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  • The city sky is, frankly, rather boring.

  • If you look up at the patches of murk between buildings,

  • you might be able to pick out The Big Dipper,

  • or perhaps, Orion's Belt.

  • But hold on.

  • Look at that murky patch again and hold out your thumb.

  • How many stars do you think are behind it?

  • Ten, twenty? Guess again.

  • If you looked at that thumbnail-sized patch of sky

  • with the Hubble Space Telescope,

  • instead of points of light, you'd see smudges.

  • These aren't stars.

  • They're galaxies, just like our Milky Way.

  • Cities of billions of stars,

  • and more than 1,000 of them are hidden behind your thumb.

  • The universe is bigger than you can see from the city,

  • and even bigger than the starry sky you can see from the countryside.

  • This is the universe as astrophysicists see it,

  • with more stars than all the grains of sand on Earth.

  • By staring up at the stars at night,

  • you've taken part in the oldest science in human history.

  • The study of the heavens is older than

  • navigation, agriculture, perhaps even language itself.

  • Yet unlike other sciences, astronomy is purely observational.

  • We cannot control the parameters of our experiments from lab benches.

  • Our best technology can send man to the moon,

  • and probes to the edge of the solar system.

  • But these distances are vanishingly small

  • compared to the yawning gulfs between stars.

  • So how can we know so much about other galaxies,

  • what they're made of, how many there are, or that they're even there at all?

  • Well, we can start with the first thing we see when we look up at night: the stars.

  • What we are trying to learn is their properties.

  • What are they made of? How hot are they? How massive? How old?

  • How far are they from Earth?

  • And believe it or not,

  • we can learn all of these things simply from the light shining in the sky.

  • We can decipher one kind of stellar message by turning starlight into rainbows.

  • When you look at a rainbow on Earth,

  • you're really looking at light from our Sun

  • being scattered through water droplets in the atmosphere

  • into all the different wavelengths that make it up.

  • And we study the light from other stars,

  • we can create rainbows on demand using not water droplets,

  • but other specific instruments that disperse light.

  • When we look at the scattered light from our sun,

  • we see something strange: dark lines in our rainbow.

  • These lines are the characteristic fingerprints of atoms.

  • Each type of atom in the solar atmosphere soaks up light at specific wavelengths,

  • and the amount of absorption depends on how many of these atoms there are.

  • So by observing how much light is missing at these characteristic wavelengths,

  • we can tell not only what elements are in the Sun's atmosphere,

  • but even their concentrations.

  • And the same idea can be applied to study other stars.

  • Make a spectral rainbow, see what's missing,

  • and figure out which elements are present.

  • Bingo. Now you know what stars are made of.

  • But we aren't restricted to just the wavelengths that our eyes perceive.

  • Consider radio waves.

  • Yes, they can bring the Billboard Top 100 to your car,

  • but they can also travel almost unimpeded through space.

  • Because they've come so far,

  • radio waves can tell us the very early history of the universe,

  • from just a few thousand years after The Big Bang.

  • We can also study the infrared light, emitted by colder objects,

  • like the gas and dust clouds in space,

  • and the ultraviolet light from the hot stars recently born from those clouds.

  • Studying different wavelengths not only gives us

  • a more complete picture of any single object

  • but also different views of the universe.

  • For this reason, astrophysicists use several different kinds of telescopes

  • covering the spectrum from the infrared to the ultraviolet to the X-ray,

  • from giant radio dishes to giant silver mirrors to space satellites,

  • detecting light that would be otherwise blocked by the Earth's atmosphere.

  • Astrophysicists don't just see

  • the billions of stars among the billions of galaxies in the universe.

  • They hear, feel and sense them through many channels,

  • each revealing a different story.

  • But it all begins with light, the kind we can see and the kind we can't.

  • Want to know the secrets of the Universe?

  • Just follow the light.

The city sky is, frankly, rather boring.

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B1 US TED-Ed universe atmosphere rainbow sky study

【TED-Ed】How do we study the stars? - Yuan-Sen Ting

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    稲葉白兎 posted on 2015/03/10
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