The Life Cycle of Solar Mass Stars

Formation of Solar Mass Stars

Solar mass stars, like our Sun, begin their lives in vast clouds of gas and dust called nebulae. Gravity causes regions within these nebulae to collapse, pulling material together.

As the gas clumps contract, they heat up and form a protostar. When the core temperature becomes high enough, hydrogen nuclei start to fuse into helium, releasing energy. This marks the birth of a star.

Note: The detailed process of star formation is covered in the "Star Formation" topic.

Main Sequence Stage

Once hydrogen fusion begins in the core, the star enters the main sequence stage, which is the longest and most stable period in a star’s life.

• Hydrogen nuclei fuse to form helium, releasing a huge amount of energy as radiation.
• This energy creates an outward pressure that balances the inward pull of gravity, keeping the star stable.
• The star shines steadily, maintaining a stable size and temperature during this phase.

For example, the Sun has been in its main sequence phase for about 4.6 billion years and will remain so for several billion more.

The balance between gravity and pressure is crucial. If fusion slows, gravity causes the core to contract, increasing temperature and pressure until fusion restarts.

For instance, if the Sun’s core temperature dropped slightly, fusion would slow, gravity would compress the core, raising the temperature and fusion rate until equilibrium is restored.

Red Giant Phase

After millions to billions of years, the hydrogen in the core runs out and fusion stops there. Without fusion pressure, gravity causes the core to contract.

As the core contracts, it heats up and causes the outer layers of the star to expand greatly, cooling as they do so. The star becomes a red giant.

• Hydrogen fusion continues in a shell around the core.
• When the core temperature reaches about 100 million kelvin, helium fusion begins, converting helium into heavier elements like carbon and oxygen.

The red giant phase is much shorter than the main sequence but is a dramatic change in the star’s size and brightness.

Final Stages and Remnants

For stars with a mass similar to the Sun, the final stages are quite gentle compared to massive stars:

• The outer layers are gradually lost into space, forming a glowing shell of gas called a planetary nebula.
• The core that remains is very dense and hot but no longer undergoes fusion; this is called a white dwarf.
• White dwarfs slowly cool over billions of years, eventually fading away.
• Solar mass stars do not explode as supernovae.

The white dwarf is supported against gravity by electron degeneracy pressure, which prevents it from collapsing further.

For instance: The Sun is expected to remain in its main sequence phase for about 10 billion years in total, illustrating the long stable period of solar mass stars.