As far as I think during the red gaint phase the outer cornea expands & in the case of sun will just vanish in the surrounding without any explosion as in the case of the neutron star formation called super nova explosion.That's the answer to ur 1st part.During the white dwarf the sun would already lost its cornea so there is no question of cornea in white dwarf case<br /><br />The answer to ur part relating to the magnetic field,I will reply it later as I have no appropriate answer at the moment.
abhinavkumar_iitr05 - Thank you for the partial answer - I am interested in further detail.<br /><br /> I assume by cornea you mean corona.<br /><br />Thanks to the cornea, I see what you mean.<br /><br />The corona will vanish? You mean due to be overtaken by the expanding surface of red giant phase?<br /><br />Please note that the corona is heated primarily by the sun's complex magnetic fields and magnetic dynamoe deep within the sun.<br /><br />Yes, I know that white dwarf phase will have less heat due to fusion - however other causes for radiation will exist and some causes for magnetic fields will continue - albeit internal ions in motion will be greatly reduced.<br /><br />Other causes would be tidal effects of planets and more importantly internal rotational differences.
I am not an expert in this subject, so please take what I say with a grain of salt:<br /><br />As far as I know, there is a class of white dwarfs known as "magnetic white dwarfs" that have magnetic fields on the order of a million gauss (the solar magnetic field strength near the surface is about a single gauss). But these are quite rare, it seems that for most white dwarfs we only have upper limits of ~10000 gauss on the field strengths. However, if you were to shut off the Sun's dynamo, it would take on the order of 10 billion years for the magnetic field to diffuse out, so it's reasonable to think that the magnetic field may persist in the white dwarf sun even though convection has ceased. If it does persist through the collapse to the white dwarf stage you can estimate that the field strength should be about 10000 gauss (for a plasma the magnetic flux is conserved, so the field strength times the surface area of the object is basically constant). However, it is possible that the planetary nebula stage carries away the field, I don't know that we really know. (I don't know anything about whether white dwarfs have "internal motion" of some sort or what its effects might be).<br /><br />As for red giants, there are models that do suggest that dynamo action can generate magnetic fields in giants. These stars do have surface convection, so I don't know that it's surprising that they would have a dynamo. They also do have a wind, but it's not like the super-hot, super-tenuous solar corona. Instead the wind seems to be slower but involve more mass. Stars may end up losing as much as 20% of their mass while they are red giants, and then even more through the thermal pulses as an asymptotic giant.<br /><br />I think the late stages of a star's life (RGB and AGB phases) is really a fascinating subject, and there's a pretty accessible paper dealing with the winds from these stars at http://xxx.lanl.gov/pdf/astro- <div class="Discussion_UserSignature"> </div>
doubletruncation - Thank you. I will respond better later - don't have much time now.<br /><br />Are you referring to magnetars?<br /><br />I don't think we know if our star is typical for main sequence star magnetic field strength and properties and also corona temperature, properties and causes.<br /><br />It seems many have assumed most main sequence stars are similar - but as you note there are some very obviously stronger magnetic fields in some stars before they enter white dwarf stage.<br /><br />I will respond better later after studying your link.<br /><br />Thank you again.
<i>Are you referring to magnetars?</i><br /><br />Actually, magnetars are a different beast - neutron stars with magnetic fields on the order of 10^15 gauss (a billion times stronger than magnetic white dwarfs). Remember that a white dwarf is ~600 times larger than a neutron star (in radius), so by our rule of thumb that the magnetic field scales as 1/radius^2 you'd expect a neutron star to have a magnetic field that is ~10^5 times stronger than a white dwarf. Typically neutron stars have magnetic fields of about 10^12 gauss, so they're actually stronger than our rule of thumb would predict. The magnetars though have mind-boggling large magnetic fields, as far as I know those are the most magnetic objects in the universe (that we know of).<br /><br /><i>I don't think we know if our star is typical for main sequence star magnetic field strength and properties and also corona temperature, properties and causes.</i><br /><br />I don't really know the answer to this (for one thing it seems that measuring magnetic field strengths on sun-like stars is very difficult since the zeeman signal is small). I suspect that most positive detections of magnetic fields are for stars with fairly strong fields. <div class="Discussion_UserSignature"> </div>