[MUSIC PLAYING] The great advances in any science tend to come in sudden leaps.

April 25th of 2018 marks the beginning of just such a leap for much of astronomy.

In the early hours of the morning, the Gaia mission's second data release dropped.

Our understanding of our own galaxy will never be the same again.

[MUSIC PLAYING] The Gaia satellite was launched in late 2013, entirely built and operated by the European Space Agency.

Its primary goal is to map the stars of the Milky Way with a scale and precision orders of magnitude greater than ever before.

Gaia's predessecor, Hipparcos, cataloged 120,000 stars.

Gaia blows this out of the water, with positions, colors, and brightnesses of nearly 1.7 billion stars.

Gaia can see orders of magnitude fainter and further away than previous missions, but its greatest superpower is its precise astrometry.

Gaia can pin down a star's position to the equivalent of a human hairs width at 1,000 kilometers.

That's 1,000 to 2,000 times smaller than the resolution of the Hubble Space Telescope.

As we'll see, this precision allows Gaia to measure true distances and true velocities for 1.3 billion of its stars.

The result is a 3D dynamical atlas of our quadrant of the galaxy.

An atlas that we can wind both forwards and backwards in time.

Let's start with distances.

How does Gaia measure these?

The spacecraft orbits the sun at Lagrange point two, tracking the Earth's orbit, but 1.5 million kilometers further from the sun.

From there, it stares outwards to the galaxy.

As it traverses its orbit, Gaia detects the tiny shifts in the positions of stars due to this motion, a phenomenon called stellar parallax.

It's just like the way your finger moves relative to the background when you wink your eyes back and forth.

And the degree of motion depends on this distance.

Gaia's winks are the size of its entire orbit.

Coupled with its incredible position measurements, this enables Gaia to measure distances to stars as far away as the galactic center.

Knowing the distance to a star is critical for determining its other physical properties.

For example, combining distance with a star's apparent brightness gives us its true to luminosity.

And Gaia measures brightness with incredible accuracy.

It measures brightness in both the red and blue parts of the electromagnetic spectrum.

Now, combining those gives us the color of the star, which, in turn, gives us its surface temperature.

The combination of stellar luminosity and surface temperature has incredible diagnostic power.

We often plot these properties against each other in a Hertzsprung-Russell or HR diagram.

Location on this diagram can tell us about a star's mass, size, fusion activity, and even its past and future evolution.

For example, stars on this diagonal band-- the so-called, main sequence-- are in the primes of their lives, fusing hydrogen into helium.

After which, lower mass stars will become red giants, before leaving behind white dwarf remnants.

So let's take a look at the Gaia HR diagram, it's pretty incredible.

For the first time, we have a complete census of the stellar population far beyond the neighborhood of the sun, compared to the HR diagram of Hipparcos.

And we now see the main sequence extending down to include extremely faint red dwarfs.

We also see the full sequence of faint white dwarfs, showing the paths they follow as they slowly fade into blackness.

We see intricate details within that sequence, an unexpectedly clear separation of evolutionary paths that may reveal the composition and past life of the white dwarf.

We see hot, newly formed white dwarfs, some of which are still embedded in the nebula of gas injected in the death of their star.

Looking at the red giants, we resolve the shape of the so-called red clump in greater detail than ever before.

These are stars near the ends of their lives, now burning helium in their.

Cores.

We can even watch variable stars dance along the HR diagram as their brightness' change.

The information Gaia provides will revolutionize our understanding of stellar formation and evolution.

We are barely getting started.

Moving on to stellar velocities, the galaxy is a dynamic place.

The stars all move in their own orbits around the galactic core.

That movement is imperceptible to the human eye, even over many years.

It's imperceptible to most telescopes, but Gaia's incredible astrometry revealed the change in positions of stars over the five years of its operation.

This gives the velocities of the stars in the plane of the sky.

A dedicated high resolution spectrograph shows the tiny Doppler shift-- the stretching or compression of the wavelength of starlight due to the motion towards or away from us.

Combining motion on the sky and Doppler shift, gives the full three-dimensional velocities for these billion stars.

And putting such complete velocity measurements together with our position data, it's now possible to model the dynamics of the Milky Way with incredible detail.

On the large scale, we see the distribution of stellar velocities through the spiral structure of our part of the Milky Way.

We can see the rotation of the Milky Way through the red and blue Doppler shift of the stars.

We can map things like stellar streams and detailed substructure that tell us about the history of our galaxies formation.

Astronomers have already found evidence in the Gaia data that our galaxy was disturbed hundreds of millions of years ago, probably by an encounter with the Sagittarius Dwarf Spheroidal Galaxy.

And mapping globular clusters and dwarf galaxy orbits also tells us about future interactions with the Milky Way.

Knowing the current velocities and positions of the stars, we can actually wind the clocks backwards and forwards, to see where they came from and where they're going.

For example, this is the field of stars of the planet hunting, Kepler telescope.

And this, is how they travel to those locations over the past half million years.

And this is where they're going over the next half million.

We can now study the kinematics of stars that have cumulatively, thousands of confirmed planets.

We can potentially, trace the origins of these stars, allowing us to find solar systems that came from the same stellar nurseries.

Also, by constraining the distances to stars, we can get better measurements on the sizes of those stars, and thus, also get better measures of the sizes of the planets around them.

Based on the new data, we've already been able to confirm a gap in planetary radius size, at around 1.9 Earth radius.

We can even potentially, detect exoplanets by looking at the star's radial velocities and measuring shifts to a planet tugging on the star.

New binary star systems can also be found with these same methods.

This dynamical information will be a powerful tool in understanding the dark matter distribution of the galaxy.

For example, we can study stretched out groups of stars, called stellar streams.

These dynamically connected flows of stars, once bound together as a globular clusters or dwarf galaxies.

It's been hypothesized that disruptions in these flows are due to clumps of dark matter, so-called sub halos.

The nature of sub halos, and other details of dark matter's distribution, could help us figure out what dark matter really is.

And on top of all of this, Gaia doesn't only study stars.

It's tracked over 14,000 asteroids and other solar system objects, and found many new ones.

This is useful for future asteroid mining missions, and to identify potentially, Earth-threatening objects.

Gaia has also mapped the position and brightnesses of over half a million quasars-- the cause of distant active galaxies.

And it'll identify 100,000 supernova.

Gaia even helps us with the pulsar timing array, a galactic scale gravitational wave observatory which we spoke about recently.

It does this by providing better distance measurements to those pulsars.

Every generation, we improve our maps-- first, our maps of the world and now, our maps of the galaxy, filling in unexplored regions.

These maps help us to better understand our place in the universe.

This new map-- Gaia's 3D atlas of the Milky Way-- fills in so many of the dark gaps in our old understanding.

Every dot of light in this picture is a star, with its past and future motion now known.

This is our Milky Way, and we just became much more familiar with our galactic home in space time.