Astrophysics (Index)About

white dwarf

(WD, degenerate dwarf)
(stellar remnant largely of electron degenerate matter)

A white dwarf (WD) is a star past its main-sequence thermonuclear stage that has expelled its exterior and only the core remains. The mass of this remaining material must be sufficiently low that electron degeneracy prevents further collapse into a neutron star, i.e., it remains as electron degenerate matter (EDM). As the remains of a star, it is classified as a stellar remnant. They are small and massive, e.g., the size of Earth with the mass of the Sun, for a density on the order of 100,000 times that of Earth:

Mass range 0.17-1.33 Solar masses
Radius range 1300-15000 km
Bulk density range 103-107 g/cc
Luminosity range about 0.1 solar for brightest (at the beginning of their WD life)

Interestingly, the more massive the white dwarf, the smaller its radius, which implies a considerable range of densities (over three orders-of-magnitude).

They generally transfer their energy to the surface by conduction (interaction of particles) rather than radiative transfer. Their luminosity is quite low, produced entirely by their very slow cooling, and only somewhat-nearby white dwarfs have been observed. Most are oxygen and carbon but under some conditions can include neon, magnesium, or helium. The limit on their mass is about 1.4 solar masses (the Chandrasekhar limit) and if an existing white dwarf grows beyond that, e.g., due to mass transfer from a binary companion, it cannot remain stable: it must collapse into a neutron star (or stellar-mass black hole), but it is presumed that often near that limit, presumably before sufficient density is achieved, a Type Ia supernova results (and no remnant remains). White dwarfs begin at the temperature left over from earlier fusion plus the effects of the subsequent gravitational collapse (increasing the temperature through the Kelvin-Helmholtz mechanism), at which time they have a far higher surface temperature than the Sun, given their much smaller surface. The term black dwarf refers to the theoretical state of a white dwarf cooled to the degree that it emits no appreciable EMR, but with their very slow cooling, the universe is not old enough for any to be near that state (one estimate of the cooling time is 1015 years). Widely different stars are labeled white dwarfs, as long as they are small and hot stellar remnants explainable as electron degenerate matter and they are reasonably be considered to be many types of objects, given their variety of constituents, structure, mass, density, and temperature. Types of white dwarfs based upon their observational characteristics:

Some white dwarf spectral classes (e.g., "DBV"):

DAJust H spectral lines
DBJust He I lines
DCNo lines
DOIncludes H II lines
DZmetal lines
DQCarbon lines
DXUnclear

Optional letter designating other features:

PPolarized
HMagnetism but no polarization
EEmission lines
VVariable

Some white dwarfs show strong magnetic fields (magnetic white dwarfs, MWDs), presumed to have become more pronounced as the progenitor collapsed, analogous to neutron stars.

A pre-white dwarf (PWD) is a star no longer harboring fusion but not yet a white dwarf, i.e., with an intermediate position between asymptotic giant branch and white dwarf on the H-R diagram. They are typically pulsating stars. There are pulsating white dwarfs as well, which can occur with certain constituents at certain temperatures.


(star type,degeneracy)
Further reading:
https://en.wikipedia.org/wiki/White_dwarf
https://astronomy.swin.edu.au/cosmos/W/white+dwarf
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/whdwar.html
https://ui.adsabs.harvard.edu/abs/2015MNRAS.446.4078K/abstract
https://esahubble.org/wordbank/white-dwarf/
https://lweb.cfa.harvard.edu/~pberlind/atlas/htmls/wdstars.html

Referenced by pages:
alpha-enhanced
asymptotic giant branch (AGB)
Black Widow Pulsar (B1957+20)
brown dwarf (BD)
cataclysmic variable star (CV)
Chandrasekhar limit
collapsar
compact object (CO)
dwarf nova (DN)
dynamical instability
EF Eridani
electron degeneracy
electron degenerate matter (EDM)
electron pressure
ELM Survey
giant star
Giclas 29-38 (G 29-38)
H-R diagram (HRD)
helium planet
helium star
J1713+0747
low mass star (LMS)
luminosity class
main sequence star (MS)
mass transfer
mass-radius relation
Messier 67 (M67)
neutrino (ν)
neutron star (NS)
NGC 3201
nova (N)
O-type star (O)
PG 1159 star
planetary nebula (PN)
plasmon
post-main-sequence star
Procyon
PSR J2145-0750
pulsating star
RAMBO
rare designator prefixes
RS Ophiuchi (RS Oph)
sedimentation
Sirius
spectral class
spectral type
star
stellar demographics
stellar designation
stellar evolution
stellar merger
stellar remnant
stellar rotation
stellar-mass black hole (stellar-mass BH)
Sun
supernova (SN)
supernova progenitor
supernova remnant (SNR)
symbiotic binary (SS)
T Pyxidis (T Pyx)
Tolman-Oppenheimer-Volkoff limit (TOV)
Type Ia supernova
Type Ia supernova problem
ultraviolet astronomy
Urca process
variable star
Villanova White Dwarf Catalog
WD J0651+2844 (J0651)
Wigner crystal
ZTF J1539+5027
[α/Fe] versus [Fe/H] diagram

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