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.