On stars

Alan Cai

May 17, 2024

Most stars in our universe (around 90% according to some estimates) are main-sequence stars. They constitute the large middle elongated cluster of stars on the Hertzprung-Russell (HR) Diagram and possess the most typical stellar qualities. The HR diagram is a star categorization diagram that charts stars' absolute magnitude against their spectral classification, color, or temperature: roughly equivalent measurements.

The main sequence stars are traditionally seen as the full life cycle of a star. Stars often form in nebulae or regions of dust and gas sometimes spanning many light-years across. Nebulae (plural: nebula) are formed from the death of stars, including supernova explosions, and are considered areas that are significantly more dense than the surrounding universe. However, most nebulas are also predicted to be much less dense than any human-made vacuum. After gravity pulls pockets of gasses together, stars are born. Once formed, newly born stars generate thermal energy in their stellar core by fusing hydrogen into helium. Some gas and dust particles that do not condense into the star can form planets or other astronomical objects. Newly born stars are often blue and very bright, gradually progressing to yellow, orange, and finally red as they age. Main sequence stars are often referred to as dwarfs but are not to be confused with white dwarfs, which are a separate auxiliary sequence.

Stars begin to reach the end of their lives when they run out of hydrogen to fuse into helium. At this point, the star most often becomes a red giant using thermonuclear fusion: rapidly expanding as its gravitational forces begin to fail to hold its outer layers intact and as it begins to fuse hydrogen into carbon through the triple-alpha process (+2 helium atoms are also called alpha particles) and oxygen. Some stars do not become red giants due to low mass and transition directly to becoming helium white dwarfs. After its lighter elements are completely ejected, red giants leave behind their carbon/oxygen cores to become white dwarfs

White dwarfs are considered the final stage in the life cycle of stars which do not have sufficient mass to create a supernova or become a neutron star or black hole. Astrophysicists estimate that this category pertains to over 95% of all stars. White dwarfs cannot sustain themselves through nuclear fusion but still exhibit limited luminosity through smaller thermal emissions. While normal main sequence stars avoid gravitational collapse (implosion due to overwhelming gravity) using nuclear fusion, white dwarfs maintain stability through the electron degeneracy pressure, which is roughly defined as the tendency for electrons to retain a nonzero volume. The Chandrasekhar limit governs the maximum size for a white dwarf.