Q&A: Difference Between a Star and a Planet

Question: What the heck is the difference between a star and a planet? To me, they all just look like tiny points of light in the sky. — GB, Washington, DC

Answer:  Stars are spheres of gas that glow from the heat of nuclear fusion in their cores, whereas planets are objects that orbit stars. Furthermore, even though you didn’t ask, moons are objects that orbit planets.

But that was the easy answer. The more you study astronomy, and the often bizarre denizens of the cosmos, the more confusing these definitions become. Ponder  these  amazing  facts …

Some stars orbit other stars. Some lone planets wander the depths of space untethered to stars. There is no known upper limit to the size of solid, Earth-like planets. There are stars smaller than the Earth; they no longer burn with fusion, but are still hot enough to keep glowing for millions of years. Although there are no known examples, it would be possible for a small star to orbit a large planet. The largest known star, epsilon (ε) Aurigae, is almost 2000 times larger than our Sun. If a gas planet (like Jupiter) gets much bigger than 10 times the size of Jupiter, pressure at its core will ignite nuclear fusion and it will become a star. In that case, any moons it had would be promoted to the technical status of planets.

Confused? Don’t feel alone. Astronomers themselves are still debating these definitions.

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