The Sun is a nuclear reactor made of mostly electrons and protons. Both fluids are heated by nuclear fusion reactions to a central temperature of about fifteen million degrees Kelvin. This creates an immense pressure that, left to its own devices, would have exploded the Sun in a few hours. We owe our existence to gravity. It keeps the Sun bound and maintains life-as-we-know it on Earth.
But here lies a puzzle that I considered in my first paper on astrophysics 35 years ago. The proton mass is 1836 times bigger than the electron mass. This means that gravity acts more strongly on protons than on electrons by a factor of 1836. If the pressure of protons is balanced by gravity, what binds electrons to the Sun?