Science: Sun Slimmer Than Expected

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Science: Sun Slimmer Than Expected

Post by Rothkko » Fri Aug 17, 2012 12:32 am

Sun Slimmer Than Expected
Science NOW | Sid Perkins | 2012 Aug 16

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Re: Science: Sun Slimmer Than Expected

Post by bystander » Fri Aug 17, 2012 10:06 pm

The Sun's Almost Perfectly Round Shape Baffles Scientists
Institute for Astronomy | Univ of Hawai'i | 2012 Aug 16
The sun is nearly the roundest object ever measured. If scaled to the size of a beach ball, it would be so round that the difference between the widest and narrow diameters would be much less than the width of a human hair.

The sun rotates every 28 days, and because it doesn't have a solid surface, it should be slightly flattened. This tiny flattening has been studied with many instruments for almost 50 years to learn about the sun's rotation, especially the rotation below its surface, which we can't see directly.

Now Jeff Kuhn and Isabelle Scholl (Institute for Astronomy, University of Hawaii at Manoa), Rock Bush (Stanford University), and Marcelo Emilio (Universidade Estadual de Ponta Grossa, Brazil) have used the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory satellite to obtain what they believe is the definitive--and baffling--answer.

Because there is no atmosphere in space to distort the solar image, they were able to use HMI's exquisite image sensitivity to measure the solar shape with unprecedented accuracy. The results indicate that if the Sun were shrunk to a ball one meter in diameter, its equatorial diameter would be only 17 millionths of a meter larger than the diameter through its North-South pole, which is its rotation axis.

They also found that the solar flattening is remarkably constant over time and too small to agree with that predicted from its surface rotation. This suggests that other subsurface forces, like solar magnetism or turbulence, may be a more powerful influence than expected.

Kuhn, the team leader and first author of an article published today in Science Express, said, "For years we've believed our fluctuating measurements were telling us that the sun varies, but these new results say something different. While just about everything else in the sun changes along with its 11-year sunspot cycle, the shape doesn't."

The Precise Solar Shape and Its Variability - J. R. Kuhn et al
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Re: Science: Sun Slimmer Than Expected

Post by neufer » Sat Aug 18, 2012 12:35 pm

bystander wrote:The Sun's Almost Perfectly Round Shape Baffles Scientists
Institute for Astronomy | Univ of Hawai'i | 2012 Aug 16
The sun is nearly the roundest object ever measured. If scaled to the size of a beach ball, it would be so round that the difference between the widest and narrow diameters would be much less than the width of a human hair.

Because there is no atmosphere in space to distort the solar image, they were able to use HMI's exquisite image sensitivity to measure the solar shape with unprecedented accuracy. The results indicate that if the Sun were shrunk to a ball one meter in diameter, its equatorial diameter would be only 17 millionths of a meter larger than the diameter through its North-South pole, which is its rotation axis.
  • 17 ppm seems rather large to me:
http://math.ucr.edu/home/baez/physics/Relativity/GR/mercury_orbit.html wrote:
Mercury's Orbital Precession, General Relativity, and the Solar Bulge
Updated by Steve Carlip 1996.
Original by Michael Weiss.

<<If the Sun were not a perfect sphere, but had an equatorial bulge (i.e., was oblate), that would cause Mercury's orbit to precess. Well, surprise: the Sun isn't a perfect sphere; it is oblate! So how good is the agreement between Mercury's orbital precession and GR?

Clifford Will devotes a chapter to this in his 1986 book "Was Einstein Right? Putting General Relativity to the Test". Dicke and Goldenberg claimed to have detected a much larger bulge than solar models predicted, large enough to destroy the agreement between GR and Mercury's orbit, but not large enough to permit a newtonian explanation. Specifically, the data looks like this:

Code: Select all

    Mercury's perihelion precession:        574 arcseconds/century

    Newtonian perturbations from
        other planets:                      531 arcseconds/century

    GR correction:                           43 arcseconds/century

    Newtonian correction from Dicke bulge:    3 arcseconds/century
So no hope for Newton, but a problem for GR if the Dicke-Goldenberg value for solar oblateness held up. The Brans-Dicke scalar-tensor theory of gravity could handle the discrepancy, via its adjustable parameter.

Subsequent observations by other groups detected much smaller solar bulges. However, the other measurements disagreed with each other. Here's the data, again from Will's book:

Code: Select all

   Amount predicted by conventional   0.2 km,  0.1 ppm
   solar models

   Dicke-Goldenberg (1966)             52 km,  40 ppm

   Hill (1973)                          2 km,   1 ppm

   Hill (1982)                         10 km,   7 ppm

   Dicke (1985)                        24 km,  12 ppm
These aren't all direct measurements, and I haven't given any of the error bars, but here's Will's bottom line: "No one has been able to resolve or understand the discrepancies between all these values for solar oblateness, other than to say that the observations are difficult to make and subject to many errors." (In case you're wondering about the Brans-Dicke theory, that ran into a bunch of other problems during the 1970s and 80s.)

However, At least two important new developments have occurred since Will's book was written.
  • 1. A new and accurate direct measurement of Solar oblateness has been performed, using the balloon-borne Solar Disk Sextant. The result is a value that agrees with general relativity (a quadrupole moment on the order of 2 x 10−7). See Lydon and Sofia, Phys. Rev. Lett. 76, 177 (1996).

    2. A new, slightly indirect but quite powerful, method has been developed to measure Solar quadrupole moment, by using helio-seismography to determine the rate of rotation of the Sun. The results again give a quadrupole moment on the order of 10−7, too small to affect the agreement between general relativity and the observed advance of Mercury's perihelion. The best reference I know is Brown et al., Astrophys. J. 343, 526 (1989), which is cited by Will in the 1993 revision of his more technical book, Theory and experiment in gravitational physics. There may be more recent results.
While it's too early to say the issue is completely settled, the most recent and most accurate results seem to be converging towards a value that makes the GR predictions agree well with observation.>>
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Re: Science: Sun Slimmer Than Expected

Post by Rothkko » Sat Aug 18, 2012 4:00 pm

neufer wrote:
  • 17 ppm seems rather large to me:
I am not scientific, but for me it is the perfection ... would it be necessary that the Sun was a perfect sphere?
No soy científico, pero para mí eso es la perfección... ¿Sería necesario que el Sol fuera una esfera perfecta?

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Re: Science: Sun Slimmer Than Expected

Post by neufer » Sat Aug 18, 2012 4:52 pm

Rothkko wrote:
neufer wrote:
  • 17 ppm seems rather large to me:
I am not scientific, but for me it is the perfection ...
would it be necessary that the Sun was a perfect sphere?
It is partly the perfection in the proper precession of Mercury explained by General Relativity.
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Re: Science: Sun Slimmer Than Expected

Post by Rothkko » Sun Aug 19, 2012 1:21 pm

Why you scientists, is much difference? Can we say that the Sun is a perfect sphere?
¿Para ustedes, los científicos, es mucha diferencia? ¿Se puede decir que el Sol es una esfera perfecta?

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Re: Science: Sun Slimmer Than Expected

Post by Ann » Sun Aug 19, 2012 4:23 pm

I find it so interesting that the Sun is so almost-perfectly round. It is by far the roundest natural object in the solar system, possibly disregarding some freak pebbles.

Other stars bulge enormously. This makes me wonder how common the Sun's amazing roundness really is.

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Sun: Grade 17

Post by neufer » Sun Aug 19, 2012 4:49 pm

Ann wrote:
I find it so interesting that the Sun is so almost-perfectly round.

It is by far the roundest natural object in the solar system, possibly disregarding some freak pebbles.
http://www.parktool.com/blog/calvins-corner/riding-on-tea-cups-ceramic-bearings wrote: Riding on Tea Cups (Ceramic Bearings)

<<Ceramic bearings are being seen on bicycles and other hobby equipment, such as fishing reels, slot cars, and roller blade wheels. Ceramic bearings are available in most sizes of bicycle cartridge bearings. The cartridge bearings usually use steel inner and outer races, with ceramic bearings between. Ceramic bearings can also be purchased as loose ball bearings.

The ceramic bearing has found uses in industrial applications where there is high speed, high load, and, consequently, high temperature. Bicycle riding simply does not result in high load, speed or temperature. A bike moving at 80 kilometers per hour (about 50 mph), will have hubs that rotate about 11 revolutions per second. An engine crank shaft, however, may turn at over 100 revolutions per second.

The quality of bearing can be defined by various ratings, such as the rating of ball roundness. A high quality steel ball bearing is considered grade 25. The grade number refers to the tolerance per millionth of an inch. A grade 25 is accurate to 25/1,000,000 of an inch. Less expensive ball bearings may be grade 300, which are less round, at 300/1,000,000 of an inch. Ceramic bearings may be grade 5, which is a rounder ball bearing. The importance of roundness, however, should not be overrated, as the bearing surfaces the balls run on are unlikely to be as accurate as the grade of the ball bearing. The ball bearing is typically not the "weak link" in the system.>>
http://en.wikipedia.org/wiki/Gravity_Probe_B#Experimental_setup wrote: <<The Gravity Probe B experiment comprises four London moment gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi), a binary star in the constellation Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.

The gyroscopes are housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins. Near-absolute zero temperatures are required to minimize molecular interference, and enable the lead and niobium components of the gyroscope mechanisms to become superconductive.

At the time, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of (4 cm diameter) ping pong balls, they are perfectly round to within forty atoms (less than 10 nm ~ Grade 0.25). If one of these spheres were scaled to the size of the earth, the tallest mountains and deepest ocean trench would measure only 8 ft high. They are composed of fused quartz and coated with an extremely thin layer of niobium. A primary concern is minimizing any influence on their spin, so the gyroscopes must never touch their containing compartment. They are held suspended with electric fields, spun up using a flow of helium gas, and their spin axes are sensed by monitoring the magnetic field of the superconductive niobium layer with SQUIDs.>>
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Re: Science: Sun Slimmer Than Expected

Post by Beyond » Sun Aug 19, 2012 7:05 pm

Ok, nufer, so you've got your bearings and you're suspended by a magnetic field while spinning wildly around because of being so light-headed from all the Helium gas comeing at you, and you notice that the niobium is being watched by (giant?) SQUIDs. What now :?: :?:
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Re: Science: Sun Slimmer Than Expected

Post by Moonlady » Mon Aug 20, 2012 12:03 am

Ann wrote:I find it so interesting that the Sun is so almost-perfectly round. It is by far the roundest natural object in the solar system, possibly disregarding some freak pebbles.

Other stars bulge enormously. This makes me wonder how common the Sun's amazing roundness really is.

Ann
Do they bulge when they are binary system? Our sun is a single one, besides having the eight and half planets orbiting.

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Re: Science: Sun Slimmer Than Expected

Post by Ann » Mon Aug 20, 2012 12:44 am

Moonlady wrote:
Ann wrote:I find it so interesting that the Sun is so almost-perfectly round. It is by far the roundest natural object in the solar system, possibly disregarding some freak pebbles.

Other stars bulge enormously. This makes me wonder how common the Sun's amazing roundness really is.

Ann
Do they bulge when they are binary system? Our sun is a single one, besides having the eight and half planets orbiting.
I'm not sure that a star's shape is strongly affected by it having a companion. Ask Chris.

But many stars are very oblate. Jim Kaler wrote this about Regulus, alpha star of constellation Leo:
The measurement of total luminosity (in which invisible ultraviolet radiation must be accounted for) is complicated by the star's extremely fast equatorial rotation speed of 317 kilometers per second, which distorts it into an oblate spheroid with an equatorial diameter of 4.3 times that of the Sun (determined through interferometry), 32 percent larger than the polar diameter.

...

Regulus is actually a quadruple star.

...

Much more intriguing is a tight fourth companion detected only spectroscopically that orbits Regulus proper with a period of a mere 40.11 days. Analysis suggests that it is white dwarf with an anomolously low mass of just 0.3 solar, far below the minimum of 0.55 allowed by current stellar evolution. Kepler's laws then give a separation of about 0.35 Astronomical Units.

Astronomers speculate that when the white dwarf was a luminous giant (far larger and brighter than Regulus is now) that it transferred much of its mass (through tidal interaction) to the star that is now Regulus, and in doing so, sped it up to its current fast rotation rate (which fits with the white dwarf scenario.
In other words, Jim Kaler says that it is the rotational speed of Regulus that makes the star so oblate, not the presence of its companion per se. However, the companion may have helped speeding Regulus up, thereby contributing to its shape.

Other well-known fast-rotating oblate stars are Altair and Achernar.

A well-known star that is weirdly shaped because of its binary nature is Beta Lyra, Sheliak. Jim Kaler wrote about Sheliak:
The two stars, both quite massive, are very close together. Tidal forces both distort the stars and cause streams of matter to flow from one star to the other and apparently in a disk around the fainter of two, making the pair both quite difficult and not very well understood. Such "mass transfer" is profoundly important in the lives of double stars and produces some of the more bizarre of celestial phenomena (including Sheliak!). In extreme cases, one star can actually orbit inside the extended envelope of an expanding, dying giant star, gradually bringing the two closer together and setting the stage for later stellar explosions. Others are so close they actually touch at their surfaces.
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Re: Science: Sun Slimmer Than Expected

Post by Moonlady » Mon Aug 20, 2012 6:11 am

Ann wrote:
Moonlady wrote:
Ann wrote:I find it so interesting that the Sun is so almost-perfectly round. It is by far the roundest natural object in the solar system, possibly disregarding some freak pebbles.

Other stars bulge enormously. This makes me wonder how common the Sun's amazing roundness really is.

Ann
Do they bulge when they are binary system? Our sun is a single one, besides having the eight and half planets orbiting.
I'm not sure that a star's shape is strongly affected by it having a companion. Ask Chris.
But many stars are very oblate. Jim Kaler wrote this about Regulus, alpha star of constellation Leo:
The measurement of total luminosity (in which invisible ultraviolet radiation must be accounted for) is complicated by the star's extremely fast equatorial rotation speed of 317 kilometers per second, which distorts it into an oblate spheroid with an equatorial diameter of 4.3 times that of the Sun (determined through interferometry), 32 percent larger than the polar diameter.

...

Regulus is actually a quadruple star.

...

Much more intriguing is a tight fourth companion detected only spectroscopically that orbits Regulus proper with a period of a mere 40.11 days. Analysis suggests that it is white dwarf with an anomolously low mass of just 0.3 solar, far below the minimum of 0.55 allowed by current stellar evolution. Kepler's laws then give a separation of about 0.35 Astronomical Units.

Astronomers speculate that when the white dwarf was a luminous giant (far larger and brighter than Regulus is now) that it transferred much of its mass (through tidal interaction) to the star that is now Regulus, and in doing so, sped it up to its current fast rotation rate (which fits with the white dwarf scenario.
In other words, Jim Kaler says that it is the rotational speed of Regulus that makes the star so oblate, not the presence of its companion per se. However, the companion may have helped speeding Regulus up, thereby contributing to its shape.

Other well-known fast-rotating oblate stars are Altair and Achernar.

A well-known star that is weirdly shaped because of its binary nature is Beta Lyra, Sheliak. Jim Kaler wrote about Sheliak:
The two stars, both quite massive, are very close together. Tidal forces both distort the stars and cause streams of matter to flow from one star to the other and apparently in a disk around the fainter of two, making the pair both quite difficult and not very well understood. Such "mass transfer" is profoundly important in the lives of double stars and produces some of the more bizarre of celestial phenomena (including Sheliak!). In extreme cases, one star can actually orbit inside the extended envelope of an expanding, dying giant star, gradually bringing the two closer together and setting the stage for later stellar explosions. Others are so close they actually touch at their surfaces.
Ann


Thanks Ann! I think Sheliak is actually not a binary system anymore with the "mass transfer", it's a cannibal now! :chomp:

Chriiiiiiiiiiiiiiiiiiiiiiiis! Ann says I shall ask you!

Why do stars bulge and are oblate spheroids? I didn't know that, that it is common!
So there is the mass, the rotating and it's speed, but what makes our sun different from the other suns who are not perfectly round?
Is there a picture which shows a significant bulging?

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SXP 1062: the perfect sphere!

Post by neufer » Mon Aug 20, 2012 1:11 pm

Ann wrote:
...many stars are very oblate.
Not neutron stars: A neutron star has a radius of just 12 km and an enormous surface gravity of ~3×1012 m/s2.

Normally, neutron stars have rapid rotation periods between about 1.4 ms to 30 seconds.

At a 0.4 second rotational period gravity so dominates centrifugal force that the neutron star is oblate by just ~1ppm or 24mm.

At a 30 second rotational period the neutron star is oblate by ~4.2 microns.

And with the slow rotation of once every 18 minutes, pulsar SXP 1062 is oblate by ~ 3.25 nanometers! Now that's round :!:
http://www.space.com/14052-mystery-pulsar-supernova-space-oddity.html wrote:
Click to play embedded YouTube video.
<<Astronomers have discovered a strange spinning star that appears to be older than the explosion that gave birth to it, scientists say. The star is a pulsar, a rotating, super-dense core left behind after a massive star goes supernova. This pulsar, known as SXP 1062, is spinning quite slowly, suggesting an advanced age. But the pulsar can't be as old as it looks, because the star probably exploded less than 40,000 years ago, researchers said. They've just now begun delving into this newly discovered cosmic mystery.

Pulsars are created after supernova explosions, when a star's remnant collapses and becomes so dense that protons and electrons squish together to form neutrons. Conservation of angular momentum causes these newly formed, city-size neutron stars to rotate, often extremely rapidly. They're called pulsars because this rotation makes their light appear to pulse at regular intervals.

Astronomers feel fortunate to have detected SXP 1062. "Not many pulsars have been observed within their supernova remnant, and this is the first clear example of such a pair in the [Small Magellanic Cloud]," study leader Vincent Hénault-Brunet, of the University of Edinburgh in the United Kingdom, said in a statement. A second team, led by Frank Haberl of the Max-Planck Institute in Germany, independently confirmed Hénault-Brunet's findings that the leftover supernova debris is between 10,000 and 40,000 years old. Hénault-Brunet's international team used NASA's Chandra space telescope and the European Space Agency's XMM-Newton observatory to spot X-rays emitted by SXP 1062.

Most pulsars spin extremely rapidly, with some of them making hundreds of revolutions per second. But SXP 1062 is spinning just once every 18 minutes or so. "The most interesting aspect of this pulsar is possibly its extremely long period — 1,062 seconds — which makes it one of the slowest pulsars on record," said Lidia Oskinova, of Germany's Institute for Physics and Astronomy, who worked with Hénault-Brunet. "Slowly spinning pulsars are particularly difficult to detect," Oskinova added. "Only a few with periods longer than a few thousand seconds have been observed to date."

Since pulsars slow down as they age, SXP 1062's sluggish rotation seems to imply an advanced age, in contrast to the supernova remnant that surrounds it. "Extremely slow rotation in pulsars normally points to old objects — something that doesn't quite agree in this case with the fairly young age of the supernova remnant," Oskinova said.

Did something cause SXP 1062 to decelerate faster than normal? Was it born with a slower spin than other pulsars? These questions remain unanswered, but the solution could lie within the information already collected, researchers said. "Our plan is to fully mine the X-ray data to study the system's variability in greater detail, and further study the optical spectra to investigate the properties of the companion star," Oskinova said. "We can't wait to see what the data tells us.">>
Last edited by neufer on Tue Aug 21, 2012 5:41 am, edited 4 times in total.
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Re: Sun: Grade 17

Post by Rothkko » Mon Aug 20, 2012 6:45 pm

neufer wrote: At the time, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of (4 cm diameter) ping pong balls, they are perfectly round to within forty atoms (less than 10 nm ~ Grade 0.25).
Can you translate in ppm?
¿Se puede traducir en ppm?

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Re: Sun: Grade 17

Post by bystander » Mon Aug 20, 2012 7:12 pm

Rothkko wrote:
neufer wrote: At the time, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of (4 cm diameter) ping pong balls, they are perfectly round to within forty atoms (less than 10 nm ~ Grade 0.25).
Can you translate in ppm?
¿Se puede traducir en ppm?
10x10-9 / 4x10-2 = 10-6 / 4 = 0.25 ppm
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Re: Sun: Grade 17

Post by Rothkko » Mon Aug 20, 2012 7:38 pm

neufer wrote: Grade 0.25
Rothkko wrote: Can you translate in ppm?
bystander wrote: = 0.25 ppm
Grade 0.25. How did not I realize? )
Grade 0.25. ¿Cómo no me di cuenta? )
0.25 vs 17...
What is there out is magnificent.
Lo de ahí fuera es grandioso.