Life Cycle of Stars and the Hertzsprung-Russell Diagram

How a star is born

A star begins inside a nebula, a huge cloud of gas (mostly hydrogen) and dust. Gravity pulls the denser clumps of the cloud together. As the material falls inward, it gets squeezed and heats up. The hot, glowing ball is called a protostar.

The protostar keeps contracting and getting hotter. When its core reaches about 10 million K, hydrogen nuclei crash together hard enough to join. This process is nuclear fusion: four hydrogen nuclei fuse to make one helium nucleus and release a huge amount of energy. The star "turns on."

Gravity wants to crush the star inward. The energy from fusion pushes outward as pressure. While these two balance, the star stays stable. This long, steady period is the main sequence. A star spends about 90% of its life there, fusing hydrogen into helium in its core.

Mass decides everything

How a star lives and dies depends mostly on its mass.

This seems backward. A massive star has more fuel, so why does it die sooner? Its stronger gravity squeezes the core much harder, so fusion runs far faster. It burns through its hydrogen like a race car burning gas. A small star sips its fuel slowly.

The end of a star's life

When the hydrogen in the core runs low, fusion slows and the core can no longer hold up the star. What happens next depends on mass.

Low-mass stars (like the Sun)

1. Nebula, then protostar.

2. Main-sequence star (fusing hydrogen).

3. Red giant: the outer layers swell and cool.

4. Planetary nebula: the star puffs off its outer layers into a glowing shell. The name is misleading; it has nothing to do with planets.

5. White dwarf: the hot, tiny core left behind. It slowly cools for billions of years.

High-mass stars (more than about 8 times the Sun's mass)

1. Nebula, then protostar.

2. Main-sequence star.

3. Supergiant: swells to enormous size.

4. Supernova: the core collapses and the star explodes.

5. Neutron star (if the leftover core is less massive) or black hole (if the leftover core is very massive).

Gravity, fusion, and the core tell the whole story. Fusion makes outward pressure, gravity pulls inward, and when the core runs out of fuel, gravity wins in the core. In a massive star, the collapse is violent enough to cause a supernova. The Sun does not have enough mass for that.

The Hertzsprung-Russell diagram

Astronomers use a graph to sort stars. The Hertzsprung-Russell (H-R) diagram plots each star as a point using two properties:

• Luminosity (how much energy it gives off), on the vertical axis. It is often measured in units of the Sun's luminosity ($L_\odot$).

• Surface temperature (in K), on the horizontal axis. Notice that temperature decreases to the right: hot stars are on the left.

Where a star falls tells you what kind of star it is:

• Main sequence: a diagonal band from hot and bright (upper left) to cool and dim (lower right). About 90% of stars, including the Sun, are here.

• Giants and supergiants: upper right. They are cool but very luminous, so they must be huge.

• White dwarfs: lower left. They are hot but very dim, so they must be tiny.

Why is size the key? A star's brightness depends on its temperature and its surface area. A cool star can outshine a hot one if it is big enough.

A star's path on the diagram

A star does not stay at one point. As it ages, it moves across the H-R diagram.

• On the main sequence, its place depends on mass: massive stars at the upper left, low-mass stars at the lower right.

• When a star like the Sun becomes a red giant, it moves up and to the right: it gets brighter and cooler.

• When it becomes a white dwarf, it drops to the lower left: very hot, but tiny and dim.

• A high-mass star leaves the main sequence for the supergiant region at the top right before its supernova.

Back to the start: the Sun is a main-sequence star about halfway through a roughly 10-billion-year life. It will become a red giant, then a planetary nebula, and finally a white dwarf.

Summary