(stars: readonly StarRecord[]): {
     columns.magnitudes[index] = star.magnitude;
     columns.colorIndices[index] = star.colorIndex ?? Number.NaN;
     columns.spectralTypeIndices[index] = spectralTypeId;
+    columns.photometry[index] =
+      (star.magnitudeBand === 'V' ? PHOTOMETRY.bandV : star.magnitudeBand === 'G' ? PHOTOMETRY.bandG : 0) |
+      (star.colorSystem === 'BP-RP' ? PHOTOMETRY.colorBpRp : 0) |
+      (star.distanceFromGaia ? PHOTOMETRY.distanceFromGaia : 0);
+    // At least one step, so an error too small to round to one is not read back as none published.
+    columns.distanceErrors[index] =
+      star.distanceError === undefined ? 0 : Math.max(1, Math.round(Math.sqrt(Math.min(1, star.distanceError)) * DISTANCE_ERROR_STEPS));
   });
 
   // A per-star column is only worth writing when the stars actually differ.
@@ -186,6 +224,9 @@ export function decodeStarCatalog(index: StarCatalogIndex, positions: Float32Arr
     const id = columns.ids[i];
     const sourceIndex = index.sourceIndices.length > 0 ? index.sourceIndices[i] : index.sources.length === 1 ? 0 : -1;
     const source = index.sources[sourceIndex];
+    const photometry = columns.photometry[i];
+    const band = photometry & PHOTOMETRY.bandMask;
+    const distanceError = columns.distanceErrors[i];
 
     stars[i] = {
       id,
@@ -196,6 +237,11 @@ export function decodeStarCatalog(index: StarCatalogIndex, positions: Float32Arr
       magnitude: columns.magnitudes[i],
       spectralType: index.spectralTypes[columns.spectralTypeIndices[i]],
       colorIndex: Number.isNaN(colorIndex) ? null : colorIndex,
+      // Set on every record, if only to undefined, so that all of them have the one shape.
+      magnitudeBand: band === PHOTOMETRY.bandV ? 'V' : band === PHOTOMETRY.bandG ? 'G' : undefined,
+      colorSystem: Number.isNaN(colorIndex) ? undefined : photometry & PHOTOMETRY.colorBpRp ? 'BP-RP' : 'B-V',
+      distanceError: distanceError === 0 ? undefined : (distanceError / DISTANCE_ERROR_STEPS) ** 2,
+      distanceFromGaia: (photometry & PHOTOMETRY.distanceFromGaia) !== 0,
       ...(source ? { source: source.id } : {})
     };
   }