Main sequence
Stars spend most of their lives on the main sequence.
The main sequence is a classification of stars that appear as a continuous and distinctive band on plots of stellar color versus brightness, known as Hertzsprung–Russell diagrams. Stars spend the majority of their lives on the main sequence, during which core hydrogen burning is dominant, and these stars are the most numerous true stars in the universe, including the Sun.
- field
- Astronomy
- known_for
- Continuous band on color-magnitude plots where stars spend most of their lives fusing hydrogen
- key_contributors
- Ejnar Hertzsprung, Henry Norris Russell, Annie Jump Cannon, Edward Charles Pickering, Bengt Strömgren, William Wilson Morgan, Philip Childs Keenan
Lore & Background
At Princeton, Henry Norris Russell independently studied spectral classification versus absolute magnitude, finding that dwarf stars followed a distinct relationship, allowing prediction of their brightness.
Reader's Guide
The main sequence is fundamental to stellar astronomy because it represents the longest stable phase of a star's life, during which hydrogen fusion in the core provides energy. A star's position on the main sequence is determined primarily by mass, but also by age and chemical composition. More massive stars have shorter lifespans on the main sequence. The sequence is divided into upper and lower parts based on the dominant fusion process: stars below about 1.5 solar masses primarily use the proton–proton chain, while those above use the CNO cycle. Metallicity—the abundance of elements heavier than helium—affects opacity and fusion rate, with higher metallicity speeding up fusion and decreasing main-sequence lifetime. The main sequence includes the Sun and is the most numerous category of true stars. Its study enabled the Vogt–Russell theorem, which relates a star's mass to its luminosity and radius, though this breaks down for non-uniform compositions. In April 2018, the most distant ordinary main-sequence star, Icarus, was detected at 9 billion light-years away.
Did You Know?
- Stars below about 0.4 solar masses undergo convection throughout their entire mass.
- The Sun's metallicity is 1.3% by mass, a typical range for similar-mass main-sequence stars.
- The most distant main-sequence star detected, Icarus, is 9 billion light-years from Earth.
Frequently Asked Questions
What is the main sequence in astronomy?
It's the prominent diagonal band you see on Hertzsprung–Russell (color-magnitude) diagrams, representing stars that are steadily fusing hydrogen into helium in their cores. It's the longest and most populated stage a star will ever occupy.
How long do stars actually stay on the main sequence?
For the vast majority of a star's total lifetime, it sits on this band. The Sun, for example, is expected to remain a main-sequence star for roughly ten billion years before it exhausts its core hydrogen.
Who are the key figures behind the main sequence concept?
Ejnar Hertzsprung and Henry Norris Russell independently plotted stellar color against brightness in the early 1900s, while Annie Jump Cannon and Edward Charles Pickering laid the groundwork for the spectral classification system. Later, Bengt Strömgren, William Wilson Morgan, and Philip Childs Keenan refined the spectral types and photometric systems we still use today.
Is the Sun considered a main sequence star?
Yes—the Sun is a textbook G-type main-sequence star, currently in the middle of its hydrogen-burning phase. In fact, main-sequence stars make up the overwhelming majority of all true stars in the universe.
What happens when a star leaves the main sequence?
Once core hydrogen is depleted, the star can no longer sustain stable hydrogen fusion at its center, so it swells and brightens, drifting off the band onto the giant or supergiant branch of the H-R diagram. This marks the beginning of the star's later, more turbulent evolutionary stages.
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