main-sequence lifetime
Every star spends the long, stable middle of its life doing one thing: fusing hydrogen into helium in its core, steadily shining on that energy. This stretch is called the main sequence, and the main-sequence lifetime is simply how long it lasts — the working life of a star before its core fuel runs low. It answers a deceptively simple question: how long does a star get to live? The surprising twist is that the biggest, brightest stars do not live longest. They die first.
The reason is a race between fuel and appetite. A star's fuel supply scales with its mass — more mass, more hydrogen. But its luminosity, how fast it spends that fuel, rises far faster than its mass: a star ten times the Sun's mass shines thousands of times brighter. Divide a slightly larger fuel tank by a hugely larger burn rate and the lifetime plummets. The Sun, burning at a modest pace, has a main-sequence lifetime of about 10 billion years and is roughly halfway through. A star ten times heavier lasts only tens of millions of years; a tiny red dwarf a tenth of the Sun's mass can last trillions of years, far longer than the present age of the universe.
This is one of the most useful facts in all of astronomy, because it turns a star's mass into a clock. The most massive stars are so short-lived that wherever we see them — in glowing nebulae, in young clusters — we know star formation happened recently. And because clusters of stars are born together, the point where their members are just leaving the main sequence (the turnoff) tells us the cluster's age. A common misconception is that big stars 'have more fuel so live longer'; in truth their reckless luminosity burns it all in a cosmic blink.
Rigel and Betelgeuse, the bright shoulders of Orion, are each over a dozen times the Sun's mass and only a few million years old — younger than the human species — yet already nearing death. The faint red dwarf Proxima Centauri, our nearest stellar neighbor, will still be quietly fusing hydrogen long after the Sun is a cold cinder.
Mass is destiny: the heaviest stars blaze briefly, the lightest endure almost forever.
Lifetime depends on core fuel, not the whole star: only the inner ~10 percent of a Sun-like star's hydrogen is ever available for fusion, because the rest never gets hot enough. So 'how much hydrogen' is less important than 'how much hydrogen in the hot core.'