Death of our Earth

[ajtribick]

Hmmm, when's the sun gonna go white dwarf and then pull off a reverse Michael Jackson and go black dwarf?

Interesting way of putting it...

Give it a few trillion years. It takes a VERY long time for a white dwarf to cool down.

Has anyone got any data on how long it takes for a white dwarf to cool down, I've seen a lot of different numbers, ranging from about 10 billion years to 10 trillion years.

The From Now Until The End Of Time... site gives about 30 billion years for the Sun to cool after its red giant phase.

Having done a bit of searching on the internet, I get a starting temperature for a white dwarf of about 100000 K, and the oldest white dwarfs are about 10 billion years old and have a temperature of 3500 K, and assuming exponential cooling, I get a time of about 30 billion years to cool to 5 K.

Of course this is a very naive way of doing the calculation, I'd like some better estimates if possible.

[Malenfant]

Well either way it's longer than the age of the universe. Which means that there aren't any 'black dwarfs' today. (besides which, how do you define "black" here? Is it when the object cools down to the background temperature of the universe?)

[Hunter Parasite]

Well either way it's longer than the age of the universe. Which means that there aren't any 'black dwarfs' today. (besides which, how do you define "black" here? Is it when the object cools down to the background temperature of the universe?)

How would a black dwarf look? its a conspiracy

[selden]

Black, of course!
Any radiation would be in the infrared or longer wavelengths, with the peak frequency depending on the body's temperature.

[WildMoon]

Dang, 30 billion years?

By what chaos sydrome said it sounds like white dwarfs become black dwarfs go black when they're 5?°k, which is very cold. Wouldn't they become black dwarfs when they are maybe room temperature? (297.04?°K)(23.89?°C)(75?°F)(just in case anyone wants these numbers, 19.11?°R[R?©aumur] & 534.67 rankine)

It's just that 5?°K is extremely close to 0?°K (absolute zero, meaning there is no energy at all and it can't get colder) and it seems to me white dwarfs would become star corpses before that.

[ajtribick]

I think we need to define what we mean by "black dwarf" here.

It could either mean when the object gets sufficiently cold so as it doesn't radiate in the visible range, or when it reaches a temperature which is essentially the same as the background, which I took in my calculation (actually, an exponential curve never reaches the background value, so I've made a bit of a fudge here )

For the former consideration, we need the temperature at which an object does not put out significant visible light, I'll take a value of 700 Kelvin, from this WikiPedia article.

Using the same starting parameters as I did earlier, I get a time of 15 billion years until the white dwarf becomes visually black, though it would still be radiating quite strongly in the infrared! I guess this explains the low end of the range of estimates I have seen.

As for what a black dwarf would look like, let's suppose that in the same system there is another light source, given the timescales involved it would probably be a red dwarf or another, younger white dwarf (formed from a less massive star).

This article suggests that a white dwarf may be composed mainly of bluish-green diamond, possibly with oxygen "snow" (though on reading it it sounds like the oxygen is snowing through the diamond layer, so you'd get an oxygen core or something). If I recall correctly, solid oxygen is blue. You might get other interesting things like liquid helium if it gets cold enough, but I'm not an expert in these kind of things. So you'd end up with something looking bluish-green I'd guess.

[julesstoop]

I suppose it will be quite a dirty object though, due to it's large gravity, and - once cooled down enough not to have a 'molten' surface - all the time it got to collect dirt.