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  • 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 thePauli 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 bywell,

  • 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!

There's plenty of stuff in the universe that defies the human urge to categorize everything:

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