According to the nebular hypothesis, what event begins the process of formation of a solar system from a cloud of gas and dust?

Introduction / Context:
This question comes from astronomy and Earth science, specifically the origin of the solar system. The widely accepted nebular hypothesis explains how a solar system forms from a huge cloud of gas and dust. Understanding the first key step in this process is important for many competitive examinations.

Given Data / Assumptions:

• The nebular hypothesis describes formation of the Sun and planets from a common gas and dust cloud.
• The options mention gravitational collapse, planet collisions, volcanic eruptions, and star breakup.
• We assume basic knowledge that solar systems do not start with ready made planets.

Concept / Approach:
According to the nebular hypothesis, a large, cold, rotating cloud of gas and dust (a nebula) becomes unstable, often due to external disturbance or internal gravitational instabilities. Gravity causes this nebula to begin collapsing toward its center. As it collapses, it spins faster and flattens into a disk. Most of the mass eventually forms a central star, while the remaining material forms planets and smaller bodies. The initial step that triggers solar system formation is therefore gravitational collapse of the nebula, not collisions of fully formed planets or eruption of magma.

Step-by-Step Solution:

Verification / Alternative check:
Astronomy textbooks and educational animations describe solar system formation starting from a giant molecular cloud or nebula. They consistently show gravity pulling material inward to form a rotating disk with a proto star at the center. Collisions between planets and volcanic activity on planets happen much later in the history of a solar system, not at the very beginning of its formation. This confirms that gravitational collapse of a nebula is the correct starting event.

Why Other Options Are Wrong:

• The collision of two already formed planets is wrong because planets do not exist before the system forms; such collisions may form moons but not start a solar system.
• The eruption of magma from a young planet is wrong because it is a surface process on a planet that already exists.
• The breakup of an existing star into smaller stars and planets is not the widely accepted mechanism for solar system formation and does not match the nebular hypothesis.

Common Pitfalls:
Some learners imagine solar systems forming from explosions or collisions only, because those events sound dramatic. However, the key idea in modern theories is slow gravitational collapse and disk formation, not violent splitting of stars. To remember this, picture a large, quiet cloud that gradually contracts and spins, rather than a sudden explosion, when thinking about how solar systems begin.

Final Answer:
The gravitational collapse of a rotating nebula of gas and dust