Main Sequence Stars

Type of physical science: Astronomy; Astrophysics

Field of study: Stars

"Main sequence stars" is a designation for stars that are in the most stable and longest part of their life cycles. These stars are in a state of stellar equilibrium, which defines a balance of energy between the star's mass, composition, and physical structure. Most of the stars in the universe are main sequence stars.

Overview

In 1907, Danish astronomer Ejnar Hertzsprung (1873-1967) devised a classification scheme for the thousands of stars being cataloged. In Hertzsprung's classification process, he plotted star types on a graph showing spectral classes, absolute magnitude, luminosity, and temperature. This diagram became known as the Hertzsprung-Russell diagram (the H-R diagram). American astronomer Henry Norris Russell (1877-1957) had devised a similar stellar classification scheme about the same time as Hertzsprung, and his name was added to the diagram.

When the stars were plotted on the H-R diagram, it was observed immediately that most stars fell along a band running diagonally across the graph from the upper left to the lower right of the plot. This band of stars became known as "main sequence stars." Hertzsprung recognized this relationship and correctly surmised that this uniform relationship (the connection between spectral class and luminosity) arose from a condition that existed in the stellar atmosphere. Later, the H-R plot would be used to describe and understand many aspects of a star's birth, lifetime, and death. The importance of the Hertzsprung-Russell discovery to the understanding of wide and varied aspects of stellar behavior and evolution cannot be understated.

The H-R diagram also shows that there are other stars that do not fit on the main sequence. The stars plotted to the upper right of the H-R diagram are giant and supergiant stars, whereas the stars plotted below the main sequence to the lower left are white dwarf stars. The sun lies on the main sequence band, as shown in the figure.

Main sequence implies stellar stability. Most stars spend most of their lives on the main sequence band of the H-R diagram. For example, the sun is estimated to be about 4.6 billion years old. It has been on the main sequence for most of that time. It will burn for about another 5 billion years, and all but a small percentage of the beginning and the end of its life will be spent on the main sequence. Hence, even stable stars such as Earth's sun are not always on the main sequence band. Stars may be plotted at different parts of the H-R plot, depending on their characteristics. Earth's sun appeared at the upper right of the H-R plot, moved to the main sequence, and will ultimately move to the lower left as it consumes its fuel. Yet, a majority of its life will be spent on the main sequence. Most stars follow similar paths across the plot during their lifetimes. Such wandering across the H-R diagram depicts, as Hertzsprung correctly ventured, a change in the physical state of the star as it evolves from birth to death.

Stars are born from great clouds of dust, hydrogen, and helium in deep interstellar space. These thick clouds are composed mostly of molecular hydrogen (they are thus called molecular clouds) and begin to collapse in on themselves as a result of the gravitational attraction of each dust grain and molecule for one another. Over time, as the core of the molecular cloud collects more and more mass, the process of collapse (or accretion) begins to accelerate.

At the beginning, the primary energy driving a star's formation is gravity. As the material begins to fall in toward the core, however, the molecules speed up and the temperature of the molecular cloud begins to rise. This effect creates a thermal pressure that resists the gravitational accretion. At this point, the unborn star begins to mimic a process that it will sustain throughout its life: thermal balance. If the gravitational pressure prevails over the thermal, the molecular cloud likely will become a star. In many of these clouds, gravity will win a gradual, long-term victory over thermal pressure. (If gravity does not prevail, the cloud will not form a star.) Eventually, the pull of gravity far exceeds that of the thermal pressure, and true gravitational collapse begins.

Even though gravity has, for a short time, endured over thermal resistance, there comes a point when the increasing density of the core has again balanced gravity at the core and further collapse of the core halts. This state of balance is called "hydrostatic equilibrium." The outer layers have yet to catch up with the dramatic collapse and balance of the core, however. These outer layers finally fall into the core in a process called nonhomogeneous collapse. At this point, the molecular cloud is no longer a cloud but a protostar. At this stage, the protostar is enveloped in a thick cloud of dust, so that any photons created by the thermal energy in the core that do escape are so altered by the dust that energy is emitted primarily in the nonvisible infrared portion of the spectrum. Soon, a new balance begins to assert itself in the young star. The central, collapsed core is emitting prodigious energy as the cooler, outer layers of the star are still collapsing. The energy is transferred from the interior to the cooler exterior in great bubbles and streams. The star has reached a new state of energy balance called "convective equilibrium."

As the outer layers continue to fall in toward the core of the protostar, the added pressure of the collapsing outer material continues to heat the core. When the external radiative temperature reaches 2,000 Kelvins, the outer layer of dust evaporates. Meanwhile, the core of the protostar begins to approach temperatures of several hundred thousand Kelvins. The convective (boiling-like) mechanism can no longer carry the heat away in an efficient manner, and a new energy balance asserts itself called "radiative equilibrium." This thermodynamic process will sustain the star for the remainder of its existence. The protostar's interior temperature continues to climb as its energy is radiated outward more efficiently and more material continues to collapse inward. Finally, the temperature of the core reaches from 4 to 10 million Kelvins and the protostar becomes a true star. Thermonuclear fusion takes over from the energy supplied by gravitational collapse. At this point, the young star relies on fusion alone to supply its energy needs, and gravitational contraction stops. The protostar required about 100,000 years to reach the state of radiative equilibrium and another 10 million years to achieve fusion temperature.

Once the star has achieved fusion, it enters the main sequence, where it remains for most of its lifetime.

As a sun-like star exhausts its primary fuel--hydrogen--it begins to contract because the energy required to maintain its diameter cannot be maintained by the fusion of hydrogen. As it contracts, however, gravitational pressure will raise the core temperature and the star will begin to fuse helium. This higher temperature reaction causes its outer layers to expand dramatically, forming a giant star and moving it off the main sequence. From here, it is not clearly understood, but as it exhausts the helium fuel, the star will become unstable and ultimately will move across the H-R plot to the lower left section, where it will become a white dwarf.

Applications

A detailed understanding of how stars are born and how they become stable over most of their lifetimes relates to many sciences. Understanding this process is not only an important link in cosmology and astronomy but also in such diverse sciences as geology and meteorology.

The cosmologist is concerned about how the universe began and how the dynamic, constantly changing universe will end. Learning how a significant proportion of the mass of the universe (in the form of stars) is retained, changed through fusion, and then returned to the universe tells cosmologists much about Earth's own star system. For example, cosmologists theorize that the sun is a second-generation star. The material that makes up the sun and the planets was spewed forth in the explosion of first-generation stars billions of years ago. According to cosmologists' speculations, this process was required to create the heavier elements that could not have come from the initial primordial material. One can also speculate about the development of other planetary systems by understanding how molecular clouds accrete material and how some of the material spins off and forms planets, while most of the material forms the star itself. With this information, the number of other planetary systems there may be in the galaxy, or even in Earth's own region of the galaxy, can be considered.

Understanding the ongoing dynamics of stellar interiors relates directly to how much energy a star emits and to a star's stability. Few people ever wonder (or worry) about a stable energy output from the sun, but the sun's energy is directly responsible for the sustenance of all life on Earth and also Earth's weather. With a change of even a few percentage points of energy output from the sun, serious consequences could result on Earth, such as radically altered weather patterns (global warming or cooling) and shifting agricultural areas. A constant radiative output by the sun is so important that even a minuscule variation could ultimately affect the world's geopolitical stability.

By a direct study of the sun's energy output and by comparing information gained from sun-like stars in Earth's region of the galaxy, cosmologists have learned much about the stability of the sun and what to expect over long periods of time. The sun's output is not expected to vary significantly so as to contribute to major problems in the short term. Nevertheless, there are adherents to theories claiming that Earth's periodic ice ages are caused by cyclic variations in the sun's total energy output.

Context

Although the theories of stellar formation and evolution are well supported by mathematical constructs, there is a shortage of observational confirmation of these ideas. It is evident that a probe will not be sent deep into the sun's interior; hence, cosmologists may never have a direct observational view of stellar interiors. Nevertheless, there are many other methods of indirect observation that scientists are applying, such as the study of solar magnetism, oscillations, and helioseismology.

No one has ever witnessed the birth of a star. There is a considerable organized effort in progress, but no candidate region has appeared. The concept of observing planetary formation is so tightly bound to stellar birth that astronomers are equally interested in visual confirmation of planetary accretion disks within the view of a newly formed star. The concepts of stellar birth are fundamental to astrophysical theory. Yet, as more information is discovered, more questions are raised. Although there is great confidence in the concepts of energy outflows and the inherent stability (if not the overt thermodynamic necessity) of these processes, scientists do not have a good grasp of how these ideas relate to what is actually observed in the stellar spectra.

Until the observational evidence blends with theoretical evidence, the concepts of star birth remain speculative, including theories of the relationship between planetary formation and star formation. Another concern is how a newly formed star rotates and, once it is rotating, how a collapsing star later slows its rotation before thermonuclear fusion begins. All these questions must relate ultimately to direct observation of candidate protostars and accretion disks. While addressing these questions and observations, the science of star birth and the sun's thermonuclear life on the main sequence will continue to have direct applications to Earth's long-term weather picture and the welfare of humanity. A study of planetary formation relates specifically to the possibility that there are other intelligent life-forms somewhere in the universe. By estimating the number of planets that form from an average star system, the number of possible habitable planets may be calculated and speculations can be formed on the possibility of the development of intelligent life. In a project called SETI (Search for Extraterrestrial Intelligence), star types are targeted by listening for radio signals. Such knowledge of star formation, stability, and manifested characteristics enables such searches to be conducted more efficiently.

Principal terms

ACCRETION: the process whereby a newly forming star system collects and consolidates the material for its stellar and planetary masses

CONVECTIVE EQUILIBRIUM: a state of stellar stability characterized by a fluid, convective transfer of energy from the hot central core to the cooler outer layers

HERTZSPRUNG-RUSSELL (H-R) DIAGRAM: a diagram used widely in astronomy and astrophysics that depicts stellar position relative to spectral class, luminosity, temperature, and magnitude

HYDROSTATIC EQUILIBRIUM: a state of stellar stability characterized by a balance between gravitational collapse and thermal pressure at the core of the forming star

MAIN SEQUENCE STAR: refers to the position of a star on the H-R plot; typically describes a stable star in steady state, consuming hydrogen by thermonuclear fusion

MOLECULAR CLOUDS: massive, very large clouds of molecular hydrogen and dust; the breeding ground of stars in deep space

PROTOSTAR: the first phase of star formation, when the molecular cloud has collapsed and is emitting prodigious amounts of thermal energy from gravitational pressure

RADIATIVE EQUILIBRIUM: when radiation becomes the predominant, balanced mechanism for carrying heat away from the interior of a newly forming star

SPECTRAL CLASS: the classification of a star's chemical characteristics based on its spectral signature

Bibliography

Asimov, Isaac. EXTRATERRESTRIAL CIVILIZATIONS. New York: Fawcett Columbine, 1979. Stellar types and classes are used to detail how life could evolve and survive around different types of stars (including giant and dwarf) found on the H-R diagram. A good example of how deeply the H-R diagram and the discussion of stellar stability has woven its way into all aspects of astronomy. Easy to read, though not illustrated.

Bok, Bart J., and Priscilla F. Bok. THE MILKY WAY. Cambridge, Mass.: Harvard University Press, 1981. The definitive book on the Milky Way written by two world experts on the subject. The book is well written, easy to digest, and includes a detailed deliberation of main sequence stars, the H-R diagram, and its relevancy to the discussion of stars near the sun. Illustrated.

Cooke, Donald A. THE LIFE AND DEATH OF STARS. New York: Crown, 1985. This very colorfully illustrated and easy-to-read book gives an in-depth account of star birth, from molecular cloud to protostar to main sequence. Gives an excellent discussion of stellar equilibrium states. For a wide audience.

Cornell, James, and Alan P. Lightman. REVEALING THE UNIVERSE. Cambridge, Mass.: MIT Press, 1982. This book discusses specific areas of observational and theoretical astronomy. A discussion of the H-R diagram is included, specifically on how the shape of stellar interiors predicts the shape of the H-R diagram. Somewhat technical, and is best approached by those with a scientific background. Illustrated.

Hartman, William K. ASTRONOMY: THE COSMIC JOURNEY. Belmont, Calif.: Wadsworth, 1978. Much less rigid than most texts. Heavily illustrated, and offers not only a detailed discussion of stellar birth but also an unbeatable comparison of the flexibility of the H-R diagram in a discussion of star birth, maturity and death. Understandable to those with a minimal background in science.

Harwit, Martin. COSMIC DISCOVERY. New York: Basic Books, 1981. Covers "the search, scope and heritage of astronomy." Hertzsprung's development of the H-R diagram, main sequence stars, and use of spectral classification data. A fascinating expedition through the history and foundation of astronomy. Illustrated.

Hertzsprung-Russell diagram

Interstellar Clouds and the Interstellar Medium

Protostars and Brown Dwarfs

Stellar Oscillations and Helioseismology

Thermonuclear Reactions in Stars