Low mass stars which began their lives with up to a few solar masses, end up as a white dwarf. At this stage, the star no longer produces energy in its interior through nuclear fusion, as the temperature does not rise enough to allow for the next step in the sequence of successive element fusion to kick in. The stellar core collapses and the star's outer layers are expelled which form a planetary nebula.

The gravitational collapse of the star is no longer prevented by radiation pressure but is halted by the electron degeneracy pressure, resulting in a very compact core, a white dwarf.

The electron degenerate pressure is a quantum physical characteristic of degenerate matter where all the electrons in the gas, due to the Pauli exclusion principle, each have to be in a unique state. The filling of all the lower energy states from the centre outward provides a pressure against further compression.

The chemical composition of the core depends on the initial mass of the star as this determines to which level of element fusion the star could progress before the final collapse of the core. Most white dwarfs consist mainly of completely ionised carbon and oxygen.

Models of the balancing of the gravitational contraction versus the electron degeneracy pressure results in a relationship between the mass M and the radius R of a white dwarf, where M×R^-3=constant.

As white dwarfs no longer produce radiation, their evolution is mainly by cooling over a period on the order of several 10¹⁰ years during which they release their residual energy and steadily decrease in luminosity. During this cooling process, the core material is thought to crystallise from the inside out, forming a compact structure.

Science Goals

Gaia's observations of white dwarfs will significantly advance the study of these compact objects:

• Testing of, and provide constraints for the mass-radius relationship 
• By comparing theoretical models with the observed properties of white dwarfs in binary systems, Gaia will be able to constrain the relation between the mass of the star prior to shedding its outer layers, and the mass of the resulting white dwarf
• Observations of white dwarfs in the Solar neighbourhood (accounting for ~5% of the stars within 10 pc) allows for accurate limits to be set on the age of the Solar neighbourhood as derived from the cooling time of white dwarfs
• Distinguish between different populations of white dwarfs in the Galaxy, thereby providing constraints on the models of galactic evolution

	Stellar Evolution
	Variable Stars