What is Stellar Evolution?

Stellar evolution is a sequence of rapid changes that a star undergoes during its life. Stars are based on mass. Their lifetimes range from the most massive stars to only a few million years to the smallest stars to hundreds of billions of years older than the age of the universe. All stars are born from the collapse of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, the protostar reached equilibrium and settled into the so-called main sequence star. Most of the life of a star is in the state of generating energy by nuclear fusion. Initially, the main sequence star fuses hydrogen into helium at the core to generate energy, and then the helium nuclei dominate the core. Stars like the sun fuse hydrogen into helium in layers of spherical shells from the core. This process will gradually increase the size of the star through the sub-giant stage until it reaches the state of the red giant. Stars with a mass of not less than half of the sun can also generate energy by fusing core hydrogen into helium, and heavier stars can sequentially produce heavier elements in concentric circles. When a star like the sun runs out of core fuel, its core will collapse into a dense white dwarf, and its outer layer will be driven away into a planetary nebula. A star with a mass about 10 times or heavier than the sun, explodes into a supernova when its inactive iron core collapses into a very dense neutron star or black hole. Although the age of the universe is not enough to allow the lowest mass red dwarfs to evolve to the end of their lives, the stellar model believes that they will gradually brighten and heat before depleting their core hydrogen fuel, and then become low mass white dwarfs [2] . Stellar changes are very slow, and no change can be detected for centuries, so observing a single star alone cannot study how the star evolves. Therefore, astronomers use other alternative methods, such as observing many stars at different life stages, and using computer simulations to infer the structure of the stars.

Stellar evolution

Low mass stars

In the early 1920s, the British astronomer ASEddington (1882-1944) researched that the internal fuel of the star was about to run out during the later stages of evolution, and the energy produced was not enough to offset the gravitational force between the materials inside the star, so the volume contracted. The density increases and it evolves into a dense white dwarf. In 1925, astronomers discovered the first white dwarf in their observations.

In 1939, the American physicist Oppenheim (1904-1967) proposed that a massive star due to its great gravity will make its final destination not white dwarf, and it will continue to shrink, atoms and nuclei. Both are crushed, and the positively-charged protons and negatively-charged electrons are combined into neutral neutrons under the action of strong gravitational forces. The huge star body shrinks into a neutron star with a small body and a very large mass and density. In the same year, Indian-American astronomer S. Chandrasekhar (1910-1995) predicted that stars with masses less than 1.44 times the sun would evolve into white dwarfs; stars with masses greater than 1.44 times the sun or in the form of large explosions After discarding part of the mass, it evolved into a white dwarf, or continued to shrink, and evolved into a denser neutron star or black hole after the supernova burst.

In 1967, the British radio astronomer A. Hewish (1924-) and his graduate student J. Bell (1943-) discovered the first neutron star.

In the 1950s, American astronomer M. Schwarzschild (1912-) predicted that a supermassive star would shrink continuously after its eruption. When its gravity is strong enough to prevent light from escaping, it will become a "black hole." . In 1974, the British theoretical physicist Hawking (S. Hawking, 1942-2018) proved that positive and negative particle pairs will be generated in black holes, and positive energy particles will escape, forming the phenomenon of "evaporation" of black holes. Based on this, astronomers have now discovered several celestial bodies that may be black holes, but it has not yet been fully confirmed.
Human research on stellar evolution has not yet been completed, and exploration will continue.