The Origin of the Earth
The Evolutionary Cycle of Stars
The key features of stars which are of interest to astronomers are their
mass, their luminosity, their surface temperature and the distance they are
away from us. These parameters are used to classify stars into types. More
importantly they are also used to place stars into an evolutionary sequence
which describes their formation, life and death. A widely used classification
diagram for stars is the Hertzsprung-Russell Diagram (the H-R diagram) which
is a plot of luminosity vs effective surface temperature. The luminosity of
a star is a function of its radius and effective temperature. (for more details
The Hertzsprung-Russell Diagram shows four main groupings of stars. These
- Main sequence stars; these define a curved trend across the centre of
the diagram which displays a relationship between mass and luminosity, such
that stars with a high luminosity have a high effective temperature.
- Red giants are stars which are cooler, but more luminous than stars on
the main trend — these are thought to be either dead stars or protostars.
- Super giants are similarly stars which are more luminous, but much cooler
than main trend stars.
- White Dwarfs are stars which are hotter and less luminous than main trend
stars and are thought to be dead stars radiating away their energy.
The cycle of birth-life-death for a star may be charted on a Hertzsprung-Russell
Most stars evolve onto the Main Trend and then into the Dwarf stage as follows:
- Development of a large cloud of increasingly dense hydrogen (and helium)
gas and dust. This giant molecular cloud contains molecular hydrogen at
low temperatures (hundreds of °K). This is a nebula
and within the nebula aggregates of gas and dust form the beginnings of
- Temperatures rise, driven by gravitational collapse and a Protostar
forms at about 2000-3000°K.
- The star subsequently evolves to be part of the main sequence
of stars. The precise position on the main sequence depends upon the mass
of hydrogen present in the star. At this point in its life temperatures
in the core of the star are as high as 107°K. Nuclear fusion takes
place within the centre of the star and helium is formed. At temperatures
above 108°K He reacts through nuclear fusion to form heavier elements.
Whilst stars are part of the Main Sequence developmental stage planetary
formation may take place. (See http://www.aao.gov.au/images.html/general/planetary_frames.html.)
- The star eventually converts into a Red Giant and expands
to up to 100 times the diameter of the original star. Red Giants develop
as the hydrogen in the core is depleted and the helium derived from it tries
to burn to carbon. A Red Giant is sometimes described as a 'bloated' star.
- The final stage of stellar evolution depends upon star size. Smaller stars
end up as the surviving cores of Red Giants which become greatly reduced
in size to the White Dwarf stage. Larger stars shed nearly all their
remaining gaseous matter and synthesize elements in a supernova (see http://www.aao.gov.au/images.html/general/supernova_frames.html) — a huge explosion marking the end of the life of the star. Within the dense
supernovae, neutron stars may form as the extremely dense endpoint in the
life of a star.
Further details of the process of star formation may be found on the following