What color is a white dwarf?

[George]

Just being my colorful self. :-?

Regardless, take the white dwarf Sirius B serious.

Its surface temp. of 25,000 K is up there with the big blue O stars. Even B stars (11,000 - 25,000 K)and A stars (7,500 K to 11,000 K)are considered "blue".

I don't see how it's size would make the difference to our eyes.

Is my thinking off here? #-o

[Ut]

The larger the surface area, the brighter the star, and the brighter the star, the easier it is to actually see colour. Then again, most stars look white to the naked eye, so... Really what I'm saying is, I have no idea, but love to hear myself talk!

[Hamlet]

A white dwarf starts out with a much higher surface temperature of around 100,000 K or more and would probably appear, to our eyes, as white light. As the WD cools, the peak color production would slowly shift from the blue end to the red end of the spectrum until its too cool to produce visible light at all. Sirius B probably started out as a white-hot ember but has now cooled to a more modest surface temperature.

I don't think its apparent color has anything to do with its size. It's small size does matter in regulating how fast it cools off. Smaller surface area to radiate heat means it takes a loooong time to cool off.

[Glom]

Smaller surface area to radiate heat means it takes a loooong time to cool off.

Surface area to volume ratio. So the bigger the white dwarf, the longer it will take to cool off because there is less surface area for the given volume.

I believe that according to the current models, the universe isn't old enough for any white dwarf to have cooled down to a black dwarf.

[George]

A white dwarf starts out with a much higher surface temperature of around 100,000 K or more and would probably appear, to our eyes, as white light. As the WD cools, the peak color production would slowly shift from the blue end to the red end of the spectrum until its too cool to produce visible light at all. Sirius B probably started out as a white-hot ember but has now cooled to a more modest surface temperature.

This doesn't help me understand. The greater the temp., the greater the blackbody radiation curve moves toward the blue and beyond. Even though it's not a true blackbody, it will still behave very similar.

One possible explanation might be the reds and yellows are still at enough intensity to stimulate our red and green color cones. However, I do not know if it's size somehow fools our eyes where a larger star (blue one) of similar temperature does not.

Smaller surface area to radiate heat means it takes a loooong time to cool off.

Interesting this would be the case. Ice broken-up absorbs heat in our drinks faster. I suppose it is the issue of surface area to mass ratio. If the mass density were the same as a normal star, it should cool faster, right?

[ngc3314]

Just being my colorful self. :-?

Regardless, take the white dwarf Sirius B serious.

Its surface temp. of 25,000 K is up there with the big blue O stars. Even B stars (11,000 - 25,000 K)and A stars (7,500 K to 11,000 K)are considered "blue".

I don't see how it's size would make the difference to our eyes.

Is my thinking off here? #-o

Basically, "white" is just a name, by now denoting a whole sequence in the HR diagram. The first ones found (and isolated enough for good color measurements) had temperatures more like main-sequence A stars, and the name stuck. The coolest ones are down somewhat cooler than 6000 K, if memory serves. Close-up, a white dwarf would look just as blue as any other star of similar temperature (and young WDs are the hottest kind of well-studied star). In both cases, there is a limit to how blue we will see anything that acts like a blackbody, since the ratio of yellow to blue reaches an asymptotic value (this is why the reddest stars are redder than the bluest stars appear blue - like the old Grook about why Scotsmen are so much more Scottish than Englishmen are English).

Visually, the biggest problem is that there are no white dwarfs brighter than about 9th magnitude, so you have to work really hard to be able to see much of any color. And the best ones are mostly close to much brighter companions (Sirius, Procyon).

[George]

Just being my colorful self. :-?

Regardless, take the white dwarf Sirius B serious.

Its surface temp. of 25,000 K is up there with the big blue O stars. Even B stars (11,000 - 25,000 K)and A stars (7,500 K to 11,000 K)are considered "blue".

I don't see how it's size would make the difference to our eyes.

Is my thinking off here? #-o

Basically, "white" is just a name, by now denoting a whole sequence in the HR diagram. The first ones found (and isolated enough for good color measurements) had temperatures more like main-sequence A stars, and the name stuck. The coolest ones are down somewhat cooler than 6000 K, if memory serves. Close-up, a white dwarf would look just as blue as any other star of similar temperature (and young WDs are the hottest kind of well-studied star).

That makes sense. Any chance you know of a "natural" color image of one near 6,000 K taken from a space scope? [I suspect there is a slight chance it too may be blue if observed above our bleaching atmosphere]

In both cases, there is a limit to how blue we will see anything that acts like a blackbody, since the ratio of yellow to blue reaches an asymptotic value (this is why the reddest stars are redder than the bluest stars appear blue - like the old Grook about why Scotsmen are so much more Scottish than Englishmen are English).

Hmmm....you're not Scott are you? :wink:

Are you saying that you will always have yellow radiance from any source (assuming near-blackbody) greater than 6000 K which would dilute the blueness but red stars do not suffer from the mid-range mixing?

Visually, the biggest problem is that there are no white dwarfs brighter than about 9th magnitude, so you have to work really hard to be able to see much of any color. And the best ones are mostly close to much brighter companions (Sirius, Procyon).

Great point! If the color cones don't "turn on" it will always be a shade of white even if it's... light is a blue-white white dwarf, right? (say it 3x's fast :wink: )

Red also has an advantage due to the greater sensitivity of the red color cone vs. our blue color cones.

I have eye data somewhere which states our color thresholds. It would be nice to know what magnitude is necessary to excite our eyes to see color. This, I suspect, is a tricky issue without a simple answer, as our eye is complicate when it comes to colors. For instance, the lens of our eye can magnify the light onto a small area if the light is parallel. Laser light is a real safety problem on this issue. Our lens also produces chromatic aberation where, I think, green is the winner for best focus ability (might be yellow, though).

I am quite curious about color images of dwarfs. It might help seal some cracks in my crack-pot idea that the Sun might be blue. :P A nagging and scientifically trival endeavor. Well....now that I've exposed myself, color images of solar twins would be nice, too. 8)

[Tobin Dax]

Hmmm....you're not Scott are you? :wink:

I am. I'm a Scot, too (well, at least some ways back).

[George]

Hmmm....you're not Scott are you? :wink:

I am. I'm a Scot, too (well, at least some ways back).

Odd, you sound English.

[Tobin Dax]

Hmmm....you're not Scott are you? :wink:

I am. I'm a Scot, too (well, at least some ways back).

Odd, you sound English.

Well, then, I guess I should be glad I don't sound American. My foreign classmates (which is basically everyone except me) would certainly think it's a good thing.

[George]

Hmmm....you're not Scott are you? :wink:

I am. I'm a Scot, too (well, at least some ways back).

Odd, you sound English.

Well, then, I guess I should be glad I don't sound American. My foreign classmates (which is basically everyone except me) would certainly think it's a good thing.

Ok, now you sound Scot.

[George]

Visually, the biggest problem is that there are no white dwarfs brighter than about 9th magnitude, so you have to work really hard to be able to see much of any color. And the best ones are mostly close to much brighter companions (Sirius, Procyon).

Another way to look at it (thoughtless pun), there should be a "sweet" spot in terms of viewing distance where it's true color is observable. Too far (as in all white dwarfs from where we are) and it will look white (light below our eye's cone threshold). Too close and it will also look white (all of our cones fully excited). In between, the difference in color intesities will be observed registering a certain color.

Naturally, this should apply to most stars with red stars as a possible exception.

[Hamlet]

I believe that according to the current models, the universe isn't old enough for any white dwarf to have cooled down to a black dwarf.

Yes, according to the models I've seen it would take greater than the age of the universe for the first white dwarfs to have cooled to where they no longer emit visible light. It is thought that WD cools very quickly at first losing energy through neutrino production. Once the WD has cooled below about 30,000K surface temperature, neutrino production stops and the WD continues to cool through radiation. As the WD cools further, it is thought that convection may play a role as well.

[Tom Mazanec]

Here's a BTW.
What is a white main sequence star?
The sun is a "yellow dwarf", so...

[tjm220]

Here's a BTW.
What is a white main sequence star?
The sun is a "yellow dwarf", so...

Spectral class A like Sirius or Vega.

[George]

Forgive me if I'm a little too "tongue-in-check", but much seems to be, literally, in the eye of the beholder. But, I can't find any beholders.

Evidence supports "blue" for Class A.

Here's my not-so-scientific evidence...

Here's my sources...
Light blue…
http://www.cartage.org.lb/en/themes/...arspectra.html

white
http://www.glyphweb.com/esky/default...ification.html

blue
http://www.starfleet-ssf.com/valhalla/classes.htm

white
http://www.ucip.org/map/SpatialPhenomena/StarClass.html

light blue [From Star Fleet Command 8) ]
http://www.star-fleet.com/gmd/rm/tid...ification.html

white (but Sun yellow)
http://outreach.atnf.csiro.au/educat...ral_class.html

light blue (but Sun peachy pink)
http://www.vendian.org/mncharity/dir3/starcolor/

white
http://victoria.rasc.ca/articles/1996/art9611.html

blue
http://www.ccdump.org/stellarclass.html

blue
http://business.fortunecity.com/rowl...fications.html

Blue wins 6 to 4.

Notice how many show the Sun as being "yellow". It isn't except at certain times of the day (and if you are not in space observing it). :-?