Sun Is Roundest Natural Object Known

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The sun is the roundest natural object ever precisely measured, astronomers say.


Astronomers have sought for centuries to see if and how our backyard star’s roundness changes. That’s because even slight changes in the sun’s shape can alter its brightness and, as a result, influence weather and climate on Earth.

But getting a read on the sun’s shape from the ground is hard, due to turbulent air that refracts light and generates inaccurate measurements.

Spacecraft eventually worked around that issue, yet poor image resolution became the next hurdle: Without a crisp view of the rotating sun, it’s difficult to improve existing measurements.

(See “Sun Is Moving Slower Than Thought.”)

“The sun is very, very round, so it’s difficult to measure any deviations in that roundness,” said study leader Jeffrey Kuhn, a solar researcher and physicist at the University of Hawaii.

“It’s only been in the last few years that we’ve been able to make decent shape observations.”

Those advances are due in part to NASA’s Solar Dynamics Observatory (SDO), which launched in February 2010 and has some of the best cameras fixed on the sun. So when Kuhn earned some time controlling the spacecraft, he got to work.

Written By: Dave Mosher
continue to source article at news.nationalgeographic.com

13 COMMENTS

  1. I would assume that large round objects are less affected by the gravitational pull of small round objects and vice versa. The sun may help in making the Earth egg shaped, but the Earth has little effect on the sun. Just an assumption.

  2. The British comedy quiz programme QI asserted in one episode that a neutron star is the roundest natural object in the universe [I realise that isn't the same thing as the "roundest object known"]

    I had supposed that the event horizon of a black hole would be the roundest, but I guess spin & magnetic fields must distort the sphere ~ I’m imagining those jets at the poles of quasars for example

    I wonder if it’s possible to have a neutron star or event horizon with zero spin ~ I expect it’s vastly unlikely ~ does anyone know if I’m right on any of the above?

  3. “Previous experiments had ruled out the possibility that gravity and rotation—the sun’s surface spins about once every 25 days at its equator—have any year-to-year, month-to-month or day-to-day effects. “

    I don’t get this. The Sun rotates…therefore it ought to be an oblate spheroid just like any other ball of gas that rotates. The rotation period would matter, but even 25 days is not exactly slow when you consider that equatorial material is travelling around a 100,000 miles a day…..about 4,000 miles per hour.

  4. Generally the “roundness” of planets and stars relates to their size/mass and rotation.

    The greater the mass the more spherical the body.  Planets, dwarf planets and planetoids become spheres when their gravity is strong enough to collapse them into spherical form against the resistance of the matter. This varies with the plasticity of the matter, but for very large objects or very hot objects, all forms of matter are plastic/fluid.

    It is only small solid rocky bodies (asteroids etc) which have irregular shapes and rapidly rotating larger bodies which are noticeably oblate spheroids.

    Some may be dynamically distorted by tides where they are affected by nearby strong gravity fields.

  5. I remember reading a Larry Niven Sci-Fi story about a spaceman and an alien squabbling over a perfectly round shiny metal sphere that was orbiting a planet. They both thought it must be a piece of advanced technology

    The alien gained the upper hand and raced off in his spaceship to collect his prize. 
    Unfortunately for him, the sphere was a fragment of a neutron star and he and his craft were captured by its immense gravity and became completely flattened on its surface.

  6.  I don’t get this. The Sun rotates…therefore it ought to be an oblate spheroid just like any other ball of gas that rotates. 

    As usual the popular summary is not the real story.  The article is here

    http://www.sciencemag.org/cont

    I can see the whole thing via work.  They are looking at two things.  Firstly the theoretical oblateness for the reasons you point out and secondly the changes in that oblateness over time.  

    The abstract is available to the non-paying public and says: 

     
    The precise shape of the Sun has not been convincingly determined, despite half a century of modern photoelectric observations. The expected deviation of the solar limb shape from a perfect circle is very small, but such asphericity is sensitive to the Sun’s otherwise invisible interior conditions as well as the solar atmosphere. From a long-running, space-based experiment, we show that, when analyzed with sufficiently high spatial resolution, the Sun’s oblate shape is remarkably constant and almost completely unaffected by the solar cycle variability seen on its surface. The solar oblateness is substantially lower than theoretical expectations by an amount that could be accounted for by a slower differential rotation in the outer few percent of the Sun. 

    Michael

  7.  As I recall, it’s the surface of the sun that rotates at that speed, but the core rotates at a lesser speed. This could (be one of the things that) work against a spread at the equator..?

  8. The Sun is most likely more spherical that other bodies in the Solar System because of its greater mass and relatively slow rotation.

    There is a widely held misconception that the Earth is a sphere, but because of variable densities, distribution of its matter, gravitational variations and its rotation, it is not!

    http://news.nationalgeographic… – Richard A. Lovett

    for National Geographic News – Published April 6, 2011
    It may look like a mangled lump of Play-Doh, but this colorful object is actually the most accurate digital model yet of Earth’s gravity field, scientists say.

    Released last week, the gravity map is what’s known as a geoid, and it was created by a European satellite called the Gravity field and steady-state Ocean Circulation Explorer, or GOCE.

    In general, most people think of Earth as a relatively smooth orb.
    But matter is not evenly distributed across the planet’s surface, and
    bodies of water are constantly shifting due to winds and currents.

    By capturing the planet’s gravity rather than its physical
    appearance, the geoid shows the shape that mean sea level would have if
    it could somehow be extended over the entire surface of the globe, said John Wahr, a geophysicist at the University of Colorado.

    http://images.nationalgeograph

  9.  By capturing the planet’s gravity rather than its physical  appearance, the geoid shows the shape that mean sea level would have if it could somehow be extended over the entire surface of the globe, said John Wahr, a geophysicist at the University of Colorado.

    So that’s what The Flood was all about. Aliens checking the earths geoid. And the silly locals like Noah thought that when they said geoid they actually said god. It all starts to make sense. 

    Michael

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