White Dwarfs
and Novae

(Section Not Complete)

Subrahmanyan Chandrasekhar predicted while a very young man in the 1930s that there was a limiting mass for white dwarf stars: no white dwarf could be stable against gravitational collapse if it exceeded this mass, which is about 1.4-1.5 solar masses, depending on the detailed composition of the white dwarf. The idea was not very well received by many established astronomers, who considered it absurd that a white dwarf could have a limiting mass. Much later, Chandrasekhar's brilliant idea was completely accepted within the astrophysics community and he eventually won a Nobel Prize for his deep theoretical contributions to astrophysics. Today, all textbooks on stars describe the Chandrasekhar Limiting Mass for white dwarfs as central to the structure of white dwarfs (Ref).

Cooling of White Dwarfs
The adjacent image shows a white dwarf embedded in a planetary nebula that may be the hottest star kn own. Its surface appears to have a temperature of about 200,000 degrees C, which is 30 times the surface temperature of the Sun (Ref). Even though the temperature is so high, this is a dying star. It has used all its nuclear fuel and can no longer produce energy by thermonuclear fusion.
The Nova Mechanism
In a nova, one star is a more normal star and the other star is a white dwarf. Matter accretes in a thin layer on the surface of the white dwarf and eventually ignites in a thermonuclear explosion under degenerate conditions.

This blows a thin surface layer off into space, causing a large rise in light output from the system. The mechanism for a nova outburst is illustrated schematically in the adjacent images.

Hubble image of white dwarfs in globular cluster

Bright White Dwarfs
Name Con R.A. Dec Mag Dist (ly)Diameter (km)
Sirius B CMA 06 45.1 -16.7 8.3 8.6 10,300
40 ERI B ERI 04 15.4 -07.7 9.5 15.7 17,000
Procyon B CMI 07 39.3 +05.2 10.7 11.4 17,500
Feige 34 UMA 10 39.6 +43.1 11.1 55-
W1346 CYG 20 34.4 +25.1 11.5 45 17,000
EG247 CAM 05 05.5 +52.8 11.8 140 23,000
He3 (EG50) AUR 06 47.6 +37.5 12.0 60 13,000
EG62 (LP 532-81) PYX 08 41.5 -32.9 12.0 30 22,000
EG368 DRA 16 48.4 +59.1 12.2 40 17,000
van Maanen's star PSC 00 49.2 +05.4 12.4 14 17,000
EG180 CAM 04 31.2 +59.0 12.4 18 16,000
AC +70 5824 UMI 13 38.9 +70.3 12.8 100 -
EG15 ARI 02 08.8 +25.2 13.2 100 18,000
Source: ftp://nic.funet.fi/pub/astro/dbases/stars/potporri.txt

Nova Persei (1901)

The following figure shows a high density of white dwarfs found near the center of the globular cluster M4 (Ref). A picture of M4 taken with a ground-based telescope is shown in the left frame and the right frame shows the blown up region marked by the lines, as imaged by Hubble. The white dwarfs in the field are circled; as expected, they are quite faint compared with the other stars. Although this is an unusual concentration, we expect to find many white dwarfs in old clusters because they represent the endpoint of stellar evolution for solar-mass stars.

The adjacent figure shows the shell ejected by Nova Cygni 1992, as imaged by the Hubble Space Telescope two years after the explosion was first observed. This nova was the brightest observed in recent years and was visible without a telescope at its peak.

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