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Thread: Do Sirius A and B have opposite spin ?

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    Default Do Sirius A and B have opposite spin ?



    Hi to all,

    I cannot find the information about Sirius A and B spin (on themselves).

    Can someone help?

    Do Sirius A and B have opposite spin ?

    regards,

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    Default Re: Do Sirius A and B have opposite spin ?

    now you have me curious as well... there's nothing about rotation in any of the texts I have. Oh well, hit some more references tomorrow, unless someone has an answer sooner?
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    Default Re: Do Sirius A and B have opposite spin ?

    Thanks for your comment. I goooogled it, found nothing. Wikipedia doesn't mention anything. I'll keep searching and if I find anything, I'll let you know.

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    Default Re: Do Sirius A and B have opposite spin ?

    I would be interested to know what means there might be for determining the direction of spin of a star. I would be surprised if we would be able to detect that at all - and for most stars, even whether we can measure any spin at all.
    The Sun (for example) has a differential rotation period, varying from about 24.5 days at the equator to 38 days at the pole, but even at the equator, the speed is only about 2km/s which in itself would be measurable for the sun by taking a spectrum from a small part of the solar limb, but in terms of the effect seen for a far distant star; it would only be detectable as a contribution to the spectral lines being slightly broader than if the star was non rotating, and there are other reasons why stars would have broad spectral lines (including temperature, oscillations in the star's atmosphere, and (I believe) magnetic effects can broaden spectral lines).

    For a White Dwarf star like Sirius B, I'd expect the rotation rate to be faster (though I'm not sure why I think this), but they also have high temperatures, so there would be more thermal broadening.
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    Default Re: Do Sirius A and B have opposite spin ?

    For distant stars measurement of the rotational broadening of spectral lines gives vsin(i) where v is the rotation speed and i is the angle of inclination of the axis of rotation to the line of sight. Since you measure vsin(i) and don't know i or v separately you don't know much.
    Last edited by not_Fritz_Argelander; 06-10-2019 at 12:55 PM.
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    Default Re: Do Sirius A and B have opposite spin ?

    Quote Originally Posted by Gfamily View Post
    I would be interested to know what means there might be for determining the direction of spin of a star. I would be surprised if we would be able to detect that at all - and for most stars, even whether we can measure any spin at all.
    The only hope of doing that is for stars close enough to have their disks resolved by interferometry for instance. Achernar is one example. There aren't many others! If any! (See link at end.)

    The Sun (for example) has a differential rotation period, varying from about 24.5 days at the equator to 38 days at the pole, but even at the equator, the speed is only about 2km/s which in itself would be measurable for the sun by taking a spectrum from a small part of the solar limb, but in terms of the effect seen for a far distant star; it would only be detectable as a contribution to the spectral lines being slightly broader than if the star was non rotating, and there are other reasons why stars would have broad spectral lines (including temperature, oscillations in the star's atmosphere, and (I believe) magnetic effects can broaden spectral lines).
    The Sun rotates slowly for a star of its spectral type. (Again see link at end.) Magnetic broadening can be distinguished from rotational broadening by the shape in favorable circumstances (high resolution spectra and strong fields). There are some stars that show Zeeman splitting like Przybylski's star. A more serious confounding effect is from micro turbulence in the stellar atmosphere.

    For a White Dwarf star like Sirius B, I'd expect the rotation rate to be faster (though I'm not sure why I think this), but they also have high temperatures, so there would be more thermal broadening.
    You might be thinking this because of conservation of angular momentum? But there are torques on the star due to mass loss and magnetic field interactions so angular momentum isn't straightforwardly conserved. Likely it's true to some extent depending on the details.

    The promised link: https://en.wikipedia.org/wiki/Stellar_rotation

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    Default Re: Do Sirius A and B have opposite spin ?

    Thanks to all of you for your explanations, it all make sense now why I cannot find the information about Sirius spin anywhere.

    I'll stop searching...


    Originally Posted by Gfamily : I would be interested to know what means there might be for determining the direction of spin of a star.
    I'm interested to see if the stars, in a system with only 2 stars, have always opposite spin or if their spin is totally random. I guess now that I'll never be able to know that answer...

    I also tried to find the same information about the double pulsar PSR J0737 -3039 but couldn't find it. But since these are two pulsars, I guess that their spin information is available somewhere. Does someone know about the spins of these 2 pulsars by any chance?

    regards,

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    Default Re: Do Sirius A and B have opposite spin ?

    Quote Originally Posted by fractalrocks View Post
    I'm interested to see if the stars, in a system with only 2 stars, have always opposite spin or if their spin is totally random. I guess now that I'll never be able to know that answer...

    I also tried to find the same information about the double pulsar PSR J0737 -3039 but couldn't find it. But since these are two pulsars, I guess that their spin information is available somewhere. Does someone know about the spins of these 2 pulsars by any chance?

    regards,
    Knowing the spin orientation of pulsars is just as difficult as knowing the spin orientation of normal stars.

    Expecting that the spins of normal stars or pulsars would be opposite is IMO unlikely to be true. More likely since they form from fragmentation of a protostellar disk angular momentum would have spins oriented in roughly the same way. There is no observational evidence on the matter though.

    You might object that planets have oddly oriented spins in the Solar System. But for stellar systems there would not be the gigantic collisions that cause spin flips except much more rarely than with planets. That's just an opinion and based on looking at star formation simulations. It's not as messy as planetary formation where comparable bodies go bump.

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    Default Re: Do Sirius A and B have opposite spin ?

    Quote Originally Posted by not_Fritz_Argelander View Post
    Knowing the spin orientation of pulsars is just as difficult as knowing the spin orientation of normal stars.
    Thanks for your explanations not_Fritz_Argelander, it helps me understand the difficulty behind the detection of spins.

    Do you think it would be possible for two observatories that are few thousands of kilometers apart to detect the spin of these 2 pulsars (what I mean is that one could perhaps detect the pulse a fraction of a second before the other one does) ?


    regards,

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    Default Re: Do Sirius A and B have opposite spin ?

    Quote Originally Posted by fractalrocks View Post
    Thanks for your explanations not_Fritz_Argelander, it helps me understand the difficulty behind the detection of spins.

    Do you think it would be possible for two observatories that are few thousands of kilometers apart to detect the spin of these 2 pulsars (what I mean is that one could perhaps detect the pulse a fraction of a second before the other one does) ?
    ,
    Tough. The difference in time of arrival would be dominated by the separation of the observatories. Remember that the pulsar radius is only ~10 km.
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