Question

Briefly summarize the life cycles of stars from birth to death. At which stage of stellar life does it seem possible that life-bearing planets could exist? Why?

Short answer

Life-bearing planets are most likely during the main sequence stage, due to stable energy output and a suitable environment for sustaining life.

Step-by-step solution

Nebula Formation

Stars begin their life as a nebula, which is a large cloud of gas and dust. Gravitational forces cause the nebula to collapse and form a protostar.

Protostar Development

During this phase, the protostar gathers mass from the surrounding nebula. Once the core temperature becomes high enough, nuclear fusion ignites, marking the next stage.

Main Sequence Stage

Once nuclear fusion begins in the core, the star enters the main sequence stage. Here, the star spends the majority of its life, steadily fusing hydrogen into helium, which provides stability and light. Stars like our Sun can sustain this for billions of years.

Red Giant/Supergiant Transition

After the hydrogen in the core is exhausted, the star expands into a red giant or supergiant. The outer layers expand while the core contracts and starts fusing helium or heavier elements.

Stellar Death

Eventually, the star can no longer sustain fusion in its core. Smaller stars shed their outer layers and form a planetary nebula, leaving behind a white dwarf. Larger stars may explode as supernovae, leaving behind neutron stars or black holes.

Key concepts

Nebula

A nebula is the birthplace of stars. It is a vast cloud composed primarily of hydrogen gas and cosmic dust. These regions in space can be tens to hundreds of light-years across.
Gravitational forces within the nebula cause it to gradually collapse under its own weight. As the gas and dust come together, they form denser regions known as protostars.

• It all begins with slight fluctuations in density that cause certain regions to start collapsing.
• The process releases gravitational energy, heating up the material and allowing it to glow faintly.

This is the initial stage of stellar evolution, setting the stage for a star to eventually enter the main sequence.

Main Sequence Star

The main sequence stage is when a star reaches a stable phase in its lifecycle. This is when nuclear fusion ignites in the core, beginning the sustained conversion of hydrogen into helium.
As a star in this phase, it achieves a balance between pressure from the heat of fusion and the gravitational forces pulling inward. This balance keeps the star stable for millions to billions of years, depending on its mass.

• During this lengthy phase, stars display a variety of brightness and colors.
• The Sun, for example, is in its main sequence phase and is predicted to remain so for about another 5 billion years.

Life-bearing planets are most likely to exist during this phase because the constant light and heat provide the stable environment necessary for life to thrive.

Red Giant

As a star consumes the hydrogen in its core, it begins the transition into the red giant phase. The core contracts and heats up, while the outer layers expand significantly.
This transformation causes the star to grow larger and cooler on the surface, appearing red in color. Inside, helium or heavier elements start to fuse.

• The core's extreme pressure and temperature allow fusion of heavier elements than during the main sequence.
• This phase is relatively short-lived compared to the main sequence.

As stars like the Sun enter the red giant phase, they become unstable but also very bright, creating a spectacle observable to great distances.

Planetary Nebula

At the end of its life, a smaller star like our Sun sheds its outer layers, forming a planetary nebula. This colorful cloud of gas is ejected into space and illuminated by the hot core left behind.

• Despite the name, this has nothing to do with planets; the term was coined as early telescopes saw these structures look like planets.
• The hot core becomes a white dwarf, gradually cooling and fading away over billions of years.

The beauty of a planetary nebula lies in its fleeting nature, marking the transition from a giant star to its eventual quiet fade as a white dwarf. These nebulae enrich the surrounding space with heavier elements that can be used in the formation of new stars and planets.