A star: an astronomical object that emits its own light and heat, like the Sun (powered by nuclear fusion).
太陽は私たちの恒星だ。
The Sun is our star (a star).
望遠鏡で恒星の光を詳しく観測した。
We observed the light of a star in detail with a telescope.
恒星の寿命は質量によって大きく変わる。
A star's lifetime varies greatly depending on its mass.