[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.
Last week, we talked
about the last stars that
will shine in our universe--
the humble, red dwarf.
Selman123 asks
about the difference
between a white dwarf
and a red dwarf.
Astronomers are not the most
imaginative at namings stuff.
A white dwarf is the remnant
core of a low to mid-mass star
after it burns out and
ejects its outer layers,
leaving only the hot core.
White dwarfs aren't
necessarily white.
They start out blue and are
expected to eventually fade,
through white to red
to black, but we always
call them, white dwarfs.
Red dwarfs are just a
very low mass stars.
They start out red,
but heat up over time.
And we expect them to
pass through white,
and some to blue, at
the ends of their lives.
But we always call
them red dwarfs.
Until they burn out, and
then, they're white dwarfs.
Makes perfect sense, right?
Many of you point
out that there will
be useful sources of
energy in the universe
long after the last
red dwarf fades away.
A popular one seems to
be, rotating black holes
or harvesting Hawking
radiation from black holes.
It's a dismal future.
Super advanced civilizations
clustered around black holes,
in an utterly dark universe.
Possible?
Sure.
With an episode?
Hell, yeah.
But if you can't wait,
Isaac Arthur's channel
goes as deep into
this stuff as you
could hope to and then some.
Craig Harrison asks
about the accuracy
of this statement, the
star that burns twice as
bright, burns half as long.
Shouldn't it be, burns
exponentially shorter?
Actually, Eldon Tyrell
had a decent grasp
of hydrostatic equilibrium
and the continuity equation.
A star that burns twice as
bright does burn half as long--
actually, slightly
longer-- because that star
will have a more massive core.
On the other hand, a star
that weighs twice as much
as the sun, burns about 25 times
brighter and burns out 10 times
faster.
Obviously, this is why the
more massive Leon beat it
before Pris and Roy.
