Theory — Star Formation, Stellar Evolution, and Star Death

A star’s whole life is a contest between gravity pulling inward and pressure pushing outward. How that contest plays out depends above all on the star’s mass.

1. Star formation

Stars form inside cold, dense molecular clouds. A clump collapses under gravity, heats up, and becomes a protostar; when its core grows hot enough to fuse hydrogen, a star is born onto the main sequence.

2. Life on the main sequence

A star spends most of its life fusing hydrogen to helium in its core. Massive stars are far more luminous and burn their fuel much faster, so they live shorter lives. The main-sequence lifetime scales steeply with mass:

Main-sequence lifetime t ≈ 1010 × (M / MSun)-2.5 years
A star twice the Sun’s mass lives only about one-sixth as long.

3. Death by mass

When the core hydrogen runs out, the star swells into a red giant. What follows depends on mass: a low-mass star (below about 8 solar masses) sheds a planetary nebula and leaves a white dwarf; a high-mass star explodes as a supernova, leaving a neutron star or, for the most massive, a black hole.

4. Clusters as clocks

All the stars in a cluster formed at nearly the same time, so a cluster is a snapshot of one age. The most massive stars leave the main sequence first, so the main-sequence turnoff, the brightest point where stars are just leaving, marks the cluster’s age: the mass at the turnoff has a lifetime equal to the cluster’s age.

Apparatus

Studying stellar lives uses tools for imaging clouds and remnants and for plotting stars by brightness and temperature. In the simulation these are modelled, but the readings match what each instrument would give.

Molecular cloud
A cold, dense cloud of gas and dust where new stars form.
Protostar
A collapsing clump heating up before hydrogen fusion begins.
Red giant
A swollen, cool, luminous star late in its life.
Planetary nebula
A shell of gas cast off by a dying low-mass star, lit by the white dwarf at its centre.
Supernova remnant
The expanding, filamentary debris of a massive star that exploded.
Cluster H-R chart
A plot of a cluster’s stars whose main-sequence turnoff reveals its age.

Instructions

Work through both tabs. Calculate first by hand, then press the button to compare.

Part A — Cluster age from the turnoff

Part B — How a star dies

Simulation

Stellar Life-Cycle BenchCalculate first, then check against the simulation.

Team Questions

1. Compared with low-mass stars, high-mass stars live:
2. A low-mass star like the Sun will end as a:
3. The main-sequence turnoff of a cluster tells us its:

Example Report

Worked example: the age of a cluster

A cluster’s main-sequence turnoff is at M = 1.5 solar masses.

t = 1010 × M-2.5 = 1010 × 1.5-2.5 = 1010 × 0.363 ≈ 3.6 × 109 years.

The cluster is about 3.6 billion years old, because stars more massive than 1.5 solar masses have already left the main sequence. Reading the turnoff mass and computing the lifetime is the calculate-then-compare core of the lab.

Practice Questions

1. A star of 25 solar masses is most likely to end as a:
2. Using t = 10^10 × M^-2.5, a 1 solar mass star lives about:
3. Stars form inside: