I'll give you the life cycle of a star (birth, life, and death cycle) summarized from one of my lessons.
Stars are huge spheres of very hot hydrogen and helium gasses. The gasses in a star are held together by the large gravitational forces created by its own mass. Thermal pressure from the heat inside the star balances this gravity and prevents it from collapsing. The center of a star can reach temperatures of 15,000,000 K. The pressure at the center of a star is over 100 Billion times the atmospheric pressure on earth and the density is 7 times denser than gold.
A star burns from nuclear fusion inside the core. Here hydrogen atoms fuse into helium releasing huge amounts of energy. This energy moves slowly outward through the layers of the sun through convection and radiation. It can take a million years for the energy to work its way through the sun. Once the energy reaches the surface of the sun, it radiates into space as light and radiation. Here, the temperature drops to 6,000 K
Because of the large numbers involved, the masses and radii of stars are usually given in terms of our sun, called solar masses and solar radii, respectively. Our Sun has a solar mass of 1.0 solar masses. A star with a solar mass of 2.1 solar masses has a mass 2.1 times the mass of our Sun.
Life and Death of Stars:
Stars are formed from gas and dust. This gas and dust begins to coalesce and collapse inward by the force of its own gravity. As it becomes larger, it draws in more gas and dust. As the gases pull in further and further under its own gravity, the pressure in the center rises. If there is enough mass, the pressure becomes large enough and heat becomes high enough for the gasses to ignite in nuclear fusion. Once nuclear fusion begins the collection of gasses and dust becomes a new star.
Any object below 0.013 solar masses (or 13 Jupiter masses) is considered a planet. If the mass of the object is just above this limit, from about 0.013 solar masses (or 13 Jupiter masses) to 0.08 solar masses (or about 80 Jupiter masses), the object is called a brown dwarf. In this range, the body can fuse deuterium (which requires less heat and pressure than hydrogen) but there is a limited amount of deuterium and it doesn’t last long. A brown dwarf may sputter for a short time, but is too small to sustain nuclear fusion and won’t become a star.
Once the core of a star begins its nuclear fusion, the outward forces produced by the fusion reactions balance the inward forces of gravity creating equilibrium. This is the main stage of star’s life. Here the star keeps burning its hydrogen into helium. Stars in this stage are called main sequence stars. The lifespan of a star in this stage varies greatly with the size of the star. A small red dwarf may live for a trillion years. A smaller to average star like our sun will live about 10 billion years. The very large and massive stars may only live 10 million years. I may seem odd that the bigger a star and the more fuel it has, the faster it will die, but it is correct. This is like building a campfire. If you have a very small campfire, you can burn a log all night and it will still be there in the morning—it burns very slowly. If you had a huge roaring campfire, however, you could burn through a whole stack of logs and still run out—it burns very hot and fast. The same thing happens with stars.
The general minimum mass required for a sphere of gas to form a star is about 0.08 solar masses (or about 80 Jupiter masses). At this size, it is a little red dwarf star. Red dwarf stars have masses up to 0.4 solar masses. Above these red dwarfs up to about 1.4 solar masses are the main sequence stars often called yellow dwarf stars. This is the most common type of star and includes our sun. With these stars, as they age they will burn up much of their hydrogen. Once this happens, the burning in the core starts to slow. This reduces the outward forces and causes the star’s gravity to begin contracting the core. This causes the core to heat up further until it gets hot enough to begin another fusion reaction burning the helium in the core into carbon and oxygen. At this time, the heat causes the remaining hydrogen in the shell of the star to begin fusion reactions outside the core. All this burning causes an outward pressure once again which causes the outer layers of the star to begin expanding. The star will expand to over 200 times it former radius and becomes a red giant star. When this happens to our sun in about 5 billion years, it will grow out until it engulfs most of the inner planets, possibly even the earth. After about 100 million to 500 million years as a red giant, the helium in the core will run out. The mass and temperature will be too small to fuse any of the carbon or oxygen. The outer layers will leave the star and the remains of the core will become a white dwarf—a small but extremely dense star about the size of the earth. The white dwarf begins to cool, and after about 10 billion years of cooling, it will become a black dwarf.
Larger stars proceed through their lives much faster than smaller stars. These are called supergiant stars. They burn their fuel much faster, and the largest ones burn so violently, they can spew off large amounts of mass and energy. These large stars burn so hot they readily burn helium into oxygen and carbon. When the Helium starts to get used up, they begin fusing the oxygen into neon and eventually fusing the remaining core material into iron. When the core becomes mostly iron, there is no further fusion possible (fusing iron requires more energy than it creates), and the fusion reactions suddenly stop. When the fusion reactions stop, there is no further outward pressure force; the gravitational force of the huge mass collapses the star rapidly. It then rebounds back in a huge shockwave that blows the outer layers of the star violently away. This enormous, very brilliant explosion is called a supernova. Supernovas are so violent, that elements heaver than iron, like gold etc., are formed. It is also possible for a supernova to form when a white dwarf collects extra mass from a companion star to increase its own mass to greater than 1.4 solar masses.
These larger stars will have different endings depending on how big it was and how much is left after the supernova. If the remaining core of the supergiant falls below 1.4 solar masses, it will become a white dwarf like its main sequence cousins (which can happen for the smaller supergiants or for larger stars with extremely violent endings). If the remains of the core have a mass of 1.4 to 3 solar masses, the star will become a neutron star. A neutron star is so dense, that the protons combine with the electrons to form almost solid neutrons, like a single, packed nucleus. These stars are more massive than our sun, but only a few kilometers across. If the remains of the supergiant have a mass over 3 solar masses, the core that is left will collapse down to something even denser than a neutron star. Its mass will cave in on itself and it will form a black hole.
a math and physics teacher with a specialization in astrophysics.