How It Works

Question Of The Day: Black dwarfs – how do these invisible stars form?

When a star becomes a white dwarf it no longer has an internal heat source, instead staying luminous only because of its retained temperature. A black dwarf is a white dwarf that has given off all its heat and cooled down to the temperature of the surrounding universe, known as the cosmic microwave background, which is about 2.7 Kelvin.

While a white dwarf gives off heat via thermal radiation it does so incredibly slowly, because the densely packed electrons that prevent it from collapsing are excellent conductors of heat. A hypothetical white dwarf born at the start of the universe 13.7 billion years ago wouldn’t have cooled down to the temperature required to form a black dwarf even today. In fact, the estimated time for a white dwarf to cool to a black dwarf would be roughly 73,000 times the age of the universe. For this reason, unlike their white, red and brown dwarf cousins, black dwarfs have not yet been observed. Indeed, as a black dwarf would emit little to no radiation, finding one would be nigh on impossible, as it would appear almost invisible to us, other than the effects of its gravity.

Answered by HIW.




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  • When you say roughly 73,000 times the Hubble time, I think this is wrong…Adams & Laughlin predicted it to be 7.3×10^(14) times the Hubble time!!! However, Tipler & Barrow state a cooling to 5K (just above the current CMB temperature, which will be in fact much colder in another Hubble time) will take 730,000 times the Hubble time, I would say you’re a factor of 10 out in your quoted figure.

    • Hi Michael, thanks for your comment. As you’re no doubt aware, this area of astrophysics is heavily theoretical. No one really knows for sure how long it’ll take for a white dwarf to cool to a black dwarf. Adams & Laughlin’s estimate was that it would take 7.3×10^14 years since the start of the universe for a black dwarf to form from the cooling of a white dwarf, which elsewhere is reported as roughly 73,000 times the age of the universe (13.7 billion x 73,000 = 10^15). However as you say it is also possible that the cooling process could take 730,000 times the age of the universe. No one is 100% certain, and I think any figure within these two boundaries could be deemed to be fairly accurate. Obviously, white dwarfs are not all the same and it is entirely possible that some may cool faster than others, but most evidence suggests that the minimum time would be approximately 73,000 times the age of the universe.

      Again, thanks for your comment. This is a fascinating area of astrophysics and it is always interesting to stir up debate and discussion around the topic.

      Kind regards,

      Jonny O’Callaghan
      How It Works