What happens when a star exhausts its core hydrogen supply?
A) Its core contracts, but its outer layer expands and the star becomes bigger and brighter
B) It contracts, becoming smaller and simmer
C) It contracts, becoming hotter and brighter
D) It expands, becoming bigger and dimmer

Hint: Stars are mainly composed of Hydrogen and helium (the two lightest elements), The shining of the stars is caused by the burning of hydrogen into helium in their cores, due to which they are very hot in nature. The fusion process later creates heavier elements.

Complete Step By Step Answer:
Star is mainly composed of hydrogen and helium, so if all the hydrogen is used up the fusion process stops. The contraction of the star begins (in the same way it contracts as a protostar before the fusion started). This contraction releases energy and this energy heats up the layer immediately leading to the contraction of the helium core.
The layer immediately above the core becomes hot enough to initiate the fusion reaction of hydrogen into helium. The layers the star now possess are:
1. Helium core: which shrinks in radius as it releases energy
2. Fusion shell: The layer in which the fusion of hydrogen to helium is taking place
3. Hydrogen envelope: which is the outermost layer that absorbs energy and expands greatly in size.
These swollen stars have now become giants or supergiants. The outer hydrogen layer cools down due to expansion, and is named as red giant as it becomes very large and very cool.
Therefore, the correct answer is Option (A).

Note:
The sun is the nearest star to earth. When the sun runs out of hydrogen, it will go though the same process and become a Red Giant (after 5 billion years later), whose radius will be 100 times more than the present radius. The temperature will drop as low as 3000K and will engulf mercury while it expands.