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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
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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.
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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.
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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?
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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.
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.
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.
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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
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.