There’s plenty of stuff in the universe that defies the human urge to categorize everything:
I mean, is Pluto a planet?
Is this a new species or subspecies?
Man or woman?
Green or blue or teal?
However, sometimes nature gives us a helping hand and provides a hard, physical line distinguishing
two things, like the line between white dwarf stars, neutron stars, and black holes.
These are the different possible kinds of corpses of dead stars, and what distinguishes
them is how they hold themselves up against the inexorable attraction of gravity.
Let’s begin, though, with what makes something a star in the first place: fusion of hydrogen.
Below about 8% of the mass of the sun, a gas ball isn’t hot and high pressure enough
for hydrogen to fuse and you get a failed star , essentially a really heavy version
of Jupiter or Saturn.
Above this dividing line are stars like red dwarfs, our sun, and so on , which use hydrogen
fusion to keep themselves hot enough to stave off gravitational collapse.
But eventually stars run out of fuel for nuclear fusion, and die.
And when a star dies, it will turn into one of three very different and physically-distinct
things . If the star is small its core will turn into a white dwarf, which is an object
about the size of the earth, almost as massive as the sun, and is held up against gravity
not by heat but by electrons repelling each other due to the “Pauli exclusion principle”
. This effect is essentially a star-sized version of how electrons fill subsequently
bigger orbitals around an atom, because of a fundamental law of the universe that no
two electrons can share the exact same state.
If the star is bigger , its core will be too heavy for this kind of electron repulsion
to hold it up against gravity, and instead it’ll collapse even more until it’s around
10 kilometers in size , all of its electrons and protons fuse together into neutrons, and
it’s held up instead by the nuclear repulsion between neutrons, as well as the neutron version
of the pauli exclusion principle.
Aka, a neutron star.
If the dying star is yet bigger still (or if two neutron stars collide and merge), its
core will be too heavy for either the strong nuclear force or pauli exclusion principle
to hold it up against gravity, and it will collapse until it’s held up by – well,
as far as we know there’s not anything left to hold up stuff this dense , and it’ll
collapse into a black hole.
So when a star dies, what it turns into depends on its mass, and what distinguishes these
star corpses from each other is what they’re made of and how they hold themselves up against
gravity (or don’t).
But enough about space objects that have nice, clean, easy definitions.
There's also a lot of stuff out there that doesn't fit nicely into boxes.
Kate (interjects): Like moons.
We supposedly have a good definition for what a moon is but our solar system is full of
objects that fall into a weird grey area between moon and not moon.
Come over to MinuteEarth to find out why our definition of "moon" might be broken!