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Non-Spherical Stars
An anonymous reader writes "Now that the large interferometers are coming on line, the stars are no longer dots. Achernar (Alpha Eridani), is a huge ellipsoid whose polar radius (due to fast spinning) is 50% smaller than the equatorial one!"
I blame large plates. And a lack of exercise.
=Brian
There is nothing so good that someone, somewhere, will not hate it.
More details at the press release:
1 4-03.html
http://www.eso.org/outreach/press-rel/pr-2003/pr-
Including more technical drawings.
-molo
Using your sig line to advertise for friends is lame.
50% smaller? Wow, this must be spinning incredibly fast. With so must mass being displaced from where it would be in a sphere, it must effect the pressure inside the star. As such, i wonder how much this effects the fusion within the star. Since fusion is driven by the compressional forces of the suns mass, the effective reduced mass must reduce the energy output of this star. RIght?
Perhps i don't really kow what i am talking about.
Hey God!
I found your rubgy ball!
Nicole Kidman or Gwyneth Paltrow are the flattest stars that can be seen with the naked eye or possibly binoculars.
You know when you take your index finger and thumb and look at something pretty far away. Then you squish them till they touch.
I think someone was doing that at the end of the telescope.
Norris/Palin 2012
Fact: We deserve leaders who can kick your ass and field dress your carcass.
If you extend this idea to very fast spinning black holes, you end up with the idea of a spinning disk which "radius" in one direction is
maybe only a few percent of the radius in the other directions.
The oblateness of Altair was measured using the Palomar Testbed Interferometer (PTI) in 1999-2000.
I really don't think the fact that the star isn't a perfect sphere is surprising. The fact that we can measure it is a breakthrough. If we look at the sun, we can see that isn't a perfect sphere. It's not very much an ellipsoid either, but you could imagine a star (much younger) that spins very fast and is more like an ellipsoid. Even Jupiter (and also the earth!) are somewhat flattened.
This site describes the telescopes that comprise the interferometer used to make the observations:
http://www.eso.org/projects/vlti/
Quote:
The Very Large Telescope Interferometer (VLTI) consists in the coherent combination of the four VLT Unit Telescopes and of several moveable 1.8m Auxiliary Telescopes. Once fully operational, the VLTI will provide both a high sensitivity as well as milli-arcsec angular resolution provided by baselines of up to 200m length.
The earth is not a sphere either. Any celestial body with a reasonable angular velocity will be slightly elliptical.
Due to its daily rotation, the solid Earth is slightly flattened...
Solid Earth? Only the surface (and part of the core) is solid, right? The rest is [Dr. Evil] liquid hot magma.
The observed flattening cannot be reproduced by the "Roche-model" that implies solid-body rotation and mass concentration at the center of the star.
I thought stars were pretty much all plasma, which is to say, a fluid. Why, therefore, should stars obey any "solid-body" rule at all?
"A great democracy must be progressive or it will soon cease to be a great democracy." --Theodore Roosevelt
Actually, I think they mean "solid-body" as "cohesive object" in this case.
While I'm getting technical, Plasma can't be considered a fluid either, as it's not a liquid, it's a different state of matter altogether.
Any physics buffs know what the largest theoretical ratio would be between a star's polar radius and equatorial radius, for the stuff that stars are made out of? Is the ratio for this star anywhere close to that?
I'd imagine one can only attain this through centrifugal force, which necessarily puts structural stress on the star, and past a certain amount of structural stress stars should disintegrate.
"A great democracy must be progressive or it will soon cease to be a great democracy." --Theodore Roosevelt
I believe a star has zero tensile strength* (it's just a fluid), so once you're spinning too fast for gravity to hold you together, it's bye-bye time.
The better question is this: how did that star form? If it was spinning too fast to hold together, how did it accrete matter with that much angular momentum at all?
* Barring magnetic fields, mind you. But you'd need an ass-kicking field to hold a star together very long, I would think.
The indicated ratio between the equatorial and polar radii of Achernar constitutes an unprecedented challenge for theoretical astrophysics, in particular concerning mass loss from the surface enhanced by the rapid rotation (the centrifugal effect) and also the distribution of internal angular momentum (the rotation velocity at different depths).
The astronomers conclude that Achernar must either rotate faster (and hence, closer to the "critical" (break-up) velocity of about 300 km/sec) than what the spectral observations show (about 225 km/sec from the widening of the spectral lines) or it must violate the rigid-body rotation.
From this I think we can conclude that the star is very close to the theoretical limit for polar/eq radius for stable stars, but that this theoretical model might be inaccurate.
Your question about how the star formed at all is interesting. IANAP, but it could be that when the star formed it wasn't spinning fast enough to break apart, but as it loses mass due to fusion, it becomes more elongated until the weakened gravity isn't able to hold it together any longer.
On the other hand, you could be right as well. It's been years since my last Physics class :)
Plasma is a collection of charged particles that have some of the properties of a gas, but is different in that it's a good conductor of electricity and can be affected by magnetic fields. It won't try to fill the space it's in, and you can't pour it.
Actually, it will try to fill the space it's in, just like any other gas. It can be impeded from doing so by magnetic fields (including fields generated by its own motion), but this is not a permanent state of affairs.
I just have to remind everyone: the Earth is mostly solid.
The crust and upper mantle are solid, 99% of the partially melted asthenospheric mantle is solid rock, the mantle beneath that is solid rock, and the inner core is solid. That really just leaves the outer core as liquid.
It's easy to go on and on with observations demonstrating this is true, to pull out stacks of earthquake wave data, etc., but I think the simplest and most convincing evidence is in the shape of the Earth itself. It has been realized since the 1800s that for the Earth to retain its nearly spherical shape as it spins, the average strength of the Earth must be greater than that of a similar sized ball of glass or iron.
So again, with the exception of the outer core, the Earth is made of solid rock -- all apologies to Dr. Evil. The next time the structure of the Earth comes up on Slashdot, as it does from time-to-time, don't forget: the mantle is also made of solid rock, not liquid magma.
Thank you for your time and attention to this sniveling detail.
- Anonymous Coward
"Although plasma includes electrons and ions and conducts electricity, it is macroscopically neutral: in measurable quantities, the number of electrons and ions are equal. The charged particles are affected by electric and magnetic fields applied to the plasma, and the motions of the particles in the plasma generate fields and electric currents from within. This complex set of interactions makes plasma a unique, fascinating, and complex state of matter."
the computer is online
i am not at it
what a waste of ressources
This exchange is about on par with "How is a liquid not a fluid?" "Because it's a liquid."
"Fluid" is not a state of matter, no one's claiming it's a state of matter, saying plasma can't be a fluid because plasma is the 4th state of matter is a category error. Liquid is the second state of matter. Gas is the third state of matter. Both are fluids.
A fluid is any substance which undergoes continuous deformation when subjected to a shear stress. The problem we're probably having is that the obvious sources for the shear stresses in the couse of, say, water being poured from a cup (normal force of the side of the cup vs gravity) are paralleled for the case of plasma by electromagnetic feilds. It just don't grok intuitively but, plasma behaves like a fluid... ergo, it is a fluid.
Simple Machines in Higher Dimensions
This complex set of interactions makes plasma a unique, fascinating, and complex state of matter.
Gases and liquids are different states of matter, both fluids.The story is about natives on the planet, but the physics alone is worth the read. It's quite a strange place.
There is nothing so silly as other peoples traditions, and nothing so sacred as our own.
A star like the Sun will only convert about 0.01% of it's mass into energy over its entire lifetime. (According to my quick, back of the envelope calculation.) Which means you still have to form very close to the breakup spin rate. Which is still quite difficult to pull off, saith the dynamicists. (The same problem comes up in fusion-formation models of moons.)
The equatorial radius is 50% larger than the polar radius. This does not make the polar radius 50% smaller than the equatorial radius.
In the more recent surveys of bright stars in a cluster, they've seen that faster rotating stars (seen indirectly by the rotational broadening of spectral lines of the star) of the same spectral type have a wider scatter of observed brightnesses. The explanation for this is that:
(i) Faster rotating stars are brighter at their poles than their equators (because of centripetal force slightly expanding the distance of the equator from the core of the star), and:
(ii) The spin axes of stars are randomly oriented with respect to telescopes on Earth, so:
(iii) For a large sample of fast rotating stars, you sample all the brightnesses from the equator to the poles, hence a large scatter in measured brightness. You can assume that all stars are effectively at the same distance if they are in a distant cluster.
Hope that's reasonably clear,
Dr Fish
thanks for not flaming. i reread the whole thread and recognised my error. (liquid fluid)
;)
that's what you get for posting before the coffee break...
the computer is online
i am not at it
what a waste of ressources
If it's a gas plasma it's quite likely that it doesn't conform to rigid-body models. We believe that the interior and exterior of Jupiter rotate at somewhat different angular velocity, which creates the red spot. Similar spots have been created in (practical) models in the laboratory where fluids are spun at differential rates.
There's no reason to believe that the outside of the lenticular star spins at the same rate as the pole. It would be interesting to get a good enough image to determine if it has a "spot
Any more insite on that, or is it complete wash?
There is nothing so silly as other peoples traditions, and nothing so sacred as our own.
Thank you!
"A great democracy must be progressive or it will soon cease to be a great democracy." --Theodore Roosevelt
"A great democracy must be progressive or it will soon cease to be a great democracy." --Theodore Roosevelt
But most people aren't physicists or classical scholars. So they use "fluid" as a synonym for "liquid". That's not a sign of rampant stupidity, it's just the way imprecise usages creep into any language.
I wonder how this will affect it's distruction, the decreased pressure would decrease the rate of fusion while the spinning would make it easier to fly apart, and how would it die off? Since, it wouldn't have much growth before the centrifugal forces rip it apart it should be hotter and more compact.
Uh, it isn't a different rotation rate in the interior that creates the Great Red Spot. It's solar and internal heat, causing convection, and the spin, causing belts and zones. Admittedly, the GRS isn't totally understood, but a differentially rotating interior doesn't seem to be required. (As for whether the interior does rotate differentially, it's kind of hard to say. Since you have the belts and zones moving at all different speeds, it turns out to be difficult to define a rotation period for the cloud level. At least 2 attempts were made to define a rotation period based on the clouds, but the rotation period cited is usually the interior rotation perdio. The interior is easy to clock, thanks to the magnetic field and the radio emissions due to it.)
That said, of course you don't expect a star to be rotating as a solid body. As you suggest, there's no reason it has to. But you can make an easier, and far better, case if you just point to the Sun. The Sun's rotation period varies from equator to poles (the equator is fastest, which doesn't help a star stay together any). The interior also rotates differentially, as determined by some of the GONG results.