<p>A black dwarf is a <em>white</em> dwarf that has cooled to the point that it is no longer emitting light. However it takes a long time for a white dwarf to cool down. The exact timescale is rather uncertain, but at least hundreds of billions of years, perhaps hundreds of trillions of years. </p>

<p>The upshot is that that no black dwarfs exist in the universe today. The Universe is only about 14 billion years old, so there hasn't been time for any black dwarfs to form.</p>

<p>The paper <a href="http://xxx.lanl.gov/pdf/astro-ph/9701131v1" rel="noreferrer">A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects</a> discusses brown dwarfs, black dwarfs, and other objects that may exist in an aging universe.</p>

CC BY-SA 3.0
2017-10-21T16:44:04.917Z
Does it really take hundreds of billions of years for a white dwarf to cool so it emits no visible light? (NB They will always emit light). The coolest white dwarf known now is below 3000K and undetectable in the visible.
ProfRob
CC BY-SA 3.0
2017-10-21T19:05:23.790Z
Wikipedia gives "3900K" as the coolest white dwarf, that may be out of date. A 3000K object would be a lot hotter than lava, hotter than molten iron. The "hundreds of trillions" is to cool to 5K. To cool to the 800K that would be cool enough not to be "dim red"  would still take a lot longer than the age of the universe.
James K
CC BY-SA 3.0
2017-10-22T07:01:33.620Z
Can you give any reference for the usage of *black dwarf* in the astronomical scientific literature? As far as I'm aware, this is not a standard term.
Walter
CC BY-SA 3.0
2017-10-22T07:10:32.457Z
I mean *common* usage.
Walter
CC BY-SA 3.0
2017-10-22T13:27:34.987Z
Yes, see the paper linked in the answer.
James K
CC BY-SA 4.0
2018-12-28T17:10:44.833Z
The paper cited does not use the term "black dwarf" at all, so can you say where you get your definition from and also where the timescale of 100s of billions of years is from?
ProfRob
CC BY-SA 3.0
2017-10-20T20:53:45.043Z
2017-10-20T20:53:45.043Z
James K
<p>A black dwarf might be defined as a cooled, compact object with a temperature and surface area that are small enough to make it invisible in the optical part of the spectrum, with most radiation emerging in the infrared. However, because there is always <em>some</em> radiation in the short wavelength Wien tail of a spectrum this definition would be distance-dependent, so instead you could define a temperature below which you could argue that so little light emerges at visible wavelengths that it would be dark. Again, this is somewhat distance-dependent, but also depends on the size of the object.</p>
<p>In practice, at astronomical distances, cool compact objects like old white dwarfs and neutron stars, or low-mass and older brown dwarfs are invisible at optical wavelengths when their temperatures fall significantly below 3000K. At 3000K, the V-K colour of such objects is <span class="math-container">$\sim 6$</span>, meaning they are 250 times as bright in the near infrared. This increases to <span class="math-container">$\sim 1000$</span> at a temperature of 2200K.</p>
<p>There <em>are</em> lots of brown dwarfs (particularly the T-dwarfs) that can only be seen in the infrared. The coldest known white dwarf is probably older than 10 billion years, has a temperature below 3000K, and has not been detected at optical (or IR) wavelengths (<a href="https://arxiv.org/abs/1406.0488" rel="nofollow noreferrer">Kaplan et al. 2014</a>).</p>
<p>There are likely about a billion or so old, cold neutron stars in the Milky Way that cannot be seen via electromagnetic radiation at all unless they interact with something else.</p>
<p>The <a href="https://en.m.wikipedia.org/wiki/Black_dwarf" rel="nofollow noreferrer">Wikipedia definition</a> of an object so cold it doesn't emit light is simply unworkable. All objects above absolute zero will emit electromagnetic radiation (light) and it is hard to see how an object could cool to below the cosmic radiation background. Thus there are either no black dwarfs in the universe at all and never will be (if you use the Wikipedia definition) or there are lots of black dwarfs (loads of very cool brown dwarfs, cold neutron stars and at least some cold white dwarfs) if you use the definition of emitting an undetectable amount of visible light.</p>

CC BY-SA 3.0
2017-10-29T14:13:32.970Z
Ok, a brown dwarf might be a so called “failed star” that does not have enough mass to start nuclear fusion whereas a black dwarf is a cooled neutron star at the end of a much larger star’s life and also probably doesn’t exist because the universe is not old enough. Unfortunate name “dwarf “ for what is probably the most common star
P Pitch
CC BY-SA 4.0
2017-10-21T17:11:04.460Z
2024-07-13T12:47:53.390Z
2024-07-13T12:47:53.390Z
ProfRob
<p>A black dwarf is what (theoretically) results from a white or brown dwarf cooling to the point of no longer emitting  visible light. Such objects are only theorized, because of the incredible cooling time (&gt; the age of the universe). I posted this once before; I'm not sure why it was deleted.</p>
<p>See <a href="http://wiki.org/black-dwarf" rel="nofollow noreferrer">http://wiki.org/black-dwarf</a></p>

CC BY-SA 4.0
2024-07-13T12:49:42.113Z
@PM2Ring both white dwarfs and brown dwarfs are cooling objects supported by electron degeneracy pressure. One generally has a carbon/oxygen composition, the other is hydrogen/helium. They have plenty in common.
ProfRob
CC BY-SA 4.0
2024-07-13T20:43:30.383Z
Do neutron stars eventually blacken as well? I've never seen the term black dwarf applied to a cooled neutron star?
Joe Peters
CC BY-SA 4.0
2024-07-15T06:20:52.600Z
Joe, maybe your previous post got deleted because it was viewed as a comment rather than an answer. Your current post could potentially be seen in the same light: the question already has two excellent answers, and this post hasn't added anything that hasn't already been said in the existing answers.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2024-07-13T00:50:02.940Z
2024-07-13T00:50:02.940Z
Joe Peters
<p>Does such a heavenly body exist called a black dwarf? It would be a Brown dwarf that has cooled to the point it no longer gives off heat or light.</p>

CC BY-SA 3.0
2017-10-22T15:44:45.970Z
It seems to me that astronomy is changing and our perception of the stars, planets and their properties is changing like it did when Galileo started looking at the heavens. Correct me if I’m wrong but now we know that there are many brown dwarf stars and some of them within 10 light years from the Earth. Before Hubble, we didn’t know there were so many of them. Now it is safe to presume that virtually every star is a solar system with planets and moons whereas 50 years ago we wondered if ther were any. I suspect that brown dwarfs are not all dying neutron stars but just a common smaller star.
P Pitch
<brown-dwarf>
Is there such a thing as a black dwarf
P Pitch
# Is there such a thing as a black dwarf

(Asked by: username_0 on 2017-10-20)

Does such a heavenly body exist called a black dwarf? It would be a Brown dwarf that has cooled to the point it no longer gives off heat or light.

**Tags:** brown-dwarf
> **Comments:**
> **username_0**: It seems to me that astronomy is changing and our perception of the stars, planets and their properties is changing like it did when Galileo started looking at the heavens. Correct me if I’m wrong but now we know that there are many brown dwarf stars and some of them within 10 light years from the Earth. Before Hubble, we didn’t know there were so many of them. Now it is safe to presume that virtually every star is a solar system with planets and moons whereas 50 years ago we wondered if ther were any. I suspect that brown dwarfs are not all dying neutron stars but just a common smaller star.  

---

## 3 answers

### Answer by username_1 on 2017-10-20. Score: 13

A black dwarf is a *white* dwarf that has cooled to the point that it is no longer emitting light. However it takes a long time for a white dwarf to cool down. The exact timescale is rather uncertain, but at least hundreds of billions of years, perhaps hundreds of trillions of years.

The upshot is that that no black dwarfs exist in the universe today. The Universe is only about 14 billion years old, so there hasn't been time for any black dwarfs to form.

The paper [A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects](http://xxx.lanl.gov/pdf/astro-ph/9701131v1) discusses brown dwarfs, black dwarfs, and other objects that may exist in an aging universe.

> **Comments:**
> **username_2**: Does it really take hundreds of billions of years for a white dwarf to cool so it emits no visible light? (NB They will always emit light). The coolest white dwarf known now is below 3000K and undetectable in the visible.  
> **username_1**: Wikipedia gives "3900K" as the coolest white dwarf, that may be out of date. A 3000K object would be a lot hotter than lava, hotter than molten iron. The "hundreds of trillions" is to cool to 5K. To cool to the 800K that would be cool enough not to be "dim red"  would still take a lot longer than the age of the universe.  
> **username_3**: Can you give any reference for the usage of *black dwarf* in the astronomical scientific literature? As far as I'm aware, this is not a standard term.  
> **username_3**: I mean *common* usage.  
> **username_1**: Yes, see the paper linked in the answer.  
> **username_2**: The paper cited does not use the term "black dwarf" at all, so can you say where you get your definition from and also where the timescale of 100s of billions of years is from?  

---

### Answer by username_2 on 2017-10-21. Score: 5

A black dwarf might be defined as a cooled, compact object with a temperature and surface area that are small enough to make it invisible in the optical part of the spectrum, with most radiation emerging in the infrared. However, because there is always *some* radiation in the short wavelength Wien tail of a spectrum this definition would be distance-dependent, so instead you could define a temperature below which you could argue that so little light emerges at visible wavelengths that it would be dark. Again, this is somewhat distance-dependent, but also depends on the size of the object.

In practice, at astronomical distances, cool compact objects like old white dwarfs and neutron stars, or low-mass and older brown dwarfs are invisible at optical wavelengths when their temperatures fall significantly below 3000K. At 3000K, the V-K colour of such objects is $\sim 6$, meaning they are 250 times as bright in the near infrared. This increases to $\sim 1000$ at a temperature of 2200K.

There *are* lots of brown dwarfs (particularly the T-dwarfs) that can only be seen in the infrared. The coldest known white dwarf is probably older than 10 billion years, has a temperature below 3000K, and has not been detected at optical (or IR) wavelengths ([Kaplan et al. 2014](https://arxiv.org/abs/1406.0488)).

There are likely about a billion or so old, cold neutron stars in the Milky Way that cannot be seen via electromagnetic radiation at all unless they interact with something else.

The [Wikipedia definition](https://en.m.wikipedia.org/wiki/Black_dwarf) of an object so cold it doesn't emit light is simply unworkable. All objects above absolute zero will emit electromagnetic radiation (light) and it is hard to see how an object could cool to below the cosmic radiation background. Thus there are either no black dwarfs in the universe at all and never will be (if you use the Wikipedia definition) or there are lots of black dwarfs (loads of very cool brown dwarfs, cold neutron stars and at least some cold white dwarfs) if you use the definition of emitting an undetectable amount of visible light.

> **Comments:**
> **username_0**: Ok, a brown dwarf might be a so called “failed star” that does not have enough mass to start nuclear fusion whereas a black dwarf is a cooled neutron star at the end of a much larger star’s life and also probably doesn’t exist because the universe is not old enough. Unfortunate name “dwarf “ for what is probably the most common star  

---

### Answer by username_4 on 2024-07-13. Score: -2

A black dwarf is what (theoretically) results from a white or brown dwarf cooling to the point of no longer emitting visible light. Such objects are only theorized, because of the incredible cooling time (> the age of the universe). I posted this once before; I'm not sure why it was deleted.

See <http://wiki.org/black-dwarf>

> **Comments:**
> **username_2**: @PM2Ring both white dwarfs and brown dwarfs are cooling objects supported by electron degeneracy pressure. One generally has a carbon/oxygen composition, the other is hydrogen/helium. They have plenty in common.  
> **username_4**: Do neutron stars eventually blacken as well? I've never seen the term black dwarf applied to a cooled neutron star?  
> **username_5**: Joe, maybe your previous post got deleted because it was viewed as a comment rather than an answer. Your current post could potentially be seen in the same light: the question already has two excellent answers, and this post hasn't added anything that hasn't already been said in the existing answers.  

---

