The Universe (2007–…): Season 1, Episode 10 - Life and Death of a Star - full transcript
Gravity plays a crucial role in both the birth and death of stars in this detailed review. Various types of stellar collisions are simulated.
In the beginning,
there was darkness... and then...
BANG! Giving birth to an
endless expanding existence
of time, space and matter.
Now, see further than we've ever imagined
beyond the limits of our existence,
in a place we call...
THE UNIVERSE.
Each star you see
twinkling in the night sky,
is luminous sphere of superheated gas,
much larger than any planet.
And each has a story to tell.
A dramatic birth,
a life on the edge.
Gravity collects the star
in the first place,
and then gravity wants
to crush it.
And a death that rattles
the heavens.
The whole thing goes off
in a blinding flash,
the biggest explosion
in the Universe.
The Universe at its most volatile
and action-packed...
Life and Death of a Star.
Like glittering cities
in the desert,
galaxies arise out of the
great darkness of the Universe.
Galaxies made of billions of
blazing lights called stars.
There are billions
and billions of stars.
In fact, in our galaxy there are
400 billion stars,
just in our galaxy.
But how were these stars born?
How will they die?
And how can it be that all
human beings on Earth
owe their lives to
the deaths of stars?
The quest for answers
begins here,
in a cloud of dust and gas
hovering in the interstellar desert.
You are looking at the Pillars
of Creation.
The Pillars of Creation are a
stellar nursery.
New stars are in the process
of being born in the central regions.
Located 7,000 light years from Earth,
the pillars are part
of the Eagle Nebula,
which is just one of billions of star
forming regions in the Universe.
The pillars are towering clouds
of dust and hydrogen gas.
If you remember the Periodic Table
of elements from chemistry class,
you have the light
elements up at the top,
hydrogen, helium,
lithium, this sort of thing.
And then the really heavy ones
as you get lower down.
It's hydrogen, the lightest, simplest
most abundant element in the Universe,
that is the key component of stars.
Within a nebula, clumps of
this gas and dust
slowly coalesce into smaller clouds
over millions of years,
pulled together by
a very familiar force.
The same force that
connects us here to the Earth,
that keeps us on the Earth,
gravity, is the same force that
pulls things together
in a way that gives us...
planets and stars and galaxies
in the Universe.
Gravity, in many senses, is the
most important force in astronomy.
And when gravity acts
in the Universe
one of the basic things
that it produces is stars.
Stars are sort of the most
basic unit of mass
that is produced when
gravity pulls mass together.
Each contracting cloud
can produce anywhere
from a few dozen
to thousands of stars.
To form a star like our Sun,
which is a million miles across,
it takes a clump of gas and dust
100 times the size of our
Solar System.
These clouds start off their lives
bitterly cold,
with temperatures hundreds
of degrees below zero Fahrenheit.
But as gravity fragments
and compresses them,
the heat begins to soar.
Within a few hundred
thousand years,
the cloud spins into
a flattened disc.
Gravity coalesces the centre
of the disc into a sphere,
where the heat rises to a
scorching 2 million degrees.
This glowing system
is now known as a "protostar".
Ten million years later,
the searing hydrogen core
of the fledgling star
soars past 18 million degrees,
and something incredible happens.
The core becomes so hot,
it can sustain thermonuclear fusion.
Thermonuclear fusion...
is a lot of syllables but it just
means it's hot there
and small atoms become big atoms.
Hydrogen atoms are moving
fast enough
that they actually will fuse together
and will form a helium atom.
It's this nuclear reaction
that produces the energy
to power the star
throughout its life,
giving it a constant source
of light and heat.
It's self-luminous,
it generates its own heat.
And that's the essence of
what makes a star a star.
If you've got fusion,
you've got a star.
Once born, a star's life will
be a constant battle,
an all-out war against gravity.
Gravity collects the star in the first
place, and then it wants to crush it.
Gravity never gives up, gravity wants
to pull everything together.
So if the star is going to have a life,
and a long life,
it has to find a way
to fight against gravity.
You feel gravity all the time.
When you try to jump
or you try to climb a rock,
there's always gravity
pulling you back down.
And in order to fight against
gravity, you have to have some way
of applying a force
which works in the opposite
direction than gravity.
So, if there's a rope, you can use
your muscles to pull on the rope,
and therefore resist,
and even overcome, gravity.
But that doesn't mean
gravity gives up.
Gravity is always working.
So you have to keep applying
this force in order to not fall off.
And if you give up or let go
or the rope breaks,
gravity immediately wins
and you fall.
The same kind of thing
happens with stars.
Stars are also trying to hold themselves
up against gravitational collapse.
Gravity wants to crush the star
down to the middle.
For stars, nuclear fusion
provides the rope,
in the form of pressure.
The heat gets all the particles
in the star moving around quickly,
and they bang outwards,
and that produces a pressure
which can actually hold the star up
against gravity.
The amount of pressure pushing out
on the star just matches
the amount of gravity
pulling in on the star.
And it can sit there and burn happily
until something changes.
A star will spend most of its life
in this state of equilibrium.
It's a phase scientists call
the "main sequence".
So, our Sun is in the main sequence,
we're very happy it's there,
it provides us the same amount
of energy almost every day,
and that's what makes life possible.
All stars in the main sequence
aren't alike.
Some are much smaller
and cooler than the Sun.
Others, much larger and hotter.
So, it turns out that how
hot something is
is related to the colour of the light
that it emits.
So, a star like the Sun,
most of the light that comes out from it
is sort of a yellow-type colour.
If the Sun were much hotter,
the predominant wavelength of
light would come out into the blue.
Or even into the ultra-violet.
And cooler stars emit
more red light.
Small, cool, red stars,
like Proxima Centauri,
the nearest star to the Sun,
are known as "Red Dwarfs".
They can be as little as
1/10 the mass of the Sun.
With surface temperatures
thousands of degrees cooler.
Red dwarfs are the most common
type of stars in the Universe.
There are many, many more of these
sort of very dim, red dwarfs
floating out in space than there are
stars like the Sun.
Of course, when you
look in the night sky,
you don't see the most
common kinds of stars,
you don't see these red dwarfs
'cause they're so faint.
You merely see the very
rare, very bright stars
that turn out to be
very, very far away.
On the opposite end
of the spectrum,
are the large blue,
main sequence stars.
Averaging a surface temperature
of 45,000 ?F.
They can be 20 times
the mass of the Sun.
And 10,000 times
more luminous.
In the life and death of a star,
size definitely matters.
Mass is the fundamental thing
which drives the life history
of a star.
The more massive stars
live much shorter lives than
the less massive stars.
And that's perhaps a little
bit strange sounding,
because the massive stars have
more fuel to burn
you'd think they'd live longer.
So, it's counterintuitive
that more massive stars
will burn through their fuel
more quickly than the
lower mass stars.
Imagine two gamblers sitting
down at a blackjack table.
You would expect the one with
the most money, the most "fuel to burn",
would last the longest.
But what if the big-time gambler
is making huge bets on every hand?
A gambler that is gambling
with a lot more money
and putting down $10,000 at a time
is gonna burn through that money
much more quickly.
So the more mass you have, the higher
temperature, the higher pressure,
the higher the fusion rate.
It goes much more quickly
with the more mass you have.
And it's always just simply the
calculation: how much fuel do you have,
and at what rate are you
converting it.
The high-mass stars live
their lives faster.
They burn the candle at both ends,
it's life in the fast lane.
A high-mass star could die
within a million years.
A star 10 times as massive
as our Sun,
might live for only
1/1000 as long.
So our Sun will live for about
10 billion years in total.
A star 10 times as massive
as our Sun,
might live only
10 million years in total.
While massive stars have lifespans
measured in millions of years,
the lowest mass stars
measure their lives
in tens of billions, if not trillions,
of years.
Every low mass star that has
ever been born in the Universe,
and the Universe has been making
stars for more than 10 billion years,
all of those stars are still
in their infancy.
No such star that's ever been born
has ever come close to dying.
But for all stars,
including our own Sun,
life on the main sequence
can't go on forever.
It can only last as long as the star
has fuel to burn.
If it runs out of fuel,
fusion stops and gravity... wins.
Gravity never gives up,
whereas fuel, of course,
can run out after a while.
And so the star and the climber
both have this terrible problem,
that if they don't maintain their fight
against gravity, they will end in death.
A cataclysmic death.
Not only does the size of a star
influence how long it will live,
it also determines how
it will die.
Massive stars explode from
the scene in violent fury,
while smaller ones are doomed
to slowly fade away.
For 5 billion years,
our Sun,
a lower mass, middle-aged star,
has been happily burning through
its supply of hydrogen fuel.
Like a gambler slowly plowing
through a pile of chips.
The gambler may sit there for
a long period of time,
just like a star burns its hydrogen
for a really long period of time.
However, at some point,
you're gonna run out of money.
Scientists predict that
5 billion years in the future,
our Sun will reach
this critical crossroads:
Its supply of hydrogen fuel
will have been completely exhausted,
nuclear fusion will cease,
and gravity will begin
to crush the star.
At that point the situation
is desperate.
In order to survive, a sun-like star
must find a new source of fuel.
It has helium on hand...
but in order to start burning helium,
the core has to be
10 times hotter than it was during
its lifetime burning hydrogen.
It won't be able to fuse that helium
into heavier elements,
like carbon and oxygen,
until the core gets sufficiently hot.
And that's because it's harder to get
the helium nuclei close enough together
for the strong nuclear force
to take over,
grab them,
and cause them to fuse together.
As it continues to contract inward,
nature throws the star a lifeline.
The core actually becomes
superheated
by the very gravitational pressure
that's trying to crush it.
When it reaches 180 million degrees,
it can start fusing helium into carbon,
in a desperate gamble to survive.
So the desperate gambler might go
take out a loan on the house
and get more money.
But in getting more money
to burn through,
it's really just delaying the inevitable,
which is to go bust.
And for a star,
the inevitable is to die.
The star which took 10 billion years
to burn through its hydrogen,
now powers through its supply
of helium in a mere 100 million years.
And then the action begins.
It runs out of hydrogen...
starts fusing helium.
Runs out of helium...
attempts to fuse carbon and will fail.
But all the action,
all the "what's going on now", happens
in the last 10% of the star's life.
The searing heat of the helium burning
actually causes the outer layers
of the star to swell.
At that point, the outer
atmosphere of our star
will be held in by gravity so weakly
that it'll start sort of
just evaporating away.
Through a series of what I call
"cosmic burps", it will actually eject
the outer envelope of gases, which
are only weakly held by gravity.
That'll send some shells
of gas outward
illuminated by the hot
central star.
And that will cause what's called
the "planetary nebula" phenomenon.
Beautiful shells of glowing gas
surrounding the dying core of our sun.
With the core unable to muster
any more nuclear fusion,
can it possibly survive gravity's
crushing grip?
As a star the size of our Sun dies,
it ejects its outer layers.
With no nuclear reactions to generate
outward pressure,
gravity gains the upper hand.
The star begins to fall in
on itself,
like a climber too tired to hold on
to his rope.
There's one possibility that the rock
climber might be able to use
if he gets too tired to hold on
to the rope any more,
and that is if he can find a ledge
on the rock that he's climbing.
Gravity can pull on him
only once
but the ledge itself will support
him against gravity.
And he doesn't have to provide
any more energy to win his fight.
There's a certain kind of star, and
our Sun is actually an example of this,
where the star finds that it has
an "out" in this fight against gravity.
The contracting star finds its ledge
in a surprising place: electrons,
tiny, negatively charged
atomic particles.
Electrons don't like being compressed
so they're very close to one another,
because electrons effectively
don't like each other.
If you compact the electrons hard enough,
the pressure of the electrons themselves
is able to hold up the star
against gravity.
When the core of our dying
Sun-like star
is crushed to about
the size of the Earth,
this so-called "electron degeneracy
pressure" takes over.
Gravity can collapse the star
no further.
It's left to slowly cool into a bizarre
stellar remnant known as a "White Dwarf".
Like this one, Sirius B,
which can be seen only faintly aside
its companion Sirius,
the brighter star in our sky.
Now, a white dwarf is a very strange
type of star.
It's very, very dense.
The white dwarf has about 300,000
times the mass of the Earth,
compressed into a volume
the size of the Earth.
If you had just a teaspoon full of
material, it would weigh several tons.
So, it's really amazing stuff.
A white dwarf is the final stage
in the life of a Sun-like star.
But it's not quite dead yet.
It will continue to shine
for billions of years
as it gradually radiates away
a lifetime of energy.
I like to call white dwarfs
"retired stars",
in the sense that all of the light
that they are shining,
is energy that they accumulated during
their normal lives as stars,
while they were fusing light elements
into heavy elements,
as our Sun is doing
right now.
So, it's spending its life savings,
it's a retired star.
That will be the fate of our Sun.
But some white dwarfs can have
one last hurrah,
thanks to a friend who lends
a helping hand.
Because, although our Sun
is a cosmic loner,
more than half of all stars travel
through life with at least one companion.
Most stars are members of binaries,
or possibly even multiple star systems.
Close binary stars
can have very different fates from
the ordinary single stars.
If a white dwarf is gravitationally
bound to another star
as part of a binary system,
it can essentially steal the lifeblood
from its companion.
The small but dense white dwarf
exerts such a strong gravitational pull
that it will start siphoning off
a stream of hydrogen gas.
If it gathers material from
a companion star,
and is able to grow in mass,
then eventually, the mass of the
white dwarf can reach an unstable limit,
roughly 40% more than
the mass of our Sun.
At that point, the white dwarf
undergoes a catastrophic explosion,
where the whole thing goes off
in a blinding flash.
What's called the "thermonuclear
runaway" of the entire star.
This mammoth explosion is known
as a Type Ia Supernova.
So, if our Sun were to do this,
and it won't,
it'll die in a relatively quiet way...
But if it were to do this,
you'd need
sunblock or supernova-block
of a few billion
in order to protect yourself
from the blinding flash.
University of California Berkeley
astronomer, Alex Filippenko,
is one of the world's most successful
supernova hunters.
His team has found over 600
of them in the past decade.
An incredible feat
considering they occur perhaps twice
per century in each galaxy.
Searching for supernovas
is akin to scanning a crowded
football stadium with binoculars,
in hopes of catching the one person
who might be taking a flash photograph
at a given point in time.
If you were to look at each person
individually, one by one,
you would have a hard time
finding the person who happens
to be taking a flash photo.
Filippenko increases his odds
by expanding his search beyond
single stars,
or even single galaxies.
To do this he enlists the help of
a very high-tech assistant.
So this is a robotic
search engine
for exploding stars,
supernovae.
It has been programmed to
robotically take photographs
of over a thousand galaxies
a night,
and over the course of a week
it does 7 or 8,000 galaxies,
and then it repeats the process
comparing the new pictures of
each galaxy with old pictures.
Usually there's nothing new in the
new picture, but occasionally,
a star blows up,
a supernova goes off.
And then you can see in the new
picture a bright point of light
that wasn't there in any
the old pictures.
Though a supernova is visually
very, very bright,
the visible light is only one percent
of one percent of the total energy.
1/10,000 of the entire energy
emitted by this colossal explosion.
Although type IA supernovas come
from exploding white dwarfs,
many others, known as
Type II supernovas,
signal the dramatic deaths
of much more massive stars,
perhaps 8 or 10 times
more massive than the Sun.
Unlike their smaller cousins,
when massive stars exhaust
their hydrogen fuel,
they have the raw power to start
fusing other elements.
The ashes of each set of nuclear
reactions become fuel for the next,
so that near the end of its life,
a massive star resembles an onion
in cross-section, with an outer layer
of the original fuel, hydrogen,
surrounding layer after layer
of heavier and heavier elements.
It goes through its normal life
fusing hydrogen into helium,
then helium into carbon and oxygen,
then oxygen into neon and magnesium,
and then silicon and sulfur...
And then, iron. The massive star
builds up a core of iron.
The fusion of iron
into heavier elements
doesn't do the star any good,
it doesn't keep the star hot inside,
because fusion of iron into
heavier elements
requires energy and absorbs energy,
it doesn't liberate energy.
So the iron core builds
up without fusing,
and eventually becomes unstable,
one it reaches something like 1.5 times
the mass of our Sun, it collapses.
And the collapse is violent.
Within half a second
a core the size of the Earth
is crushed into an object roughly
10 miles across.
For a moment the collapsing
core rebounds,
smashing into the outer layers
of the star,
and kicking off one of the most massive
explosions in our Universe
since the Big Bang.
The collapse of the iron core
blows apart the rest of the star
in a colossal explosion.
It's truly an amazing,
incredible event.
Scientists are convinced that supernovas
mean much more to the Universe
than spectacular light shows.
They are in fact the source
of the heavy elements
that make up everything
around us.
All of the iron in this foundry
came from exploding stars,
from gigantic explosions.
All of it. All the iron you see
everywhere came from exploding stars.
And, in fact, all the elements
heavier than iron
directly or indirectly were
made by exploding stars.
And those elements were ejected into the
cosmos by these gargantuan explosions.
As material from
these explosions
spread out through the Universe,
it became the stuff of
planets, moons, new stars and something
even more extraordinary...
If you could trace your ancestry
back to its earliest reaches,
you would find an exploding star
in your family tree.
We are essentially made of
star stuff, or stardust,
as Carl Sagan used to say.
The elements in your body, not
just generically, but specifically,
the elements in your body heavier
than hydrogen and helium,
came from long-dead stars.
The calcium in your bones,
the oxygen that you breathe,
the iron in your red bloodcells,
the carbon in most of your cells...
all those things were created in stars
through nuclear reactions,
and then ejected
by supernovae.
And the heaviest elements,
iron and above,
were produced by the explosions
themselves, by the supernovae.
While the explosion of
a Type II supernova
showers the Universe with
heavy elements,
the core of the exploding
star is left intact.
Destroying that is gravity's job.
But to crush the core any smaller
than the size of a white dwarf,
it will have to overcome that strange
force, electron degeneracy pressure.
Gravity actually finds a way
of defeating
that tendency the electrons
have to push each other apart,
by combining the electrons with the
protons and turning them into neutrons.
You now have an object which is made
almost entirely out of neutrons,
and gravity wins, it now allows
the system to collapse further,
there're no longer
electrons stopping that,
and gravity seems to win.
Except... neutrons, it turns out,
also don't like each other,
and you end up with a new
stable object even smaller,
even more dense
called a "Neutron Star".
Compared to normal stars,
neutron stars are cosmic pebbles.
They can be as small as
10 miles across.
So imagine that you take a star about
1.5 times the size of our Sun
and then you compress all that
material down into a very small space,
about the size of Manhattan.
You just made yourself
a neutron star.
Squeezing that amount of mass
into such a small space
makes for an extremely
dense object.
One teaspoon full of neutron star
material would weigh a billion tons.
Neutron stars are some
of the most exciting
and weird objects in the Universe
that astronomers study.
If a human being were to stand
on a neutron star,
it would be a somewhat
uncomfortable experience.
On Earth, if they weighed about 150 lbs.
on a neutron star they would weigh
something like 10 billion tons.
Our biology can't stand that
amount of pressure
and so, a human being would
essentially be squashed flat
against the surface of the star.
In addition to that, neutron stars
are spinning at an incredibly high rate.
Hundreds of times per second
in some cases.
It's this rapid spin
that enabled astronomers to first
identify neutron stars.
Some neutron stars are
spinning really rapidly,
and they have a really amazingly
high magnetic field.
That magnetic field,
together with the spin,
forces a bunch of charged
particles, electrons,
to go along the axis
of the magnetic field.
And those accelerated electrons
give off light,
they produce a very
focussed beam of light.
Now, this is like a lighthouse
whose beam is always on,
but you only see it
when the lighthouse beam intersects
your line of sight.
In a similar way, we might see
the shining neutron star
only when the beam
points at us.
That object is called a "Pulsar".
Some stars are so massive, perhaps
25 or 40 times the mass of the Sun,
that not even a neutron star can hold up
under the weight of their collapse,
and gravity will crash them
even further,
into an object of infinite density
and almost equally limitless
fascination:
a Black Hole.
In some sense, a black hole
represents the ultimate death of a star.
A black hole is basically gravity's
victory over mass.
It is complete collapse of a star,
a very massive star.
This collapse creates a region
of space
where matter is compressed
into such a high density
that its gravitational field
is inescapable.
Black holes are remarkable
and nothing can escape from them,
not even the fastest moving
thing we know of, which is light.
You shine a flashlight beam up
and even it won't leave,
the beam will curve back around.
So, you won't be able
to see it from the outside.
Hence the name "black hole".
A common misperception
is that black holes just go sucking up
everything in the Universe.
Like cosmic vacuum cleaners
sucking up everything
in their vicinity.
That's actually not true.
Now, objects that are very close to
black holes do get sucked in,
but if you're comfortably far away,
with the proper trajectory
you won't get sucked in.
Scientists have long suspected that
there is yet another class of supernova
involving even bigger stars
and even more powerful explosions.
Stars that collapse so catastrophically
that they leave behind
no remnant, not even a black hole.
But no one had ever seen one
until now.
Even after billions of years,
the Universe is still surprising us
with its raw power.
In the fall of 2006,
astronomers observed
the largest stellar explosion
ever witnessed by Man.
240 million light years away
from Earth,
a massive star blew itself apart.
Alex Filippenko and his team at the
University of California, Berkeley,
were amazed at the power
of the explosion.
And the total energy emitted
was 100 times as much
as the energy of a normal
massive explosion.
It's an amazing,
really powerful explosion.
A normal supernova comes from
the explosion of a star
10 times more massive
than our Sun.
Incredibly, supernova 2006GY,
as astronomers have dubbed it,
seems to have signalled the
death of a star
150, or even 200 times
more massive.
That's about as massive
as a star can get.
Scientists are still studying
the aftermath of the explosion,
but they think supernova 2006GY
has a lot to teach us
about the first stars that populated
our Universe.
We actually think that the first
generation of stars
tended to be really massive.
And they probably exploded
by this mechanism.
It's these mega-explosions
that likely seeded the early Universe
with heavy elements.
These extremely massive stars
are the largest iron factories
in the Universe.
A single star, 150 times the
mass of the Sun,
can produce 20 or 25 solar
masses of iron.
It's incredible.
In the cycle of life, not only here
on Earth but in the Cosmos,
as stars die, particularly those that
die spectacular deaths,
the high mass stars that
manufactured
heavy elements in their cores,
those give the seeds of the next
generations of stars that then...
increased the likelihood
that that next generation
will have planets,
and planets that contain
ingredients of life itself.
Supernovas aren't the only energetic
events in the life and death of a star.
Right now, across the Universe,
there're a thousand pairs of stars
engaged in brilliant dances of fire.
For some this dance
will end in catastrophe.
Astrophysicist Joshua Barnes
of the University of Hawaii,
studies what happens when
stars collide.
We don't have the luxury
of watching stars collide.
A pair of stars as they
draw close enough to collide
would just be a single dot of light,
even in the largest
telescopes that we have.
So, we need to investigate
these things with a computer.
Using computer models,
astrophysicists can take
any two types of stars
and find out what happens if they
become involved in a stellar smash-up.
The models pose hypothetical
situations and then see what happens.
And you can sort of imagine
this is like studying collisions of cars,
and you were taking them out and smashing
them together in the parking lot,
one after the other to see
what came out of that.
Among the most explosive
collisions modelled by astrophysicists
is the clash of two orbiting
neutron stars.
Typically, they're bound together
as a pair orbiting one another
and as they orbit they disturb
the space-time* around them
and create waves of energy.
And the energy to do
that slows the stars down,
so they get closer and closer together.
As they get really close
together, they're orbiting around
hundreds or even thousands
times per second.
The final event
is very dramatic.
When two neutron stars collide, they're
moving at nearly the speed of light.
Although the final collision takes only
a fraction of a second,
it unleashes more energy than the
Sun will generate in its entire lifetime.
Thanks to computer modelling
we can also predict what would happen
if a highly dense white dwarf collided
with our Sun.
It would be a frightening collision.
When it got close enough, the
gravitational field of the white dwarf
would start to distort the Sun,
so it would no longer remain a sphere,
it would turn into an egg-shape
as this thing came close.
As the white dwarf ploughs into
the Sun at supersonic speed,
its gravity would send an enormous
shockwave throughout the star.
And that would produce so
much thermonuclear energy
to, essentially, explode the Sun.
Amazingly, it would take
only about an hour
for the white dwarf to plough
through the Sun and annihilate it.
If this scenario came to pass,
life on Earth would be doomed.
Fortunately, the chances of this
happening are slim,
because the Sun is in a very
uncrowded part of the Milky Way.
Individual stars are
kind of jostling and weaving
as they make their great circuit
around the galactic centre.
So, it's a complicated
traffic situation,
but because the space between
the stars is so great
there's not much chance
of a collision.
If you were to wait out here on
this beach until you saw the collision
between the Sun and another Star,
you would wait a long time.
Even over its entire life,
the Sun has probably
a billion in one chance
of colliding with another star.
But there are places within galaxies
where the odds of a collision
are much greater.
Regions where hundreds of thousands
or even millions of stars
are crowded together by gravity
into a globular cluster.
Compared to the spiral arms
of the Milky Way,
a globular cluster
is like a demolition derby.
The odds of two stars colliding in the
spiral arms of our galaxy
are only about one in a billion.
But within a globular cluster,
stars are packed
a million times more densely than
elsewhere in the Milky way.
In the Milky Way everybody is
pretty much going in the same direction,
but in a globular cluster
there's no organized motion.
They're basically all orbiting
around the centre
on orbits that are aligned
in all sorts of different directions,
so some are going one way,
some are going the opposite way...
In these crowded, chaotic
conditions stars collide on average
once every 10,000 years.
Every star in a cluster
was born at roughly the same time,
so when astronomers look at
an old cluster
they don't expect to see
any young stars,
but strangely a globular cluster usually
conceals some mysterious strangers.
Large blue stars, far younger than
the small dim stars surrounding them.
These seemingly impossible stars
are known as
"Blue Stragglers".
The mystery of blue stragglers
is that they're,
in some sense, younger than
they have any right to be.
All of the stars of that mass
and that luminosity
would have died off billions
of years ago in these clusters,
so the puzzle is, where do these
things come from,
how did they get into the
star clusters.
Astrophysicist Joshua Barnes
thinks he knows the answer.
He believes blue stragglers are
the result of collisions
between older and dimmer
main sequence stars.
A collision of two
main sequence stars,
two Sun-like stars,
is actually relatively gentle.
The mutual gravity of the stars
locks them in a spiral.
They've lost energy of
motion and they will come back
and have multiple subsequent passages.
They heat up and swell up and
kind of spiral around each other,
making several passes,
each closer than the last one,
until they finally come
together and the stars merge.
In the end, rather than
triggering a catastrophe,
the two stars merge to form
one more massive star.
What you're basically doing is
taking to small old stars,
piling* them together to make
one star now which is twice as massive,
and therefore being more massive
it's brighter and bluer
than the rest of
the stars in the cluster.
So it seems to be straggling
behind the rest of the stars.
While the mystery of the blue
stragglers seems to have been solved,
the heavens are bursting
with unusual objects
that dare science to explain them.
Black holes, neutron stars
and white dwarfs,
all represent the end of
remarkable stellar lives.
But there are other strange celestial
objects that never got a chance to shine.
Not quite planets, not quite stars,
these are the brown dwarfs.
A brown dwarf is basically a
failed star.
University of Hawaii astronomer Michael
Liu, searches for these elusive objects.
Stars produce a lot of light, they're
very easy to see a long way away.
The brown dwarfs are
very low temperature
so they emit very, very little light.
Because they're so dim,
it means
we can only see them
if they're very close to us.
A brown dwarf has the same
ingredients as a star,
but it simply doesn't have enough
mass to sustain nuclear fusion.
It's something that's borne
with less than 1% the mass of the Sun,
so it can't produce
its own energy,
it's essentially a failed star.
Without fusion, these failed stars
start to act more like planets.
If you were flying in a spaceship
across the surface of the star,
you wouldn't really see
anything that looked like
clouds or mountains
or anything like that.
When you go to a brown dwarf
things begin to change.
We think their atmospheres
in some ways might be similar
to things like very massive
versions of the planet Jupiter.
If you're familiar
with pictures of Jupiter
you see Jupiter has also a banding
structure and clouds on its surface.
Although we've never taken a picture
of the surface of a brown dwarf,
we think brown dwarfs may also have
a similar cloud structure.
These aren't normal kinds of clouds
like we know about on the Earth,
you have iron vapour
making these clouds,
and then the clouds
may get thick enough
that you get iron droplets
raining out of the clouds.
Obviously a person wouldn't want
to be there 'cause these are molten iron.
To date astronomers have located
only a couple hundred brown dwarfs,
and they still have many questions
about these elusive objects.
For one, they know some
brown dwarfs
have discs of dust and gas
around them.
Might those discs form
into planets?
That's just one of many
mysteries yet to be solved
as we continue to probe the stars.
But already, science has revealed
the Universe to be a magical realm
of dwarfs and giants,
stragglers and supernovas,
and hidden within the
explosive life story of stars
they have found the very
history of the Cosmos,
and a key to understanding
our own origins.
there was darkness... and then...
BANG! Giving birth to an
endless expanding existence
of time, space and matter.
Now, see further than we've ever imagined
beyond the limits of our existence,
in a place we call...
THE UNIVERSE.
Each star you see
twinkling in the night sky,
is luminous sphere of superheated gas,
much larger than any planet.
And each has a story to tell.
A dramatic birth,
a life on the edge.
Gravity collects the star
in the first place,
and then gravity wants
to crush it.
And a death that rattles
the heavens.
The whole thing goes off
in a blinding flash,
the biggest explosion
in the Universe.
The Universe at its most volatile
and action-packed...
Life and Death of a Star.
Like glittering cities
in the desert,
galaxies arise out of the
great darkness of the Universe.
Galaxies made of billions of
blazing lights called stars.
There are billions
and billions of stars.
In fact, in our galaxy there are
400 billion stars,
just in our galaxy.
But how were these stars born?
How will they die?
And how can it be that all
human beings on Earth
owe their lives to
the deaths of stars?
The quest for answers
begins here,
in a cloud of dust and gas
hovering in the interstellar desert.
You are looking at the Pillars
of Creation.
The Pillars of Creation are a
stellar nursery.
New stars are in the process
of being born in the central regions.
Located 7,000 light years from Earth,
the pillars are part
of the Eagle Nebula,
which is just one of billions of star
forming regions in the Universe.
The pillars are towering clouds
of dust and hydrogen gas.
If you remember the Periodic Table
of elements from chemistry class,
you have the light
elements up at the top,
hydrogen, helium,
lithium, this sort of thing.
And then the really heavy ones
as you get lower down.
It's hydrogen, the lightest, simplest
most abundant element in the Universe,
that is the key component of stars.
Within a nebula, clumps of
this gas and dust
slowly coalesce into smaller clouds
over millions of years,
pulled together by
a very familiar force.
The same force that
connects us here to the Earth,
that keeps us on the Earth,
gravity, is the same force that
pulls things together
in a way that gives us...
planets and stars and galaxies
in the Universe.
Gravity, in many senses, is the
most important force in astronomy.
And when gravity acts
in the Universe
one of the basic things
that it produces is stars.
Stars are sort of the most
basic unit of mass
that is produced when
gravity pulls mass together.
Each contracting cloud
can produce anywhere
from a few dozen
to thousands of stars.
To form a star like our Sun,
which is a million miles across,
it takes a clump of gas and dust
100 times the size of our
Solar System.
These clouds start off their lives
bitterly cold,
with temperatures hundreds
of degrees below zero Fahrenheit.
But as gravity fragments
and compresses them,
the heat begins to soar.
Within a few hundred
thousand years,
the cloud spins into
a flattened disc.
Gravity coalesces the centre
of the disc into a sphere,
where the heat rises to a
scorching 2 million degrees.
This glowing system
is now known as a "protostar".
Ten million years later,
the searing hydrogen core
of the fledgling star
soars past 18 million degrees,
and something incredible happens.
The core becomes so hot,
it can sustain thermonuclear fusion.
Thermonuclear fusion...
is a lot of syllables but it just
means it's hot there
and small atoms become big atoms.
Hydrogen atoms are moving
fast enough
that they actually will fuse together
and will form a helium atom.
It's this nuclear reaction
that produces the energy
to power the star
throughout its life,
giving it a constant source
of light and heat.
It's self-luminous,
it generates its own heat.
And that's the essence of
what makes a star a star.
If you've got fusion,
you've got a star.
Once born, a star's life will
be a constant battle,
an all-out war against gravity.
Gravity collects the star in the first
place, and then it wants to crush it.
Gravity never gives up, gravity wants
to pull everything together.
So if the star is going to have a life,
and a long life,
it has to find a way
to fight against gravity.
You feel gravity all the time.
When you try to jump
or you try to climb a rock,
there's always gravity
pulling you back down.
And in order to fight against
gravity, you have to have some way
of applying a force
which works in the opposite
direction than gravity.
So, if there's a rope, you can use
your muscles to pull on the rope,
and therefore resist,
and even overcome, gravity.
But that doesn't mean
gravity gives up.
Gravity is always working.
So you have to keep applying
this force in order to not fall off.
And if you give up or let go
or the rope breaks,
gravity immediately wins
and you fall.
The same kind of thing
happens with stars.
Stars are also trying to hold themselves
up against gravitational collapse.
Gravity wants to crush the star
down to the middle.
For stars, nuclear fusion
provides the rope,
in the form of pressure.
The heat gets all the particles
in the star moving around quickly,
and they bang outwards,
and that produces a pressure
which can actually hold the star up
against gravity.
The amount of pressure pushing out
on the star just matches
the amount of gravity
pulling in on the star.
And it can sit there and burn happily
until something changes.
A star will spend most of its life
in this state of equilibrium.
It's a phase scientists call
the "main sequence".
So, our Sun is in the main sequence,
we're very happy it's there,
it provides us the same amount
of energy almost every day,
and that's what makes life possible.
All stars in the main sequence
aren't alike.
Some are much smaller
and cooler than the Sun.
Others, much larger and hotter.
So, it turns out that how
hot something is
is related to the colour of the light
that it emits.
So, a star like the Sun,
most of the light that comes out from it
is sort of a yellow-type colour.
If the Sun were much hotter,
the predominant wavelength of
light would come out into the blue.
Or even into the ultra-violet.
And cooler stars emit
more red light.
Small, cool, red stars,
like Proxima Centauri,
the nearest star to the Sun,
are known as "Red Dwarfs".
They can be as little as
1/10 the mass of the Sun.
With surface temperatures
thousands of degrees cooler.
Red dwarfs are the most common
type of stars in the Universe.
There are many, many more of these
sort of very dim, red dwarfs
floating out in space than there are
stars like the Sun.
Of course, when you
look in the night sky,
you don't see the most
common kinds of stars,
you don't see these red dwarfs
'cause they're so faint.
You merely see the very
rare, very bright stars
that turn out to be
very, very far away.
On the opposite end
of the spectrum,
are the large blue,
main sequence stars.
Averaging a surface temperature
of 45,000 ?F.
They can be 20 times
the mass of the Sun.
And 10,000 times
more luminous.
In the life and death of a star,
size definitely matters.
Mass is the fundamental thing
which drives the life history
of a star.
The more massive stars
live much shorter lives than
the less massive stars.
And that's perhaps a little
bit strange sounding,
because the massive stars have
more fuel to burn
you'd think they'd live longer.
So, it's counterintuitive
that more massive stars
will burn through their fuel
more quickly than the
lower mass stars.
Imagine two gamblers sitting
down at a blackjack table.
You would expect the one with
the most money, the most "fuel to burn",
would last the longest.
But what if the big-time gambler
is making huge bets on every hand?
A gambler that is gambling
with a lot more money
and putting down $10,000 at a time
is gonna burn through that money
much more quickly.
So the more mass you have, the higher
temperature, the higher pressure,
the higher the fusion rate.
It goes much more quickly
with the more mass you have.
And it's always just simply the
calculation: how much fuel do you have,
and at what rate are you
converting it.
The high-mass stars live
their lives faster.
They burn the candle at both ends,
it's life in the fast lane.
A high-mass star could die
within a million years.
A star 10 times as massive
as our Sun,
might live for only
1/1000 as long.
So our Sun will live for about
10 billion years in total.
A star 10 times as massive
as our Sun,
might live only
10 million years in total.
While massive stars have lifespans
measured in millions of years,
the lowest mass stars
measure their lives
in tens of billions, if not trillions,
of years.
Every low mass star that has
ever been born in the Universe,
and the Universe has been making
stars for more than 10 billion years,
all of those stars are still
in their infancy.
No such star that's ever been born
has ever come close to dying.
But for all stars,
including our own Sun,
life on the main sequence
can't go on forever.
It can only last as long as the star
has fuel to burn.
If it runs out of fuel,
fusion stops and gravity... wins.
Gravity never gives up,
whereas fuel, of course,
can run out after a while.
And so the star and the climber
both have this terrible problem,
that if they don't maintain their fight
against gravity, they will end in death.
A cataclysmic death.
Not only does the size of a star
influence how long it will live,
it also determines how
it will die.
Massive stars explode from
the scene in violent fury,
while smaller ones are doomed
to slowly fade away.
For 5 billion years,
our Sun,
a lower mass, middle-aged star,
has been happily burning through
its supply of hydrogen fuel.
Like a gambler slowly plowing
through a pile of chips.
The gambler may sit there for
a long period of time,
just like a star burns its hydrogen
for a really long period of time.
However, at some point,
you're gonna run out of money.
Scientists predict that
5 billion years in the future,
our Sun will reach
this critical crossroads:
Its supply of hydrogen fuel
will have been completely exhausted,
nuclear fusion will cease,
and gravity will begin
to crush the star.
At that point the situation
is desperate.
In order to survive, a sun-like star
must find a new source of fuel.
It has helium on hand...
but in order to start burning helium,
the core has to be
10 times hotter than it was during
its lifetime burning hydrogen.
It won't be able to fuse that helium
into heavier elements,
like carbon and oxygen,
until the core gets sufficiently hot.
And that's because it's harder to get
the helium nuclei close enough together
for the strong nuclear force
to take over,
grab them,
and cause them to fuse together.
As it continues to contract inward,
nature throws the star a lifeline.
The core actually becomes
superheated
by the very gravitational pressure
that's trying to crush it.
When it reaches 180 million degrees,
it can start fusing helium into carbon,
in a desperate gamble to survive.
So the desperate gambler might go
take out a loan on the house
and get more money.
But in getting more money
to burn through,
it's really just delaying the inevitable,
which is to go bust.
And for a star,
the inevitable is to die.
The star which took 10 billion years
to burn through its hydrogen,
now powers through its supply
of helium in a mere 100 million years.
And then the action begins.
It runs out of hydrogen...
starts fusing helium.
Runs out of helium...
attempts to fuse carbon and will fail.
But all the action,
all the "what's going on now", happens
in the last 10% of the star's life.
The searing heat of the helium burning
actually causes the outer layers
of the star to swell.
At that point, the outer
atmosphere of our star
will be held in by gravity so weakly
that it'll start sort of
just evaporating away.
Through a series of what I call
"cosmic burps", it will actually eject
the outer envelope of gases, which
are only weakly held by gravity.
That'll send some shells
of gas outward
illuminated by the hot
central star.
And that will cause what's called
the "planetary nebula" phenomenon.
Beautiful shells of glowing gas
surrounding the dying core of our sun.
With the core unable to muster
any more nuclear fusion,
can it possibly survive gravity's
crushing grip?
As a star the size of our Sun dies,
it ejects its outer layers.
With no nuclear reactions to generate
outward pressure,
gravity gains the upper hand.
The star begins to fall in
on itself,
like a climber too tired to hold on
to his rope.
There's one possibility that the rock
climber might be able to use
if he gets too tired to hold on
to the rope any more,
and that is if he can find a ledge
on the rock that he's climbing.
Gravity can pull on him
only once
but the ledge itself will support
him against gravity.
And he doesn't have to provide
any more energy to win his fight.
There's a certain kind of star, and
our Sun is actually an example of this,
where the star finds that it has
an "out" in this fight against gravity.
The contracting star finds its ledge
in a surprising place: electrons,
tiny, negatively charged
atomic particles.
Electrons don't like being compressed
so they're very close to one another,
because electrons effectively
don't like each other.
If you compact the electrons hard enough,
the pressure of the electrons themselves
is able to hold up the star
against gravity.
When the core of our dying
Sun-like star
is crushed to about
the size of the Earth,
this so-called "electron degeneracy
pressure" takes over.
Gravity can collapse the star
no further.
It's left to slowly cool into a bizarre
stellar remnant known as a "White Dwarf".
Like this one, Sirius B,
which can be seen only faintly aside
its companion Sirius,
the brighter star in our sky.
Now, a white dwarf is a very strange
type of star.
It's very, very dense.
The white dwarf has about 300,000
times the mass of the Earth,
compressed into a volume
the size of the Earth.
If you had just a teaspoon full of
material, it would weigh several tons.
So, it's really amazing stuff.
A white dwarf is the final stage
in the life of a Sun-like star.
But it's not quite dead yet.
It will continue to shine
for billions of years
as it gradually radiates away
a lifetime of energy.
I like to call white dwarfs
"retired stars",
in the sense that all of the light
that they are shining,
is energy that they accumulated during
their normal lives as stars,
while they were fusing light elements
into heavy elements,
as our Sun is doing
right now.
So, it's spending its life savings,
it's a retired star.
That will be the fate of our Sun.
But some white dwarfs can have
one last hurrah,
thanks to a friend who lends
a helping hand.
Because, although our Sun
is a cosmic loner,
more than half of all stars travel
through life with at least one companion.
Most stars are members of binaries,
or possibly even multiple star systems.
Close binary stars
can have very different fates from
the ordinary single stars.
If a white dwarf is gravitationally
bound to another star
as part of a binary system,
it can essentially steal the lifeblood
from its companion.
The small but dense white dwarf
exerts such a strong gravitational pull
that it will start siphoning off
a stream of hydrogen gas.
If it gathers material from
a companion star,
and is able to grow in mass,
then eventually, the mass of the
white dwarf can reach an unstable limit,
roughly 40% more than
the mass of our Sun.
At that point, the white dwarf
undergoes a catastrophic explosion,
where the whole thing goes off
in a blinding flash.
What's called the "thermonuclear
runaway" of the entire star.
This mammoth explosion is known
as a Type Ia Supernova.
So, if our Sun were to do this,
and it won't,
it'll die in a relatively quiet way...
But if it were to do this,
you'd need
sunblock or supernova-block
of a few billion
in order to protect yourself
from the blinding flash.
University of California Berkeley
astronomer, Alex Filippenko,
is one of the world's most successful
supernova hunters.
His team has found over 600
of them in the past decade.
An incredible feat
considering they occur perhaps twice
per century in each galaxy.
Searching for supernovas
is akin to scanning a crowded
football stadium with binoculars,
in hopes of catching the one person
who might be taking a flash photograph
at a given point in time.
If you were to look at each person
individually, one by one,
you would have a hard time
finding the person who happens
to be taking a flash photo.
Filippenko increases his odds
by expanding his search beyond
single stars,
or even single galaxies.
To do this he enlists the help of
a very high-tech assistant.
So this is a robotic
search engine
for exploding stars,
supernovae.
It has been programmed to
robotically take photographs
of over a thousand galaxies
a night,
and over the course of a week
it does 7 or 8,000 galaxies,
and then it repeats the process
comparing the new pictures of
each galaxy with old pictures.
Usually there's nothing new in the
new picture, but occasionally,
a star blows up,
a supernova goes off.
And then you can see in the new
picture a bright point of light
that wasn't there in any
the old pictures.
Though a supernova is visually
very, very bright,
the visible light is only one percent
of one percent of the total energy.
1/10,000 of the entire energy
emitted by this colossal explosion.
Although type IA supernovas come
from exploding white dwarfs,
many others, known as
Type II supernovas,
signal the dramatic deaths
of much more massive stars,
perhaps 8 or 10 times
more massive than the Sun.
Unlike their smaller cousins,
when massive stars exhaust
their hydrogen fuel,
they have the raw power to start
fusing other elements.
The ashes of each set of nuclear
reactions become fuel for the next,
so that near the end of its life,
a massive star resembles an onion
in cross-section, with an outer layer
of the original fuel, hydrogen,
surrounding layer after layer
of heavier and heavier elements.
It goes through its normal life
fusing hydrogen into helium,
then helium into carbon and oxygen,
then oxygen into neon and magnesium,
and then silicon and sulfur...
And then, iron. The massive star
builds up a core of iron.
The fusion of iron
into heavier elements
doesn't do the star any good,
it doesn't keep the star hot inside,
because fusion of iron into
heavier elements
requires energy and absorbs energy,
it doesn't liberate energy.
So the iron core builds
up without fusing,
and eventually becomes unstable,
one it reaches something like 1.5 times
the mass of our Sun, it collapses.
And the collapse is violent.
Within half a second
a core the size of the Earth
is crushed into an object roughly
10 miles across.
For a moment the collapsing
core rebounds,
smashing into the outer layers
of the star,
and kicking off one of the most massive
explosions in our Universe
since the Big Bang.
The collapse of the iron core
blows apart the rest of the star
in a colossal explosion.
It's truly an amazing,
incredible event.
Scientists are convinced that supernovas
mean much more to the Universe
than spectacular light shows.
They are in fact the source
of the heavy elements
that make up everything
around us.
All of the iron in this foundry
came from exploding stars,
from gigantic explosions.
All of it. All the iron you see
everywhere came from exploding stars.
And, in fact, all the elements
heavier than iron
directly or indirectly were
made by exploding stars.
And those elements were ejected into the
cosmos by these gargantuan explosions.
As material from
these explosions
spread out through the Universe,
it became the stuff of
planets, moons, new stars and something
even more extraordinary...
If you could trace your ancestry
back to its earliest reaches,
you would find an exploding star
in your family tree.
We are essentially made of
star stuff, or stardust,
as Carl Sagan used to say.
The elements in your body, not
just generically, but specifically,
the elements in your body heavier
than hydrogen and helium,
came from long-dead stars.
The calcium in your bones,
the oxygen that you breathe,
the iron in your red bloodcells,
the carbon in most of your cells...
all those things were created in stars
through nuclear reactions,
and then ejected
by supernovae.
And the heaviest elements,
iron and above,
were produced by the explosions
themselves, by the supernovae.
While the explosion of
a Type II supernova
showers the Universe with
heavy elements,
the core of the exploding
star is left intact.
Destroying that is gravity's job.
But to crush the core any smaller
than the size of a white dwarf,
it will have to overcome that strange
force, electron degeneracy pressure.
Gravity actually finds a way
of defeating
that tendency the electrons
have to push each other apart,
by combining the electrons with the
protons and turning them into neutrons.
You now have an object which is made
almost entirely out of neutrons,
and gravity wins, it now allows
the system to collapse further,
there're no longer
electrons stopping that,
and gravity seems to win.
Except... neutrons, it turns out,
also don't like each other,
and you end up with a new
stable object even smaller,
even more dense
called a "Neutron Star".
Compared to normal stars,
neutron stars are cosmic pebbles.
They can be as small as
10 miles across.
So imagine that you take a star about
1.5 times the size of our Sun
and then you compress all that
material down into a very small space,
about the size of Manhattan.
You just made yourself
a neutron star.
Squeezing that amount of mass
into such a small space
makes for an extremely
dense object.
One teaspoon full of neutron star
material would weigh a billion tons.
Neutron stars are some
of the most exciting
and weird objects in the Universe
that astronomers study.
If a human being were to stand
on a neutron star,
it would be a somewhat
uncomfortable experience.
On Earth, if they weighed about 150 lbs.
on a neutron star they would weigh
something like 10 billion tons.
Our biology can't stand that
amount of pressure
and so, a human being would
essentially be squashed flat
against the surface of the star.
In addition to that, neutron stars
are spinning at an incredibly high rate.
Hundreds of times per second
in some cases.
It's this rapid spin
that enabled astronomers to first
identify neutron stars.
Some neutron stars are
spinning really rapidly,
and they have a really amazingly
high magnetic field.
That magnetic field,
together with the spin,
forces a bunch of charged
particles, electrons,
to go along the axis
of the magnetic field.
And those accelerated electrons
give off light,
they produce a very
focussed beam of light.
Now, this is like a lighthouse
whose beam is always on,
but you only see it
when the lighthouse beam intersects
your line of sight.
In a similar way, we might see
the shining neutron star
only when the beam
points at us.
That object is called a "Pulsar".
Some stars are so massive, perhaps
25 or 40 times the mass of the Sun,
that not even a neutron star can hold up
under the weight of their collapse,
and gravity will crash them
even further,
into an object of infinite density
and almost equally limitless
fascination:
a Black Hole.
In some sense, a black hole
represents the ultimate death of a star.
A black hole is basically gravity's
victory over mass.
It is complete collapse of a star,
a very massive star.
This collapse creates a region
of space
where matter is compressed
into such a high density
that its gravitational field
is inescapable.
Black holes are remarkable
and nothing can escape from them,
not even the fastest moving
thing we know of, which is light.
You shine a flashlight beam up
and even it won't leave,
the beam will curve back around.
So, you won't be able
to see it from the outside.
Hence the name "black hole".
A common misperception
is that black holes just go sucking up
everything in the Universe.
Like cosmic vacuum cleaners
sucking up everything
in their vicinity.
That's actually not true.
Now, objects that are very close to
black holes do get sucked in,
but if you're comfortably far away,
with the proper trajectory
you won't get sucked in.
Scientists have long suspected that
there is yet another class of supernova
involving even bigger stars
and even more powerful explosions.
Stars that collapse so catastrophically
that they leave behind
no remnant, not even a black hole.
But no one had ever seen one
until now.
Even after billions of years,
the Universe is still surprising us
with its raw power.
In the fall of 2006,
astronomers observed
the largest stellar explosion
ever witnessed by Man.
240 million light years away
from Earth,
a massive star blew itself apart.
Alex Filippenko and his team at the
University of California, Berkeley,
were amazed at the power
of the explosion.
And the total energy emitted
was 100 times as much
as the energy of a normal
massive explosion.
It's an amazing,
really powerful explosion.
A normal supernova comes from
the explosion of a star
10 times more massive
than our Sun.
Incredibly, supernova 2006GY,
as astronomers have dubbed it,
seems to have signalled the
death of a star
150, or even 200 times
more massive.
That's about as massive
as a star can get.
Scientists are still studying
the aftermath of the explosion,
but they think supernova 2006GY
has a lot to teach us
about the first stars that populated
our Universe.
We actually think that the first
generation of stars
tended to be really massive.
And they probably exploded
by this mechanism.
It's these mega-explosions
that likely seeded the early Universe
with heavy elements.
These extremely massive stars
are the largest iron factories
in the Universe.
A single star, 150 times the
mass of the Sun,
can produce 20 or 25 solar
masses of iron.
It's incredible.
In the cycle of life, not only here
on Earth but in the Cosmos,
as stars die, particularly those that
die spectacular deaths,
the high mass stars that
manufactured
heavy elements in their cores,
those give the seeds of the next
generations of stars that then...
increased the likelihood
that that next generation
will have planets,
and planets that contain
ingredients of life itself.
Supernovas aren't the only energetic
events in the life and death of a star.
Right now, across the Universe,
there're a thousand pairs of stars
engaged in brilliant dances of fire.
For some this dance
will end in catastrophe.
Astrophysicist Joshua Barnes
of the University of Hawaii,
studies what happens when
stars collide.
We don't have the luxury
of watching stars collide.
A pair of stars as they
draw close enough to collide
would just be a single dot of light,
even in the largest
telescopes that we have.
So, we need to investigate
these things with a computer.
Using computer models,
astrophysicists can take
any two types of stars
and find out what happens if they
become involved in a stellar smash-up.
The models pose hypothetical
situations and then see what happens.
And you can sort of imagine
this is like studying collisions of cars,
and you were taking them out and smashing
them together in the parking lot,
one after the other to see
what came out of that.
Among the most explosive
collisions modelled by astrophysicists
is the clash of two orbiting
neutron stars.
Typically, they're bound together
as a pair orbiting one another
and as they orbit they disturb
the space-time* around them
and create waves of energy.
And the energy to do
that slows the stars down,
so they get closer and closer together.
As they get really close
together, they're orbiting around
hundreds or even thousands
times per second.
The final event
is very dramatic.
When two neutron stars collide, they're
moving at nearly the speed of light.
Although the final collision takes only
a fraction of a second,
it unleashes more energy than the
Sun will generate in its entire lifetime.
Thanks to computer modelling
we can also predict what would happen
if a highly dense white dwarf collided
with our Sun.
It would be a frightening collision.
When it got close enough, the
gravitational field of the white dwarf
would start to distort the Sun,
so it would no longer remain a sphere,
it would turn into an egg-shape
as this thing came close.
As the white dwarf ploughs into
the Sun at supersonic speed,
its gravity would send an enormous
shockwave throughout the star.
And that would produce so
much thermonuclear energy
to, essentially, explode the Sun.
Amazingly, it would take
only about an hour
for the white dwarf to plough
through the Sun and annihilate it.
If this scenario came to pass,
life on Earth would be doomed.
Fortunately, the chances of this
happening are slim,
because the Sun is in a very
uncrowded part of the Milky Way.
Individual stars are
kind of jostling and weaving
as they make their great circuit
around the galactic centre.
So, it's a complicated
traffic situation,
but because the space between
the stars is so great
there's not much chance
of a collision.
If you were to wait out here on
this beach until you saw the collision
between the Sun and another Star,
you would wait a long time.
Even over its entire life,
the Sun has probably
a billion in one chance
of colliding with another star.
But there are places within galaxies
where the odds of a collision
are much greater.
Regions where hundreds of thousands
or even millions of stars
are crowded together by gravity
into a globular cluster.
Compared to the spiral arms
of the Milky Way,
a globular cluster
is like a demolition derby.
The odds of two stars colliding in the
spiral arms of our galaxy
are only about one in a billion.
But within a globular cluster,
stars are packed
a million times more densely than
elsewhere in the Milky way.
In the Milky Way everybody is
pretty much going in the same direction,
but in a globular cluster
there's no organized motion.
They're basically all orbiting
around the centre
on orbits that are aligned
in all sorts of different directions,
so some are going one way,
some are going the opposite way...
In these crowded, chaotic
conditions stars collide on average
once every 10,000 years.
Every star in a cluster
was born at roughly the same time,
so when astronomers look at
an old cluster
they don't expect to see
any young stars,
but strangely a globular cluster usually
conceals some mysterious strangers.
Large blue stars, far younger than
the small dim stars surrounding them.
These seemingly impossible stars
are known as
"Blue Stragglers".
The mystery of blue stragglers
is that they're,
in some sense, younger than
they have any right to be.
All of the stars of that mass
and that luminosity
would have died off billions
of years ago in these clusters,
so the puzzle is, where do these
things come from,
how did they get into the
star clusters.
Astrophysicist Joshua Barnes
thinks he knows the answer.
He believes blue stragglers are
the result of collisions
between older and dimmer
main sequence stars.
A collision of two
main sequence stars,
two Sun-like stars,
is actually relatively gentle.
The mutual gravity of the stars
locks them in a spiral.
They've lost energy of
motion and they will come back
and have multiple subsequent passages.
They heat up and swell up and
kind of spiral around each other,
making several passes,
each closer than the last one,
until they finally come
together and the stars merge.
In the end, rather than
triggering a catastrophe,
the two stars merge to form
one more massive star.
What you're basically doing is
taking to small old stars,
piling* them together to make
one star now which is twice as massive,
and therefore being more massive
it's brighter and bluer
than the rest of
the stars in the cluster.
So it seems to be straggling
behind the rest of the stars.
While the mystery of the blue
stragglers seems to have been solved,
the heavens are bursting
with unusual objects
that dare science to explain them.
Black holes, neutron stars
and white dwarfs,
all represent the end of
remarkable stellar lives.
But there are other strange celestial
objects that never got a chance to shine.
Not quite planets, not quite stars,
these are the brown dwarfs.
A brown dwarf is basically a
failed star.
University of Hawaii astronomer Michael
Liu, searches for these elusive objects.
Stars produce a lot of light, they're
very easy to see a long way away.
The brown dwarfs are
very low temperature
so they emit very, very little light.
Because they're so dim,
it means
we can only see them
if they're very close to us.
A brown dwarf has the same
ingredients as a star,
but it simply doesn't have enough
mass to sustain nuclear fusion.
It's something that's borne
with less than 1% the mass of the Sun,
so it can't produce
its own energy,
it's essentially a failed star.
Without fusion, these failed stars
start to act more like planets.
If you were flying in a spaceship
across the surface of the star,
you wouldn't really see
anything that looked like
clouds or mountains
or anything like that.
When you go to a brown dwarf
things begin to change.
We think their atmospheres
in some ways might be similar
to things like very massive
versions of the planet Jupiter.
If you're familiar
with pictures of Jupiter
you see Jupiter has also a banding
structure and clouds on its surface.
Although we've never taken a picture
of the surface of a brown dwarf,
we think brown dwarfs may also have
a similar cloud structure.
These aren't normal kinds of clouds
like we know about on the Earth,
you have iron vapour
making these clouds,
and then the clouds
may get thick enough
that you get iron droplets
raining out of the clouds.
Obviously a person wouldn't want
to be there 'cause these are molten iron.
To date astronomers have located
only a couple hundred brown dwarfs,
and they still have many questions
about these elusive objects.
For one, they know some
brown dwarfs
have discs of dust and gas
around them.
Might those discs form
into planets?
That's just one of many
mysteries yet to be solved
as we continue to probe the stars.
But already, science has revealed
the Universe to be a magical realm
of dwarfs and giants,
stragglers and supernovas,
and hidden within the
explosive life story of stars
they have found the very
history of the Cosmos,
and a key to understanding
our own origins.