The main sequence is made up of every star that is actively fusing hydrogen into helium in its core, spanning seven spectral classes from the hottest blue O-type stars down to the coolest red M-type stars. This band dominates the Hertzsprung-Russell diagram, running diagonally from upper left to lower right, and contains roughly 90% of all stars at any given time. What separates these stars from one another is mass, which dictates their temperature, color, brightness, and lifespan.
How Stars Land on the Main Sequence
A star joins the main sequence once its core reaches at least 3 million degrees Kelvin and hydrogen fusion ignites. The minimum mass for this to happen is about 0.08 solar masses (roughly 75 times Jupiter’s mass). Anything lighter never gets hot enough and becomes a brown dwarf, a failed star stuck between planet and star. At the upper end, stars can reach 100 solar masses or more before radiation pressure starts tearing them apart. Between those two boundaries, every hydrogen-fusing star sits somewhere along the main sequence, classified by its surface temperature into one of seven spectral types: O, B, A, F, G, K, and M.
The Seven Spectral Types
O-Type Stars
These are the rarest and most extreme main sequence stars. Surface temperatures exceed 25,000 K, reaching above 50,000 K for the hottest examples. They appear blue-white and are staggeringly luminous, with some putting out more than 800,000 times the Sun’s energy. O-type stars burn through their hydrogen fuel so fast that they last only about a million years on the main sequence for a 40-solar-mass star. That’s a blink in cosmic terms.
B-Type Stars
B-type stars range from about 10,000 to 25,000 K at the surface. They’re blue to blue-white and still very bright, from around 60 to 20,000 times the Sun’s luminosity depending on where they fall within the class. Rigel, one of the brightest stars in the night sky, started its life as a B-type main sequence star before evolving off the sequence. These stars live longer than O-types but still only manage tens of millions of years.
A-Type Stars
With surface temperatures between about 7,500 and 10,000 K, A-type stars appear white to blue-white. Sirius A, the brightest star in Earth’s sky, is an A-type main sequence star. So is Vega, one of the most studied stars in astronomy. A-type stars are roughly 10 to 60 times as luminous as the Sun and live several hundred million years on the main sequence. Their spectra are dominated by strong hydrogen absorption lines, which is actually what first defined this class historically.
F-Type Stars
F-type stars sit in the 6,000 to 7,500 K range, appearing yellow-white. They’re modest by comparison to the hotter classes, shining at roughly 1.5 to 9 times the Sun’s luminosity. Their spectra begin to show signatures of metals like calcium and iron alongside hydrogen. These stars live a few billion years, long enough that planets orbiting them could potentially develop complex chemistry.
G-Type Stars
This is the Sun’s class. G-type stars range from about 5,000 to 6,000 K, appearing yellow. The Sun sits at roughly 5,800 K with a luminosity of 1 solar unit by definition. Stars at the cooler end of this class drop to about 0.7 solar luminosities. G-type stars live around 10 billion years on the main sequence, and their spectra show prominent metal lines along with hydrogen. They are common enough and long-lived enough to be prime candidates in the search for habitable planets.
K-Type Stars
K-type stars range from about 3,500 to 5,000 K, glowing orange. They’re dimmer than the Sun, putting out between 0.1 and 0.5 solar luminosities, but they compensate with exceptionally long lifetimes, stretching into tens of billions of years. Their cooler surfaces allow more complex molecules to form in their atmospheres. Alpha Centauri B, one of the Sun’s nearest stellar neighbors, is a K-type main sequence star.
M-Type Stars (Red Dwarfs)
M-type stars are the coolest, faintest, and by far the most common main sequence stars. Surface temperatures fall below 3,500 K, with some as low as 3,200 K or less. They appear red and produce as little as 0.03 solar luminosities. Proxima Centauri, the closest star to the Sun, is classified as M5 V. What M-type stars lack in brightness they make up for in longevity: a 0.2-solar-mass red dwarf has an estimated main sequence lifetime of 560 billion years, roughly 40 times the current age of the universe. No M-type star that has ever formed has yet had time to leave the main sequence.
Why Mass Controls Everything
A star’s mass at birth determines where it lands on the main sequence and how long it stays there. The relationship between mass and luminosity follows a rough power law: luminosity scales with mass raised to an exponent that varies across the mass range. For Sun-like stars, luminosity increases with roughly the 4.7th power of mass, meaning a star twice the Sun’s mass would be about 26 times brighter. For very massive stars around 100 solar masses, the exponent drops closer to 1.6, while for small red dwarfs around 0.1 solar masses it sits near 2.7.
This steep relationship between mass and energy output explains the enormous range of lifetimes. A massive star has more fuel, but it burns through that fuel disproportionately faster. A 40-solar-mass O-type star exhausts its hydrogen in roughly a million years, while the Sun will last about 10 billion. Red dwarfs sip their fuel so slowly they will shine for hundreds of billions of years.
How Stars Produce Their Energy
Not all main sequence stars fuse hydrogen the same way. Stars below about 1.5 solar masses, including the Sun, rely primarily on the proton-proton chain, a process where individual hydrogen nuclei collide and combine step by step to form helium. It’s relatively slow and operates at core temperatures starting around 3 million K.
Stars above 1.5 solar masses use a different pathway called the CNO cycle, which kicks in once core temperatures reach about 14 million K. This process uses carbon, nitrogen, and oxygen atoms as catalysts, cycling through them as hydrogen is converted to helium in a six-stage reaction. The end product is the same (helium), but the CNO cycle is far more temperature-sensitive, which is why it dominates in hotter, more massive stars and produces energy at a much higher rate.
What Happens When a Star Leaves
A main sequence star’s departure begins when it runs out of hydrogen in its core. For stars between about 0.8 and 8 solar masses, gravity starts winning over radiation pressure and the core contracts. This heats a thin shell of hydrogen surrounding the now-helium core enough to ignite fusion there. The extra energy pushes the outer layers outward, and the star swells dramatically. As the surface expands, it cools and reddens, and the star moves off the main sequence toward the red giant branch of the HR diagram.
More massive stars (above roughly 8 solar masses) follow a similar initial path but evolve into supergiants, transitioning from their original blue-white O or B spectral type to red as they grow. Along the way, some pass through a Cepheid variable stage, pulsating with periods of 1 to 70 days. Each time a new element ignites in the core, the star grows larger and redder, building an onion-like structure of fusing shells before eventually ending in a supernova.
The lightest red dwarfs face no such dramatic fate. Stars below about 0.8 solar masses will simply burn through their hydrogen over timescales far longer than the universe has existed, eventually fading into dim, cooling remnants without ever becoming giants.