The Lives of Stars — why mass decides everything

In the images of this gallery they all stand side by side: blue stars that set entire nebulae aglow, yellow ones like our Sun, red midgets you cannot even see, and the debris of a star that no longer exists. Behind it lies an astonishingly simple order — a single dial decides almost everything about a star's life: its mass at birth.

One family, sorted by temperature

Stars are divided by their spectra into the classes O B A F G K M — a temperature ladder from above 30,000 K (O, blue-white) down to below 3,700 K (M, deep red). A star's color is therefore not decoration but a thermometer: a blue star is hot, a red one is cool, just as white-hot iron is hotter than red-hot iron. With its surface temperature of 5,772 K, the Sun sits as a G2 star right in the middle of this ladder.

The frequencies, however, are extremely lopsided: roughly three out of four stars are red M dwarfs — and yet not a single one of them appears in the visible night sky, because they are too faint. O-type stars, on the other hand, are so rare that there is only one for every million ordinary stars; that this gallery shows several of them is because they are precisely the ones that ignite emission nebulae. The inconspicuous truth in between: the Sun is brighter than about nine out of ten stars in its neighborhood — it is a completely average star only in the sense that nothing special happens to it.

The Hertzsprung-Russell diagram, the map of the stellar world: temperature decreasing toward the right (the historical convention), luminosity logarithmic upward. Whoever burns hydrogen in the core lives on the main sequence — and where exactly is determined by mass alone. Giants, supergiants and white dwarfs are not other kinds of star but phases of life away from this line.

Hot inside, different outside

Every main-sequence star does the same thing: in its core it fuses hydrogen into helium. In the Sun this happens at a core temperature of about 15 million K — the 5,500 °C of the visible surface are almost room temperature by comparison. Massive stars reach more than 30 million K in the core and switch to a more aggressive burning mechanism whose throughput grows steeply with temperature.

From this follows the fuel paradox, perhaps the most important number in this article: luminosity grows roughly with the 3.5th power of mass. A star with ten times the mass therefore has ten times the tank, but burns through it about three thousand times faster. More fuel means a shorter life. A red dwarf eases through trillions of years, the Sun manages about ten billion, an O-type star is finished after a few million — which is why blue stars in the sky always mark places where stars have formed only just now.

Three life tracks, sorted by birth mass. The Sun's road is the middle one: red giant, planetary nebula, white dwarf — not a bang, but a fading out.

What is packed into those three rows:

Red dwarfs are practically immortal. They are fully convective — their entire hydrogen supply is gradually mixed into the core and used up — and so frugal that their main-sequence lifetime runs into trillions of years. The universe is 13.8 billion years old: nowhere has a red dwarf ever yet died a natural death.
The Sun ends quietly. In a good seven billion years it will swell into a red giant, swallow Mercury and Venus and then cast off its envelope as a planetary nebula. What remains is a white dwarf the size of Earth containing half a solar mass — a teaspoon of it weighs tons. It fuses nothing any more, it only cools down, for the rest of time.
Massive stars build onions and die at iron. After hydrogen they ignite helium, carbon, neon, oxygen, silicon — each shell faster than the one before. At iron the energy balance flips: fusing it costs energy instead of delivering any. The core collapses in fractions of a second, the star explodes as a supernova and leaves behind a neutron star or a black hole.
Almost everything about us comes out of these furnaces. Carbon, oxygen, nitrogen, iron — bred in stars, spread by stellar winds, planetary nebulae and supernovae. The astronomers' catchphrase “we are stardust” is not poetry but bookkeeping.
You can see the ladder with the naked eye. Betelgeuse in Orion shines visibly orange-red (a cool supergiant), Rigel diagonally opposite blue-white (a hot B-type star). The color coding of the HR diagram hangs above the horizon on every clear winter evening.