Main sequence
The main sequence is the dominant stage in a star’s life. A star is said to be on the main sequence when it generates its energy through the fusion of hydrogen into helium in its core. This is the longest and most stable phase of a star’s life: the Sun has been in this phase for 4.6 billion years and will remain there for another 5 billion years. Approximately 90% of observable stars are on the main sequence.
Hydrostatic equilibrium
A main-sequence star is in hydrostatic equilibrium: the radiation pressure produced by nuclear fusion exactly balances the gravitational force that tends to cause the star to collapse. As long as this equilibrium is maintained, the star remains stable. It is hydrogen fusion that provides the energy necessary for this equilibrium.
Mass and Position on the Main Sequence
A star’s position on the main sequence is determined by its mass. Massive stars are hot, blue, and extremely bright, but they burn through their hydrogen quickly—in just a few million years. Low-mass stars, such as red dwarfs, are cold, reddish, and dim, but can burn for hundreds of billions of years. The Sun, which has an intermediate mass, has a total lifespan of about 10 billion years.
Importance in Astronomy
The main sequence is the standard tool for classifying and understanding stars. Its position on the Hertzsprung-Russell diagram makes it possible to determine the age of a star cluster, calibrate distances, and understand stellar evolution. It also serves as the benchmark for understanding why stars leave this sequence as they age.
Concrete example
The Sun is in the middle of the main sequence, with a surface temperature of 5,778 K and a reference luminosity (1 solar mass). Stars like Sirius (type A, 10,000 K) are much hotter and brighter. Red dwarfs like Proxima Centauri (3,000 K) are much cooler and less luminous.
Frequently Asked Questions
What happens when a star leaves the main sequence?
When a star’s core exhausts its hydrogen, fusion stops, the corecore contracts, and the outer envelope expands—the star becomes a red giant. It then leaves the main sequence to enter the final stages of its evolution.
Do all stars pass through the main sequence?
Almost. Substellar objects (brown dwarfs) are not massive enough to trigger hydrogen fusion and do not reach the main sequence. Stars with sufficient mass (above 0.08 solar masses) all pass through it.
Is the Sun a typical main-sequence star?
The Sun is a star of intermediate mass and luminosity, often used as a reference. In terms of frequency, very low-mass red dwarfs are far more numerous in the galaxy. The Sun therefore ranks toward the upper end in terms of mass and luminosity compared to the statistical average of stars.