Birth, Life and Death of the Sun: Chronicle of an Average Star

An ordinary star with an extraordinary destiny

The Sun is a G2V spectral type star, approximately 4.57 billion years old, located at an average distance of 1 AU (≈ 149,597,870 km) from Earth. It is a plasma sphere about 1,392,700 km in diameter, primarily composed of hydrogen (≈ 73.5%) and helium (≈ 24.9%), with a small proportion of heavier elements called metals.

The Sun's internal equilibrium results from the balance between two fundamental forces: the radiation pressure generated by the fusion of hydrogen into helium in its core, and gravity, which tends to collapse it. This equilibrium condition, called hydrostatic equilibrium, ensures the star's stability on the main sequence of the Hertzsprung-Russell diagram (1905-1969).

The solar core, with a radius of ≈ 0.25 R_☉, reaches temperatures close to \(1.5×10^7\,K\) and densities of about 150 g/cm³ (or 150,000 kg/m³), about 25 times denser than Earth's core. Nuclear fusion reactions occur there via the proton-proton chain, producing energy of \(3.8×10^{26}\,W\). This colossal power, transported to the surface first by radiation and then by convection, is converted into electromagnetic radiation observed from Earth.

The solar mass represents 99.86% of the Solar System, exerting a decisive gravitational influence on all planets, asteroids, and comets. Its average energy flux at Earth's level, called the solar constant, is about \(1,361\,W/m^2\), governing Earth's climate, photosynthesis, and atmospheric dynamics.

N.B.:
A star of spectral type G2V belongs to the class of yellow dwarfs. The letter "G" indicates a surface temperature between 5,300 K and 6,000 K, while the number "2" specifies a hotter subclass within type G. The suffix "V" indicates it is a main-sequence star, i.e., in the stable phase of fusing hydrogen into helium. The Sun, with an average photospheric temperature of 5,778 K and a luminosity of one Sun (\(L = 1 L_\odot\)), serves as the reference for this classification.

Birth: The collapse of a nebula

The Sun was born about 4.6 billion years ago in a vast molecular cloud in the Orion Arm of the Milky Way. Under the combined action of gravity and a shock wave, likely from a nearby supernova, part of the cloud underwent gravitational collapse. Matter began to concentrate at the center of a dense region (the solar protostar) while an accretion disk formed around it.

During this collapse, the conservation of angular momentum caused accelerated rotation of the disk and progressive flattening of the structure. Temperature and density increased considerably in the protostar's core; when the central temperature reached about \(10^7\,K\), proton collisions became frequent enough to trigger thermonuclear fusion via the proton-proton chain.

The dominant reaction, first described by Hans Bethe (1906-2005), can be summarized as: \( 4\,^1H \rightarrow\, ^4He + 2e^+ + 2\nu_e + 26.7\,\text{MeV} \)

This conversion of mass into energy, expressed by Albert Einstein's (1879-1955) equation \(E = mc^2\), released enough energy to halt the young Sun's gravitational contraction. The star then entered a phase of thermal stability, marking its placement on the main sequence.

The remnants of the accretion disk gave rise to the primitive matter of the Solar System: planets, satellites, asteroids, and comets. This phase, lasting an estimated tens of millions of years, set the initial conditions for the future evolution of our planetary environment.

Life: Stability and magnetic cycle

For most of its existence, the Sun remains a stable star on the main sequence. This stability results from a balance between the pressure exerted by radiation from nuclear fusion and the gravitational force that tends to compress matter. This state of hydrostatic equilibrium ensures a quasi-stationary structure for about 10 billion years.

Structure of the Sun: Four main stratified zones

Differential rotation and transition zone

The Sun's differential rotation, faster at the equator (≈ 25 days) than at the poles (≈ 35 days), causes shear in the transition zone called the tachocline. These shears amplify and twist magnetic field lines, generating a complex and variable field via the dynamo effect.

Magnetic cycle and solar activity

This magnetic field is responsible for sunspots, flares, and the solar wind. Their activity follows an average 11-year cycle, identified in 1843 by Heinrich Schwabe (1789-1875) and further studied by George Ellery Hale (1868-1938) through the discovery of solar magnetism.

Heliospheric influence and terrestrial effects

This cycle influences the entire heliosphere, modulating the amount of energetic particles reaching Earth and thus affecting the ionosphere, radio communications, and even the formation of polar auroras. The Sun's activity is therefore a major astrophysical variable in space weather.

Death: Transformation into a red giant and white dwarf

Core collapse and the beginning of the red giant phase

In about 5 billion years, the hydrogen in the Sun's core will be exhausted, leading to the cessation of central nuclear fusion. Deprived of the radiation pressure needed to counterbalance gravity, the core will begin to collapse. The central heating will trigger the fusion of helium into carbon and oxygen via the triple-alpha process. The outer layers will expand, turning the Sun into a red giant. Its radius could reach Earth's current orbit.

Mass loss and planetary nebula formation

During this phase, the Sun will undergo thermal pulsations and lose a significant portion of its mass through intense stellar winds. The ejection of the outer layers will form a planetary nebula, enriching the interstellar medium with carbon and other light elements.

Formation of the white dwarf and cooling

The residual core will contract under gravity until it becomes a white dwarf. Its mass will be about 0.6 M_☉ and its radius comparable to Earth's. At this stage, no nuclear fusion will occur, and the star will radiate only residual energy, cooling slowly over billions of years until it eventually becomes a black dwarf.

Stellar chronology of the Sun

Main stages in the Sun's life

Source: NASA – Solar Physics and Harvard ADS.