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Fortunate
2007-Nov-22, 01:49 AM
Astronomers have discovered white dwarf stars with pure carbon atmospheres. The discovery could offer a unique view into the hearts of dying stars.

http://www.physorg.com/news114873541.html


When I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones," Dufour said. "But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn't agree with the SDSS data," Dufour said.

In May 2007, "out of pure desperation, I decided to try modeling a pure-carbon atmosphere. It worked," Dufour said. "I found that if I calculated a pure carbon atmosphere model, it reproduces the spectra exactly as observed...."

Jerry
2007-Nov-22, 03:40 AM
Very curious...would such a star ever supernova? How would the meltdown proceed, what would the lightcurve and spectra demonstrate?

Fortunate
2007-Nov-22, 03:50 AM
There sure is a lot of diversity. Fascinating and fun.

Kaptain K
2007-Nov-22, 04:48 AM
Very curious...would such a star ever supernova? How would the meltdown proceed, what would the lightcurve and spectra demonstrate?
These stars are white dwarfs. The only way a white dwarf can supernova is if it is accreting matter from a close companion. No close companion, no supernova.

Kullat Nunu
2007-Nov-23, 04:05 PM
Regarding the title of this thread, there are true carbon atmosphere stars, namely R Coronae Borealis variables. These enigmatic objects may be the end result of white dwarf collisions.

Noclevername
2007-Nov-23, 04:37 PM
These stars are white dwarfs. The only way a white dwarf can supernova is if it is accreting matter from a close companion. No close companion, no supernova.

And if it does that, it'll no longer have a pure-carbon atmosphere.

jimmarsen
2007-Nov-23, 06:44 PM
Does this tell us anything about the age of these objects? How is the age of a white dwarf determined?

Noclevername
2007-Nov-23, 07:25 PM
Does this tell us anything about the age of these objects? How is the age of a white dwarf determined?

Cut it in half and count the rings.

Jerry
2007-Nov-24, 02:56 AM
These stars are white dwarfs. The only way a white dwarf can supernova is if it is accreting matter from a close companion. No close companion, no supernova.

White dwarfs are not expected to have carbon atmospheres - these stars are off of-the-map, the evolution not understood. Many of the predictions of how white dwarfs should behave have been stood on their heads in the last ten years.

Kaptain K
2007-Nov-24, 03:32 AM
Jerry,

You are comparing apples and oranges!

Jerry
2007-Nov-25, 12:14 AM
???

If a White dwarf is an apple, what is the orange here? White dwarfs are considered main sequence stars, but they are not suposed to have carbon atmospheres. So if you are questioning the logic of calling these things white dwarfs, that is reasonable. Given that recent events have broadened both the magnitude and expansion envelope of white dwarf events, I don't think anyone can predict how these stars supernova with any reasonable certainty. For example, a white dwarf with a high rate of spin may have an effective mass that does not supernova until the spin rate retards...with or without a companion star.

Kaptain K
2007-Nov-25, 01:11 AM
White dwarfs are not main sequence!


...they are not supposed to have carbon atmospheres.
Make that "were not expected to have..." and I'll agree. Finding the "unexpected" is expected. It's what makes science grow, rather than be handed down from on high like religion.

Jerry
2007-Nov-25, 10:13 PM
They show up on the H-R diagram as type VII, and are generally considered the 'end of the road' for many stars; or as Wiki puts it "White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our Galaxy". I guess that is not main sequence if you do not consider the final state part of the main sequence; although I would hate to answer the question that way on a test without having it spelled out in my lecture notes.

Kullat Nunu
2007-Nov-26, 04:48 PM
Main sequence is a very specific part of the HR diagram (or a star's life cycle). WDs occupy the great relatively empty lower left region in the diagram and are nowhere near the main sequence stars (except of course when they're moving from red giants to white dwarfs).

antoniseb
2007-Nov-26, 05:37 PM
"White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our Galaxy".
Here is a place where Wiki might be off by a bit. I suspect that the vast majority of stars in the galaxy are not massive enough to become white dwarfs. They are not massive enough to make it to a Helium burning stage, and so will simply fade from red to dark (many billions of years from now).

I guess that is not main sequence if you do not consider the final state part of the main sequence;
Main Sequence doesn't simply mean ordinary. It means ordinary and relatively new. Ordinary stars move off the main sequence as they consume Hydrogen in their cores. The final state is not a main sequence state. Note that the term sequence often implies progression through time, but is not applied that way here.

Kullat Nunu
2007-Nov-26, 07:41 PM
Red dwarfs will become white dwarfs. The process is a little bit slow. And without a red giant phase and such, just a steady build-up of helium.

antoniseb
2007-Nov-26, 08:20 PM
Red dwarfs will become white dwarfs. The process is a little bit slow. And without a red giant phase and such, just a steady build-up of helium.
I'm not saying you are wrong, but I'd like to see a source on this. I have the impression that there are thresholds for mass required for:
- Lithium burning
- Deuterium burning
- Hydrogen burning
- Helium burning

And that there is a pretty large separation between the mass of a minimum red dwarf and the mass of a star that will eventually be able to fuse Helium. Now, it is possible that M1 red dwarfs are massive enough, but I doubt that anything smaller than an M3 has a chance. Do you have a source that tells what the threshold is?

Jerry
2007-Nov-26, 08:25 PM
If you were asked on a test to step through the entire life of a main sequence star, and you did not include a white dwarf step, you would not likely receive full credit for the answer.

Kullat Nunu
2007-Nov-26, 08:30 PM
I didn't say that helium will be burned because it won't. The amount of helium steadily rises until no hydrogen is available and the star has become a white dwarf. No idea how the brightness of a red dwarf changes over time. Because red dwarfs have completely convective atmospheres, they can practically use all of their hydrogen unlike solar-like and more massive stars which run into problems when hydrogen is depleted in their cores. That plus the low intensity are the reasons why red dwarfs live so long.

antoniseb
2007-Nov-26, 08:36 PM
The amount of helium steadily rises until no hydrogen is available and the star has become a white dwarf.
Except that:
1. The star is not made of Carbon, Oxygen and Neon
2. The star is never hot enough to be called white (or even Orange).

antoniseb
2007-Nov-26, 08:54 PM
If you were asked on a test to step through the entire life of a main sequence star, and you did not include a white dwarf step, you would not likely receive full credit for the answer.
Come on Jerry, please don't work to try and change the meaning of words with precise meaning. The opening sentence of the Wikipedia article about Main Sequence says:
The main sequence of the Hertzsprung-Russell diagram (http://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram) is the curve (http://en.wikipedia.org/wiki/Curve) along which the majority of stars (http://en.wikipedia.org/wiki/Star) are located. Stars on this band are known as main-sequence stars or dwarf stars.

If you look at the H-R diagram, there is a curve labeled "Main Sequence", and this is where the zero age main sequence stars are. As they age, they "move off the main sequence".

If you ask someone to describe the entire life of a main sequence star, you should include white dwarf for many stars. Similarly if you are asked to describe the entire life of a human baby, you should probably mention old age... but an old human is not a baby, and a white dwarf is not a main sequence star.

You're a smart well educated guy, and I have to believe that you know this, so why try to confuse other people with this abuse of the language?

baric
2007-Nov-26, 10:53 PM
If you were asked on a test to step through the entire life of a main sequence star, and you did not include a white dwarf step, you would not likely receive full credit for the answer.

Whether a star on the main sequence ends as a white dwarf is mass-dependent.

Some are too small, some are too big, but others are just right. But "white dwarf" is no more a part of the main sequence than neutron stars or black holes.

Romanus
2007-Nov-27, 03:13 PM
Red dwarfs do become white dwarfs...they just skip the red giant phase, and slowly heat up and shrink over trillions of years.

The following excerpt is from the November 1997 issue of Sky and Telescope, page 22, "Why the Smallest Stars Stay Small"; the results were derived with computer modeling:


...[Gregory Laughlin et al.] found that a star with one-tenth our Sun's mass takes two billion years merely to settle down to life as a hydrogen fusion reactor. And there it stays for nearly six trillion (orig. italics) years...Finally, when its unbelievably long life at last draws to a close, a 0.1-solar-mass star skips red gianthood altogether...A few hundred billion years later, it joins the stellar graveyard in the form of a helium-rich white dwarf.

baric
2007-Nov-27, 07:30 PM
Red dwarfs do become white dwarfs...they just skip the red giant phase, and slowly heat up and shrink over trillions of years.

The following excerpt is from the November 1997 issue of Sky and Telescope, page 22, "Why the Smallest Stars Stay Small"; the results were derived with computer modeling:

Well, if a computer modeled what will happen in 5 trillion years, then it must be true.

Kullat Nunu
2007-Nov-27, 10:47 PM
Except that:
1. The star is not made of Carbon, Oxygen and Neon

So what? They will become helium white dwarfs.


2. The star is never hot enough to be called white (or even Orange).

That's not a problem. Oldest white dwarfs are no longer white, and all of them will be black in the end. White dwarfs are not necessary white just like red giants are typically light orange instead of "red". It's just a term.

Kullat Nunu
2007-Nov-27, 10:58 PM
...[Gregory Laughlin et al.] found that a star with one-tenth our Sun's mass takes two billion years merely to settle down to life as a hydrogen fusion reactor. And there it stays for nearly six trillion (orig. italics) years...Finally, when its unbelievably long life at last draws to a close, a 0.1-solar-mass star skips red gianthood altogether...A few hundred billion years later, it joins the stellar graveyard in the form of a helium-rich white dwarf.

I've got the book by Laughlin and Adams about the end of the universe. Pretty interesting read, especially the part about the time after the last of the original red dwarfs have gone. According to the book, there will be still a few (literally a few) red dwarfs in our Galaxy--they're results of randomly colliding massive brown dwarfs which have never become stars and therefore have retained their original hydrogen but after colliding have the required mass. That time is incredibly distant in the future, as such events are equally incredibly unlikely.

The funniest part is that although he book is not even ten years old it is already utterly outdated. And so is the above speculation I wrote.

antoniseb
2007-Nov-27, 11:10 PM
So what? They will become helium white dwarfs.
Is this a new term? I agree that they will be stellar cinders, but the fact that they are all Helium and not Carbon/Oxygen and never blew off outer layers, or were hot sources of UV and xrays make them substantially different from what we have been identifying as white dwarfs. Do you have some well accepted glossary of astronomical terms that defines White Dwarf in such a way that these cool balls of Helium could be included in the category? I've never seen one.

Jerry
2007-Nov-28, 04:12 AM
The funniest part is that although he book is not even ten years old it is already utterly outdated. And so is the above speculation I wrote.
There are a lot of us who are outdated. I poled a half dozen physicists, two considered White Dwarfs part of the main sequence, two said no, but with qualifications, and two said I don't know...but I will find out.

When I was introduced to astronomy, the white dwarf was sitting right underneath the red giant, firmly in a series that had to be memorized for the test...but this was before black holes. Reality has kinda smeared the populations of WDs around. The latest HR diagram I looked at is a lot fuzzier than it was 20 years ago.

Kullat Nunu
2007-Nov-28, 02:52 PM
Is this a new term? I agree that they will be stellar cinders, but the fact that they are all Helium and not Carbon/Oxygen and never blew off outer layers, or were hot sources of UV and xrays make them substantially different from what we have been identifying as white dwarfs. Do you have some well accepted glossary of astronomical terms that defines White Dwarf in such a way that these cool balls of Helium could be included in the category? I've never seen one.

I was talking about still hypothetical objects.1 I don't see any reason why the end result of a red dwarf could not be called a white dwarf. The formation mechanism is much less dramatic, but the end result is not vastly different. They're much cooler and much larger than "real" white dwarfs in size, but after all the more massive a white dwarf the smaller it is.

1 And yet there may actually be real extant helium white dwarfs (http://adsabs.harvard.edu/abs/2004ApJ...606L.147L)!

Kullat Nunu
2007-Nov-28, 03:01 PM
When I was introduced to astronomy, the white dwarf was sitting right underneath the red giant, firmly in a series that had to be memorized for the test...

How's that possible? Their temperature and luminosity place them well below the main sequence in the Hertzsprung-Russell diagram (http://www.atlasoftheuniverse.com/hr.html). And that's why they were classified as class VII stars in the Yerkes classification, below subdwarfs (which were classified as type VI).

antoniseb
2007-Nov-28, 05:50 PM
I don't see any reason why the end result of a red dwarf could not be called a white dwarf. ...

1 And yet there may actually be real extant helium white dwarfs (http://adsabs.harvard.edu/abs/2004ApJ...606L.147L)!

Ah, so there is no current definition that includes these objects, and the terminology is still open for discussion (we have only a few hundred billion years to get the terms right).

Concerning the 'helium white dwarfs' mentioned in your cited paper, they did not form this way, they were heavier stars that had their non-core layers stripped away, and were still hot. That is really my argument against calling these helium cinders of former red dwarfs 'white dwarfs'. They aren't hot and never were.

Kullat Nunu
2007-Nov-28, 09:27 PM
Concerning the 'helium white dwarfs' mentioned in your cited paper, they did not form this way, they were heavier stars that had their non-core layers stripped away, and were still hot. That is really my argument against calling these helium cinders of former red dwarfs 'white dwarfs'. They aren't hot and never were.

And my argument was that in the common terminology a "white dwarf" is the end product of any main sequence star that is not a neutron star or black hole (or a vampirized substellar remnant). Not all existing white dwarfs are white, yet there is no reason to call them otherwise. And if such an object consists of helium, it is then a helium white dwarf in the same manner as a white dwarf consisting of carbon and oxygen is a carbon-oxygen white dwarf. We're splitting hairs here. :whistle:

antoniseb
2007-Nov-29, 02:11 AM
...in the common terminology a "white dwarf" is the end product of any main sequence star that is not a neutron star or black hole (or a vampirized substellar remnant). .... We're splitting hairs here.

Perhaps we are splitting hairs here. I said something and you said I was wrong. Your argument is based on your definition. In the case of a type L star, it will burn until the Lithium is all gone, and then be a cold Hydrogen White Dwarf, using your terminology. I agree that it is a stellar cinder, but I have to say that like the helium cinder, it is not a white dwarf.

On the other hand, if we both agree that the terminology for these remnants is not universally agreed upon, I'm willing to let it go.

Maddad
2007-Nov-29, 11:34 AM
The term main sequence refers to a star that is still fusing hydrogen in its core. Once the star runs out of core hydrogen, then it is no longer main sequence. A star of any mass starts out as main sequence. The lower the initial mass of the star, the longer it stays on main sequence. Low mass stars, lighter than our Sun, can stay on the main sequence for a quarter trillion years. We will stay there for ten billion. Massive stars, a few dozen times the mass of our Sun, will move off main sequence in thirty million years. Truly gargantuanly massive stars, stars that the early universe typically birthed, will stay main sequence for as little as a human lifespan.

Kullat Nunu
2007-Nov-29, 08:53 PM
In the case of a type L star, it will burn until the Lithium is all gone, and then be a cold Hydrogen White Dwarf, using your terminology. I agree that it is a stellar cinder, but I have to say that like the helium cinder, it is not a white dwarf.

You mean deuterium? The type L brown dwarfs burn deuterium, not lithium. In addition, the most massive type L dwarfs are proper stars that burn hydrogen and therefore are very cool red dwarfs.

Can type L dwarfs be called stars when they burn deuterium?


Perhaps we are splitting hairs here. I said something and you said I was wrong. Your argument is based on your definition.

On the other hand, if we both agree that the terminology for these remnants is not universally agreed upon, I'm willing to let it go.

I had the impression that "helium" (or just) white dwarf in the case of red dwarf end product was an established term. If I was wrong, I'm ready to admit it although I still don't see any reason to call them otherwise.

Jerry
2007-Dec-03, 04:59 AM
How's that possible? Their temperature and luminosity place them well below the main sequence in the Hertzsprung-Russell diagram (http://www.atlasoftheuniverse.com/hr.html). And that's why they were classified as class VII stars in the Yerkes classification, below subdwarfs (which were classified as type VI).
Like I said, in my memory WDs were the end of the main sequence, and I polled physicists who learned basic astronomy in the same decade I did, and they got the answer wrong, too. I can't find a text old enough to verify this - the oldest I have kicking around is about 1985, and you may have to go back to the 40s. [I'm not that old, but my instructors were.]

Kaptain K
2007-Dec-03, 11:12 AM
Jerry,
I would be most interested in a citation for this. I have been interested in astronomy since the 50s and studied it since the 60s, and not once did I ever read, nor was I ever taught that white dwarfs were considered main sequence!

neilzero
2013-Apr-01, 01:10 AM
It appears there are several separate meanings for white dwarf and they are radically different from each other. This caused the confusion in this thread. The most common meaning of white dwarf is a K or G star after the red giant stage. It has about one solar mass and is about the size of Earth. They are very hot and thus have a blue tint, unless they have been cooling for billions of years.
The least used definition is a F star, fusing hydrogen, which appears white viewed though Earth's atmosphere instead of the yellow tint of half as massive G stars like our Sun. F white dwarf have slightly larger radius of their photosphere and about the same average density of our Sun.
The third definition of white dwarf is low mass stars, which will (none have yet) burn up almost all their hydrogen to helium causing fusion to cease, without fusing significant amounts of helium. These will cool very slowly, and shrink to about half their present radius = Sometimes called helium stars, but it is bad to call them stars if negligible fusion is occuring. These helium stars are a bit larger than Jupiter, but much lower density than the first type of white dwarf. They are also white. On second thought; they are red or orange and may not get hotter when fusion stops) They cool to Infrared in several billion years.
The 4th, 5th,and 6th definition are slightly more massive stars, that stop fusing after nitrogen, oxygen, neon or carbon, before they get to iron. These are likely very rare and also cool very slowly so they may be white until they cool to yellow. A 7th possible definition is new born brown dwarfs which are white hot, due to gravitational compaction, but they do not have enough mass to fuse hydrogen. Brown dwarfs also cool to yellow in about one billon years. Likely I have some of this wrong so please correct. More details about each type would also be helpful. Neil

Don Alexander
2013-Apr-05, 01:29 AM
Well now, that's a thread necro if I ever saw one...

Anyway, here's my take as a professional astronomer who is, though, not working in the field of stellar physics.

To my best knowledge, there are THREE types of stars that are named "dwarfs":

- All main sequence stars. These are stars which glean their energy from the fusion of hydrogen to helium in their cores. No gravitational contraction (those are still protostars following the Hayashi track), no deuterium burning, no helium or higher burning. They are called dwarfs to discern them from GIANT stars, which are all kinds that are burning helium or higher elements, and are pretty much always (exceptions would be Wolf-Rayet stars, for example) larger than when they were still on the main sequence. The most well-known main sequence dwarf moniker are red dwarfs, since these stars are, in addition, actually really small (well, for stars). But even a 100 solar mass star, as long as it is on the main sequence, is known as a blue dwarf. The Sun is a yellow dwarf. Therefore, I guess one could call AF stars white dwarfs, but I've never heard the term used like that.
- The white dwarfs that are the actual topic of this thread are the remnant cores of main sequence stars which were not massive enough to explode as supernovae (roughly < 8 M_o Zero-Age-Main-Sequence), nd are not surrounded by planetary nebula anymore. Depending on the mass of the progenitor, they can be of different composition. Usual white dwarfs are mostly carbon with some helium (that's what I'll sun turn into). More massive stars near the SN boundary yield oxygen-neon-magnesium white dwarfs. All these remnants are compact (roughly the size of the Earth), are held stable by a fully degenerate electron gas, and have no real energy source (since the electron gas prevents further contraction), therefore they slowly cool over time, reaching a theoretical endpoint in black dwarfs.
- Brown dwarfs are substellar objects which are not massive enough to fuse hydrogen via the Proton-Proton chain. in the mass span of 13 to 80 Jupiter masses (0.013 to 0.08 solar masses) they are able to fuse deuterium, but this energy source can't persist very long. A lower mass limit for brown dwarfs is not known. There's a rough definition beyond the deuterium fusion criterion that states that brown dwarfs are formed like stars, while possibly similar-massed gas giants are formed out of accretion disks of newly formed stars.

Perhaps this helps for anyone stumbling over this.