Stars are born, stars grow old, stars die. The life cycle of a star is actually its 'struggle to live' fighting the gravitational pull and internal pressure. A majority of the life of a star is spent in the main sequence stage. Most stars, specifically our Sun, fuse hydrogen into helium and helium into heavier elements like carbon, oxygen up to even iron and nickel.
Stars exhaust their energy during this process. Stars go through a series of changes during their lifespan. The process is known as stellar evolution, during which they change in their structure, composition, and appearance. Very massive stars live for a few million years, while those with lesser mass live for trillions of years.
It won't be wrong to say that a star is a sizzling mass of gas. It is composed of the inner core where the process of fusion takes place and an outer gaseous shell. The core is hot and dense, acting as the gravitational center of a star. The outer shell, made of hydrogen and helium, facilitates the transfer of heat from the core of the star to its surface. Light and heat energy is released into space from the surface of the star. Here's more on the stages in the life cycle of a star.
Nebulae may be formed due to gravitational collapse of gas in the ISM. Gravitational collapse refers to inward fall of a body under its own gravity. Some nebulae are formed from supernova. Here, the particles thrown during the explosion ionize and come together, forming a nebula. Clouds of dust and gas (majorly hydrogen) may be stirred by a passing star, which causes the particles to come closer together. This causes the matter in the nebula to concentrate towards one central point, which becomes the center of mass of the new star. Depending on the amount of matter, a dwarf or a new star is formed. The critical mass for the formation of a new star is around 80 times the mass of Jupiter. Nebulae are of different types. Emission nebulae emit light (electrons from hydrogen atoms combine with protons, giving out red light in the process). Reflection nebulae glow (dust particles in them reflect light from the stars). Many just remain suspended in space for years, while others are able to see a new star born!
Gravitational forces make the particles in a nebula spin. As they spin faster, the velocities cause particles to clump together forming a cloud-like structure. This is when a 'protostar' is born. If large clumpy structures break into small clouds, a cluster of protostars may be formed. The gravitational forces in the particles cause contraction and heating of the star. Physicist Viktor Ambartsumian, for the first time, proposed the existence of a protostar. A protostar is formed as a result of contraction out of the gas of giant molecular clouds in the ISM. A protostar starts accreting mass, which means addition of atoms to its center. Due to accretion, a protostar is unable to achieve equilibrium. The process ends with the formation of a T Tauri star. A protostar may take 100, 000 years to reach the main sequence stage in its life cycle.
When gas pressure inside the star equals gravity, the star attains a stable state and begins entering the main sequence phase. Nuclear fusion occurs and it begins to glow. The star contracts and becomes stable. It is now called the main sequence star or stable star. Stable stars exhibit the condition of equilibrium. Equilibrium is achieved when the force pushing out from the center equals the gravitational force that pulls the atoms inward. As the stars contract, the temperature, density and pressure at the core continue to rise. For a major part of its life span, a star stays in its main sequence phase. The conversion of hydrogen to helium takes place during this stage in the life cycle of a star.
The temperature at the core of the star slowly rises because the star emits energy. Hydrogen gets converted into helium by the process of nuclear fusion. When the hydrogen in the core depletes, the core loses stability. The temperature and pressure continue to rise. The star then starts glowing red, thus entering the red giant phase. Very large red giant stars are known as Super Giants. They are magnanimous in size (have diameters about 1000 times that of the Sun) and have very high luminosities. The path taken by a star after this phase depends on its mass. It will become a neutron star, a white dwarf, black dwarf or a black hole.
Stars with a smaller mass become white dwarfs. Their core shrinks to become a white dwarf while their outer layers are planetary nebulae. White dwarf stars are composed of electron degenerate matter. They are usually formed out of carbon and oxygen. If their temperature has fused carbon to neon, an oxygen-neon-magnesium white dwarf is formed. The electron degeneracy pressure causes white dwarfs to become dense. As they lack any source of energy, these initially hot stars, on radiating all their energy, cool down. White dwarfs have a mass comparable to that of the Sun and volume comparable to that of the Earth. After the depletion of all its energy, a white dwarf enters the 'dark dwarf' stage.
Supernova is an explosion of a star accompanied by emission of radiation and light. There are two basic types of supernovae. One is when a carbon-oxygen white dwarf reaches a critical density value, leading to uncontrolled fusion of carbon and oxygen, further leading to explosion. The second type of supernova is formed towards the end of a massive star's life cycle. When all the fuel in a star is exhausted, the iron core collapses with an explosion forming supernova.
A star in the red giant phase takes a different life cycle path. Fusion causes the helium atoms to form carbon atoms. They are further pulled together due to gravity, which results in the formation of oxygen, nitrogen and finally iron atoms. Iron starts absorbing energy that leads to an explosion. During this stage in the life cycle of a star, it is known as the neutron star. Cores of massive stars collapse, converting proton-electron pairs into neutrons. A neutron star might spin speedily giving off light and X-rays. The highly magnetized spinning star appears to be pulsing, and is known by the name 'Pulsar'. These stars pulsate with surprising regularity.
Very massive stars become black holes. There is no nuclear fusion, the star core shrinks down to a point and the star gets swallowed by its own gravity, thus becoming a black hole. A black hole, as you might know, is a region through which nothing can pass. It swallows everything that comes its way and grows in size. Black holes are formed when heavy stars collapse in a supernova at the end of their lifespan.
Black holes thus formed grow bigger by absorbing the surrounding mass, swallowing other stars and merging with other black holes. This leads to the creation of super massive black holes. This is where a star's life cycle comes to an end.