Illustrated Physics
physics, drawn
Concept

Chandrasekhar limit — where it appears

The largest mass a cold star supported by electron degeneracy pressure can have, about 1.4 solar masses. Above it the electrons become relativistic, their pressure stiffens too slowly, and no equilibrium exists.

Named by 4 essays across one field — each of them below, with the objects they name alongside it.

Cold matter, and the mass above which nothing holds it up. The radius of a cold, degenerate star against its mass, obtained by integrating the equations of hydrostatic support outward from 11 different central densities with the exact degenerate equation of state, and nothing else. Heavier means smaller — the opposite of every ordinary object, and the direct consequence of a pressure that comes from counting states rather than from heat. The curve turns over and runs into a vertical asymptote at 1.452 solar masses, against 1.456 from the limiting polytrope, whose own constant 2.0182 is integrated here as well. That is Chandrasekhar's limit. It exists because the electrons become relativistic: once they are, the pressure goes as the four-thirds power of the density, and for that exponent alone the mass of a self-gravitating ball is independent of its radius — so squeezing it harder produces no more support and there is exactly one mass such a star can have. The horizontal line is the Earth's radius, which the curve crosses near a solar mass: a white dwarf of the Sun's mass is the size of a planet, and the ones close to the limit are a few thousand kilometres across. What the model leaves out is what actually happens at the top: at those densities electrons begin to be captured onto nuclei, which removes the very pressure holding the star up, so the collapse starts slightly below the line rather than at it.

The mass no cold matter can hold up

A white dwarf gets smaller as it gets heavier, which no ordinary object does. Follow that curve upward and the radius reaches zero at 1.46 solar masses — because once the electrons are relativistic the pressure goes as the four-thirds power of the density, and for that exponent alone the mass of a self-gravitating ball does not depend on its radius at all.

astrophysics · Self-gravity The heaviest star free neutrons can hold up. Mass against radius for cold stars of free neutrons, each point a different central density, held up by the pressure of the neutrons' exclusion alone: in general relativity and, with the same gas, in Newton's gravity. Relativity's curve reaches a highest mass of 0.71 solar masses at a radius of 9.5 km and turns back; Newton's keeps rising towards 5.8 solar masses as the stars shrink. The measured masses of neutron stars reach about 2.08 (dashed), three times what free neutrons allow, so the matter inside has to be stiffer than a gas of free neutrons: the repulsion between nucleons at short range is holding them up.

The pressure that weighs down what it holds up

In Newton's gravity a star is held up by pressure, and stiffer matter can hold up more: a star of free neutrons could reach almost six solar masses. In general relativity the pressure itself has weight. Every layer squeezed harder to hold up the layers above adds to the pull it is resisting, and past a point pressing harder makes things worse. The same gas of neutrons then tops out at 0.71 solar masses. Neutron stars twice as heavy exist, which says what their insides must be like — and no matter of any kind can hold a static body smaller than nine-eighths of its Schwarzschild radius.

astrophysics · Self-gravity The redshift of a white dwarf weighs it. The gravitational redshift at the surface of a cold white dwarf, expressed as the velocity that would give the same Doppler shift, against the star's mass; computed from the radius each mass is forced to have. The Sun's surface redshift is 636 m/s. A white dwarf of 0.6 solar masses shifts its light by 30.1 km/s, one of 1.0 by 77.6, of 1.2 by 128 and of 1.35 by 223. Because the radius is not free, the curve is single-valued and steepening: a measured shift returns one mass. The cold model puts Sirius B, at 1.018 solar masses, at 80.8 km/s; its measured mass and radius together give 80.0.

The redshift that weighs a dead star

Light leaving the surface of an ordinary star is reddened by gravity, but the shift alone says little, because a star's radius is free and the shift depends on mass over radius. A white dwarf has no free radius. Degenerate electrons fix its size from its mass, so one measured line shift returns one mass — and the gauge grows sharper the closer the star is to collapse. The first measurement of it, in 1925, was hailed as a confirmation of general relativity and was wrong by a factor of four, in agreement with a forecast that was wrong by the same factor.

astrophysics · Gravitational redshift Change the strength of gravity and the window slides, whole. The floor and ceiling of the mass range of stars, as functions of the coupling αG with everything else fixed. The floor comes from a temperature, the ceiling from a pressure ratio, and both scale exactly as αG^(−3/2). So the width of the window, a factor of 4898, does not change at all; the whole of it slides. A universe with gravity a thousand times stronger would have stars 3 × 10⁴ times less massive. The stars' count is the reciprocal of gravity's weakness to the three-halves power, and no number in the Sun's own structure is needed.

The number of protons that makes a star

Every star ever observed holds between about 10⁵⁶ and 2 × 10⁵⁹ protons, and none of the theory of stars is needed to get near that number. Gravity between two protons is 5.9 × 10⁻³⁹ of what it could be, and that one ratio raised to the minus three halves is 2 × 10⁵⁷. The floor comes from a temperature and the ceiling from a pressure, and they move together.

astrophysics · Planck scale

Named alongside it

The objects these essays reach for when they reach for this one.

Degeneracy pressureHydrostatic equilibriumNeutron starSelf-gravityWhite dwarfCompactnessDegenerate matterDimensional analysisEddington limitEquation of stateExclusionGeneral relativity

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