WEBVTT - BrainStuff Classics: Why Are Stars and Planets Nearly Spherical?

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<v Speaker 1>Welcome to brain Stuff production of I Heart Radio. Hey

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<v Speaker 1>brain Stuff, Lauren Bogle bomb here with another classic episode

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<v Speaker 1>from our archives. This one deals with one of the

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<v Speaker 1>most simple sounding yet cosmically impressive concepts that we humans

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<v Speaker 1>can contemplate. Why are stars and planets all nearly spherical? Hi,

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<v Speaker 1>brain Stuff, Lauren bog O bomb here. It's something we

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<v Speaker 1>kind of take for granted. Roses are red and planets

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<v Speaker 1>are spherical. That's just the way things are, right, after all,

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<v Speaker 1>Building model solar systems would be way more challenging if

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<v Speaker 1>instead of using little foam balls we had to make

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<v Speaker 1>a bunch of do decahedron shaped planet models. But have

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<v Speaker 1>you ever wondered why planets look like this? Why are

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<v Speaker 1>they basically spherical and not say, cylindrical or cube shaped?

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<v Speaker 1>We should kick off this discussion by calling a spade

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<v Speaker 1>a spade. None of the planets in our solar system

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<v Speaker 1>are perfect spheres, nor, for that matter, is our Sun.

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<v Speaker 1>All those bodies could be more accurately described as oblate

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<v Speaker 1>spheroids objects with the shape bulge slightly around the middle.

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<v Speaker 1>To borrow an analogy from the astronomer philled plate, they

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<v Speaker 1>look like a basketball that someone is sitting on put

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<v Speaker 1>More technically, in a celestial body with an oblate spheroid shape,

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<v Speaker 1>the polar circumference will be smaller than the equatorial one.

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<v Speaker 1>So here on Earth, if you were to travel from

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<v Speaker 1>the north pole to the South pole and back, you'd

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<v Speaker 1>have walked a grand total of twenty four thousand, eight

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<v Speaker 1>hundred and twelve miles that's thirty nine thousand, nine hundred

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<v Speaker 1>and thirty one kilometers. On the other hand, a complete

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<v Speaker 1>trip around the equator would be a bit longer. That's

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<v Speaker 1>because the circumference of Earth's equator is twenty four thousand,

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<v Speaker 1>nine hundred miles or forty thousand and seventy kilometers. As such,

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<v Speaker 1>when you stand at sea level on the equator, you're

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<v Speaker 1>further away from the center of our planet than you

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<v Speaker 1>would be at either the north or South pole. On

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<v Speaker 1>some other planets. This bulge is even more pronounced. Just

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<v Speaker 1>look at Jupiter. Earth is only zero point three percent

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<v Speaker 1>wider at the equator than it is from poll to poll,

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<v Speaker 1>but Jupiter's measurements showcase a much bigger disparity. Astronomers have

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<v Speaker 1>found that this plus sized planet is a full seven

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<v Speaker 1>percent wider at its equator than it is between the polls.

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<v Speaker 1>The oblate spheroid shape is the result of two main factors,

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<v Speaker 1>gravity and rotation. Troy Carpenter, director of Washington State's Goldendale Observatory,

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<v Speaker 1>recently discussed the matter with us in an email exchange.

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<v Speaker 1>They explain, everything which has mass experiences gravity, and gravity

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<v Speaker 1>attempts to crush an object inward in all directions. That's

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<v Speaker 1>because all objects experience self gravity, a force which pulls

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<v Speaker 1>their atoms toward a common center. As the mass of

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<v Speaker 1>an object increases, so too does its self gravitational pull.

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<v Speaker 1>After it exceeds a certain mass, the pull gets overpowering

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<v Speaker 1>to the point where the object collapses into itself and

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<v Speaker 1>becomes spherical. Little items, like say a banana or a

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<v Speaker 1>lug wrench, can resist this fate because their self gravity

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<v Speaker 1>is relatively weak, allowing them to retain non spheroid shapes. However,

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<v Speaker 1>in planets, suns, and other truly massive bodies, the force

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<v Speaker 1>is so strong that they can't avoid being distorted into

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<v Speaker 1>sphere roids. But Carpenter said gravity is not the whole story.

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<v Speaker 1>While gravity conspires to render the planet's spherical, the speed

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<v Speaker 1>of their rotation is simultaneously trying to flatten them. The

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<v Speaker 1>faster a celestial body spins, the more disproportionate it's equatorial

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<v Speaker 1>bulge gets. Carpenter tells us this is why there are

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<v Speaker 1>no perfect spheres in our Solar system, only oblate spheroids.

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<v Speaker 1>The Sun is almost a perfect sphere due to its

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<v Speaker 1>immense gravity and relatively slow rotation rate of twenty five days.

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<v Speaker 1>A significant percentage of stars in the sky rotate much

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<v Speaker 1>faster and bulge noticeably at their equators. One such star

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<v Speaker 1>is all Tear, located just sixteen point eight light years

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<v Speaker 1>away from our home planet. It's among the brightest objects

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<v Speaker 1>in the night sky. All Tear is also notable for

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<v Speaker 1>spinning very very fast. It completes a full rotation on

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<v Speaker 1>its axis every ten point four Earth hours. Accordingly, astronomers

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<v Speaker 1>estimate that All Tears at least fourteen percent wider at

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<v Speaker 1>the equator than it is from poll to poll. Rotational

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<v Speaker 1>speed also explains Jupiter's bulge. After all, a day on

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<v Speaker 1>this gas giant is a brisk nine point nine earth

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<v Speaker 1>hours long. Other forces act upon the stars and planets

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<v Speaker 1>as well, altering their shapes. Although Earth is an oblate spheroid,

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<v Speaker 1>it certainly isn't a perfect one. The gravitational pull of

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<v Speaker 1>the Sun and Moon both influence the planet's shaped to

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<v Speaker 1>a degree. For that matter, so do Earth's own plate

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<v Speaker 1>tik toonics. Consequently, the mass of our home world isn't

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<v Speaker 1>evenly distributed. In fact, it's sort of lumpy. Still, it

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<v Speaker 1>looks a good deal rounder than Jupiter and Saturn. In turn,

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<v Speaker 1>the planets in our universe appear way more spherical than

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<v Speaker 1>some of their moons do. Mars, for instance, has two

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<v Speaker 1>small satellites, neither of which has a self gravity to

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<v Speaker 1>be pulled into an oblate spheroid. Instead, their appearance is

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<v Speaker 1>often described as potato shaped. In conclusion, will say this

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<v Speaker 1>much for our home planet. It may not be flawless,

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<v Speaker 1>but at least the place is fairly well rounded. Today's

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<v Speaker 1>episode was originally produced by Tristan McNeil and is based

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<v Speaker 1>on the article why our planets almost sphere on how

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<v Speaker 1>stuff Works dot Com, written by Mark Vancini. Brain Stuff

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<v Speaker 1>is production of iHeart Radio in partnership with how stuff

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<v Speaker 1>Works dot Com, and is produced by Tyler Clang. For

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