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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!"
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.
Quick and dirty calculations:
.25 km/s
Earth spins about
Said star spins 220-300 km/s
Obviously the star isn't a body of a uniform density. Possibly not conforming to known ideas regarding rotating solid masses in general.
I wish I had a better physics comprehension in times like this...
The oblateness of Altair was measured using the Palomar Testbed Interferometer (PTI) in 1999-2000.
The arguement fails for the same reason black holes are called black. Once the anything is inside the event horizon, it's impossible for it to stop falling inwards, no matter how fast it is moving. So no matter how fast it spins, it will collapse to a point (in theory).
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.
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.
Who would've thought we could start to resolve the diameters of other stars within our lifetimes??
Considering that Albert Michelson (yes, that Michelson) made the first measurement of another star (not the Sun) in 1920 (about a third or the way down the page for that detail), the question is probably more like how old are you? My parents weren't born yet when that happened.
"Bite me, it's fun!" - Crowe T. Robot
Er, I don't beleive so, no. You're restricting yourself to the Schwartzschild solution, there. Schwartschild assumed that the black hole wasn't spinning and was uncharged. So of course it's spherically symmetrical, there's nothing to break the symmetry.
Real black holes are likely to be spinning. And then they aren't spherical, as I recall. Also, their horizons start to seperate. Things get a *leeetle* bit weird from there on out.
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.
Simple Machines in Higher Dimensions
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
The modern interferometers, besides having very long observing baselines, also make such a large number of baseline observations that they can actually do an inverse transform and get an image.
If you are interested, some nice info is found here, and the best collection of stellar interferometry links is found here.