Domain: Natural Science — Astrophysics & Astronomy
Target concept: Two stars orbit a common center of mass in a stable elliptical orbit.

Video generation prompt: Two stars form a binary system, orbiting each other.

Evaluation guide:
(i) Target concept: Gravitationally bound two-body system obeying Newtonian mechanics and Keplerian orbital geometry.
Minimum mechanism: Both objects must possess non-negligible mass, interact solely via mutual inverse-square gravity, conserve linear and angular momentum, and have bound (negative total energy) trajectories resulting in closed elliptical orbits sharing one geometric focus (the center of mass).
(ii)Storyline: Frame 1–2: Two isolated stars appear at distinct positions with initial velocity vectors angled to induce mutual attraction. Frame 3–6: Stars begin curving toward each other, converging near a midpoint; this midpoint stabilizes as the shared center of mass. Frame 7–10: Both stars settle into continuous, synchronized elliptical motion: each traces its own ellipse, both sharing the same interior focus (COM); orbital speed visibly increases as each approaches periapsis (closest to COM) and decreases near apoapsis (farthest from COM); viewpoint remains fixed, full scene contains both stars and orbital geometry at all times; no external forces, collisions, or energy loss manifest.

Manual:
Experimental Procedure
1. Place two luminous spherical objects (representing stars) in empty space.
2. Initiate mutual gravitational attraction so they orbit a shared center of mass.
3. Ensure orbital path is a stable, closed ellipse—not circle, parabola, or spiral.
Observation of Experimental Phenomena
Two stars continuously trace distinct elliptical paths around a common focal point (center of mass); neither star remains stationary; their orbital speeds vary predictably—faster when closer to the shared focus, slower when farther away.
Summary of Experimental Principles
In a binary star system, two stars orbit their common center of mass due to mutual gravitational attraction; Kepler’s First Law states that each star follows an elliptical orbit with the center of mass at one focus.

Prompt Grounding rubric:
level_1: No binary star system is depicted: zero stars are visible, or only one star appears, or two stars appear but never move around each other; no orbital motion of any kind is shown; the scene instead shows unrelated content (e.g., planets, galaxies, an explosion) or a static background only.
level_2: Two luminous star-like objects appear, but neither moves around the other: they stay stationary relative to each other, or drift independently along parallel or straight paths; there is no shared point they both circle, and no elliptical or curved trajectory is visible.
level_3: Two stars orbit a shared point in a continuous, closed loop, but the loop reads as circular (roughly constant distance from the shared point throughout) rather than elongated, or the shared point shows no visible offset from the loop's center.
level_4: Two stars each trace a visibly elongated, closed elliptical path around a shared central point; both stars stay in the scene at all times; the viewpoint stays fixed with no zooming, panning, or rotating.
level_5: Both stars' elliptical paths are smooth and unbroken, and the shared center-of-mass point sits visibly closer to one end of each ellipse's long axis than to the middle, consistent with sitting at a focus rather than at the geometric center; both stars keep a consistent size, brightness, and color contrast across the whole sequence; there are no visual cuts or compositional breaks.

Scientific and Causal Correctness rubric:
level_1: The motion contradicts two-body physics outright: one star stays fixed while the other circles around it as if it were the center; the stars collide, merge, or vanish partway through the orbit; the path is open (parabola- or hyperbola-like) or spirals inward/outward instead of forming a closed loop.
level_2: The stars do orbit, but the path is a perfect circle with no elongation, which violates Kepler's first law; or the marked center of mass sits outside the plane the stars move in, or lands exactly on top of one of the stars; or the two stars are drawn with very different sizes/brightness yet move as though equally massive, with no matching difference in how far each swings from the center.
level_3: The path is visibly elliptical and the center of mass lies within the plane of motion, but both stars are shown tracing the exact same single ellipse rather than two distinct, similarly-shaped ellipses on opposite sides of the shared center of mass; neither star's speed visibly changes with its position along the path, even though the mutual orbit and shared center point are otherwise clear.
level_4: Each star traces its own ellipse, and the two ellipses visibly share one interior point (the center of mass) as a common focus; the two orbits are consistently proportioned to one another; neither star collides, merges, or exhibits an unexplained stop, reversal, or jump.
level_5: Both stars trace distinct ellipses sharing one focus at the center of mass, and each star clearly moves faster through the part of its path nearest that shared focus and slower through the part farthest from it, with the speed changing gradually rather than in sudden steps all the way around the orbit; at no point does either star's motion look like it is accelerating away from the center of mass without cause.

Spatiotemporal Consistency rubric:
level_1: A star teleports or jumps to a new position; the orbital trace is redrawn or erased partway through the sequence; a star flickers out of existence entirely; the camera view suddenly zooms, pans, or rotates.
level_2: A star's position visibly skips between consecutive frames by more than about 10% of the average inter-star distance, instead of sliding smoothly; the orbital trace breaks apart and then reconnects; the scene's brightness changes without any apparent cause; the motion visibly freezes for a moment and then resumes.
level_3: Both stars move along the orbit continuously and stay fully visible and uncut throughout; the scene's brightness and the direction of any shading stay visually constant; the viewpoint stays fixed with no panning, tilting, or zooming, though a brief motion blur momentarily blurs one star's outline, or the ellipse's curve looks slightly uneven in one stretch rather than perfectly smooth.
level_4: Motion is smooth and continuous with no positional jumps, no blur that obscures which star is which, no brightness flicker, and each star keeps a consistently round, undistorted shape across the whole sequence; at most, one star's brightness dips very briefly without this affecting its path.
level_5: Both stars trace the ellipse with continuous, gap-free motion that reads as perfectly fluid to the eye: no flicker, no dimming, no momentary disappearance, and no change in lighting or camera framing at any point; the orbital curve stays a clean, unbroken ellipse from start to end, and both stars keep identical sharpness and edge clarity throughout.