Astronomy Practice Exam

What is a star called that expands and cools after depleting its hydrogen fuel?

1. White dwarf

2. Red giant

3. Supernova

4. Neutron star

The correct answer is: Red giant

A star that expands and cools after depleting its hydrogen fuel is known as a red giant. This phase occurs in the life cycle of a star when it exhausts the hydrogen in its core and starts to fuse helium or heavier elements. During this process, the core contracts under gravity as it becomes more dense, while the outer layers expand dramatically. The star's surface temperature decreases, resulting in a reddish appearance, which is why these stars are termed "red giants." They can swell to a size large enough to engulf nearby planets, including potentially the inner planets of a solar system. Understanding this phase is crucial because it represents a significant transition in stellar evolution, ultimately leading to the end stages of a star's life, which may further evolve into different forms based on its initial mass.