I Is it possible to have a black star?

[bbbl67]

I don't mean a black hole, I mean an actual functioning star that is black to our (human) senses. What that would mean is that a star that is so hot, that it produces hardly any radiation in the visible part of the spectrum, nothing lower than ultraviolet. I mean it's possible from the other end of the temperature scale, like brown dwarfs are so cool, that they don't produce anything higher than infrared radiation, so they would look black to us, unless they were reflecting visible light off of a nearby main sequence star. But it's debatable whether brown dwarfs are actual stars.

 

[Bandersnatch]

What that would mean is that a star that is so hot, that it produces hardly any radiation in the visible part of the spectrum, nothing lower than ultraviolet.

No, it's not possible. If you look at Planck's law graphed for different temperatures, e.g.:

(source: http://www.eumetrain.org/data/3/30/print_5.htm)
You can see that as temperatures rise, the peak emissions move towards the shorter wavelengths, but for any given wavelength the emissions always increase with temperature.
This means that hotter black body radiators (like stars) are always brighter, even though most of their emissions might be outside the visible spectrum.

 

[phyzguy]

White dwarfs, when they have cooled long enough, will eventually become cool enough that they emit no detectable visible radiation. However, the universe is not yet old enough for this to have happened. Maybe in another 100 billion years or so...

 

[Gigaz]

Not everything emits radiation like a black body. Stars are relatively good black bodys (ironically) but for most stuff here on Earth you would actually have Plancks law only as an upper limit for the radiation at each particular wavelength. Ultimately the specific material properties determine the thermal radiation.
I think that no one really knows what a Neutron star looks like in visible light, they are usually found from their X-rays and they are kind of rare. And they are presumably a billion Kelvin hot, with a surface that in some ways resembles a crystalline solid body. Noone knows the exact properties, but anyways a Neutron star could be quite dark in visible wavelengths I think. It's possible and no one really knows. If you see an image, it's an artists impression.

 

[Drakkith]

I think that no one really knows what a Neutron star looks like in visible light, they are usually found from their X-rays and they are kind of rare. And they are presumably a billion Kelvin hot, with a surface that in some ways resembles a crystalline solid body. Noone knows the exact properties, but anyways a Neutron star could be quite dark in visible wavelengths I think. It's possible and no one really knows. If you see an image, it's an artists impression.

We have at least one picture of a neutron star in visible light: https://upload.wikimedia.org/wikipedia/commons/0/01/IsolatedNeutronStar.jpg
I found the picture here: https://en.wikipedia.org/wiki/Neutron_star#History_of_discoveries

It looks just like any other really hot object would look unless you do some spectroscopic measurements to see the shift in wavelengths due to its strong gravity.

 

[Nik_2213]

"... hot..."
I thought that might be the case, given a neutron star's spin, magnetic field and gravity interact with interstellar medium. Also, if the neutron star has escaped from a binary, it may have a high velocity...

 

[bbbl67]

White dwarfs, when they have cooled long enough, will eventually become cool enough that they emit no detectable visible radiation. However, the universe is not yet old enough for this to have happened. Maybe in another 100 billion years or so...

I thought about that too, but let's say in the trillion years or so it takes for a white dwarf to cool down to a black dwarf, the background radiation is also cooling down. So let's say a white dwarf cools down to 2.7K (today's CMBR temperature), by then the actual CMBR will be in the micro-Kelivins. So wouldn't a white dwarf always quite a bit hotter than the surrounding space? Thus it would always be a white dwarf?

 

[Chronos]

A definition may be useful here. Apparently - and Interestingly enough - the international astronomical union [IAU], the usual authority for all things astronomical, has not adopted a standard definition for a star. While this may seem shocking, note that the IAU did not adopt a standard definition for a planet until the year 2000. Even that simple task became an epic adventure in bureacracy that still leaves scientists muttering among themselves. Perhaps the IAU learned a valuable lesson from that experience and has elected to leave common terms to the linguists for debate. Aopealing to Merriam Webster, we find the following definition: a :a natural luminous body visible in the sky especially at night
b :a self-luminous gaseous spheroidal celestial body of great mass which produces energy by means of nuclear fusion reactions The common denomination appears to be the term 'luminous' and luminous, according to this same source means a :emitting or reflecting usually steady, suffused, or glowing light

• luminous objects—the nebula. the stars, the planets
• —Lincoln La Paz

b :of or relating to light or to luminous flux
Digging further into the mire, Webster elaborates
Definition of luminous flux
:radiant flux in the visible-wavelength range usually expressed in lumens instead of watts
So, we are left with the notion that a star is a distant, massive body that emits a radiant flux in the visible wavength range by means of nuclear fusion.
By this token, it appears a 'black star', as a non-luminous body, is an oxymoron. Personally, I would not hold out in hopes of being rescued by the IAU any time soon.

 

[Drakkith]

"... hot..."
I thought that might be the case, given a neutron star's spin, magnetic field and gravity interact with interstellar medium.

What do you mean?

 

[phyzguy]

I thought about that too, but let's say in the trillion years or so it takes for a white dwarf to cool down to a black dwarf, the background radiation is also cooling down. So let's say a white dwarf cools down to 2.7K (today's CMBR temperature), by then the actual CMBR will be in the micro-Kelivins. So wouldn't a white dwarf always quite a bit hotter than the surrounding space? Thus it would always be a white dwarf?

Once it has cooled below a few hundred K, it is no longer emitting any visible radiation. At this point, I would consider it a "black dwarf", even though it is still emitting radiation in the IR and microwave. If you take something on the Earth which is black, do you consider it white just because it is radiating in the IR?

 

[bbbl67]

Once it has cooled below a few hundred K, it is no longer emitting any visible radiation. At this point, I would consider it a "black dwarf", even though it is still emitting radiation in the IR and microwave. If you take something on the Earth which is black, do you consider it white just because it is radiating in the IR?

I understand, but in the several trillions of years it takes for white dwarfs to cool down to these levels, it seems likely that most main sequence stars will have disappeared and all we'll be left with are these slowly dissipating embers of light called white dwarfs and neutron stars. So likely the most common source of light in that future would be sub-IR. So we'll have given up our eyes, and replaced them with microwave sensors. So the definition of white will have shifted down to those wavelengths.

 

[_PJ_]

No real surprise definitions took so long coming.
There simply wasn't any need for a more formal definition of planets until the number of comparable sized TNO's etc. discovered made for the debate. Before then (and given exoplanets were not confirmed either) everybody was content and agreed that the big things that weren't the sun were planets, and the little things that orbited them were moons. If there was study or discussion of, say, shepherd satellites of Saturn's rings, it would be so contextually specific that whether they counted as "moons" was not a relevant consideration.

As for stars, bear in mind that even since the identification of main sequence evolution and original classifications, as more and more features and phenomena particular to certain stellarvarieties has lead naturally to a far richer classification, T Tauri, Wolf Rayet, Carbon (N-type) stars etc. But again, stars were siply the bright objects that were NOT nebulae, galaxies or what we now know to be neutron stars etc. - planets beyond the solar system were just not resolvable so there was no risk of any visible dot in the sky being a planet if you had eliminated the solar system)
There was simply no need for any formal classification at least for the sake of distinguishing a star from a non-star. Now, however, with advances and consideration between the massive extrasolar gas giants and failed stars, as the line blurs more concrete distinctions are required.

 

[Nik_2213]

"... hot..."
I thought that might be the case, given a neutron star's spin, magnetic field and gravity interact with interstellar medium.
What do you mean?
==
Even when the interior of the object cools, there will be some in-fall to produce surface effects...
Or have I misread the possibilities ??

 

[Tish62]

I would believe that if there was a monochromatic energy source that was exclusively in the far UV/ X-ray/ gamma end of the spectrum that it would still "glow" from the incandescence it would cause on the particles and gases in the immediate vicinity of the star.