Does The Sun Spin?

Despite being unimaginably huge balls of gas, Jupiter and Saturn rotate just like the Earth and moon do, like rigid solids. In fact, Jupiter rotates the fastest among all the planets: it rotates at an unprecedented rate of roughly 10 days per rotation! Why would we expect anything different from the sun, which is only 10 times bigger than Jupiter? The sun also rotates, but just like Jupiter and Saturn, in a somewhat unusual manner.

The Formation

Discerning whether the sun rotates or not is quite difficult because it, unlike other planets, appears homogeneous from a distance, like water rotating in a crystal ball… except that it’s not. Galileo was the first person to discover that the sun spins after he observed the movement of ‘spots’ on it with his newly built telescope. These spots are known to be the result of the sun’s magnetic field interacting with its plasma. The spots often lead to solar flares, the most powerful explosions in the solar system.

Sun flare Sun's atmosphere, the corona, erupted out into space

Solar flares. (Photo Credit: NASA Goddard Space Flight Center / Wikimedia Commons)

According to the Core Accretion model, around 4.6 billion years ago, the newly formed solar system was a hot soup of dust and gas abundant in hydrogen whirling around haphazardly in space. This vagrant matter is believed to have been produced in a supernova, the explosive death of a star. The dense cloud gradually contracted under its own gravity. The compression then made it immensely hot and heavy around the center. This dense region became the Sun. The matter on the outskirts of this nebula was pushed outward into space by powerful solar winds. This matter ultimately aggregated to form the planets.

As the gas that formed the sun contracted, the matter, to conserve angular momentum, rotated progressively faster. This is analogous to an ice skater whose rotational velocity increases as she pulls her hands inward: because angular momentum must be conserved, a decrease in radius is compensated with an increase in velocity. This is why the sun and all the planets rotate, but not all of them spin the same way.

skater moment of inertia

Differential Rotation

If Jupiter were any more massive, it would have become a star. Its enormous gravity would have forced its hydrogen to coalesce and form helium atoms or undergo nuclear fission. This process is replicated inside hydrogen bombs and quite evidently produces tremendous amounts of heat and light. This process is what powers the core of an ebullient star.

The sun, however, is a star, its hydrogen does undergo nuclear fusion to produce tremendous heat and light. The infernal temperatures don’t just produce gas, but also plasma. The sun then, unlike Jupiter and Saturn, is not just gas, but a mixture of this gas and viscous, boiling plasma. Nonetheless, because the three celestial bodies are essentially liquids, they exhibit what is called differential rotation: different regions on them rotate at different velocities! However, because the sun is more viscous, its regions rotate much slower than Jupiter’s or Saturn’s.

sun rotation

As the picture demonstrates, the sun’s equator rotates faster than its poles. On average, the sun takes 27 days to spin once on its axis. Differential rotation is speculated to be driven by differences in temperature. The heat produced by the core moves outward and disrupts the movement of the mass above. The trembling spurs a redistribution of angular momentum in the form of a wave that spreads outward, often so far that some of the momentum is said to be lost in stellar winds.

Sun magnetic field

A computer simulation of the sun’s incredibly complex magnetic field. (Photo Credit: NH2501 / Wikimedia Commons)

The same phenomenon is speculated to fuel the sun’s incredibly complex and capricious magnetic field, which – every now and then, for some unknown reason – reverses its polarity! But, again, all this is speculation; the exact cause of the star’s differential rotation is yet to be discovered.

References

  1. Wikipedia
  2. LiveScience
The short URL of the present article is: http://sciabc.us/j18dE
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About the Author:

Akash Peshin is an Electronic Engineer from the University of Mumbai, India and a science writer at ScienceABC. Enamored with science ever since discovering a picture book about Saturn at the age of 7, he believes that what fundamentally fuels this passion is his curiosity and appetite for wonder.

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