MPS: Distant Star Is Roundest Object Ever Observed

[bystander]

Distant Star Is Roundest Object Ever Observed in Nature
Max Planck Institute for Solar System Research | 2016 Nov 16

Scientists measure the shape of a pulsating star with unprecedented precision.

[img3="The star Kepler 11145123 is the roundest natural object ever measured in the universe. Stellar oscillations imply a difference in radius between the equator and the poles of only 3 km. This star is significantly more round than the Sun. © Laurent Gizon et al. and the Max Planck Institute for Solar System Research, Germany. Illustration by Mark A. Garlick."]http://www.mps.mpg.de/4771282/standard_ ... 223558.jpg[/img3][hr][/hr]

Stars are not perfect spheres, several mechanisms can change their shape. One mechanism is rotation: the more quickly a star rotates, the more flat it becomes due to the centrifugal force. Since distant stars appear as points in the sky, measuring their shape is a challenging task. A team of researchers led by Prof. Dr. Laurent Gizon from the Max Planck Institute for Solar System Research (MPS) and the University of Göttingen succeeded in measuring the oblateness of a slowly rotating star. In their study, which is published on 16 November, 2016 in the journal Science Advances, they determine for the first time stellar oblateness with unprecedented precision using asteroseismology – the study of the oscillations of stars. The technique is applied to a star 5000 light years (47,000,000 billion kilometers) away from Earth and reveals that the difference between the equatorial and polar radii of the star is only 3 kilometers – a number that is astonishing small compared to the star’s mean radius of 1.5 million kilometers.

All stars rotate and are therefore flattened by the centrifugal force. The faster the rotation, the more oblate the star becomes. Our Sun rotates with a period of 27 days and has a radius at the equator that is 10 km larger than at the poles; for the Earth this difference is 21 km. Gizon and his colleagues selected a slowly rotating star named Kepler 11145123. This hot and luminous star is more than twice the size of the Sun and rotates three times more slowly than the Sun.

Gizon and his colleagues selected this star to study because it supports purely sinusoidal oscillations. The periodic expansions and contractions of the star can be detected in the fluctuations in brightness of the star. NASA’s Kepler mission observed the star’s oscillations continuously for more than four years. Different modes of oscillation are sensitive to different stellar latitudes. For their study, the authors compare the frequencies of the modes of oscillation that are more sensitive to the low-latitude regions and the frequencies of the modes that are more sensitive to higher latitudes. This comparison shows that the difference in radius between the equator and the poles is only 3 km with a precision of 1 km. “This makes Kepler 11145123 the roundest natural object ever measured, even more round than the Sun” explains Gizon. ...

Shape of a Slowly Rotating Star Measured by Asteroseismology - Laurent Gizon et al

• Science Advances 2(11):e1601777 (16 Nov 2016) DOI: 10.1126/sciadv.1601777

[neufer]

When a massive object--such as a star--spins, how is its gravitational field affected?

<<By assuming that the sun was spherical, Albert Einstein (father of the Theory of General Relativity) explained with precision a variation in the orbit of Mercury, the so-called precession of the perihelion--a phenomenon that Newton's theory of gravity could not explain.

But in the 1960s, the American physicist Robert H. Dicke of Princeton noted that if the interior of the sun were rotating rapidly--compared to a slower speed observed at the surface--then the non-spherical component of the sun's gravitational field could produce up to 10 percent of the effect Einstein had computed, in which case, General Relativity would be an incomplete theory of gravity.

Ultimately, careful observations and analyses of the sun by Dicke, his students and colleagues could not confirm the idea. Thus, Dicke's conjecture and subsequent studies have tended to affirm Einstein's theory.>>

[Ann]

I'm so hugely interested in what kind of star this extremely round object might be. Well, I want to know about its spectral class, of course. It would be such a blast if it was a B-type star, but I guess not. B-type stars live very short lives, and Kepler 11145123 must be old enough to have spun down from its wild youth.

Besides, the caption says that the star is more than twice the size of the Sun. To me, this suggests that Kepler 11145123 is an A-type star, not a B-type one. Although judging from the illustrations below, spectral class A1V Sirius does seem too puny to be a match for Kepler 11145123. So maybe the latter is spectral class A0V, or perhaps even B9V?

Click to view full size image

Fast-rotating oblate Vega, spectral class A0V, and the Sun.
Illustration: RJ Hall.

Click to view full size image

Sirius, spectral class A1V, and the Sun. Source: Unknown.

[Ann]

Indeed, KIC 11145123 is really an A-class star!

Monica Young wrote:

“This is in many regards a very surprising star,” says Jorgen Christensen-Dalsgaard (Aarhus University, Denmark), who was not involved in the study. As an A-class star, it’s more than twice as wide as the Sun. But unlike fellow A stars Altair and Vega, it spins slowly, completing a rotation every 100 days, compared to the Sun’s roughly 26-day equatorial period. The slow rotation may be a sign of its age.

So indeed, KIC 11145123 is an A-class star. But that doesn't tell us what kind of A-class star it is. Jorgen Christensen-Dalsgaard compared it with Vega (spectral class A0V) and Altair (spectral class A5V). According to Jim Kaler, the radius of Altair is 1.8 or 1.7 times that of the Sun, whereas the polar radius of Vega is 2.26 times that of the Sun, while its equatorial radius is as much as 2.75 times that of the Sun.

Interestingly, again according to Jim Kaler, the radius of Sirius (spectral class A1V) is only 1.75 times solar.

So when it comes to size, KIC 11145123 is more like Vega than Sirius or Altair.

Whatever that tells us about how hot it is!

Ann

[neufer]

Kepler Star Is The Most Spherical Object Ever Seen In Nature
by Astrobob

<<Our Sun rotates with a period of 27 days, causing it to bulge 20 km more around its middle than from top to bottom... Prof. Laurent Gizon of the Max Planck Institute in Germany and his colleagues selected a slowly rotating star named Kepler 11145123. This hot and luminous star is more than twice the size of the Sun and rotates three times more slowly than the Sun. Naturally, the slower a star rotates, the less flattened it becomes.

Kepler 11145123 is the roundest natural thing known. Comparing the frequencies of low-latitude regions vs. those of high latitudes, they were able to show that the difference in the star’s radius — the distance from center to edge — is only 3 km with a precision of 1 km. One more surprise: the star is even less oblate than implied by its rotation rate, the reason the authors propose that a magnetic field at low latitudes could “sphere up” the star even more.>>