The White DwarfsW. J. Luyten

University of Minnesota, Minneapolis, Minn., U.S.A.

Appears like the DawnFair as the MoonBright as the Sun

Majestic as the Stars

by Nari Jeong 2007. 4. 2

Introduction White dwarfs have been known for slightly more than fifty years; with

their planet like diameters, and stellar-like masses their densities are enormously much higher than those of ‘normal’ stars. They thus appear to represent a state of matter totally unknown and possibly unattainable on earth.

• Their low luminosity the follow-up observation to determine their astrophysically important properties are difficult and generally require very large telescopes.

• Based on the fact they are of low luminosity and generally of high temperature, therefore blue or white in color.

Methods of Discovery• First, faint stars with large proper motions : and then subsequently d

etermining the color and thus select those object that are whiter then the ordinary main-sequence stars.

• Second, led to the publication of the famous list of 48 Faint Blue Stars by Humason & Zwicky 1) fifteen stars found in the Hyades region 2) 8700 of them jointly with Haro 3) as old plate material 4) faint blue stars

• Faint blue stars the percentage of white dwarfs increases from virtually zero at m=13 to about 10% at m=15 or 10

• Very blue stars one could expect to find increasingly larger numbers of quasi-stellar objects

Colors and Spectra• Determination of accurate photo-electric colors.

• While the spectra of the first few white dwarfs found showed some similarity to those of main-sequence A stars it soon became apparent that the very high values of the surface gravity as well as possible Strark and magnetic effects would cause the spectroscopic features of most white dwarfs to be very different from those of ordinary stars.

• Followed by the usual, B,A,F,G of K -> no degenerate object which would deserve the classification DM has been identified.

Parallaxes and Luminosities• Parallaxes are perhaps the most urgently needed data for White Dwa

rfs.• Number of white dwarfs have been identified as belonging : 20 in the

Hyades, 4 in Praesepe, and 1 each in the Pleiades and M67• H-R diagram is made up for these stars it is seen that degenerate st

ars occupy a broad area to the lower left of the diagram -> proper motion surveys it is not yet possible to make up such a diagram -> full uncertainty in our knowledge concerning the kinematics of white dwarfs would come into play is such a diagram

• Yellow degenerate stars can be identified : DM stars -> yellow sub dwarfs with extraordinary high velocities -> peculiar distribution of velocities.

• Neutron strars.

Masses and Red shifts• The two white dwarfs discovered, Sirius B and o2 Eridani B were bot

h components of binaries• Companion to Sirius has often been suspected of being a close doub

le, and its mass is rather larger than expected -> obtain reliable values for its apparent magnitude, color, and spectrum

• Eridani B -> mass, luminosity color, and spectrum, hence also surface temperature are known with reasonable accuracy

• Large telescopes -> binaries stars -> motions , masses• Values for the masses of degenerate stars con be determined• we fall back again -> Kepler’s laws for the reliable determination of

mass for degenerate stars.• Real red shifts cannot be determined for single white dwarfs -> many

of the most interesting : spectra in which red shifts cannot be reliably determined.

White Dwarfs in Clusters & Frequency in Space

• Present theory suggests there should be more white dwarfs in old clusters such as M67 the Hyades and Praesepe than in young cluster such as the Pleiades and hand X persei.

• White dwarfs constitute only a few percent -2.3% +,-0.3% of all stars in space while theoreticians have generally derived much higher values up to 10%

• Should indeed turn out to be high-velocity sub dwarfs then the theoretical estimate would have to be substantially reduced.