White Dwarfs: Stars at the End of Their Life

White Dwarfs

White dwarfs are the compact remnants of low- to intermediate-mass stars (<8 M☉), after they have exhausted their hydrogen and gone through the giant phases. They no longer produce energy through nuclear fusion and radiate their residual heat. These stars typically have a mass of 0.5 to 1.4 M☉ concentrated in a radius comparable to that of Earth (~0.01 R☉), resulting in extremely high densities.

Mass: 0.5 to 1.4 M☉
Radius: 0.008 to 0.02 R☉ (~7,000 to 14,000 km)
Surface temperature: 5,000 to 100,000 K depending on age
Cooling time: several billion years

Internal Structure

The internal structure of white dwarfs is very different from that of active stars:

Core: mainly composed of carbon and oxygen (or helium for the lightest)
Atmosphere: thin outer layer of hydrogen or helium
Electron degeneracy pressure: prevents gravitational collapse

Stellar Evolution

White dwarfs result from the evolution of stars like the Sun:

Main sequence: stable hydrogen fusion
Red giant phase: expansion and helium fusion
Loss of outer layers: formation of a planetary nebula
Formation of the white dwarf: dense, hot star but without nuclear fusion

Examples of Studied White Dwarfs

Characteristics of Famous White Dwarfs
Star	Spectral Type	Mass (M☉)	Radius (R☉)	Temperature (K)	Remarks
Sirius B	DA2	1.02	0.008	25,000	Companion of Sirius A
Procyon B	DQZ	0.6	0.012	7,740	Companion of Procyon A
Van Maanen 2	DZ7	0.67	0.012	6,220	Isolated white dwarf near the Sun
GD 165	DA	0.63	0.011	12,000	White dwarf with a substellar companion
EG 21	DB	0.58	0.012	14,500	Helium-type star with detected pulsations
RE J0317-853	DAH	1.35	0.007	50,000	Highly magnetized white dwarf
40 Eridani B	DA4	0.6	0.012	16,500	Companion of the triple system 40 Eridani

Sources: NASA – Stellar Data (new window), International Astronomical Union (new window).

Spectral Notation of White Dwarfs

Understanding White Dwarf Classification (DA, DB, DQ…)
Notation Element	Example	Meaning	Physical Details
DA	DA2	Hydrogen-rich atmosphere	Spectrum dominated by hydrogen Balmer lines
DB	DB3	Helium-rich atmosphere	Helium lines visible, hydrogen absent
DQ	DQ7	Presence of carbon	Carbon lines or molecular bands in the atmosphere
DZ	DZ7	Presence of metals	Signatures of heavy elements in the atmosphere

Source: International Astronomical Union – White Dwarf Classification (new window)