The Main Sequence

After stars form over a period of a few to a few tens of millions of years, they reach a steady state of hydrogen burning that can last for 10 billion years or more before changing significantly again. Stars on the main sequence plot on a fuzzy line on a diagram

Steady Star Main Sequence

Hubble image of bright blue newly formed stars that are blowing a cavity in the center of star-forming region N90 (NASA)

depicting the relationship between solar luminosity and surface temperature (called a Hertzsprung-Russell or H-R diagram [see page 698]), indicating a balance between a star's temperature, luminosity, and mass during the hydrogen-burning stage. once a star uses up its hydrogen fuel, these relationships change, the star is out of equilibrium, and it leaves the main sequence, plotting in different places on the H-R diagram, depending on its mass. The time that a star leaves the main sequence represents the beginning of its end, and its lifetime after departure from the main sequence will be relatively short.

Low-mass stars burn so slowly that most still exist, as they have not yet exhausted their hydrogen fuel supply. In contrast high-mass stars burn more quickly; some of the largest stars stayed on the main sequence for only a few tens of millions of years. Most of the very massive stars that were created in the history of the universe have left the main sequence, gone through their death throes, and moved to the end states of existence. stars of intermediate mass are experiencing or will experience intermediate fates. In general low-mass stars have a gentle end of their existence as bright objects, whereas high-mass stars generally have an explosive ending. The exact boundary between high- and low-mass is fuzzy but ranges somewhere between five and ten solar masses.

While a star is burning hydrogen on the main sequence, it maintains a balance or equilibrium between gravitational forces that tend to draw all the atoms together to the center of the star and gas pressure from the hydrogen burning inside the star pressing outward against gravity. This equilibrium is maintained for about 90 percent of a star's lifetime, and as long as it is maintained nothing spectacular will happen to the star; it will simply continue to burn hydrogen in fusion reactions, converting it into helium. The nuclear fusion process is a multistep process in which four hydrogen nuclei (protons) combine to produce one helium 4 nucleus, emitting gamma ray radiation and two neutrinos. over this time period the star surface may occasionally erupt, forming giant solar flares or sunspots, and is constantly emitting large amounts of photons and other particles from the nuclear reactions inside. A star typically grows in luminosity during its history on the main sequence. The Earth's sun, for instance, is now about 30 percent more luminous than when the Earth, sun, and solar system formed nearly 5 billion years ago.

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