The Life Cycle of Larger Stars

Formation of Larger Stars

Larger stars begin their lives in a stellar nebula, which is a vast cloud of gas and dust in space. Gravity pulls this gas and dust together, causing the cloud to collapse and form a dense region called a protostar.

As the protostar contracts, its temperature rises due to the conversion of gravitational potential energy into heat. When the core temperature becomes high enough (millions of degrees Celsius), nuclear fusion starts. This is the process where hydrogen nuclei combine to form helium, releasing a huge amount of energy.

The start of nuclear fusion marks the birth of a true star, as the energy produced creates an outward pressure that balances the inward pull of gravity.

Main Sequence Phase

During the main sequence phase, larger stars fuse hydrogen into helium in their cores. This process releases energy that provides a stable outward pressure.

The star remains stable because of a balance between two forces:

• Gravity pulling the star’s mass inward
• Radiation pressure from nuclear fusion pushing outward

This balance keeps the star’s size and brightness steady for millions of years.

Red Supergiant Stage

Eventually, the hydrogen in the core of a larger star runs out. Without hydrogen fusion to support it, the core contracts under gravity, causing the outer layers to expand hugely and cool down. The star becomes a red supergiant, much larger and cooler on the surface than during the main sequence.

Inside the core, the temperature rises enough to start fusing helium into heavier elements like carbon and oxygen. This helium fusion produces energy but is less efficient than hydrogen fusion.

The star’s outer layers are now very large and red in colour due to their cooler temperature, but the core is very hot and dense.

Final Stages of Larger Stars

After the red supergiant phase, the star undergoes further fusion of heavier elements in its core, creating elements up to iron. Fusion beyond iron does not release energy, so when iron builds up, the star can no longer support itself against gravity.

The core collapses rapidly, causing a massive explosion called a supernova. This explosion blasts the outer layers of the star into space, spreading heavy elements throughout the universe.

What remains after the supernova depends on the original mass of the star:

• Neutron star: If the core is between about 1.4 and 3 times the mass of the Sun, it collapses into an extremely dense neutron star, made mostly of neutrons. This threshold of about 1.4 solar masses is known as the Chandrasekhar limit.
• Black hole: If the core is more massive, gravity crushes it into a black hole, a point with gravity so strong that not even light can escape.

The heavy elements formed during the star’s life and supernova are essential for creating planets and life.

For instance, the iron in your blood was originally formed inside a massive star and spread across space by a supernova billions of years ago.

Example: A larger star with a mass about 10 times that of the Sun will have a much shorter lifetime than the Sun because it burns its fuel faster due to higher core temperatures.