Keep every star the naked eye sees, Rigel, Deneb and Alnilam among them

The 250 pc cutoff took 1 543 of HYG's 8 920 stars of V 6.5 or brighter:
1 339 that both surveys put past it and 204 HYG has no distance for. Five
of the fifty brightest stars in the sky were gone (Rigel, Deneb, Alnilam,
gamma-2 Vel, Wezen), and Naos, Sadr, Aludra and Arneb with them, while
11th-magnitude Gaia stars at the same distance were drawn. The cutoff
bounds a download, not what the sky shows.

placementDistancePc now keeps a star of V 6.5 or brighter at any distance,
at the better of its two distances as before: Gaia's where the archive's
Hipparcos cross-match has a usable parallax, Hipparcos's otherwise. Gaia
saturates on the brightest, so Rigel (264.6 pc), Deneb (432.9), Alnilam
(606.1), Wezen (492.6), Naos, Sadr, Aludra and Arneb sit at their
Hipparcos distance. 1 502 HYG rows come back, 165 of the 206 without a
Hipparcos distance among them because Gaia measured them; 36 fold into a
Gaia entry. 41 naked-eye stars stay out because neither survey gives them
a distance: beta Phe, Polis, Mu Cep, Rho Cas, Eta Car, Alp Cam, Phi Cas,
Chi Aur, Psi-1 Aur, theta-1 Ori, Omi-1 Cen, 66 Ori, 16 Sgr, 10 Sge and 27
HD stars.

Measured on the rebuilt catalogue: 425 117 stars (+1 466), 8 301 of them
past 250 pc (+1 466), the same 336 within 10 pc and 3 652 within 25 pc.
Merge gate: 12 352 HYG rows without a Gaia counterpart (was 10 886; the
ceiling of 15 000 is unchanged, and its comment now counts the naked-eye
stars among the survivors) and the same 23 cross-catalogue pairs under an
arcsecond. Five more exoplanets find their host: HD 81817 b and c,
HD 158996 b, HD 208527 b, HD 220074 b. gzip -9 sizes: stars.bin
4 711 922 -> 4 728 315 B, stars-meta.bin 2 576 213 -> 2 587 977 B,
stars-index.json 3 724 855 -> 3 731 763 B.

The brief behind this also asked to cut once on the best distance, which
would drop the 6 835 stars Hipparcos puts inside 250 pc and Gaia outside.
Those already sit at Gaia's distance (4eb61ff), so nothing is misplaced,
and they stay.

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

src/app/shared/astro/star-merge.spec.ts

@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest';
 
 import { raDegDecDistanceToXyz } from './coordinates';
 import { StarRecord } from '../models/star.model';
-import { directionCosine, isSameStar, MERGE_ANGULAR_TOLERANCE_DEG, mergeStarCatalogues, placementDistancePc } from './star-merge';
+import { directionCosine, isSameStar, MERGE_ANGULAR_TOLERANCE_DEG, mergeStarCatalogues, NAKED_EYE_MAGNITUDE, placementDistancePc } from './star-merge';
 
 /** A star at a given sky position and distance, which is how catalogues actually report them. */
 function at(id: number, raDeg: number, decDeg: number, distancePc: number, overrides: Partial<StarRecord> = {}): StarRecord {
@@ -240,32 +240,44 @@ describe('mergeStarCatalogues', () => {
 
 describe('placementDistancePc', () => {
   it("draws a star both surveys measured at Gaia's distance", () => {
-    expect(placementDistancePc(120, 118.4, 250)).toBe(118.4);
+    expect(placementDistancePc(120, 118.4, 8, 250)).toBe(118.4);
   });
 
   // The case the old cut got wrong: Hipparcos inside, Gaia outside. Kept, at the distance Gaia
   // gives, rather than at one a third short or dropped for having been misplaced.
   it('keeps a star Hipparcos put inside the cutoff, where Gaia puts it, even past the cutoff', () => {
-    expect(placementDistancePc(200, 306, 250)).toBe(306);
+    expect(placementDistancePc(200, 306, 8, 250)).toBe(306);
   });
 
   // The mirror image: Hipparcos outside, Gaia inside. The Gaia download already holds the star,
   // and keeping the HYG row is what lets the merge give that entry its name.
   it('keeps a star only Gaia puts inside the cutoff', () => {
-    expect(placementDistancePc(262, 241, 250)).toBe(241);
+    expect(placementDistancePc(262, 241, 8, 250)).toBe(241);
   });
 
   it('keeps a star Gaia measured and Hipparcos gave no distance for', () => {
-    expect(placementDistancePc(undefined, 180, 250)).toBe(180);
+    expect(placementDistancePc(undefined, 180, 8, 250)).toBe(180);
   });
 
   it('falls back to Hipparcos where Gaia has no usable distance', () => {
-    expect(placementDistancePc(90, undefined, 250)).toBe(90);
+    expect(placementDistancePc(90, undefined, 8, 250)).toBe(90);
   });
 
   it('drops a star both surveys put outside, or neither measured', () => {
-    expect(placementDistancePc(300, 410, 250)).toBeNull();
-    expect(placementDistancePc(300, undefined, 250)).toBeNull();
-    expect(placementDistancePc(undefined, undefined, 250)).toBeNull();
+    expect(placementDistancePc(300, 410, 8, 250)).toBeNull();
+    expect(placementDistancePc(300, undefined, 8, 250)).toBeNull();
+    expect(placementDistancePc(undefined, undefined, 8, 250)).toBeNull();
+  });
+
+  // Rigel: Hipparcos 265 pc, and no Gaia distance, since Gaia saturates on it. The cutoff bounds a
+  // download, not what the sky shows, and a map without the middle of Orion's belt is not the sky.
+  it('keeps a star the naked eye sees at any distance, at the better one', () => {
+    expect(placementDistancePc(265, undefined, 0.18, 250)).toBe(265);
+    expect(placementDistancePc(433, 802, NAKED_EYE_MAGNITUDE, 250)).toBe(802);
+    expect(placementDistancePc(433, 802, NAKED_EYE_MAGNITUDE + 0.01, 250)).toBeNull();
+  });
+
+  it('still drops a naked-eye star no survey gives a distance for', () => {
+    expect(placementDistancePc(undefined, undefined, 3.3, 250)).toBeNull();
   });
 });

src/app/shared/astro/star-merge.ts

@@ -67,6 +67,9 @@ export const MERGE_BRIGHTER_TOLERANCE = 1;
  */
 export const MERGE_DISTANCE_RATIO_TOLERANCE = 0.5;
 
+/** The faintest star, in V, the naked eye sees under a dark sky: the traditional limit of 6.5. */
+export const NAKED_EYE_MAGNITUDE = 6.5;
+
 /**
  * Where to draw a star Hipparcos and Gaia both measured, and whether the map keeps it at all.
  *
@@ -77,15 +80,18 @@ export const MERGE_DISTANCE_RATIO_TOLERANCE = 0.5;
  * Gaia counterpart, and at the median Hipparcos had them at two-thirds of Gaia's distance.
  *
  * Now a star either survey places inside `cutoffPc` is kept, and every kept star sits where the
- * better measurement puts it, inside the cutoff or not. `null` for a star neither survey places
- * inside, or that no survey gives a distance for.
+ * better measurement puts it, inside the cutoff or not. So is every star the naked eye sees, at
+ * any distance: the cutoff took 1 543 of HYG's 8 920 stars of V 6.5 or brighter, Rigel, Deneb
+ * and Alnilam among them, while 11th-magnitude Gaia stars at the same distance were drawn. Gaia
+ * saturates on the brightest of them, so those sit at their Hipparcos distance. `null` for a star
+ * kept by neither rule, or that no survey gives a distance for.
  */
-export function placementDistancePc(hipparcosPc: number | undefined, gaiaPc: number | undefined, cutoffPc: number): number | null {
+export function placementDistancePc(hipparcosPc: number | undefined, gaiaPc: number | undefined, magnitude: number, cutoffPc: number): number | null {
   const best = gaiaPc ?? hipparcosPc;
   if (best === undefined) {
     return null;
   }
-  const inside = best <= cutoffPc || (hipparcosPc !== undefined && hipparcosPc <= cutoffPc);
+  const inside = magnitude <= NAKED_EYE_MAGNITUDE || best <= cutoffPc || (hipparcosPc !== undefined && hipparcosPc <= cutoffPc);
   return inside ? best : null;
 }