What is a main-sequence star?

A main-sequence star is a star that produces its energy by fusing hydrogen into helium in its core. It is the longest and most stable phase of stellar life, and about 90% of all stars — including the Sun — are in it.

How long does a star stay on the main sequence?

It depends almost entirely on mass. A massive blue O-type star burns out in a few million years; the Sun gets roughly 10 billion years; a small red dwarf can keep fusing for trillions of years, far longer than the current age of the universe.

What are the spectral classes O, B, A, F, G, K and M?

They are a temperature ordering of stars, from hottest to coolest. O stars exceed 30,000 K and glow blue; M stars sit near 3,000 K and glow red. The Sun is class G, around 5,772 K at the surface.

What happens when a main-sequence star runs out of hydrogen?

The core contracts and the outer layers swell: the star leaves the main sequence and becomes a red giant. What comes after that depends on mass — a white dwarf for stars like the Sun, a supernova leaving a neutron star or black hole for the heavyweights.

Main-Sequence Starとは何か？

A main-sequence star is a star in the long, stable phase of its life, fusing hydrogen into helium in its core. Roughly 90% of the stars you can see are on the main sequence, including the Sun — and a star stays here for the overwhelming majority of the time it exists.

The defining feature is a standoff. Gravity pulls every gram of the star toward the centre; the pressure of fusion-heated plasma pushes back out. When the two balance exactly, the star is in hydrostatic equilibrium and barely changes for billions of years. That balance is self-correcting: squeeze the core and fusion speeds up, which reheats and re-inflates it. This thermostat is why stars are among the most stable objects in nature.

One number at birth sets everything else: mass. A star ten times heavier than the Sun is not simply bigger — it burns hotter, bluer, and thousands of times brighter, and it exhausts its fuel in a few million years instead of ten billion. That is the main sequence: a single line running from hot, massive, blue, short-lived stars at one end to cool, small, red, near-immortal ones at the other. Astronomers label positions along it with the spectral classes O, B, A, F, G, K and M. The Sun is a G2V star, sitting almost exactly in the middle.

The model here shows a Sun-like photosphere: convection cells the size of continents rising, cooling and sinking on a timescale of minutes, darker magnetic spots where field lines choke off the heat flow, and loops of plasma arcing above the limb. Nothing here is painted on — the surface is generated by noise fields evaluated on the sphere, the same way the real granulation pattern is generated by convection.