What happens to a star when it runs out of hydrogen fuel?

When a star runs out of hydrogen fuel, it undergoes significant changes in its structure and energy production processes. This transition typically leads the star to expand and cool, forming what is known as a red giant. During this phase, the core of the star contracts under gravity, raising its temperature until nuclear fusion of helium begins in the core. The outer layers of the star expand dramatically, and the star appears red because its surface temperature decreases.

The formation of a red giant is particularly characteristic of stars that are similar to our Sun or less massive. The exact sequence can vary depending on a star's mass, but for many stars, hydrogen depletion marks a transition to the red giant phase as they begin to fuse heavier elements.

In contrast, the other scenarios such as forming a black hole, collapsing into a neutron star, and becoming a white dwarf are outcomes typically associated with stars at different stages of their life cycle and particularly at the end stages for more massive stars. They occur after significant evolutionary progress beyond the red giant phase, especially for those that are massive enough to proceed to those final stages of stellar evolution.