Problem 7 - Astronomy Journey to the Cosmic Frontier

Question

What halts the collapse of a protostar?

Step-by-Step Solution

Answer

Nuclear fusion creates outward pressure that stabilizes the protostar at hydrostatic equilibrium.

Step 1: Understanding Protostar Formation

A protostar forms from a cloud of gas and dust in space, contracting under the force of gravity. As it contracts, the density and temperature in the core increase, leading to the conditions necessary for nuclear fusion.

Step 2: Initiation of Nuclear Fusion

Once the temperature in the core of the protostar reaches around 10 million Kelvin, nuclear fusion reactions begin. Hydrogen nuclei start combining to form helium, releasing energy in the process.

Step 3: Energy Balance and Hydrostatic Equilibrium

The energy released from nuclear fusion creates outward radiation pressure, counteracting the inward gravitational pull. When these two forces balance out, it results in hydrostatic equilibrium, halting the collapse of the protostar.

Key Concepts

Nuclear Fusion in Protostars

Nuclear fusion is the process that powers stars, including protostars. When a protostar forms, it is primarily composed of hydrogen gas. As the core contracts due to gravity, the temperature rises dramatically. Once the core temperature reaches around 10 million Kelvin, conditions become ripe for nuclear fusion.

During nuclear fusion in stars, hydrogen nuclei (protons) collide and fuse to form helium. This process releases an enormous amount of energy as radiation. The energy is what ultimately causes the protostar to shine.

Key points about nuclear fusion:

• It requires extremely high temperatures.
• Results in the conversion of hydrogen into helium.
• Releases energy that powers the protostar's light.

Achieving Hydrostatic Equilibrium

Hydrostatic equilibrium is the condition where a protostar reaches a balance between two opposing forces. Once nuclear fusion begins in a protostar, it generates a significant amount of energy. This energy exerts an outward pressure in the form of radiation.

The radiation pressure pushes outwards, counteracting the squeezing effect of gravity that tries to pull the star inward. When these two forces - the inward pull of gravity and the outward push of radiation pressure - are equal, hydrostatic equilibrium is achieved.

Important aspects of hydrostatic equilibrium:

• It stops further gravitational collapse.
• Involves a balance between radiation pressure and gravitational forces.
• Helps maintain the star's size and stability.

The Role of Gravitational Collapse

Gravitational collapse is the initial phase that leads to the formation of a protostar. It begins when a massive cloud of gas and dust in space starts to condense under its own gravity. This gravitational attraction is the primary force causing the cloud to collapse.

As the cloud contracts, it grows denser and hotter, especially at the core. The continuous inward pull of gravity keeps adding pressure and heat, which are crucial for nuclear fusion to eventually ignite.

Essential points on gravitational collapse:

• It triggers the birth of a protostar.
• Increases core temperature and density.
• Provides necessary conditions for nuclear fusion to start.