Astronomy Practice Exam

What typically happens to a red giant once it runs out of hydrogen fuel?

1. It shrinks and becomes a nebula

2. It expands and cools

3. It undergoes a supernova

4. It remains stable indefinitely

The correct answer is: It expands and cools

When a red giant exhausts its hydrogen fuel in the core, it undergoes significant changes in its structure and behavior. The core contracts under the force of gravity as hydrogen fusion ceases, leading to an increase in temperature. This temperature rise allows helium to begin fusing into heavier elements like carbon and oxygen, rapidly altering the conditions within the star. As a result of these processes, the outer layers of the star expand dramatically, causing the star to grow in size and take on a cooler temperature, which gives it the characteristic reddish hue associated with red giants. This expansion and decrease in surface temperature are why the statement about the star expanding and cooling is correct. The star can ultimately develop into a more evolved state, such as a yellow giant or a supergiant, depending on its mass. The other scenarios presented in the choices are different types of stellar evolution. While some stars may eventually result in a supernova after going through various phases, particularly massive stars, many red giants do not reach that endpoint. Instead, they end their lives in a more benign fashion, often shedding their outer layers to form planetary nebulae, leaving behind a dense white dwarf, which is not aligned with the options that suggest shrinking into a nebula or remaining stable