Life and death of stars

The life of a star begins with an agitated phase, known as T-Tauri, which lasts about one million years. During this phase, the star – which is still in its cocoon of gas and dust – will send out bursts of radiation and particles which will severely disrupt the circumstellar disc.

The star enters the “Main Sequence”, where it will spend the majority of its existence (9 billion years for a star like the Sun). When the hydrogen (H) is exhausted in the core, contraction resumes and the temperature at the centre of the star rises. The rest of the scenario, which describes the end of the star’s life, will depend on its mass:

• For low-mass stars ($M < 0.3 M_{\text{Sun}}$), contraction ceases and the star will be “extinguished”.
• For more massive stars ($M > 0.3 M_{\text{Sun}}$), which accounts for most stars, the temperature at its centre reaches $10^8\text{K}$, initiating the fusion of He (helium) in the core. In the shell, the temperature rise allows the resumption of the fusion of hydrogen (H) into helium (He). The brightness, thus the radiation pressure, and therefore the radius of the star, increases significantly. At the same time, the shell swells, cools and the centre becomes denser. The star becomes a red giant. In the centre, the combustion of helium (He) into carbon (C) and oxygen (O) does not last very long. When the helium (He) is exhausted, the star experiences its second energy crisis. Once again, its mass will determine its evolution:
• f the mass is less than $1.4M_{\text{Sun}}$, the diluted envelope slowly expands to form a “planetary nebula” (for example Lyra). The core “extinguishes” very slowly to form a very small ($R \sim 3,000 \space \text{km}$) and very dense ($\sim 10^{10}{\text{kg/m}}^{3}$) white dwarf ^? , which is initially very hot but which will slowly cool to become a black dwarf.
• For stars of a mass greater than $1.4M_{\text{Sun}}$, contraction continues. The fusion of C (carbon), O (oxygen), Si (silicon), Mg (manganese), Ne (neon) … Fe (iron), is very fast and releases little energy. Once it has reached Fe (iron), the most stable element in terms of thermonuclear reactions, no more fuel is available. Contraction resumes. The temperature in the center increases, leading to the fusion of elements heavier than Fe (iron). But these fusion reactions consume energy whereas the fusion of the elements lighter than Fe (iron) releases energy), further accelerating the contraction process!

Then comes the fusion of electrons and protons ^? into neutrons. The star’s core then goes into free-fall, until it reaches a radius of $\sim 10\text{km}$ and a density of $10^{17}{\text{kg/m}}^{3}$. This is followed by an internal rebound and a shockwave creating a supernova ^? . After the explosion, an extremely dense central object remains, either a neutron star ^? , or a black hole.