The Birth of A Star

Stars do not shine for ever. Stellar evolution is the process by which stars are ‘born’, start to shine, continue shining in a stable state until eventually (after a time depending on their mass) they change, ending up as a variety of different stellar objects (again depending on their mass). Compared with the age of the Universe, high-mass stars can have relatively short lifetimes – possibly just a few million years – whereas the least massive stars can have extremely long lifetimes – exceeding the current estimated age of the Universe. The changes in stars as they evolve occur too slowly for us to detect. Instead, astrophysicists observe numerous stars at different points in their lifetimes and construct computer models of their structure and evolution.

Stars are born in the ‘space’ between stars called the interstellar medium, which contains molecular clouds (Figure 1) that are mostly made up of cold hydrogen gas in the form of atoms, molecules and ions at temperatures of 10–50K and densities of 108–1015 molecules per cubic metre. About 1% of this material is ‘dust’ in the form of silicates and graphite material. Molecular clouds have masses many times greater than the mass of a single star, and contain fragments of varying masses which clump together under gravitational attraction. The irregular clumps tend to rotate, and a combination of the action of gravity and the conservation of angular momentum spins them inwards to form a denser spherical centre, forming a protostar. It is surrounded by a rotating at disc of material called the circumstellar disc, where planets may form Infalling matter from the cloud fragment causes the protostar to increase in size, and the density and temperature also increase. It begins to shine dimly in the infrared, the energy source being the gravitational energy of the infalling material. After a time that may be as much as a few million years, the temperature of the star is such that the mutual electrostatic repulsion between hydrogen nuclei can be overcome and nuclear fusion reactions begin in its core. A strong outward stellar wind is produced, which opposes the infall of material. It starts to shine in the visible part of the electromagnetic spectrum, and is now known as a pre-main-sequence star.

#LessonsFromScience