Q:
How long might a white dwarf exist before becoming a brown dwarf?
A:
A white dwarf and a brown dwarf as astronomers usually use these terms are 2 completely different objects. A brown dwarf star is a "failed star" with a mass not large enough to ignite significant nuclear burning, which means it emits very little visible light (its mass must be less than about 80 times the mass of Jupiter, or 0.08 of the mass of the Sun). A white dwarf star is the end state of a star like our Sun, whose nuclear fuel supply has been exhausted.
Please see our Chandra X-ray Astronomy Field Guide to white dwarfs:
http://chandra.harvard.edu/xray_sources/white_dwarfs.html
brown dwarfs:
http://chandra.harvard.edu/xray_sources/browndwarf_fg.html
and stellar evolution:
http://chandra.harvard.edu/xray_sources/stellar_evolution.html
and also a NASA site which gives a good explanation of white dwarfs:
http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html
Revised: September 15, 2008