Which definition describes a ball of hot gas, primarily hydrogen and helium, that undergoes nuclear fusion?

• A. Star
• B. Planet
• C. Comet
• D. Meteor

Answer: (A)

That description matches a star. A star is a hot ball of gas, mostly hydrogen and helium, that produces energy through nuclear fusion in its core. Hydrogen nuclei fuse to form helium, releasing a tremendous amount of energy as light and heat. The extreme temperatures and pressures inside a star make fusion possible, keeping it shining for millions to billions of years. Planets aren’t hot enough to fuse elements and simply orbit stars; comets are icy bodies with tails, and meteors are space rocks that burn up in Earth's atmosphere. So the defining idea here is a star—a luminous object powered by nuclear fusion.