Stardust and eternity – 3.1.2

Stellar evolution

Stars have their own life: they are born, evolve over time and finally die, sometimes in a quite dramatic way. Their lifetime depends mainly on their mass, ranging from a few million years in the case of very massive stars to trillions of years in the case of low-mass stars. The cradle of stars lies in very large and dense molecular clouds, made up of interstellar gas and dust inside galaxies; they sometimes constitute actual stellar nurseries. When these gigantic clouds (about 100 light-years) collapse on themselves as an effect of their own gravity, they break up into smaller pieces. As they compress, these fragments begin to spin on themselves and heat up, first forming rotating disks and eventually condensing near their centers into rotating spheres, the “protostars”. As these newborn stars continue to grow in mass, they become “pre-Main-Sequence stars”. Once they reach their limit mass, they settle into the “Main Sequence”, where their further evolution is entirely driven by their mass.

Only stars with masses greater than 0.08 solar masses (M_☉) are capable of reaching core temperatures above 10 million °C, which are necessary to trigger thermonuclear reactions in their cores. Stars with masses below the limit are doomed to become substellar objects known as “brown dwarfs” and slowly cool down over hundreds of millions of years, whereas those with masses above the limit are able to initiate the nuclear fusion of hydrogen into helium. The mass of the newborn helium nucleus is slightly less (only 0.7%) than that of the 4 hydrogen nuclei needed to create it and is converted into energy according to the famous Einstein’s formula E = mc². The energy released through nuclear fusion exerts an outward pressure that pulls the stellar mass outwards, preventing it from collapsing under its own weight. The balance between outward pressure and inward gravity is called “hydrostatic equilibrium”; the star is said to settle in the Main Sequence phase of its evolution, where its temperature, radius and luminosity remain virtually unaltered.

On the Main Sequence, highly massive and hottest stars quickly burn out their fuel, thus lasting only a few million years. Conversely, low-mass red dwarfs – which are smaller and colder fuse hydrogen more slowly and remain in hydrostatic equilibrium for hundreds of billions of years, more than the age of the Universe (about 13.7 billions years). Our Sun lies at an intermediate stage: it is neither too massive nor too small and is going to last for another 5 billions years, after having already spent as much time in the Main Sequence phase.

When a star is old enough, it begins to run out of hydrogen and begins its journey out of the Main Sequence phase. This happens because the decreased outward pressure in the star’s core is now unable to counter the weight of heavy layers that begin to contract again under the force of gravity. However, as the star’s particles are pressed tightly together, there is a limit to the density the core can reach. As for low-mass stars (below about 0.5 M_☉), electrons resist being packed together (about 1 ton per cm³) and produce an “degeneration pressure”, which pushes outwards against gravity and prevents the core from further collapsing. Despite the lack of nuclear reactions, these stars are stable and end up as “helium white dwarfs”.

Stars with intermediate masses like the Sun, after having exhausted hydrogen fuel in their core – now transformed into helium – begin to burn hydrogen in a shell at the edge of their core. Thus, an energy is produced that inflates the star into a “red giant”. At the same time, the helium core contracts and reaches about 100 million °C, thus allowing the nuclei of helium to react and form carbon. Towards the end of its life, as the helium fuel is also exhausted, its outer layer is ejected in a shell of gas – called “planetary nebula”. The core, about the size of the Earth, becomes a carbon “white dwarf” only sustained by electron degeneracy.

As for stars with masses above 8 M_☉, at the much higher temperatures reached in their cores, additional nuclear reactions become possible. They go through consecutive nuclear burning phases, producing progressively heavier elements in their layers – such as neon, oxygen, and silicon – and leading to an onion-like structure. At this stage, the formation of an iron core stops its thermonuclear evolution, as no further exothermic fusion is possible and the core violently collapses on itself; the star explodes into a “supernova”, leaving a “neutron star” or a “black hole” as a remnant – depending on the core mass. Neutron stars form when the core has a mass between about 1.5 to 3 M_☉: they have densities about 100 trillion times that of water and a radius of only 10 km. Stars that are too massive to form neutron stars produce the so-called “black holes”: their gravity is so strong that even light cannot escape them.

Tookit

Stardust and eternity

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A star is born: Image by Андрей Сидоренко – Pixabay

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Images

Process of star formation (Bill Saxton/NRAO/AUI/NSF)