main-sequence turnoff
/ MAIN SEE-kwens TURN-off /
Imagine a marathon where the fastest runners burn out first, then the next fastest, then the rest — and you could read how long the race had been going simply by seeing how far down the field the burnout had reached. Star clusters offer exactly this clock. The main-sequence turnoff is the point on a cluster's Hertzsprung-Russell diagram where its brightest stars have just started to peel off the main sequence, and it tells you the cluster's age.
Here is the logic. Every star in a cluster was born at the same time, but they burn through their hydrogen fuel at wildly different rates: the most massive, luminous stars race through it in a few million years, while low-mass stars dawdle for billions. As a cluster ages, the most massive stars leave the main sequence first, swelling into giants; then less massive ones follow, and the 'turnoff' point creeps steadily down the main sequence from the hot, bright end toward cooler, fainter stars. The temperature and luminosity at which the turnoff sits tells you the mass of the stars just now leaving — and the main-sequence lifetime of stars with that mass is the cluster's age.
The main-sequence turnoff is one of the most powerful age-dating tools in astronomy. It is how we know that open clusters are typically tens to hundreds of millions of years old, while globular clusters are 12 to 13 billion years old. That last fact is profound: it sets a firm lower bound on the age of the galaxy and the universe itself. When stellar turnoff ages and the age inferred from cosmic expansion are compared, they agree to within their uncertainties — a striking, independent confirmation that the universe is roughly 13.8 billion years old.
In a young open cluster the main sequence reaches all the way up to hot blue stars, so the turnoff is high — the cluster is young. In an ancient globular cluster the blue stars are long gone and the turnoff has crept down to Sun-like stars, betraying an age of over 12 billion years.
How far down the main sequence has emptied out tells you the cluster's age.
The turnoff dates the cluster, not any single star — it works only because all the cluster's stars share one birth time. Turnoff ages also depend on stellar models, so they carry real uncertainties, but they agree well with the universe's age from independent methods.