As helium burning continues,
it leaves in its place a core of mostly carbon and oxygen.
For medium-mass stars like our sun,
this carbon-oxygen core will never become hot enough to ignite.
Instead, helium continues to burn in a shell around this core, and
a similar shell of hydrogen burns around the helium.
Both shells slowly move out from the star center, consuming what fuel is left and
driving off the star's outer layers.
Without a fuel source, the core of the star can no longer produce energy
to prevent itself from further collapse.
However, rather than collapsing entirely, the star becomes a white dwarf, a hot,
dense sphere of carbon and
oxygen that's held up by the quantum mechanical properties of its electrons.
Stars more than eight times the mass of the Sun take a much more dramatic path.
Following the helium burning phase, the core contracts, reaching temperatures of
up to a billion degrees Kelvin, hot enough to trigger the fusion of carbon.
This begins a sequence of fusion reactions,
building ever heavier nuclei towards the center of the star
in a series of concentric shells, culminating in the production of iron.
Iron has the most tightly bound nucleus of all elements.
With no lower energy state to go to,
it can't release energy through nuclear reactions.
The growing core of iron is therefore unable to create energy to support itself,
and it's destined to collapse.
After millions of years of stable energy production,
the massive core of the star collapses in just a few thousandths of a second,
triggering an intense burst of energy that rips the star apart
in one of the most violent and energetic phenomena in the universe, a supernova.
Supernovae are staggeringly energetic.
At their peak brightness, the light they generate can outshine an entire galaxy of
stars, radiating as much energy as our sun will over its entire lifetime.
Yet despite this huge release of energy, the dense core of the star remains intact.
Just as a white dwarf is held up against gravity by the quantum mechanical
properties of electrons, so the remnant core of the massive star is prevented from
collapsing further by the same quantum mechanical laws.
But this time, applied to an even more densely packed mass of nuclear particles,
a neutron star is born.
For stars more than 20 times heavier than our sun,
the explosive energy triggered by the rapid collapse of the iron core
may still not be enough to overcome the star's overwhelming gravitational pull.
In this case, even the forces and
laws that govern atomic nuclei cannot resist the force of gravity.
In nature's ultimate example of collapse, the star, once the brightest of its kind,
is crushed into an infinitely dense point in space, forming a black hole