Classification – Astronomy terms: pick the odd one out. Three are celestial bodies; one is a region or expanse, not a discrete body. Which option does not belong? Options: Sky, Star, Planet, Comet.

Introduction / Context:
Astronomy classifications distinguish between specific celestial bodies (star, planet, comet) and broader expanses or observational backdrops (sky). The sky is a perceived region, not an individual body.

Given Data / Assumptions:

• Star: luminous celestial body.
• Planet: non-luminous body orbiting a star.
• Comet: icy small Solar System body with coma/tail near perihelion.
• Sky: the vault/expanse as seen from Earth; not a single object.

Concept / Approach:
Classify by ontological status: discrete object vs backdrop/region.

Step-by-Step Solution:
Step 1: Tag “star/planet/comet” as bodies.Step 2: Recognize “sky” as a region of observation.Step 3: Therefore, Sky is the odd one out.

Verification / Alternative check:
Cross-check definitions in basic astronomy: only three are bodies with mass and trajectories as individual objects.

Why Other Options Are Wrong:
Star/Planet/Comet: All are discrete celestial bodies.

Common Pitfalls:
Do not confuse what we see (the sky) with objects we see within it.

Final Answer:
Sky