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Stars are a bit like human beings:

they can be warm or cold, they come
in all kinds of shapes and sizes,

and, let’s face it,
they can be dim or bright.

And recent discoveries suggest that
the number of stars in our galaxy alone,

the Milky Way, may exceed 200 billion.

Just what are these citizens
of the night sky?

"Days are numbers, count the stars."

It’s the first line from a popular song,
but the one after is more relevant.

"We can only see so far," it says.

And it sums up perfectly our relationship
with the stars

that matter so much in our lives.

What is a star in the first place?

Well, a star is to the cosmos
what a key player is to a team:

a ball of energy just waiting
to be unleashed.

Stars are essentially bodies
of hot gas arising within a nebula

which, itself, is simply a cloud
of dust and gas within a galaxy.

Most famous, perhaps, is the Eagle Nebula
with its stunning "Pillars of Creation".

Each nebula is like a cosmic kindergarten

from which bright young things are just
bursting to escape.

We call them "stars".

Suggestions are that seven new stars form
each year within our Milky Way alone.

What trips the wire
to kickstart the formation of a star?

Usually it will be one of three events:

the effect of an explosion
from a nearby supernova,

the nebulas moving through a
particularly crowded pocket of space,

or a flirtation with another passing star.

At some stage,
an area of high density within a nebula

will resolve itself
into a globule of gas and dust

which will then contract
under the force of its own gravity.

This condensing matter heats up.

As the density increases,
this protostar,

that is to say, the first iteration
of the new heavenly body,

starts spinning around a central axis.

A new star exists in what scientists
call "hydrostatic equilibrium":

the inward force of gravity is balanced by
the outward pressure from the star’s core.

If there is sufficient matter,
a nuclear reaction will take place,

releasing a huge burst
of energy which must find its way

from the new body’s core to its surface.

This process takes place
over an enormous timespan

through a combination
of radiation and convection.

Next time you go to the beach,

imagine trying
to count every grain of sand,

not just the ones you see
but all the others below.

Then turn that idea on its head
and imagine trying to count the stars,

all those we see and those we don’t.

ESA’s "Gaia" telescope is making
that seemingly impossible task

more like a reality.

Its full map of the night sky is due
for completion this year,

but already, preliminary data covering
two million stars has been released,

and the scientific world is very excited.

We want
to measure a huge number of stars,

where they are, and how they are moving.

So we can answer two questions
at the same time:

"What is the structure of our galaxy?"

But also, how it is evolving.

Or we can also look back in time:

"How did the stars move to come
into the place where they are now?”

The mission’s technical measurement
principle is there are two fields of view,

two cameras basically looking at the sky
at a very constant and fixed angle,

and it rotates along the sky,
so it traces a path along the stars.

It then uses these measurements
to determine the position of stars

relative to each other, and then
you can get to extreme accuracies,

also, for the absolute position
of these objects.

The data comes down
to the ESA antennas

on the ESTRACK network
in Argentina, in Spain, and in Australia.

From there, it goes to Darmstadt,
who control the spacecraft,

and then it goes to our central
data processing hub near Madrid in Spain,

which is an ESA center.

And from there, it goes
to the data processing consortium

which then slices it up in different parts
and processes this into science products.

Our first release will contain positions
of one billion stars.

So that will allow us to look
what does the night sky really look like when you would look at...
in random direction with a telescope,

which can see very faint stars.

And a subset of two million stars,

we will have the distance and the motion.

So that is really the basis
for astronomical studies.

People can really look
into details of these sources

and study the behavior of the stars.

And in that... in addition, we have
light curves for about 3,000 stars,

so how they have been varying over time,

to analyze better the internal structure
of these stars.

Eventually,
it will plot position and movement

of a billion stars in our galaxy,
the Milky Way.

Astrometrists still have
their work cut out,

but thanks to Gaia, the job of counting
the stars just got a whole lot easier.

Something remarkable happened
in November 2016:

astronomers discovered a new way
of witnessing a star’s formation.

Adding to the familiar methods of transit,
gravitational lensing, and direct imaging,

they found a new "little friend",
quite literally.

Chandra,
the world’s most powerful X-ray telescope,

is part of the Great Observatory
that includes Kepler and Spitzer.

It orbits the Earth
some 140,000 kilometers out

and is capable of fine definition
of hot, turbulent areas in space.

Little Friend acted as a mirror,
deflecting X-rays from Cygnus X-3

towards Earth to help astronomers
identify stars coming into being.

In conjunction with the Smithsonian’s
Submillimeter Array system,

it detected carbon monoxide
and the outflow of gases

which suggest a new star in formation.

This is the first time scientists
have been able to use X-rays

to peer into a Bok globule:
one of the focal points of star formation.

The nomenclature
of stars derives from their size,

which is, in part, a function of the phase
of their life they are going through.

A life, incidentally,
which may extend to trillions of years.

The range goes from red hypergiant
at one end of the scale

to white dwarf at the other, smaller end.

Those at the large end
are far larger than our Sun.

They may be billions
of times greater in volume.

Dwarf stars abound.
The Sun is a yellow dwarf, for example,

with a surface temperature
of 5,500 Celsius.

While red dwarfs, like Proxima Centauri,

are stars on their way
to becoming white dwarfs:

what remains of giant stars whose light,
to put simply, is failing.

The process of a star’s birth culminates
in the fusion

of a hydrogen, at its core, into helium:

a process called the "main sequence"
to which the majority of stars belong.

Red dwarfs are not only the most common,
they are the most durable.

They burn at the low end of the surface
temperature range at around 3,500 Celsius.

In massive stars, the hydrogen
to helium conversion is much faster.

Paradoxically, the bigger the star,
the shorter its life.

How else are stars classified?

Basically by two criteria:
brightness and color.

Stars are catalogued by their magnitude:
either apparent or absolute.

Apparent magnitude, as its name suggests,

refers to the luminosity
of stars as seen from Earth.

This may vary, of course,
according to the mass of the star itself

and especially its distance from us.

Absolute magnitude corrects that
by establishing the star’s luminosity,

as detected from a standard distance.

Paradoxically again, the brightest carry
the lowest orders of magnitude.

A century ago, working independently
on opposite sides of the Atlantic,

Herzsprung and Russell came up
with the same basic methodology

for classifying stars within a
spectroscopic range according to the light

generated by their wavelengths.

The scale runs from O to M,
and, to cite some examples,

from the blue of Zeta Puppis
to the red of Betelgeuse or "Beetlejuice".

Made any holiday plans recently?

If you’re a stargazer,

then NASA’s own travel bureau
may have just the thing for you.

While a trip to another world may not be
within your budgets just yet,

astronomers are making us increasingly
aware of the heavenly bodies above us

and planning on getting us there.

When set alongside exotic places
like Monaco, or Morocco, or wherever,

the holiday destinations advertised

in NASA’s graphic travel bureau

may not seem too enticing.

But they are certainly, to use a
travel agency cliché, "out of this world."

The striking images, genuine "postcards
from the edge," we might call them,

include HD40307g.

That’s the very "down-to-Earth" name
for an exoplanet

which astronomers call a "super-Earth".

One of those revealed by the Kepler space
telescope on its so-called "K2 mission"

when it bounced back
from a mechanical failure in 2014.

Could there be at least one
planet orbiting every star in the galaxy?

Already more than 3,000
of them have been confirmed

with almost the same
number awaiting confirmation.

The nearest to us is Proxima Centauri b,
a mere four light years away from Earth

in the triple-star system
of Alpha Centauri.

Proxima Centauri b, excitingly,
is an Earth-sized planet

in the star’s habitable zone:

the distance at which liquid water
may form on its surface.

Astronomers
have found clear evidence

of a planet orbiting the star,
Proxima Centauri.

This alien world is the closest possible
abode for life outside the solar system.

The idea of celestial harps is not new,

but in real, "down-to-Earth" life,
there is indeed a "HARPS"

at the center of the search
for life elsewhere in our skies.

It’s ESO’s High Accuracy Radial Velocity
Planet Searcher or HARPS for short.

Which is a spectrographic instrument
attached to the 3.6 meter telescope

at La Silla in Chile.

Reflecting our connected age,
in 2016, ESO invited members of the public

to follow live as it embarked
on a determined search for proof that,

circling Proxima Centauri,
there was, as suspected, an exoplanet:

the "pale red dot" that gave its name
to the program.

Not just any exoplanet,
"Proxima b", as it had been labeled,

is the likeliest one so far discovered
with a chance of playing host to life.

In late summer 2016
came the thrilling news.

Close examination of the
gravitational pull of the exoplanet

and its wobble effect on its host produced
what ESO calls "clear evidence"

for a potentially habitable world,
1.3 times the size of Earth

and in an 11.2 day orbit around its star.

Ultraviolet and x-ray radiation levels
on its surface appear high,

and the exoplanet is much nearer
to its host than we are to our Sun.

But ESO next plans to use its forthcoming
Extremely Large Telescope, the ELT,

and later, interstellar probes to get even
closer to solving the enigma of Proxima b.

Where do stars go when they die?

That depends on their size or rather,
on their mass.

Stars of high mass will follow one
of two paths. Nearing the end of their life,
as the core of the star collapses,

it will give rise to a supernova:
the gigantic explosion triggered when the output of energy
at its core suddenly ceases.

Scientists are predicting
that Beetlejuice,

the red giant in Orion whose mass may be
as much as 20 times that of our Sun,

is on its way to a supernova
within the next million years.

The end product of a supernova will be
either a new, different kind of star

or that great unknown: a black hole.

On one hand,
the core may survive as a neutron star,

which may measure the remarkably small
diameter of ten to twenty kilometers.

Not only that,
but they are of extraordinary density.

A teaspoon of their substance would weigh
in the millions of tons here on Earth.

Neutron stars often act as the lighthouses
of the sky as well.

Because of their strong,
magnetic field and fast rotation,

they emit polar radiation beams,

discernible when the beam is directed
towards Earth,

just as the beam from a lighthouse will be
visible out at sea,

only in fleeting, cyclical movements.

The other possible fate for a dying star
is to become a black hole.

While there is a plurality
of objects in our sky,

black holes bring us face-to-face
with a singularity:

the point at which matter is compressed.

The singularity will be either a point
of infinite density

or adopt the shape of a ring.

Either way, its gravitational pull
is so strong that nothing, not even light,

can resist or escape it.

Its boundary is described
as the "event horizon".

Ninety percent of black holes
in the universe don't have a lot

of hot material orbiting around them.

They don't form these accretion disks,
and so we can't observe them.

Tidal disruption events,

where the stellar debris causes the
formation of a temporary accretion disk,

offers us a way to probe this population
of super massive black holes.

One tool astrophysicists use
to stare into the abyss

is X-ray reverberation mapping.

X-ray reverberation mapping
has been very successful

at probing the accretion flow in
well-established accretion disk structures

but had never been used
to look at tidal disruption events.

My collaborator at the university in
Maryland and I were having lunch one day,

and she says, "Has anyone ever looked
at tidal disruption events

with X"

That night, I stayed late at the office
and just tried it out on this data

from SWIFT J1644 and, much to my surprise,
the result was amazing. And I could see that we were looking at
the structure of the inner accretion flow

around a normally dormant black hole
for the first time.

It's not like a normal accretion flow
in an active galaxy that's a flat disk.

This is something that is extremely puffy,
very turbulent,

and we are measuring flashes
of X-ray emission

deep within this newly formed
accretion disk.

Previously, astronomers had thought
that the X-ray emission is coming

from far out in a jet.

But what we're
finding with these observations is that

the X-ray emission is coming from flares
very close to the super massive black hole

and we can use these observations to probe
properties of the black hole itself.

For instance, we found that the mass of
the black hole is something on the order

of a million times the mass of the Sun.

The Milky Way,
which contains our solar system,

is, itself, part
of a so-called "Local Group" of galaxies,

including, for example, Andromeda, which
in turn belong to the Virgo Supercluster,

one hundred million light years across.

When we’ve had a bump on the head,
we say we are "seeing stars."

Looking at it
from an astronomical point of view,

the number of stars out there is enough
to make anyone’s head spin.