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White Dwarf Stars

Abstract

The defining characteristics of white dwarf stars are presented and interpreted in an evolutionary context. Some occurences are listed with their known physical properties.

Keywords: white dwarf stars

Introduction

White dwarfs are the cooling naked cores of the most highly evolved stars which have mostly consumed their fusionable elements and undergone gravitational collapse. None is bright enough to observe with the naked eye. The most famous example is Sirius B, but a far easiler object for amateur viewing is undoubtedly Omicron2 [= 40] Eridani B.

Characteristics

Spectra

White dwarfs are not classifiable by ordinary rules.For example, more massive white dwarfs are usually less luminous because their higher gravity contracts the stars, reducing the surface area. They are chemically differentiated into two broad groups: hydrogen rich (e.g. Sirius B) and helium rich (e.g. HZ-29).  Colour does not correlate with temperature as each type spreads out across the whole temperature sequence from early O-type to M equivalents.

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Mass and Luminosity

All white dwarfs are necessarily less massive than the Chandrasekhar limit, approximately 1.4 M¤. Above this, electron degeneracy pressure is no longer able to prevent the objects further collapse.

Variability/Mass Loss

General

Some white dwarfs have enormously powerful magnetic fields – in the order of 108 times the earth's field – whereas others have no detectable field at all. This phenomenon is not well understood.

Interpretation

White dwarfs are the cooling naked cores of the most highly evolved stars which have mostly consumed their fusionable elements and undergone gravitational collapse. Being insufficiently massive to collapse into a neutron star, further collapse is prevented by electron degeneracy pressure.
However, the gravitational field is sufficiently great that, where there is any hydrogen, the helium is dragged down to lower depths and the hydrogen floats on top, resulting in a pure hydrogen atmosphere and a DA star, no matter what the temperature. Thus a DB star can have no hydrogen at all, having lost the entire hydrogen envelope during evolution from the AGB. We do not yet know why one star loses all its hydrogen, whereas another does not.
The cooling time for white dwarfs is enormously long. The time required to cool to the right of the ZAMS on an HR diagram is longer than the age of the universe: i.e. none has done so yet.
The AM CVn types are close binary systems with orbital periods so small that both stars in the system must be degenerate objects. Typical periods are between 15 and 45 minutes. For more information about AM CVn itself, refer to the Center for Backyard Astrophysics.
Astrophysical modelling indicates that the interiors of white dwarfs are non-convective. "This is for two reasons: first, degenerate matter is highly conductive, that is, its effective opacity is very low, and secondly, no stable nuclear burning is possible under degenerate conditions ..., implying that such stars must be inert" (Prialnik 2000, pp. 102-103).

Occurrence/Examples

"As the end products of the intermediate main sequence, white dwarfs are everywhere. Huge numbers roam free in the Galaxy, as will the degenerate descendant of the Sun. They do not obviously populate the sky only because they are so faint; even the brightest is only eighth magnitude." (Kaler 1992, pp. 165-166.)

A long list is provided by Burnham 1989a, pp. 417-425; a few of the more prominent or better-known examples are listed below (Table 1).
Star Coordinates Distance Class/Magnitude Reference
a CMa B 6:45:8.87
-16:42:57.8		 2.7 pc A
B		 A0V
DA5		 -1.42
8.65		 Burnham 1989a, p. 399
a CMi B 7:39:18.11
+05:13:30.1		 3.5 pc A
B		 F5IV
?		 0.35
10.8		 Burnham 1989a, p. 450
o2 (= 40) Eri B
HD 26976		 4:15:16.32
-07:39:08.9		 2.9 pc A
B
C		 [K1V]
[DA]
[dM4e]

4.48
~9.6
10.8

 Burnham 1989b, p. 893
HZ 21       DO 14.2 Kaler 2001, p. 150
HZ 29
AM CVn		 12:34:54
+37:37:43
   – DBp 14.0 Link
Wolf 28 (Van Maanen’s Star) 00:49.2
+05.4		 4.2 pc – [DF
-
DG]		 12.4 Burnham 1992, pp. 1474-77
ZZ Ceti       DA 11.9 Kaler 2001, p. 150
Table 1: Table of some white dwarf stars. Magnitudes given are apparent magnitudes.
For observerving with amateur equipment, o2 (= 40) Eri and Wolf 28 (Van Maanen’s Star) are the best choices. Omicron2 [= 40] Eridani B is "the only white dwarf which can honestly be called an easy object for the small telescope" (Burnham 1989b, p. 891).

References

Burnham, Robert 1989a: Burnham’s Celestial Handbook. Volume 1 – Andromeda to Cetus. Dover.

— 1989b: Burnham’s Celestial Handbook. Volume 2 – Chamaeleon to Orion. Dover.

— 1992: Burnham’s Celestial Handbook. Volume 3 – Pavo to Vulpecula. Dover.

Kaler, James B. 1992: Stars. Scientific American Library.

Kaler, James B. 2001: Extreme stars at the edge of creation. Cambridge, 236 pp.

Prialnik, Dina 2000: An introduction to the theory of stellar structure and evolution. Cambridge, 261 pp.
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