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Most of the stars in the universe will end their lives as white dwarfs, the class of star that’s just a remnant of the star’s former self when all the nuclear fuel in the star’s core has burned. Studying these white dwarfs gives astronomers an important view of the endpoint most stars. [...]
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I am always pleased when a way to probe something previously untested comes along. Hopefully we'll be able to find a few of these, and get the budget required to make a good close look at the nature of the pulsations. This might not tell us the sort of thing that makes us rewrite the textbooks, but it is very cool to find new details.
Forming opinions as we speak
Order of Kilopi
Are these perecentages the outer layer percentages? [I thought they were higher for the respective elements.]Until recently, astronomers knew of only two types of white dwarf stars: those that have an outer layer of hydrogen (about 80 percent), and about those with an outer layer of helium (about 20 percent), whose hydrogen shells have somehow been stripped away
Order of Kilopi
My reading was that they represented the percentages of observed white dwarfs with each kind of composition.
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Yes, this is right (I'm one of the author's of the study). The gravity of white dwarfs is so high (about 100,000 to 1 million that on Earth's surface) that all but the lightest element in an atmosphere is pulled down out of sight in very short time scales, as short as a few days, or as long as a few centuries.Originally Posted by Disinfo Agent
So, in 80% of white dwarfs, we see atmospheres of pure hydrogen. In 20% of white dwarfs, we see helium atmospheres, meaning that somehow the hydrogen has been stripped away from the star (or completely burned up). And, in 0.1% of white dwarfs, we see carbon atmospheres, meaning that the hydrogen AND helium have somehow disappeared.
If you were to look at the composition of the entire star, though, roughly 99% of the white dwarf is made of carbon and oxygen, 1% is made of helium, and the hydrogen (where it exists) is less than .01% of the star (by mass).
We glossed over some details in the press release, and of course things tend to get garbled the more they get processed, but I'm sorry about the confusion.
Kurtis Williams
Order of Kilopi
Yes, I susepect you are correct [Disinfo Agent]. [I missed kurtisw's post] As I understand it, based on little pieces here and there, a DA dwarf is essentially all hydrogen outer shell because the helium and heavier elements have settled toward the "bottom". The DB dwarfs are the helium shell dwarfs.
I did enjoy seeing the image Fraser gave of this carbon variable dwarf appear blue, and not white. Most white dwarfs are bluish-white. If we could only get them reclassified as blue dwarfs, then there is a certain "yellow" dwarf that could subsequently reclassified.[I won't bet a sundae that this will ever happen.
]
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These carbon-atmosphere white dwarfs are anomolously blue because carbon is very opaque in ultraviolet light, where these hot stars output most of their light. So that light gets pushed into optical blue light, while optical red is unaffected.I did enjoy seeing the image Fraser gave of this carbon variable dwarf appear blue, and not white. Most white dwarfs are bluish-white. If we could only get them reclassified as blue dwarfs, then there is a certain "yellow" dwarf that could subsequently reclassified.
There used to be a fight over whether white dwarfs should really be called "degenerate dwarfs," and some people still use the latter term (which is more descriptive, but harder to type). A colleague of mine has an old bumper sticker from the Astronomical Society of the Pacific that says, "I love degenerate dwarfs." I'd be afraid to put that on my car, personally.
Order of Kilopi
Any idea what percent are O, B, and A class dwarfs? I assume the vast majority are extremely hot embers.
Ah, you're one of the authors.We glossed over some details in the press release, and of course things tend to get garbled the more they get processed, but I'm sorry about the confusion.
Congratulations on y'all's accomplishment!
Did you first wonder if you were seeing a very fast transiting planet?
[Fraser: The UT link does not work.]
Here is an alternative link.
I will never forget looking at the Eskimo nebula through the Otto Struve telescope. The ring was a distinctively blue-white color. [I was a tourist.]
Order of Kilopi
Are you saying they are a rather saturated blue (though not as much as in the image, no doubt)? Is their SED close to a Planck distribution, except in the UV?
Are their any of these stars with apparent magnitudes brighter than around 9 that you might be aware of? I would like to attempt some progressive defocus imaging of one.
Yep, you're in Texas!There used to be a fight over whether white dwarfs should really be called "degenerate dwarfs," and some people still use the latter term (which is more descriptive, but harder to type). A colleague of mine has an old bumper sticker from the Astronomical Society of the Pacific that says, "I love degenerate dwarfs." I'd be afraid to put that on my car, personally.
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I don't know off hand. Most would be in the A-class or even F-class main sequence star temperature range, because hot white dwarfs cool very quickly down to cooler temperatures. But we call all hydrogen-atmosphere white dwarfs "DA" class, regardless of temperature.
Thank you very much. It was a lot of hard work and a LOT of luck.
Congratulations on y'all's accomplishment!
Did you first wonder if you were seeing a very fast transiting planet?
Kurtis
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They are actually bluer than a Plank distribution with the measured temperature because of all the carbon obscuration in the ultraviolet. But once you get redder than about 5000 Angstroms (greenish light), the SED looks pretty much like a blackbody distribution.
Alas, no. All of the known carbon-atmosphere white dwarfs are fainter than 18th magnitude. They're so incredibly rare that you have to look a long way before you can find any.Are their any of these stars with apparent magnitudes brighter than around 9 that you might be aware of? I would like to attempt some progressive defocus imaging of one.
Kurtis
Order of Kilopi
Yes. I have a side interest in their color class, however. I'm a heliochromologist. Ok, make that more of a degenerate heliochromologist.
Part of the humor, and science, regarding the Sun's color is found in the fact that the Sun is not a yellow dwarf, but is a white star only, thus it is more appropriately, yup I'm gonna say it,.... a G2V white dwarf.
Since many are amateurs and students, if you get a chance, please share some of you and your team's experiences. I, and many others here, would love to hear how the slight variability was found in this dwarf, as well as, what headaches were encountered with that work.Thank you very much. It was a lot of hard work and a LOT of luck.
I'm unclear as to what you are describing as blue. Since we see essentially nothing after about 3800A, the UV levels would make no difference on a visual color determination, or does carbon create some stronger emissions in the blue band?They are actually bluer than a Plank distribution with the measured temperature because of all the carbon obscuration in the ultraviolet.
[I should state that hot "white dwarfs" will appear as white since their surface brightness will exceed the upper threshold of our eye's color cones. Proper neutral attenuation, however, is necessary whenever I refer to star color, except some T-class maroon (burnt orange?) stars, perhaps. [Added: Of course, we can see the color of most stars due to their low apparent magnitude. ]]
Last edited by George; 2008-May-02 at 11:54 AM. Reason: grammar
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I write a little about it on my own blog, http://www.professor-astronomy.com/blog It's not as nice of a blog as Phil's Bad Astronomy blog, but I try and throw in some of the "fun" of professional research. Hopefully I haven't turned too many aspiring astronomers off.
I was trying to tip-toe around a more detailed explanation. But when I say "color," I am really referring to the difference in magnitudes of an object taken through two different filters. For a blackbody spectrum that peaks substantially blueward of the two filters (so that the filters are looking at the red tail of the Plank spectrum), the color of a blackbody in those filters is independent of temperature.I'm unclear as to what you are describing as blue. Since we see essentially nothing after about 3800A, the UV levels would make no difference on a visual color determination, or does carbon create some stronger emissions in the blue band?
If you compare the colors of real stars to blackbody colors, you find that most stars (and especially blue stars) are redder than a blackbody in optical colors. This is mainly because there are many more atomic transistions in the blue than in the red, so blue light is suppressed. And the energy that is blocked by those transitions still has to escape from the star; this energy will come out at wavelengths where the opacity (opaqueness) is lower. In most stars, this is toward the red. So, in a normal star, the blue optical bands are a little fainter than a blackbody, and the red optical bands are a little brighter, so the star looks redder than the blackbody at the same temperature.
In these pure-carbon atmospheres, the peak of the blackbody spectrum (and thus the maximum energy release) is in the ultraviolet. But carbon has a LOT of opacity in the ultraviolet, and so the energy comes out in the nearest relatively clear spectral region, which is optically blue light. So, the blue optical wavebands are brighter than the blackbody, and the red optical wavebands are about the same as a normal blackbody. So, the star looks blue compared to a blackbody.
As I said a couple paragraphs ago, this is unusual for stars. So, in color diagrams, these stars stick out as exceptionally blue. And, in the color picture from the Sloan Survey, it therefore will look bluer than anything else in the field. We thought it made for a pretty picture to have the many contrasting colors and the very blue color.
Kurtis
Order of Kilopi
Howdy Mr. W, welcome and nice work.
I hope you don't mind a question from the the cheap seats, but from what you are saying, all the dwarf stars mentioned formed normally for their types and where then were stripped down to thier present layer by unknown and/or differing forces?
So, in even lower numbers, should there be oxygen or even carbon atmosphere dwarf stars?
Or would the process of stripping the dwarf star down that far simply disrupt it all together?
Order of Kilopi
There is good general information to be had
from whitedwarf.org I note. I still fondly
remember how I had a full reply to my request
for the distances of the nearest 100 dwarfs a
few years ago. Within 2 hours and on the
second of January!
Order of Kilopi
Very nice. That was just what I was hoping for. Thanks.
Ah, the U-B and B-V values, I presume.I was trying to tip-toe around a more detailed explanation. But when I say "color," I am really referring to the difference in magnitudes of an object taken through two different filters.
By redder, you mean shifted to the redder end, though for your star is visually more blue. This logic is really more colorful than I thought it would be.If you compare the colors of real stars to blackbody colors, you find that most stars (and especially blue stars) are redder than a blackbody in optical colors.
This explains why the blue end is so strong in the Sun's sp. irr. data. I had thought it was mainly due to the center to limb variation (CLV) of the Sun which has a peak central temp. of 6390K (4550A blue peak, assuming a bb distribution). However, when integrated with the full range of distributions that extend down to the SED at the limb of 5000K, then the amount of blue energy observed is more than I could account for.So, in a normal star, the blue optical bands are a little fainter than a blackbody, and the red optical bands are a little brighter, so the star looks redder than the blackbody at the same temperature.
[Attached shows that the actual sp. irr. peak is on the edge of blue next to violet (4505A, Solar 2000 and Wiehrli '85), which is noticeably higher than the Solar Planck temperature of 5850K (effective temp. is 5777K). The 5850K line drawn in the attachment should be reasonably accurate in its superimposed placement; I don't have time to do the actual plot on this more color accurate sp. irr. drawing.]
This is interesting, and it sounds a lot like the "dressed state" hang-up time that allows blue light to be held longer than the more red photons which have to "borrow" more energy to be captured, thus the reds are more quickly released. Is this even close to what you are describing? [Grant had recently mentioned this Uncertanity Principle application for a great explanation for refraction.]This is mainly because there are many more atomic transistions in the blue than in the red, so blue light is suppressed.
Yep, I get an 850A peak for your star.In these pure-carbon atmospheres, the peak of the blackbody spectrum (and thus the maximum energy release) is in the ultraviolet.
Is this like a rush of water being blocked by the dam and spilling out over the spillway (a lower level)?But carbon has a LOT of opacity in the ultraviolet, and so the energy comes out in the nearest relatively clear spectral region, which is optically blue light.
I see. A blue star has even greater blue augmentation the hotter it gets. This seems to be observable with progessive defocusing techniques, but I have not had time to do much of this imaging. (see attached, with permission given from Stefan Seip)So, the blue optical wavebands are brighter than the blackbody, and the red optical wavebands are about the same as a normal blackbody. So, the star looks blue compared to a blackbody.
Interestingly, I calculated the bb temperature needed to get the same ratio of blue to red as found in the sky. It was somewhere around 10 million to 20 million degrees; so the Solar core is sky blue in color, assuming proper attenuating and cooling mechanisms.
Yes, good choice. I'd bet most of these stars are blue white dwarfs in an oxymoronic way, and as we trod down the color conundrum path.As I said a couple paragraphs ago, this is unusual for stars. So, in color diagrams, these stars stick out as exceptionally blue. And, in the color picture from the Sloan Survey, it therefore will look bluer than anything else in the field. We thought it made for a pretty picture to have the many contrasting colors and the very blue color.
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