Star Formation

Molecular Clouds and Nebulae

Stars begin their lives in molecular clouds, also called nebulae. These are vast regions of cold, dense gas and dust found in galaxies. The gas is mostly hydrogen, with some helium and tiny amounts of other elements.

Molecular clouds are extremely cold, typically just a few degrees above absolute zero, which allows atoms to stick together and form molecules. The density is much higher than the surrounding space, making these clouds ideal places for star formation.

Within these clouds, gravity can cause parts of the cloud to collapse. This happens when the gravitational pull overcomes the internal pressure pushing outwards. The collapse forms dense clumps that will eventually become stars.

Protostar Formation

As a region within a molecular cloud collapses, the material gathers into a dense core called a protostar. The gravitational energy released during collapse heats the core, causing its temperature to rise.

At this stage, the protostar is not yet a true star because nuclear fusion has not started. Instead, the protostar is a hot, dense ball of gas contracting under gravity.

Eventually, the pressure inside the protostar builds up enough to balance the inward pull of gravity. This balance is called hydrostatic equilibrium, and it stops the protostar from collapsing further.

The protostar continues to heat up as it contracts, preparing for the next stage of star formation.

For instance, if a protostar's core temperature rises from 10 K to 1,000,000 K due to contraction, this heating is caused by gravitational energy converting into thermal energy as the gas particles move faster.

Nuclear Fusion Ignition

When the core temperature of the protostar reaches around 10 million kelvin, hydrogen nuclei begin to fuse together in a process called nuclear fusion. This fusion releases a huge amount of energy, producing light and heat.

The start of nuclear fusion marks the birth of a true star. The star enters the main sequence phase of its life, where it remains stable for millions or billions of years.

During the main sequence, the outward pressure from fusion energy balances the inward pull of gravity, maintaining the star's size and temperature.

Star Life Cycle Overview

The mass of a star determines how it evolves after the main sequence phase. More massive stars burn their fuel faster and have shorter lifetimes, while smaller stars burn fuel slowly and live longer.

During the main sequence, stars are stable because fusion energy balances gravity. After this phase, stars may expand into red giants or supergiants, or collapse into white dwarfs, neutron stars, or black holes depending on their mass.

The detailed life cycle of stars is covered in other topics, but star formation is the crucial first step that sets the stage for all later evolution. For more information on the later stages of star evolution, see the topic "The Life Cycle of Solar Mass Stars".

Example: Calculating the Temperature Increase During Protostar Contraction

Suppose a protostar contracts so that its core temperature increases from 100 K to 10 million K. Calculate the factor by which the temperature has increased.

The factor increase is:

$$\text{Factor} = \frac{10,000,000}{100} = 100,000$$

This shows the core temperature increases by 100,000 times during contraction before fusion starts. This large increase is essential to reach the temperatures needed for nuclear fusion to begin.