Star life cycles

Stars are born. They live, age and then die. The dust and elements that are thrown out by dying big mass stars can get recycled - and this material can go on to form new stars in the future. The phases that stars go through as they age are shown by the star’s life cycle diagram.

A diagram showing the life cycle of a star. A Protostar can become either a Black dwarf or Supernova. If it becomes a Supernova it can then develop into either a Black hole or a Neutron star.

Stars begin as protostars. Following the protostar stage and the start of nuclear fusion, all stars enter their main sequence.

When stars are in their main sequence the forces on them balance. Outward radiation and gas pressure forces are balanced by gravity forces.

A diagram showing a star in its main sequence. Red arrows pointing away from the centre show how gas pressure pushes outward. Green arrows pointing inward show how gravity pulls material inward.

Over time, the forces acting on the star become unbalanced. When the inward gravitational forces are less than the outward radiation pressure forces, the star swells and cools, thus turning red. High mass stars become red supergiants, low mass stars become red giants.

The forces become unbalanced when the hydrogen begins to run out. The star begins to fuse helium and then increasingly heavier elements to maintain fusion. When iron is formed in the core of the star, nuclear fusion stops and the star contracts under its gravity. What happens next depends on the mass of the star.

Red giant stars collapse to form a white dwarf star that gradually cools over time. Red supergiants quickly collapse, producing a giant explosion called a supernova.

High mass red supergiants form neutron stars where the core of the supergiant (about 1.5 times the mass of the Sun) has collapsed into a space with a radius of about 12 km.

The very high mass supergiants collapse with such force that they form black holes, a point of mass with such high density that the force of gravity is so large that not even light can escape from its surface.

During the collapse and supernova explosion, elements heavier than iron (such as uranium) are formed and are ejected into space. The ejected material joins up with other dust and hydrogen and begins the process of forming new stars. These stars could have planets that also form around the star. This is recycling on a big scale. The uranium found on Earth has come from this process – so we are made from the material that came from an earlier supernova.