Are White Dwarfs Stars? Understanding the Classification and Characteristics

[Jamison Lahman]

Simple enough question, are white dwarfs stars?

I know white dwarfs are the remnants of former stars, but is it correct to classify them as a current star? My understanding was that nuclear fusion was a deciding factor into whether or not an object was a 'star' and white dwarfs were void of any fusion. Wikipedia, however, has an article defining White Dwarfs as 'compact stars;' https://en.wikipedia.org/wiki/Compact_star. Additionally, when a white dwarf is in a binary system, they are often referred to as binary star systems.

 

[rootone]

I guess they are classified as stars on the basis that they can only be the remnant of previously active stars.
Similar to the way that a dead person still is considered to be a person.

 

[Ken G]

Yes, the term "star" is actually pretty ambiguous, much like the definition of a "planet." Certainly, white dwarfs are studied by people who regard themselves as stellar astronomers, just as Pluto is studied by people who regard themselves as planetary astronomers. The most common definition of "star" requires it to have nuclear fusion going on, to distinguish stars from brown dwarfs and planets, but it's kind of silly to say that the Sun ceases to be a star when its fusion ceases, or that it only becomes a star when fusion initiates. Indeed, if you look up definitions of "star" on the web, you will get all kinds of nonsense, such as this Cambridge dictionary definition http://dictionary.cambridge.org/us/dictionary/english/star:
"a large ball of burning gas in space that is usually seen from earth as a point of light in the sky at night:"
That's a poor definition from the point of view of an astronomer, because stars aren't "burning" and saying what it looks like from Earth doesn't really tell us much about what it is.
Even this astronomical definition from Sky and Telescope http://www.skyandtelescope.com/astronomy-resources/what-is-a-star/ is quite poor :
"A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements."
That's essentially the definition of a main-sequence star, but there are many stars that don't have fusion going on in their cores, such as when the Sun is a red giant. Many other definitions also mention fusion in the core, so are wrong by any astrophysical standard. Other definitions say that stars are spheres of gas, so that's wrong too, because rapid rotators are quite clearly not spherical. Basically, a star is any object that shares enough characteristics with the Sun, be it past, present or future, such that it is useful to group said objects into the same category as the Sun and create an astrophysical subcommunity that shares these common interests. What exactly are the properties of the Sun that make it useful to generate this subcategory of objects is really a subjective matter that is difficult to pin down, and you will be hard pressed to find a good definition as a result!

 

[Jamison Lahman]

It's kind of silly to say that the Sun ceases to be a star when its fusion ceases, or that it only becomes a star when fusion initiates.

Why?

Even this astronomical definition from Sky and Telescope http://www.skyandtelescope.com/astronomy-resources/what-is-a-star/ is quite poor :
"A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements."
That's essentially the definition of a main-sequence star, but there are many stars that don't have fusion going on in their cores, such as when the Sun is a red giant. Many other definitions also mention fusion in the core, so are wrong by any astrophysical standard. Other definitions say that stars are spheres of gas, so that's wrong too, because rapid rotators are quite clearly not spherical. Basically, a star is any object that shares enough characteristics with the Sun, be it past, present or future, such that it is useful to group said objects into the same category as the Sun and create an astrophysical subcommunity that shares these common interests. What exactly are the properties of the Sun that make it useful to generate this subcategory of objects is really a subjective matter that is difficult to pin down, and you will be hard pressed to find a good definition as a result!

What is the astrophysical standard? I would imagine the IAU would have the most authoritative definition of a star, but I am unable to find one from them.

 

[Ken G]

Why?

Because astrophysicists still regard red giants as stars, and so will any humans alive when the Sun is one.

What is the astrophysical standard? I would imagine the IAU would have the most authoritative definition of a star, but I am unable to find one from them.

The standard is that stars are what stellar astronomers commonly refer to as stars. So although white dwarfs are not stars by some official definition, they are almost always regarded as stars by the people who actually study them. The point being, the official definition of a star is as useless in the practice of astronomy as is the official definition of a planet-- official definitions have little value and are rarely actually used.

 

[phinds]

A white dwarf is a white dwarf. Why worry beyond that?

 

[newjerseyrunner]

I always just considered them an exotic object like neutron stars, quark stars, black holes... to me a star implies an active core.

We do not consider dead people to still be people. Once their neural activity dies, they stop being a person and become a body. The corpse of Lenin isn't a person. It WAS a person, similar to how a white dwarf was a star.

 

[bahamagreen]

The Hertzsprung–Russell diagram looks like it tried to capture all stars' development beginning to end, but since it is indicating luminosity vs temperature, there are some things that don't have a place in the diagram (things without much or any luminosity or temperature but are clearly the beginnings or endings of stars).

 

[Ken G]

/

I always just considered them an exotic object like neutron stars, quark stars, black holes... to me a star implies an active core.

So red giants are not stars? And a ball that looks just like the Sun, but fusion hasn't quite yet begun, is not a star either?

There's only one purpose for classifying objects like "stars," and that is to group together unifying concepts that can be discovered in one of the objects and be useful toward understanding another. So the only important question is, are there things we learn about stars like the Sun that are useful for understanding white dwarfs, or does such an association only lead us astray? The fact that there are a body of people calling themselves "stellar astrophysicists" that study white dwarfs argues that there probably are significant enough overlaps that it is useful to regard white dwarfs as stars, regardless of the official definition. The same holds for the planetologists that study Pluto, they really don't care what the official definition is, they care that their planetary expertise is really useful in understanding Pluto.

 

[Jamison Lahman]

Because astrophysicists still regard red giants as stars, and so will any humans alive when the Sun is one.The standard is that stars are what stellar astronomers commonly refer to as stars. So although white dwarfs are not stars by some official definition, they are almost always regarded as stars by the people who actually study them. The point being, the official definition of a star is as useless in the practice of astronomy as is the official definition of a planet-- official definitions have little value and are rarely actually used.

I'm not arguing the uselessness of the nomenclature, just curious about what classifications that presently exist. Some people hate semantics, and I get that, but it fascinates me about as much as the science itself.

Correct me if I'm wrong, but red giants are still driven by fusion in their cores albeit it's not hydrogen fusion. Moreover, there is a shell outside the core which is still fusing hydrogen which contributes to its luminosity. Therefore, it is possible to have definitions which would include red giants and exclude white dwarfs.

 

[Jamison Lahman]

A white dwarf is a white dwarf. Why worry beyond that?

I don't disagree. I could ask you why have a classification system at all? Why not call everything, 'an object'? It only makes sense to group similar things together, and if the defining future of a star is the process of fusion, then it would be ignorant to call it a star, no?

 

[Jamison Lahman]

/
So red giants are not stars? And a ball that looks just like the Sun, but fusion hasn't quite yet begun, is not a star either?

This is another curious thing about stars. We have words for pre-Sun like stars, namely protostars and pre-main-sequence stars. However, the term used as soon as the star reaches the MS is zero-age main-sequence star. I imagine they're considered stars much the same as fetuses are referred to as people. Nevertheless, protostars and PMSs lack any fusion.

 

[Ken G]

Yes, we have main-sequence stars, and pre-main-sequence stars, and the only difference is that fusion is going on in the core-- otherwise, they are very similar, and are labeled as though they are types of stars. So the question really is, is there enough similarities between a pre-main-sequence star, and a main-sequence star ,such that it is beneficial to group them together under the heading "stars", or is there so much difference that collecting them that way is not beneficial? The way they are treated in the field is to group them together, as understanding one helps understand the other, so the field doesn't really care what the official definition is.

 

[_PJ_]

Definition in atronomy arent necessarily strict or entirely unambiguous. Just ask Pluto.
Generally this is result of (in case of planets, stars and nebulae etc.) histori al tradition and limited knowledge/experience.
Until any greater detail or variety or alternatives are identified, a well acknowledged and practical classification is sufficient.

Much literature and therefore ongoing education exists following tradition and includes White Dwarfs as stars.

I still refer to tomatoes and peas as vegetables, since that works and is clearly understood in context.

With repect to White Dwarfs in particular, I see no harm in considering them stars in general context of such objects as stars including possibly magnetars but likely excluding Black holes and brown dwarfs.

Should the context require a more specific categorisation to distinguish White Dwarfs then adding the epithet Compact might serve this end similarly to adding the word "Dwarf" to a planet, or politically correct "exo".

Ultimately what shojld matter is the underta ding and basis for furthering that knowledge. Is the distinction so readily impacting upon the physics and measurenments of processes /properties to warrant such division?

Classification division for division's sake can become absurd - just consider how some regard musical style/genre - until every party has their own system whereby almost individual examples exist in a unique classification of their own - ultimately defeating the point of GROUPING via classification.

 

[Jamison Lahman]

With repect to White Dwarfs in particular, I see no harm in considering them stars in general context of such objects as stars including possibly magnetars but likely excluding Black holes and brown dwarfs.

Should the context require a more specific categorisation to distinguish White Dwarfs then adding the epithet Compact might serve this end similarly to adding the word "Dwarf" to a planet, or politically correct "exo".

Ultimately what shojld matter is the underta ding and basis for furthering that knowledge. Is the distinction so readily impacting upon the physics and measurenments of processes /properties to warrant such division?

See, I have no problem with them being stars either, however, I would imagine white dwarfs would have more in common with neutron stars and black holes, and possibly even brown dwarfs than with actual stars. It is a curious idea to me to include some and exlcude others without having some formal definition.

Classification division for division's sake can become absurd - just consider how some regard musical style/genre - until every party has their own system whereby almost individual examples exist in a unique classification of their own - ultimately defeating the point of GROUPING via classification.

A fair point. I think it's a little different in this case because it is more quantifiable. Moreover, I don't mind if the sciences are somewhat pedantic. Nevertheless, I like listening to a new song and being able to ask, "is this genre liquid drum and bass or jungle?" and knowing exactly what subgenre it is though I would refer to both as drum and bass colloquially.

 

[Ken G]

To me, it doesn't matter so much if one decides to count white dwarfs as stars, or if one decides not to count them, what matters is that they realize they are manipulating categories for the purposes of serving their understanding-- not because they want to know if a white dwarf "really is a star or not." It's almost as though some people don't understand that all these categories are arbitrarily invented by us. I'm not criticizing the question, it's a good question because it points out just how arbitrary these categories are. Our classifications should always be our servants, never our masters.

 

[triclon]

Most astronomers call White Dwarfs stars. Star is a very generic term for anything hot and massive. If an astronomer wants to specify what type of star they are talking about, they will usually give it's spectral classification (https://en.wikipedia.org/wiki/Stellar_classification). For example, the Sun is a G2V star. The G2 refers to the what the Sun's spectrum looks like (roughly it's temperature) and the V means it is a "main sequence" star, burning hydrogen into helium in its core. A white dwarf would be called a "D" star for "degenerate". Different types of white dwarfs have different surface compositions. A "DA" star would be a white dwarf showing mostly hydrogen in its outermost layer.

 

[Jamison Lahman]

Most astronomers call White Dwarfs stars. Star is a very generic term for anything hot and massive. If an astronomer wants to specify what type of star they are talking about, they will usually give it's spectral classification (https://en.wikipedia.org/wiki/Stellar_classification). For example, the Sun is a G2V star. The G2 refers to the what the Sun's spectrum looks like (roughly it's temperature) and the V means it is a "main sequence" star, burning hydrogen into helium in its core. A white dwarf would be called a "D" star for "degenerate". Different types of white dwarfs have different surface compositions. A "DA" star would be a white dwarf showing mostly hydrogen in its outermost layer.

a degenerate star seems like an oxymoron. Moreover, I doubt the hydrogen of a DA star is burning since the temperature is far lower than needed.

 

[phinds]

a degenerate star seems like an oxymoron.

What, you've never heard of a star marrying its first cousin?

 

[Jamison Lahman]

What, you've never heard of a star marrying its first cousin?

Are you referring to Type 1a Supernova? hahaha

 

[Steelwolf]

Are all White Dwarfs Stars? No, but Disney did hire a bunch for decades, and the circuses regularly hired them, but there have been many non-white dwarfs end up being stars as well...

I know this is 'off topic', but with the way the question was worded I could not help it.

 

[stefan r]

We have never been in contact with any star except the sun. Even with the sun it is obviously observation from a distance except for the solar wind, flairs, and nuetrinos.

What we do have is many photons. Photons are collected in telescopes and then the wavelengths are measured. After lots of measurement from various angles and after factoring in some missing lines and extra lines astronomers(and cosmologists) have come to the conclusion that some type O light sources are very different from other type O light sources. We have a nice model that shows white dwarfs as very different things than luminous blue variables. However, the light from a particular white dwarf can have more in common with the light from a LBV than the light from another white dwarf. Figuring out which one you are looking at requires some detail.

The nuclear fusion standard fits fine with white dwarfs. If hydrogen builds up on a white dwarf fusion happens. Sometimes helium too. The fusion standard is usually used to distinguish red dwarf star from brown dwarf star. The presence of deuterium lithium and He₃ give nice sub categories. There has to be some cut off between "small star" and "big planet".

Just to confuse things, if you want to you can think of the sun as two objects: a core and a convective shell. Stellar cores eventually burn their hydrogen and become compact cores then become white dwarfs. Convective shells eventually inflate as red giants and blow away as stellar winds and planetary nebula. This definition of objects does not work very well because the compact cores accrete and burn material from shells. Also there is a radiative zone which is not burning hydrogen and also not convecting. The red dwarfs would still be one object. Labeling everything inside the corona as one thing, a star, is more convenient. The measured part is the photo sphere. In white dwarf stars the photo sphere is very close to the core.

Nuetron stars are also called stars. We have not collected light from a black hole. We do have light from stars that are falling in. Galaxies, Quasars, and clusters are objects that contain large numbers of stars. Even the name "Quasar" is an abbreviation of quasi stellar object. The quasar is (or was when named) a point source of light same as stars are point sources of light.

Technically a spectroscopic binary is one thing. Our model suggests that the spectroscopic binary would be 2 things if we could get an image from a better angle or closer to the source or if we built a better telescope. It is one source of light with a spectrum that has characteristics of binary stars.

 

[stefan r]

a degenerate star seems like an oxymoron. Moreover, I doubt the hydrogen of a DA star is burning since the temperature is far lower than needed.

https://en.wikipedia.org/wiki/RS_Ophiuchi Spectral type M2. Hydrogen lights up from the bottom.

Oxydwarf:
https://www.sciencenews.org/article/odd-white-dwarf-found-mostly-oxygen-atmosphere

 

[Drakkith]

In the end, does it matter if you call a white dwarf a star or not? It changes nothing except categorization. It's like moving a book about space-faring elves around in a library between the Fantasy and Science Fiction sections. It changes nothing important in my opinion. As Ken G said early in the thread:

The point being, the official definition of a star is as useless in the practice of astronomy as is the official definition of a planet-- official definitions have little value and are rarely actually used.

 

[rootone]

"This is not a parrot, it's an ex-parrot, it is deceased"

 

[Ken G]

So it's settled-- a white dwarf is a star pining for the fjords.

 

[Jamison Lahman]

In the end, does it matter if you call a white dwarf a star or not? It changes nothing except categorization. It's like moving a book about space-faring elves around in a library between the Fantasy and Science Fiction sections. It changes nothing important in my opinion. As Ken G said early in the thread:

My inquiry was never about the importance of the definition of a star, it was about the most authoritative definition available.

 

[Ken G]

My inquiry was never about the importance of the definition of a star, it was about the most authoritative definition available.

But that's just it-- it depends on what you regard as "authoritative"-- what they tell elementary school students by looking it up in an encyclopedia, or what professional stellar astronomers mean when they say they study "stars"? The former generally says the object must be undergoing fusion, but this is largely stoked by the misconception that only when there is fusion is the object bright, which is utterly untrue. So your question really comes down to, which "authority" is important to you, the authority of an encyclopedia that could be used to get the answers right on an exam in some class that is only interested in superficial understanding, or the authority of someone who actually knows what they are talking about? Professional astronomers care more about what they can learn about one object by comparing it to another, and in that case, whether or not there is fusion might be a very secondary consideration for what they regard as a "star."

 

[Ken G]

I should probably also mention that a key reason why fusion gets mentioned in "authoritative" definitions of a star is to distinguish stars from gas giant planets, with brown dwarfs somewhere in the middle (and if we want to count brown dwarfs as stars, we'll include deuterium fusion in the definition of a star, and if we don't, we won't. After all, is a brown dwarf more like a star or a gas giant?). But although that's a perfectly valid distinction to make, because the Sun is quite a bit different from Jupiter, for example, it also has some unwanted connotations, like that if we hold that fusion can "initiate" (which is already an idealization), then the definition holds the Sun was not a star just before "ignition", even though its properties were very similar before and after the onset of fusion. Also, high-mass stars have the entire zone where fusion is occurring being highly convective, so the hydrogen runs out everywhere it is fusing, and such stars must then have a brief interval when they are not fusing anything, followed by a period of shell fusion, but it would be pretty silly to say it ceases to be a star and then starts being a star again, all while the object as a whole is going through very gradual changes in structure. This is why the professionals don't bother with the "authoritative" definition, it's pretty useless to them except as a way to distinguish stars and planets. At the end of the day, what is really the most authoritative definition is "what experts mean when they use the word," but it's hard to put that in an encyclopedia! Still, if we take that as the most authoritative definition, I would say that white dwarfs are stars, because you will often hear an expert refer to a "white dwarf star" without apology to the encylopedias.

 

[Jamison Lahman]

But that's just it-- it depends on what you regard as "authoritative"-- what they tell elementary school students by looking it up in an encyclopedia, or what professional stellar astronomers mean when they say they study "stars"? The former generally says the object must be undergoing fusion, but this is largely stoked by the misconception that only when there is fusion is the object bright, which is utterly untrue. So your question really comes down to, which "authority" is important to you, the authority of an encyclopedia that could be used to get the answers right on an exam in some class that is only interested in superficial understanding, or the authority of someone who actually knows what they are talking about? Professional astronomers care more about what they can learn about one object by comparing it to another, and in that case, whether or not there is fusion might be a very secondary consideration for what they regard as a "star."

I would regard the IAU as the most authoritative but I could not find a definition from them.

 

[Ken G]

Puzzling, isn't it? IAU stands for "international astronomical union", which translates literally to "group involving many countries that is concerned with the laws pertaining to stars." You'd think a group like that would have a clear definition of what a star is, wouldn't you? But this is the whole problem-- cut-and-dried definitions are not actually very conducive to research efforts. Instead, what is effective is having a somewhat flexible working meaning of these terms. That's what flies everywhere except in the tests they give in elementary school classes, so beware of "authoritative definitions."

 

[Jamison Lahman]

Puzzling, isn't it?

Yes, that is why I asked.

 

[Jamison Lahman]

http://iopscience.iop.org/article/10.1086/307414/pdf

"For objects this cool, the presence of lithium proves that they are substellar."

"Some of these dwarfs certainly are substellar but such a designation cannot be tied uniquely to any particular spectroscopic trait."

"Because some of these objects are substellar and thus not truly stars, the entire collection should be referred to as 'L dwarfs,' not 'L stars.'"

https://arxiv.org/pdf/astro-ph/0608359.pdf

"3. Definitions:
(1) A “primary” body is a star or substar formed by core accretion from an
interstellar cloud, not by secondary accretion from a disk.
(2). A “substar” is a body with less than 80 Jupiter masses, the lower limit for
stellar hydrogen fusion."

 

[Ken G]

Yes, so none of that tells you if a white dwarf is a star. But as said above, the answer to that is, no if you look in a dictionary or encyclopedia, yes if you go to an astronomy meeting.