**Gravitational Force and Inward Force**

The gravitational force is an inward force because it always pulls objects towards each other. This force is a fundamental force of nature that exists between any two objects with mass. According to Newton's law of universal gravitation, the force of gravity is directly proportional to the product of the masses of the objects and inversely proportional to the square of the distance between their centers.

**Explanation of Contraction**

When you mention contraction in relation to the gravitational force, it is likely referring to the phenomenon of gravitational contraction. Gravitational contraction occurs when a massive object, such as a star, undergoes a collapse due to its own gravity.

**Gravitational Contraction in Stars**

In astronomy, stars are born from vast clouds of gas and dust called molecular clouds. These clouds are made up of particles that are attracted to each other due to gravitational forces. Over time, the gravitational force causes the particles to come closer and closer together, increasing their density and temperature.

As the density increases, the gravitational force becomes stronger, pulling more matter towards the center of the cloud. Eventually, the pressure and temperature at the core of the cloud become so high that nuclear fusion reactions begin. This marks the birth of a star.

**Contraction and Nuclear Fusion**

During the process of nuclear fusion, hydrogen atoms combine to form helium, releasing an enormous amount of energy. This energy counteracts the force of gravity, preventing further collapse. However, this balance between gravity and fusion reactions is delicate.

As the star exhausts its nuclear fuel, the fusion reactions decrease, and the outward pressure diminishes. Gravity then dominates, causing the star to contract. This contraction leads to an increase in density and temperature, which can ignite new fusion reactions in heavier elements (depending on the star's mass). This cycle continues until the star exhausts all of its nuclear fuel and undergoes its final stages of stellar evolution.

**Conclusion**

The gravitational force is an inward force because it always pulls objects towards each other. In the context of astronomy, gravitational contraction refers to the collapse of a massive object, such as a star, due to its own gravity. This contraction leads to an increase in density and temperature, triggering nuclear fusion reactions and balancing the force of gravity. The interplay between gravity and fusion reactions determines the life cycle of stars in the universe.