Space Physics: The Life Cycle of Stars

Star Formation

Stars are born in nebulae, which are vast clouds of gas and dust located in galaxies.
The force of gravity pulls the gas and dust together to form a dense hot core called a protostar.
As the temperature and pressure in the core increase, nuclear fusion of hydrogen nuclei to form helium starts, releasing energy in the process.

Main Sequence Stars

When gravitational forces and outward pressure from the nuclear fusion are balanced, the protostar becomes a main sequence star, like our Sun.
The length of a star’s main sequence phase depends on its size. Larger stars have shorter main sequence lives because they burn through their nuclear fuel more quickly.

Red Giants and Red Supergiants

When a main sequence star exhausts its hydrogen fuel, it expands to become a red giant or a red supergiant depending on its initial mass.
In these stages, heavier elements up to iron are formed by fusion.

White Dwarfs and Supernova

Small stars, like the Sun, will shed their outer layers and leave behind a hot dense core called a white dwarf.
More massive stars will undergo a more dramatic change, exploding in a brilliant event called a supernova.

Neutron Stars and Black Holes

After a supernova explosion, a small but extremely dense neutron star might be left behind.
For the largest red supergiants, the supernova explosion can lead to the formation of a black hole, an area of space with gravity so strong that not even light can escape.

Stellar Evolution

The cycle of star birth and death results in the recycling of stellar material, as the material ejected during the supernova explosion will contribute to the creation of new stars and planets.
This life cycle of stars is believed to be responsible for the distribution of different elements throughout the universe.

Light and Spectral Analysis

The light from stars can be analysed by a spectroscope revealing a star’s spectrum.
The pattern of spectral lines can provide information about a star’s chemical composition and temperature.
Astronomers can use red shift to determine whether a star is moving towards or away from us, and at what speed. It can also provide evidence of the expansion of the Universe.

The Hertzsprung-Russell Diagram

The Hertzsprung-Russell (H-R) diagram is a tool that scientists use to illustrate the life cycle of stars, plotting their absolute magnitude or luminosity against their temperature or colour.
Main sequence stars fall along a band diagonally across the diagram, with red giants located in the upper right, white dwarfs in the lower left, and supergiants found at the top end of the scale.
The H-R diagram helps show the relationship between a star’s temperature, colour, luminosity and size.