Main sequence stars

A main sequence star is one that is fusing hydrogen into helium in its core. This is the longest-lasting stage in a star’s energy-producing life. Most stars, including the Sun, are in this main sequence stage.

Overview

The Hertzsprung-Russell diagram (or H-R diagram) is a graph of stellar luminosity versus surface temperature. When stars are plotted on an H-R diagram, most lie along a band running diagonally across the graph from the upper left (luminous, hot, blue stars) to the lower right (faint, cool, red stars). This band is known as the main sequence because so many stars lie along it in the H-R diagram, and the stars along it are called main sequence stars. Computer models of stellar interiors show that main sequence stars generate their energy by fusing hydrogen into helium in their cores. They are in a state of hydrostatic equilibrium, meaning the pressure due to gas and radiation trying to expand them is balanced by their self-gravity trying to contract them. Most stars spend most of their energy-producing lives on the main sequence.

Our solar system’s Sun, for example, is a main sequence star. It is estimated to be about 4.6 billion years old. It has been on the main sequence, fusing hydrogen into helium in its core, for most of that time, all but the first few tens of millions of years of its existence. It has enough hydrogen in its core to continue doing so for approximately five billion more years.

Stars are born in interstellar clouds of gas and dust called nebulae. These nebulae are composed mostly of hydrogen, with some helium and very small amounts of other elements. A typical nebula contains enough matter to form hundreds or thousands of stars. A portion of a nebula will begin to contract gravitationally. This contraction might be triggered by shock waves from a nearby supernova or by the nebula’s passage through a spiral-arm density wave as the nebula orbits the center of its galaxy. As the gas contracts, half the energy released gravitationally goes to raising its temperature, and the other half is radiated away; it starts to shine as a protostar. The central part of the protostar heats the most, and collisions between the atoms there strip away their electrons, ionizing the gas. When the central temperature of the contracting protostar reaches a few million kelvins, it becomes hot enough to begin fusing hydrogen nuclei (the most abundant element in stars initially) into helium nuclei in its core. The initiation of hydrogen fusion in the core stops the gravitational contraction, and the protostar becomes a main sequence star.

Nuclear fusion reactions release energy because the total mass of the lighter nuclei that are fused together exceeds slightly the mass of the heavier nucleus that is produced. The small excess mass is converted into energy according to the famous Einstein relation between mass and energy, E = mc². In hydrogen fusion, which supplies the energy for main sequence stars, four hydrogen nuclei fuse into one helium nucleus.

For fusion to occur, the positively charged nuclei must overcome their electrostatic repulsion for each other; this requires high temperatures (so the nuclei are moving fast) and high densities (so the nuclei are reasonably close together). Only the cores of main sequence stars have high enough temperatures and densities to sustain fusion reactions, so that is where hydrogen fusion occurs, and the energy is produced.

The energy flows from the core to the surface via two possible mechanisms: convection and radiation. Convection transports energy as rising bubbles of hot, low-density gas while surrounding cooler, denser gas sinks. Radiation transports energy as a flow of photons of electromagnetic radiation. Radiation always occurs throughout the interiors of main sequence stars whether convection also occurs or not. Low-mass main sequence stars (about 0.1 times the Sun’s mass) have convection occurring throughout the interior. Main sequence stars like the Sun have a radiative central region and a convective outer envelope. High-mass stars (about ten times the Sun’s mass) have a convective central region and a radiative outer envelope.

As a main sequence star consumes the hydrogen in its core, it becomes slightly brighter, although it remains near the main sequence. When the hydrogen in the core of a main sequence star is exhausted, the core contracts and heats up; basically, the core is “searching” for a new nuclear fusion reaction to tap. Hydrogen fusion is transferred to a shell still rich in hydrogen surrounding the shrinking helium core. The outer part of the star expands, and as it does so, it cools off and becomes redder. (Notice the two simultaneous but opposite behaviors: the core contracts and heats up while the outer layers expand and cool off.) The star becomes larger and redder. It leaves the main sequence and becomes a red giant or red supergiant.

The length of time a star spends on the main sequence depends on its mass. Massive stars have more fuel, but they consume it much more quickly, so their main sequence lifetimes are short—perhaps as little as a few million years, compared to about ten billion years for a star like the Sun. Low-mass stars have less fuel, but they consume it very slowly, so their main sequence lifetimes are long—perhaps as long as a trillion years. This is much longer than the age of the universe, which means that every low-mass star that ever formed is still a main sequence star.

Applications

Most stars, including our Sun, are main sequence stars. Understanding the structure and stability of main sequence stars is important in many ways. The Sun has immediate effects on the solar system. Any changes in the Sun will produce changes on Earth. The Sun’s energy output is directly responsible for Earth’s weather and the sustenance of all life on Earth. A small change in the energy output from the Sun could result in serious consequences for Earth, such as radically altered weather patterns, climate change (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 Earth’s geopolitical stability. There is evidence that the Sun’s activity (and output) might vary over timescales of decades to centuries, and these variations may be linked to recorded changes in climate. When the Sun finally does leave the main sequence and become a red giant, it will be so bright that Earth’s temperature will rise dramatically. Our oceans will boil away, our atmosphere will escape into space, the surface rocks will at least partly melt, and Earth will become uninhabitable.

Since the mid-1990s, planetary systems have been discovered around a number of other stars. Few of the planets detected so far seem at all Earth-like, but it is interesting to speculate about the possibility of extraterrestrial life and, especially, intelligent life. The only known example of the development and evolution of life (and intelligence) is on Earth, but if the timescale here is typical, then only main sequence stars of about one solar mass or less would have stable main sequence lifetimes long enough to allow life, and possibly intelligence, to develop on planets orbiting them. In a project called the Search for Extraterrestrial Intelligence (SETI), stars are monitored with radio telescopes to try to find modulated radio signals that might be coming from intelligent civilizations on planets orbiting those stars. The stars targeted for such monitoring, for the most part, are main sequence stars of one solar mass or less because they are thought to be the best candidates to have planets on which life might have developed.

Context

The interior structure of stars cannot be observed directly but instead is studied by constructing computer models based on the physics that we think applies to different interior regions of different stars. Main sequence stars have comparatively simple interiors, and astrophysicists have a fairly good theoretical understanding of them. Consequently, main sequence stars provide an important link between the theoretical physics of stellar interiors and the stellar properties that are observed and measured directly.

One way of confirming theories about hydrogen fusion in main sequence stars, generally and the Sun in particular, is to try to detect the neutrinos that should be produced in the fusion reactions. Neutrinos do not interact readily with most matter, and so they should pass directly out of the Sun’s interior. Beginning in the 1970s, sensitive neutrino detectors picked up only about one-third as many neutrinos as models of the Sun’s interior predicted. This discrepancy was finally resolved in 2003, when the Sudbury Neutrino Observatory (SNO), deep underground in a nickel mine in Sudbury, Ontario, Canada, detected the predicted number of neutrinos, but of all three types and not just the one type produced in hydrogen fusion for which the early detectors were designed. This confirmed a hypothesis from particle physics that the three types of neutrinos could convert from one type to another and proved that neutrinos do have mass (albeit a very small mass) since only if they have mass can they change between types.

While addressing the astronomically interesting questions of stellar structure, the study of main sequence stars will continue to have direct application to our Sun’s stability and its consequent effects on Earth’s long-term climate and the welfare of humanity.

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