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I Are White Dwarfs Stars?

  1. Mar 22, 2017 #1
    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.
     
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  3. Mar 22, 2017 #2
    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.
     
  4. Mar 24, 2017 #3

    Ken G

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    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!
     
  5. Mar 24, 2017 #4
    Why?

    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.
     
  6. Mar 24, 2017 #5

    Ken G

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    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.
     
  7. Mar 24, 2017 #6

    phinds

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    A white dwarf is a white dwarf. Why worry beyond that?
     
  8. Mar 24, 2017 #7
    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.
     
  9. Mar 24, 2017 #8
    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).
     
  10. Mar 24, 2017 #9

    Ken G

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    /
    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.
     
  11. Mar 24, 2017 #10
    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.
     
  12. Mar 24, 2017 #11
    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?
     
  13. Mar 24, 2017 #12
    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.
     
  14. Mar 24, 2017 #13

    Ken G

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    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.
     
  15. Mar 25, 2017 #14
    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.
     
  16. Mar 26, 2017 #15
    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.
    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.
     
  17. Mar 26, 2017 #16

    Ken G

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    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.
     
  18. Apr 15, 2017 #17
    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.
     
  19. Apr 15, 2017 #18
    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.
     
  20. Apr 16, 2017 #19

    phinds

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    What, you've never heard of a star marrying its first cousin?
     
  21. Apr 16, 2017 #20
    Are you referring to Type 1a Supernova? hahaha
     
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