Main sequence

The main sequence is a fundamental classification of stars in astronomy. It represents a continuous and distinctive band seen on plots of stellar color versus brightness.

These stars are the most numerous true stars in the universe. Our own Sun is a member of this group. Stars spend the majority of their lives in this stage. During this period, the process of core hydrogen burning is the dominant source of energy. This stage is often called the "zero-age main sequence," or ZAMS, when stars first begin this stable phase of life.

A star's journey begins within a gaseous nebula. As the nebula undergoes gravitational collapse, the material becomes very dense.

This collapse creates high pressure and high temperature at the core. Eventually, these conditions trigger the nuclear fusion of hydrogen into helium. This fusion releases massive amounts of thermal energy. This energy radiates outward from the hot, dense core, creating a strong pressure gradient. This outward pressure counters the inward pull of gravity. This balance is known as hydrostatic equilibrium.

A star's position on the main sequence is determined primarily by its mass. Age and chemical composition also play important roles. Astronomers often discuss a star's metallicity, which is the abundance of elements heavier than helium. For example, the Sun is composed of 74.9% hydrogen and 23.8% helium by mass. This leaves a metallicity, or mass fraction of other elements, of 1.3%. Higher metallicity can increase a star's opacity. This allows energy production to remain concentrated in the core. A hotter environment can speed up nuclear fusion and decrease the star's time on the main sequence.

Main-sequence stars are divided into different types based on their energy processes. The Sun and stars with lower mass primarily use the proton–proton chain. In this process, hydrogen atoms fuse together in several stages to form helium.

Stars with higher mass, specifically those above 1.3 to 1.5 times the Sun's mass, use a different method. They primarily use the CNO cycle, where carbon, nitrogen, and oxygen act as intermediaries. In these massive stars, the CNO cycle causes convection in the core regions. This convection stirs up newly created helium to maintain the fuel needed for fusion. Stars with lower mass have radiative cores and convective zones near their surfaces.

Our understanding of these stars grew through many important discoveries. In 1901, Annie Jump Cannon and Edward Charles Pickering developed the Harvard Classification Scheme.

In 1906, Ejnar Hertzsprung noticed that red stars belonged to two groups: giants and dwarfs. He published the first plots of color versus luminosity. Around the same time, Henry Norris Russell studied the relationship between spectral types and absolute magnitude. He found that dwarf stars followed a distinct relationship that allowed their brightness to be predicted. In 1933, Bengt Strömgren named the resulting chart the Hertzsprung–Russell diagram.

The classification of stars is organized by temperature and luminosity. The spectral types follow a sequence from hottest to coolest: O, B, A, F, G, K, and M.

These colors range from blue to red. The luminosity class for main-sequence stars is V.

Massive O-type stars are hot and brilliant, while smaller M-type stars are known as red dwarfs. This mass-luminosity relationship is a core concept in stellar physics. It helps astronomers deduce a star's mass and radius if its composition and position are known.

Mass has a direct impact on how long a star lives. Massive stars are much more luminous and have much shorter lifespans.

They consume their hydrogen fuel very rapidly. Once the hydrogen fuel in the core is consumed, the star evolves away from the main sequence. It may become a supergiant, a red giant, or a white dwarf. Astronomers have even found extremely distant main-sequence stars. In 2018, they detected a star named Icarus at a distance of 9 billion light-years. This discovery shows how main-sequence stars serve as vital tools for measuring the distant universe.