Keep the star visible at the distance that frames its system

Framing the whole system pushed the camera far enough back that the star
at the centre became a speck — about a pixel across for the Sun.

The cause is a constraint that cannot be tuned away. A star is sized
against its system's innermost orbit, because it must never swallow its
closest planet, while the camera is placed to frame the outermost ring.
In the solar system those differ by a factor of a hundred: at the distance
that fits Pluto in view, a disc that stays clear of Mercury is a pixel
across. No radius satisfies both, because the information genuinely does
not fit on one screen at that zoom.

So the disc stays honest to the orbits and the halo carries the
visibility. Light is not a surface: a glow that reaches past the innermost
orbit says the star is bright, not that it is large. Its extent is still a
multiple of the star — so a compact system keeps exactly the corona it had
— but floored against the framed radius, which is what the wide systems
needed.

The disc grows a little too: it may now reach 45% of the innermost orbit
rather than 35%, which still leaves clear space between the star's limb
and the closest orbit.

Also makes createGlowSprite take the extent it will draw rather than a
radius and a multiplier. The two were only ever multiplied together, and
how large a star's halo should be is not a property of the star — it
depends on how its system is framed, which is a decision that belongs with
the framing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaySiNst4HhDXBHnMy8p5G

src/app/features/galaxy-system/galaxy-system-scene.component.ts

@@ -19,7 +19,7 @@ import { CameraRigController } from './camera-rig-controller';
 import { DeepSkyRenderer } from './deep-sky-renderer';
 import { galacticNormal, PolarGridPlane, TetherField } from './grid-plane';
 import { MilkyWayRenderer } from './milky-way-renderer';
-import { starMarkerRadiusAu, systemFramingDistanceAu, systemViewDirection } from './system-framing';
+import { starGlowExtentAu, starMarkerRadiusAu, systemFrameRadiusAu, systemFramingDistanceAu, systemViewDirection } from './system-framing';
 import { HudReadout, StarmapHudComponent } from './starmap-hud.component';
 import { colorIndexToRgb, StarFieldRenderer } from './star-field-renderer';
 import { LabeledPoint, StarLabelOverlay } from './star-label-overlay';
@@ -27,7 +27,8 @@ import { SystemOrbitsRenderer } from './system-orbits-renderer';
 
 /** HYG catalog id for the Sun itself — the only star we have a real close-up photo of. */
 const SOL_STAR_ID = 0;
-const SUN_GLOW_SCALE = 3.2;
+/** Stars drawn from a colour rather than a photograph get a more restrained halo. */
+const DIM_STAR_GLOW_SCALE = 0.6;
 
 /** Stars closer than this to the camera get a name label (always includes the selection). */
 const LABEL_MAX_DISTANCE_PC = 20;
@@ -698,6 +699,14 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     this.systemRenderer = new SystemOrbitsRenderer(systemBodies, systemExoplanets, { x: star.x, y: star.y, z: star.z }, hostLuminosity);
     this.systemGroup.add(this.systemRenderer.object);
 
+    // Framed against the grid's outer ring rather than the outermost orbit — the ring is always
+    // the wider of the two — and against the camera this scene actually has, so the margin holds
+    // whatever the window shape. Computed before the star, because how far away the star will be
+    // seen from is what decides how big its halo has to be to stay visible.
+    const viewport = { fovDegrees: camera.fov, aspect: camera.aspect };
+    const framingDistance = systemFramingDistanceAu(this.systemRenderer.gridOuterRadiusAu, viewport);
+    const frameRadiusAu = systemFrameRadiusAu(framingDistance, viewport);
+
     // Sized against this system's innermost orbit, so the star never swallows its own planets.
     const starRadiusAu = starMarkerRadiusAu(this.systemRenderer.minTopLevelSemiMajorAxisAu);
     this.starMarkerGeometry?.dispose();
@@ -710,10 +719,10 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
       // other point in the galaxy view is far too distant to be resolved as a disk.
       starMarkerMaterial.map = loadCachedTexture(SUN_TEXTURE_PATH);
       starMarkerMaterial.color.set(0xffffff);
-      this.starGlow = createGlowSprite(0xfff2c0, starRadiusAu, SUN_GLOW_SCALE);
+      this.starGlow = createGlowSprite(0xfff2c0, starGlowExtentAu(starRadiusAu, frameRadiusAu));
     } else {
       starMarkerMaterial.color.copy(starColor);
-      this.starGlow = createGlowSprite(starColor, starRadiusAu, SUN_GLOW_SCALE * 0.6);
+      this.starGlow = createGlowSprite(starColor, starGlowExtentAu(starRadiusAu, frameRadiusAu, DIM_STAR_GLOW_SCALE));
     }
     this.starMarker = new THREE.Mesh(this.starMarkerGeometry, starMarkerMaterial);
     this.systemGroup.add(this.starMarker, this.starGlow);
@@ -733,10 +742,6 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
 
     this.rig!.setImmediate({ position: direction.clone().multiplyScalar(SYSTEM_ENTRY_DISTANCE_AU), target: new THREE.Vector3(0, 0, 0) });
 
-    // Framed against the grid's outer ring rather than the outermost orbit — the ring is always
-    // the wider of the two — and against the camera this scene actually has, so the margin holds
-    // whatever the window shape.
-    const framingDistance = systemFramingDistanceAu(this.systemRenderer.gridOuterRadiusAu, { fovDegrees: camera.fov, aspect: camera.aspect });
     // Arrives along whichever direction the approach came from, then swings round to look down
     // on this system's own orbital plane as it settles — so the swap stays continuous but the
     // system is not presented edge-on. See `systemViewDirection`.

src/app/features/galaxy-system/system-framing.spec.ts

@@ -5,6 +5,7 @@ import { eclipticToEquatorial, OBLIQUITY_J2000_DEG } from '../../shared/astro/co
 import {
   bodyMarkerRadiusAu,
   DEFAULT_STAR_MARKER_RADIUS_AU,
+  starGlowExtentAu,
   starMarkerRadiusAu,
   systemFrameRadiusAu,
   systemFramingDistanceAu,
@@ -112,6 +113,75 @@ describe('systemFramingDistanceAu', () => {
   });
 });
 
+describe('starGlowExtentAu', () => {
+  /** Apparent size on screen, as a fraction of the frame's half-height. */
+  function apparentFraction(innermostAu: number, outermostAu: number, glowScale = 1): number {
+    const rings = systemGridRingsAu(outermostAu);
+    const distance = systemFramingDistanceAu(rings[rings.length - 1]);
+    const frame = systemFrameRadiusAu(distance);
+    // The sprite's extent is its full width, so half of it is what reaches out from the star.
+    return starGlowExtentAu(starMarkerRadiusAu(innermostAu), frame, glowScale) / 2 / frame;
+  }
+
+  it('scales with the star for a compact system, where the star is already big enough', () => {
+    // A tight frame relative to the star, so the star's own multiple is what decides.
+    const marker = 0.02;
+    const tightFrame = 0.5;
+    expect(starGlowExtentAu(marker, tightFrame)).toBeCloseTo(marker * 3.2, 9);
+    expect(starGlowExtentAu(marker * 2, tightFrame)).toBeCloseTo(marker * 2 * 3.2, 9);
+  });
+
+  it('floors against the frame once the star would otherwise vanish into it', () => {
+    // A star sized against a close-in orbit, framed from far enough out to hold a wide system:
+    // the multiple of the star is nothing, so the frame decides instead.
+    const tinyStar = 0.001;
+    const wideFrame = 56;
+    expect(starGlowExtentAu(tinyStar, wideFrame)).toBeGreaterThan(tinyStar * 3.2 * 100);
+  });
+
+  it('keeps the Sun visible at the distance that frames the solar system', () => {
+    // The case that prompted this: the solar system spans a factor of a hundred from Mercury to
+    // Pluto, so a disc that stays clear of Mercury is about a pixel across once Pluto is in view.
+    expect(apparentFraction(0.387, 39.288)).toBeGreaterThan(0.015);
+  });
+
+  it('holds the floor across every system scale the datasets contain', () => {
+    // A compact system's star is genuinely large relative to its own system and keeps the bigger
+    // halo; the floor is not there to equalise them, only to stop the wide ones disappearing.
+    for (const [innermost, outermost] of [
+      [0.387, 39.288],
+      [0.035, 0.204],
+      [0.01154, 0.06189],
+      [1.2, 12.4]
+    ]) {
+      expect(apparentFraction(innermost, outermost)).toBeGreaterThan(0.015);
+    }
+  });
+
+  it('does not blot out the system it sits in', () => {
+    for (const [innermost, outermost] of [
+      [0.387, 39.288],
+      [0.035, 0.204],
+      [0.01154, 0.06189]
+    ]) {
+      expect(apparentFraction(innermost, outermost)).toBeLessThan(0.2);
+    }
+  });
+
+  it('dims for a star drawn from a colour rather than a photograph, but never below the floor', () => {
+    // Above the floor the multiplier applies...
+    expect(starGlowExtentAu(1, 10, 0.6)).toBeLessThan(starGlowExtentAu(1, 10, 1));
+    // ...and at the floor it cannot dim a star into invisibility.
+    expect(starGlowExtentAu(0.001, 56, 0.6)).toBe(starGlowExtentAu(0.001, 56, 1));
+  });
+
+  it('falls back to the star alone when there is no frame to measure against', () => {
+    for (const frame of [0, -1, Number.NaN]) {
+      expect(starGlowExtentAu(0.2, frame)).toBeCloseTo(0.2 * 3.2, 9);
+    }
+  });
+});
+
 describe('the grid and the framing together', () => {
   /** What the scene actually composes: rings from the orbits, then a distance from the rings. */
   function fit(outermostOrbitAu: number, viewport?: SystemViewport): { ring: number; frame: number } {

src/app/features/galaxy-system/system-framing.ts

@@ -25,7 +25,25 @@ export const DEFAULT_STAR_MARKER_RADIUS_AU = 0.2;
  * space between the star's limb and the closest orbit, rather than the orbit grazing or
  * disappearing inside it.
  */
-const STAR_RADIUS_TO_INNERMOST_ORBIT = 0.35;
+const STAR_RADIUS_TO_INNERMOST_ORBIT = 0.45;
+
+/**
+ * Halo extent as a multiple of the star's own radius, and the floor on that extent as a
+ * fraction of the framed radius.
+ *
+ * The floor is what keeps a star visible. A system's star is sized against its *innermost*
+ * orbit — it must never swallow its closest planet — while the camera is placed to frame the
+ * *outermost* ring, and those differ by a factor of a hundred in the solar system. At the
+ * distance that fits Pluto in view, a disc that stays clear of Mercury is about one pixel
+ * across; there is no radius that satisfies both, because the information genuinely does not
+ * fit on one screen at that zoom.
+ *
+ * The halo resolves it, because light is not a surface: a glow that reaches past the innermost
+ * orbit does not claim the star is that large, it claims the star is bright. So the disc stays
+ * honest to the orbits and the halo is floored against the frame.
+ */
+const STAR_GLOW_TO_MARKER = 3.2;
+const MIN_STAR_GLOW_TO_FRAME = 0.035;
 
 /**
  * Clear space left around the framed radius, as a fraction of it. The camera backs off this
@@ -125,6 +143,20 @@ export function systemFrameRadiusAu(distanceAu: number, viewport: SystemViewport
   return distanceAu * tightHalfExtent(viewport);
 }
 
+/**
+ * Extent (AU) of the star's glow sprite — how wide it is drawn, not its radius.
+ *
+ * Normally a multiple of the star's own radius, so a compact system keeps the corona it has.
+ * Floored against the framed radius, so a star framed from far enough out to hold its whole
+ * system still reads as a bright point rather than disappearing into it. `glowScale` lets a
+ * caller dim the halo for stars drawn without a real photograph.
+ */
+export function starGlowExtentAu(markerRadiusAu: number, frameRadiusAu: number, glowScale = 1): number {
+  const fromStar = markerRadiusAu * STAR_GLOW_TO_MARKER * glowScale;
+  const fromFrame = Number.isFinite(frameRadiusAu) && frameRadiusAu > 0 ? frameRadiusAu * MIN_STAR_GLOW_TO_FRAME : 0;
+  return Math.max(fromStar, fromFrame);
+}
+
 /**
  * Distance (AU) to settle the camera at so that `framedRadiusAu` fits in view with a margin
  * around it.