Life Stages of a High-mass Star
A protostar forms when a dense gas clump breaks loose from the cloud core and collapses. As the clump gets denser, it becomes opaque. This is followed by a rise in pressure and temperature at the center as the eluding infrared radiation is being trapped. This becomes a stable protostar at a point when the incursion of more gas is halted by pressure. In the beginning, the protostar contains only 1 % of the final mass, but the covering of the star keeps on growing. Thermonuclear fusion begins in the core after a few million years and a robust stellar wind forms. This puts to an end the in-fall of the new mass and thus, changes the protostar into a young star since it has a fixed mass. If a protostar accumulates a mass more than 0.08 solar masses, then a high-mass star is formed.
A high-mass star has five main stages in its life cycle. In the first phase, there is burning of the hydrogen found in the core. Hydrogen fusion is the source of energy. All main sequence stars undergo the fusion of hydrogen into helium in the core. The rate at which this occurs is dependent on the mass of the star. Mass is critical in the determination of the size, lifespan, and the luminosity of the main sequence star. These stars do not change for long durations of time. The second stage is characterized by the contraction and instability of the core due to lack of hydrogen. The helium combines with carbon, creating sulfur, magnesium, iron, and neon in the core. The core then changes into iron and the burning stops. Following this, the outer part of the star starts to enlarge. The following stage can take a million years during which there is a formation of various substances in shells surrounding the iron core. The fourth stage involves the collapsing of the core causing an explosion known as a supernova. This explosion causes a shockwave and thus, leads to the explosion of the outer layers. The core then changes into a black hole or a neutron star in the fifth stage if it survived the supernova. The black holes are darker, more compact and have a higher density than the neutron stars. In addition, the neutron stars are stable while the dark holes are not stable since all the matter collapses during the supernova.
The supernovae occur as a result of the core collapsing. When the temperature in the core reaches 1010 K, the iron can photo-disintegrate. As a result, gamma photons, protons, and He nuclei are released. The reaction between the neutrons and the electrons clears all the electrons from the core and as a result, the core continues to collapse. The neutrons degenerate when the temperature gets to 1012 K and the neutron degeneracy pressure stops the core from collapsing inward. However, the surrounding material continues to collapse, and a huge shockwave is formed. Energy is released in form of neutrinos, with only 1% being light. The density of the material that is collapsing and the high number of neutrinos, together with the shockwave, put a lot of pressure on the substance collapsing onto the core and it is blown off into space. Supernovae can also occur when a star is pushed over the Chandrasekhar Limit of 1.4 solar masses, resulting into an explosion that can lead to the star’s destruction, releasing a significant amount of radioactive nickel, which decays into iron and cobalt.