Life and Death of Stars

Generally speaking, there are two main life cycles for stars, and almost all stars can fit into one or the other. The determining factor for categorizing the star is its mass. Any star less than about three solar masses (one solar mass is the mass of our sun) will spend almost all of its life transiting what is called the "Main Sequence." About 90% of all stars are like this. If a star is above the crucial value of three solar masses when it is born, it will spend much less time on the Main Sequence, have a much shorter life span, and it will die much more violently, creating either a neutron star or black hole.

During the major part of their lives, most stars on the Main Sequence will create their energy by the process of hydrogen fusion - the process of fusing two hydrogen atoms to create one helium atom *. Energy is created because a helium atom weighs slightly less than the two hydrogen atoms, and the excess mass is converted into energy, as related by Einstein's famous equation E=m*c². Our sun is currently in this stage of converting hydrogen to helium.

Nuclear Fusion Schematic In the picture to the right, two protons join together to form a deuterium nucleus, which is also known as "heavy water." A positron and a neutrino are released as by-products. The deuterium nucleus is bombarded by another proton, creating a helium-3 nucleus. The by-product of this is a photon in the form of a gamma ray (a very high-energy form of light). Then, the helium-3 nucleus in bombarded by another helium-3 nucleus, creating a normal helium-4 nucleus. The by-product of this are two protons, which are free to start the whole process over again. The positron will be destroyed and form another gamma ray; the energy from this in the form of gamma rays is radiated out of sun's core.

Since our sun is currently in this stage, the numbers here are for it alone, although since it is like most other stars, the are representative of how all stars work. Each second, the sun converts 500 million metric tons of hydrogen to helium. In turn, every second 5 million metric tons of excess material is converted into energy. This means that every year, 157,680,000,000,000 metric tons are converted into energy. The material from one second energy is about 1x10²⁷ (one octillion thousand) watts of energy. On Earth, we receive about 2/1,000,000,000 (two billionths) of that energy, or about 2x10¹⁸ (two quintillion) watts. This is enough energy to power 100 average light bulbs for about 5 million years -- longer than humans have been standing upright.

*There are actually two major processes that are used by stars. The one presented here is a simplification of one part of the main one. If you are interested, the other cycle, called the "CNO Cycle," is presented in the Advanced version of this web site, as is a more-in-depth version of this, which is called the "pp Chain."

After about ten billion years, a main sequence star has converted approximately 10% of its hydrogen to helium. Although this might seem as though it could still undergo hydrogen fusion for another 90 billion years, this is not the case. Remember that there are immense pressures at the core of stars, and it is only because of these pressures that the fusion can occur -- in a fixed volume, increased pressure leads to increased heat. Outside of the range of pressures there is still mostly hydrogen, but it cannot be used because the pressures are not high enough to initiate fusion.

The helium core begins to contract, and the outer layers expand and cool, glowing redder. The star is now called a red giant. At this point, helium fusion begins. The star was previously unable to fuse the helium; however, now that the core has contracted, the added pressure is enough to fuse helium into heavier elements. Simultaneously, hydrogen fusion also occurs at this point in a shell around the helium core, for pressures there have also increased enough for hydrogen to fuse. Life expectancy from here on is about 100,000,000 years.

After this time, the red giant is made of mostly carbon. The next fusion process would be to fuse the carbon into iron. The problem in this star is that there is not enough pressure in the core to do this. Because the outward pressure of energy is no longer maintained, the core collapses and sends a shockwave outwards, and the star's outer layers are cast off in a planetary nebula, with the resulting core becoming a white dwarf. The core is made almost of pure carbon (like coal), and glows white because it still possesses a lot of left-over heat. It now also possess much less mass because it has shed its outer layers, and any planets it has would move to much farther orbits or be completely ejected from the system, if they had not been engulfed by the star in the expanded red giant phase.

The white dwarf is destined to drift in space for millennia as it slowly cools. Most have an approximate size of the Earth (8,000 km (7,500 miles) diameter), and has a density such that a matchbox's worth would weigh about as much as an elephant. It has a maximum weight of 1.4 solar masses. As it cools, it will grow dimmer, and will eventually become a black dwarf - a frozen lump of carbon floating though space.

Supernova Remnant - N132D in LMC A star with a mass much greater than that of the sun will form, live, and die more quickly than a main sequence star. The reason for this is its greater mass, for the resulting gravity squeezes the star's core and creates greater pressures, resulting in a faster fusion rate.

After about 10 to 15 million years (versus over 10 billion for a main sequence star), a supergiant's core has turned to carbon and has swollen into a red supergiant (Betelgeuse, a bright star forming the constellation Orion's shoulder, is an example of a star in this stage). The reason it glows red is that since its outer layers have expanded, it has a much greater volume to heat, yet is only producing the same amount of energy. Thus, it is naturally cooler, and glows red.

The difference at this point between a supergiant and a main sequence star is that a supergiant has the pressures needed to fuse carbon into iron. However, this fusion process takes energy, rather than gives it. So, as energy is lost, the star no longer possesses the balance between outward pressure and gravity pushing in. As a result, gravity wins out, and the core collapses in a violent explosion called a supernova. (One supernova explosion on A.D. July 4, 1054, was so bright that it could be seen in broad daylight for 23 days. The nebula it created is called the Crab Nebula, and is the picture to the left.)

The path here is also divided as to what the final outcome of the star will be. If the star is less than about nine (but more than 1.4) solar masses, the core will collapse into a neutron star - a star made entirely of neutrons. If the star possesses more mass, it will continue to collapse into a black hole - a point of theoretically infinite density that possesses such a strong gravitational pull that not even light can escape its pull.

Although the sun is technically considered a main sequence star, it also falls under the classification of a yellow dwarf. On this page as with the rest of the site, however, the term "dwarf" is used to refer to stars smaller than the sun in order to avoid confusion.

Red dwarfs are the only active (undergoing hydrogen fusion) type of dwarf (other types are brown, white, and black). Red dwarfs range between 1/3 and 1/12 the sun's mass, and shine only 1/100 to 1/1,000,000 as brightly. Proxima Centauri, Earth's closest extrasolar star, is a red dwarf 1/5 the size of the Sun, and if it were to trade places with the Sun, it would shine on Earth only 1/10 as much as the sun currently does on Pluto.

Red dwarfs, because of their small size, undergo fusion much less quickly than a solar mass star. Therefore, they use up their supply of hydrogen much less quickly than a main sequence star, and can live for more than 100 trillion years.

When they die, they simply wink out of existence, for they do not have enough pressure to fuse helium. Thus, they simply grow dimmer and cooler as they float through the void of space.