In article ,
Chris L Peterson wrote:

On Fri, 13 Jul 2007 19:45:36 -0000, Radium wrote:

Hi:

When the sun becomes a white dwarf, approximately how long will it
take for it to cool off to a cold black dwarf that does not emit
anymore heat or light [or other energies] than the surrounding space?

That would take forever. But a few tens of billions of years should be
long enough that the temperature drops low enough that no photons in the
visible range are produced, which is generally what defines a black
dwarf.

That would take forever too!!! According to the Planck blackbody
radiation curve, any blackbody hotter than absolute zero radiates
photons over all wavelengths. Yes, you and I radiate visible light,
X-rays and even gamma rays, although at an extremely low rate - but
still at a rate slightly above zero. And if a cool body would emit,
say, one gamma ray photon over a billion years, is that "no gamma
radiation" or not?

So if you say "that would take forever" about cooling down until no
more energy is radiated, you must say the same about radiation at
visible wavelengths, or any other wavelength rate. Only if you set some
level near but above sero as "insignificant" you can assign a finite
cooldown time span.


Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/