The life cycle of a star begins and ends in an immense gas cloud, or interstellar cloud. Since the gas cloud is spread out over a huge area, its density initially very small, much smaller than that needed for the formation of a star. The cloud therefore needs outside help, which can take the form of a gravitational force or of energy waves that push some of the cloud matter together, creating a body the gravity of which will attract the rest of the cloud. As this body picks up mass, becoming heavier and heavier, it becomes a proto-star, a star which puts out energy solely as its constituent matter is compressed by gravity. The core of the proto-star is the densest and hottest area; once the center reaches the critical temperature of 10 million degrees Kelvin the star proper is born as thermonuclear reactions begin, turning hydrogen into helium through fusion. The star can now produce its own energy, which will serve to fight its own gravity, keeping the star from collapsing in on itself.
For the rest of its life, the star will fight a constant battle against its own mass so as not to collapse upon itself. This battle consists of the star’s gravity compressing the body’s peripheral gas inwards towards the core (heating the gas) versus the star’s thermonuclear fusion, which generates a massive amount of energy that radiates outwards, opposing the gravitational force. Fusion is necessary because the thermal energy produced by gravity’s compression of the star’s constituent matter isn’t on its own enough to prevent the star from collapsing. The fragile equilibrium of gravity and fusion is maintained through the star’s life and assures its survival as a normal star until the post main sequence. In this stage of its life the star can become one of two things depending on its mass. The first is a giant star which is the product of a small mass and which has a long life. The second is a supergiant star, which is the product of a very massive star and which has a relatively shorter life span.
Post Main Sequence (smaller stars)
In all stars the hydrogen “burnt” by fusion is converted into helium in the core throughout the life cycle. However in smaller stars (approximately the mass of our sun) the temperatures inside the core are not high enough to fuse helium into carbon. Instead a core of inert helium, which does not engage in fusion, forms around the core, creating a region which is not producing any thermonuclear energy and which is not combating the force of gravity. Therefore gravity begins to slowly compress the star, increasing the core temperature; the thermal energy produced slows the compression for a while. However there comes an inevitable point when there is no more hydrogen in the core as it consists entirely of helium. The thermonuclear force is no longer countering gravity enough and the star begins to collapse. Gravity compresses the star, increasing the temperature of the gases. In the layer next to the core, where there is still some hydrogen, this increase in temperature causes fusion picks up, dilating the stellar envelope. Meanwhile the core contracts until the helium atoms become degenerated (a state of matter in which pressure and temperature are independent from one another, detailed by the Pauli exclusion theorem). The star has become a red giant.
The temperature of the core increases progressively. When it reaches around 100 million degrees Kelvin the fusion of helium into carbon begins to occur. This fusion of helium, a phenomenon called helium flash, occurs very quickly because of the degenerated state of the helium atoms. After a very short period of time (relative to a star’s life span) the outflow of energy becomes more regular and the gas envelope retracts slightly. So for a star roughly the mass of our sun, helium is turned into carbon but after that the star incapable of generating the temperatures necessary to begin carbon fusion. The same process detailed above therefore begins again: the core contracts while the stellar envelope expands. However this time a fresh round of thermonuclear fusion does not start, instead the stellar envelope becomes less and less attached to the core until it is ejected, forming a planetary nebula (the birth place of planets). The carbon core is left, naked, to slowly burn out as a white dwarf.
Post Main Sequence (larger stars)
In the case of supergiant stars (the mass of which is at least 8 times the mass of our sun) the entire process is quite similar except for the fact that it continues past carbon, producing heavier and heavier elements. When the helium core contracts under the force of gravity it rapidly reaches temperatures of the order of 600 million degrees Kelvin. At this temperature even the carbon atoms cannot resist and begin fusion, forming neon. This reaction releases a huge amount of thermal energy which allows the heavier neon to begin fusion, producing even heavier elements, and so on. This chain of fusion reactions produces an immense amount of energy and the speed of the fusion reactions increases exponentially: 600 years for carbon fusion, one year for neon, 6 months for oxygen, and finally as short a time as one day for silicon. The stellar envelope of course expands at each link in this chain, giving birth to a red supergiant.
However this chain also leads to an impasse: iron, the inert element which cannot engage in thermonuclear fusion. The chain ends suddenly, with a very dense but dead core. The star’s structure looks like an onion at this stage: at the center, iron, and then successive layers of silicon, oxygen, neon, carbon, helium, and finally hydrogen in the outermost layer. Fusion continues in these outer layers, producing more and more iron which is drawn towards the core. When the iron core reaches the limit of 1.44 solar masses, it can no longer resist its own gravity and collapses.
Gravitational Collapse (larger stars only)
At this stage in the life cycle of larger stars there are multiple possibilities, again depending on the mass of the star in question. If the mass of the core is inferior to 1.4 solar masses, the star will turn into a white dwarf, slowly burning itself out. If the mass of the core is between 1.4 and 3.2 solar masses the star’s fate is that of a becoming neutron star. Finally if the mass of the core is higher than 3.2 solar masses the process of gravitational collapse will continue and the star will either explode violently into a supernova or become a black hole.