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  • Of all the spectacles mankind has viewed through a telescope,

  • there are few lovelier than a spiral galaxy.

  • Majestic whirlpools of stars, they rotate in a stately and predictable dance.

  • The fact that we see many billions of them in our telescopes

  • tells us they are both common and stable.

  • It is perhaps surprising that it is relatively easy to understand the inner workings of these cosmic pinwheels.

  • By combining physical principles worked out by Sir Isaac Newton in the late 17th century,

  • with the observed amount of mass in a galaxy,

  • scientists can calculate the rates at which these galaxies rotate.

  • Using these techniques, astronomers predict how fast stars at different distances from the center of the galaxy should move.

  • Stars very close to the center move slowly.

  • That's because there is very little mass between them and the center of the galaxy to pull them along.

  • Stars a bit further away move faster,

  • because they are being pulled by all of the stars in between them and the center.

  • As we get really far away, the stars are predicted to move slowly again.

  • Their great distance reduces gravity to a gentle tug,

  • so they move leisurely in their orbits.

  • Knowing this, scientists looked at the galaxies and measured how fast stars were moving.

  • To their surprise, they found that while the stars closer to the center of the galaxy behaved as predicted,

  • those further away moved far too quickly.

  • This observation was devastating to the tradtional theories of gravity and motion.

  • If the stars were moving as fast as their measurement suggested,

  • galaxies should have torn themselves apart.

  • It was a crisis, and astronomers and physicists scrambled to find a mistake in their calculation.

  • Was Newton's theory of gravity wrong?

  • Was his theory of motion wrong?

  • Or was it possible that astronomers had incorrectly measured the galaxy's mass?

  • All options were investigated, and all were ruled out. Except one.

  • Today, scientists believe that the answer lies in a previously unknown kind of matter, called dark matter.

  • This dark matter can be envisioned as a cloud which surrounds most galaxies.

  • This matter is very unusual. It is affected by gravity,

  • but it is invisible to visible light and all other forms of electromagnetic radiation.

  • The name "dark matter" originates in this form of matter's inability to emit or absorb light.

  • Dark matter adds to the gravity of the galaxy

  • and explains the orbital speed of stars far from the galactic center.

  • Dark matter has not yet been directly observed,

  • but scientists believe that it is likely to be real,

  • mostly because the other options have been ruled out.

  • Using dozens of approaches, astronomers and physicists continue to search for direct evidence

  • that would prove that the dark matter hypothesis is true.

  • This question is one of the most important physics research questions of the 21st century.

Of all the spectacles mankind has viewed through a telescope,

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B2 TED-Ed dark matter galaxy matter dark gravity

【TED-Ed】Dark matter: How does it explain a star's speed? - Don Lincoln

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    VoiceTube posted on 2013/06/27
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