The Universe-8.
Stars and planets are born, live their lives and die.
The Universe also lives its life in distinct stages.
It began 13.75 billion years ago with the Big Bang, and in this embryonic period,
known as the Primordial Era, the Universe was a place without the light from the stars.
The swirling hot matter would have blown as brightly as a sun.
For the first 100 million years, the conditions were far too violent for stars to form.
This changed when the Universe had expanded and cooled sufficiently for the weak force of gravity,
to begin to clump the primordial dust, gas and dark matter into galaxies.
With this came the dawning of the second great epoch in the life of the Universe:
the Stelliferous Era, the age of stars.
The moment the first stars were born is one of the most evocative milestones,
in the evolution of the Cosmos.
It signals the end of a alien time when the Universe was without a structure -
a formless void.
The beginning of the Stelliferous Era marks the beginning of the age of light.
This is the moment when the Universe would have been recognisable by us.
The sky would have been become black, punctuated with the glowing mist of the galaxies,
and the sharp sliver of the stars.
This is our Universe today -
a place where starlight decorates our nights, and illuminates our days.
Our Sun is one of at least 200 billion stars in our galaxy.
It is one of the 100 billion galaxies in the observable Universe.
Despite the fact that the Universe is over 13 billion years old, we are still just at the beginning.
We are living close to the beginning of the Stelliferous Era,
an era of astonishing beauty and complexity.
But the Cosmos isn’t static and unchanging.
It won’t always be this way because as the arrow of time plays out,
it produces a Cosmos that is as dynamic as it is beautiful.
In 2009, NASA’s Swift satellite detected one of the most distant cosmic explosions -
a gamma ray burst that lasted 10 seconds.
The Swift satellite was designed and built with the intention that it would aid,
the study of a rare type of event known as a gamma ray burst.
These events, which last only a few seconds,
are the most energetic and powerful emitters of radiation in the known Universe.
It is thought that gamma ray bursts occur in supernova explosions -
as the dying act of the most massive stars as they collapse to form black holes.
Minutes after the burst had faded away, the UK’s Infrared Telescope, UKIRT,
in Hawaii saw the glowing ember of the explosion.
As the day progressed, the largest telescope across the world focused on the event,
as it appeared above the horizon.
The afterglow was observed for several hours,
but after 5 days the event had faded completely from view.
The blob of the fading remains of GRB090423 was once one of the brightest stars in the Universe.
The poetically named GRB090423 was once a Wolf-Rayet star.
Named after two French astronomers who discovered the first one in 1867,
Wolf-Rayet stars are massive.
They are over 20 times the mass of the Sun.
Because they are so massive, and burn so brightly, they are extremely short lived.
When these stars run out of nuclear fuel , after only a few hundred thousand years,
they die in a dramatic fashion.
They collapse in an instant, and release more energy in one second,
than our Sun will produce in its entire 10 billion year lifetime.
GRB090423 was a big Wolf-Rayet star.
It was 40 to 50 times the mass of our Sun.
However, this is not the only thing that is interesting about it.
It is not just the story of the death of this star, revealed by the brief appearance of the pale red dot,
that captivated astronomers.
The light from this dot had travelled a very long way across the Universe to reach us,
and has taken a very long time to do it.
When we look at the afterglow of this explosion,
we are looking at a event that happened a long time ago, in a galaxy very far away.
In fact, this light has been travelling towards us for almost the entire history of the Universe.
GRB090423 died over 13 billion years ago.
This is just over 600 million years after the Universe began.
This is incredibly early in the Universe’s history.
GRB090423 is one of the oldest single object ever seen.
Another galaxy called UDFy-38135531, when discovered,
in 2010, has a light travel time of over 13 billion years.
Allowing for the expansion of the Universe,
UDFy-38135531 is more than 30 billion light years away from Earth.
It is a discovery of GRB090423, and the sight of the explosive death,
of one of the first stars in the Universe, that gives us a glimpse of the grandest timescale of them all.
Today, 13.7 billion years after the Universe began, we are living through the most productive era,
that our Universe will ever know.
The Stelliferous Era is a time of life and death,
with the constant dance between gravity and nuclear fusion creating a dynamic,
ever changing landscape in the heavens.
For a human being, for whom a century is a lifetime, the changes may appear slow.
But there is no doubt that you are part of the Universe at its most vibrant.
Stars like GRB090423 play themselves out in the night sky.
We have seen at first hand that no star can last forever.
Every star has a destiny, including our own Sun.
The Sun was formed 4.57 billion years ago from a collapsing cloud of hydrogen and helium,
and a sprinkling of heavier elements.
For the tiniest fraction of this time, humans have marked the passing of the days, as it rose and set.
They surely considered it to be an eternal presence.
It was only during the 20th century that we discovered the Sun’s fires must one day dim.
At the moment the Sun is in the middle of its life, fusing hydrogen and helium,
at the rate of 600 million tonnes every second.
It will continue to do this for another 5 billion years.
But eventually it won’t simply fade away.
As the stores of hydrogen runs dry, the Sun’s score will collapse and momentarily,
as helium begins to fuse into oxygen and carbon,
a last release of energy will cause its outer layers expand.
Imperceptibly at first, the extra heat of the Sun will extend towards us,
its diameter will increase by around 250 times.
The fiery surface of our star will move beyond Mercury, towards Venus,
and onwards to our fragile world.
The effects on our planet will be as catastrophic as they are certain.
Gradually, the Earth will become hotter.
As the surface of the Sun encroaches, our oceans will boil away.
The molecules in our atmosphere will be agitated off into space.
The memory of life on Earth will fade into history.
Long after life has disappeared, the Sun will fill the horizon.
It may extend beyond Earth itself.
This swollen stage in a star’s life is known as the Red Giant phase.
It is marked by the final release of energy, and the beginning of a long, long decline.
In six billion years time, in a beautiful display of light and colour,
our Sun will shed its outer layers into space to form a planetary nebula.
We know this because we have seen the sequence of events, in the final phases of distant stars.
Written across the night sky in filamentary patches of colour are the echoes of our future.
In the far future, somewhere in the Universe,
astronomers on a world not yet formed gaze through a telescope at our planetary nebula,
and reflect on its beauty.
They may glimpse at its heart a faintly glowing ember.
All that remains of the Sun we once thought of as magnificent.
She will be smaller than the size of Earth, less than a millionth of her current volume,
and a fraction of her brightness.
Our Sun will have become a white dwarf - the destiny of almost all the stars in our galaxy.
It will be a fading, dense remnant, momentarily masked by a colourful cloud.
If our planet survives, little more than a scorched and barren rock will remain,
silhouetted darkly against the fading embers of a star.
Sirius, the brightest star in our sky, sits at just over eight light years away.
This makes it one of our nearest neighbours.
It is so bright that on occasion it can be observed during bright twilight,
partly because of its proximity, but also because it is twice as big as our Sun,
and twenty-five times as bright.
It is therefore not surprising that observations of Sirius have been recorded,
in the oldest of astronomical records.
For thousands of years we looked up at this beacon and assumed it was a single star.
In 1862, American astronomer Alvan Graham Clark observed a sister star,
hidden in the glare of Sirius’s light.
It took so long to notice Sirius’s companion because,
as the photograph taken by the Hubble Space Telescope reveals,
it is so much dimmer than its sibling.
Shining faintly, the small dot of light is an image of the white dwarf star Sirius B.
This is one of the larger white dwarf stars discovered by astronomers.
It has a mass similar to our Sun, that is packed into a sphere the size of Earth.
With no fuel left to burn, white dwarfs like Sirius B glow faintly with residual heat,
of their extinguished furnaces.
Like most white dwarfs, Sirius B is made primarily of oxygen and carbon.
This is the remnants of helium fusion, packed tightly with a density of million times,
of that a younger, living star.
This is the future of our Sun; a vision of Sun’s death.
Slowly cooling in the freezing temperatures of deep space,
it is estimated that our Sun will reach this phase in six billion years time.
From Earth, if indeed there is an Earth at that time,
our Sun will shine no brighter than a full moon on a clear night.
Death must come to all stars.
One day every star in the night sky will fade, and the Cosmos will be plunged into eternal night.
This is the most profound consequence of the arrow of time.
This structured Universe that we inhabit, along side all of its wonders, the stars,
the planets and the galaxies, cannot last forever.
As we move through the age of stars, through the aeons ahead,
countless billions of stars will live and die.
Eventually, though there will be only one type of star,
that will remain to illuminate the Universe in its old age.
Although relatively young now, the Sun like every other star in the Universe must die one day.
In five billion years time, the Sun’s store of hydrogen will run dry, and the Star will begin its long, dramatic swansong.
During this lengthy goodbye, the last dying bursts of extra heat, will extend towards us,
passing Mercury and Venus on the way, and leaving a trail of destruction in its wake.
Long after life has disappeared on Earth, the Sun will continue to fill the horizon,
as it swells in the Red Giant Phase until 6 billion years time,
our Sun will shed its outer layers of gas and dust into space, exposing its core,
which will fade into a white dwarf.
The nearest star to our Sun is Proxima Centauri.
Although only 4.2 light years away, it is not visible to the naked eye.
The reason for this is that Proxima Centauri is small, very small when compared to our Sun.
It has just 12% of our Sun’s mass.
So to our eyes it will appear to shine 18,000 times less brightly than our Sun.
Proxima Centauri is a red dwarf star.
This is most common type of star in our Universe.
Red dwarfs are diminutive and cold, with surface temperatures in the region of 4000 degrees.
They do have one advantage over their more luminous and magnificent stellar brethren.
Because they are so small, they burn their nuclear fuel extremely slowly.
Consequently they have life spans of trillions of years.
This means that stars like Proxima Centauri will be the last living stars in the Universe.
The rate of fusion reactions in the cores of these red dwarfs,
that is needed to provide the thermal pressure,
to resist the inward pull of their weak gravity is very low, which enables them to live longer.
Even so, they are still active stars.
There surfaces are whipped into turmoil by the turbulent convective currents,
that constantly churn their interiors.
Amongst all this activity, explosive solar flares occur almost continuously,
blasting bursts of light and X-rays into space.
Ultimately, though, the frugality of these stars, is no defence against the arrow of time.
Four trillion years from now, Proxima Centauri’s fuel resource will finally run out.
The star will slowly collapse into a white dwarf.
After trillions of years of stellar life and death,
only white dwarfs and black holes will remain in the Universe.
In 100 trillion years time, this age of the stars will draw to a close,
and the cosmos will enter its next phase.
This is the Degenerate Era.
And yet, even after 100 trillion years of light,
the vast majority of the Universe’s history still lies ahead.
Bleak, lifeless and desolate, our Universe will go on, as it enters the dark.
In the far future of the cosmos, the last remaining beacons of light,
will no more be permitted to evade the second law of thermodynamics.
Even the white dwarfs must fade, as the laws of physics methodically dismantle the Universe.
Slowly, the glowing embers of the last stars, will cease to emit visible light.
After trillions of years, the final beacons burning in the cosmic sky, will turn cold and dark.
Their remnants are known as black dwarfs.
Black dwarfs are dark, dense, decaying balls of degenerate matter.
Nothing more than the ashes of stars, they take so long to form, that after 14 billion years,
the Universe is currently too young to contain any at all.
Yet despite never seeing one , our understanding of fundamental physics,
allows us to make concrete predictions, about how they will end their days.
It is thought that the matter inside the black dwarfs, the last matter in the Universe,
will eventually evaporate, and be carried out into the void as radiation, leaving nothing behind.
The process by which matter might, given enough time, decay, or not understood.
Scientists need more advanced theory, known as the Grand Unified Theory,
to speak with certainty about the behaviour of protons,
neutrons and electrons over trillion years timescales.
There are reasons to expect that such a theory may exist,
and that a mechanism for even the most stable subatomic particles to decay into radiation,
might be present in nature.
For this reason, experiments to measure the lifetime of protons,
are ongoing in laboratories around the world.
As of now, nobody has observed proton decay.
We are therefore in the realm of speculation.
But here is one possible, and given our understanding of physics today, probable,
story of how our Universe will end.
With the black dwarfs gone, there will not be a single atom of matter left in the Universe.
All that will remain of our once rich cosmos will be particles of light and black holes.
After an unimaginable expanse of time it is thought that even the black holes will evaporate away,
and the Universe will consists of a sea of light.
They are photons all tending to the same temperature, as the expansion of the Universe cools them,
towards absolute zero.
When we say unimaginable period of time, we mean 10 to the power of 100 years.
That is a very big number indeed.
If we were to start counting with a single atom representing one year,
there wouldn’t be enough atoms in all the stars, in all the galaxies in the entire Universe,
to get anywhere near that number.
Once the last remanence of the last star have decayed away to nothing,
and everything reaches the same temperature, the story of our Universe will finally come to an end.
For the first time in its life, the Universe will be permanent and unchanging.
Entropy finally stops increasing because the cosmos cannot get anymore disorganised.
Nothing happens, and it keeps not happening forever.
This is known as the heat death of the Universe.
It is an era where the cosmos will remain vast, cold, desolate and unchanging for the rest of time.
There is no way of measuring the passing of time, because nothing in the cosmos changes.
Nothing changes because there are no temperature differences,
and therefore no way of moving energy around to make things happen.
The arrow of time has simply ceased to exist.
This is an inescapable fact, written into the fundamental laws of physics.
The cosmos will die.
Every single one of the hundreds of billions of stars, in the hundreds of billions of galaxies,
in the Universe will expire.
Can we build a Universe in a different way?
Can we build the Universe such that it didn’t have to descend from order to chaos?
The answer is ’no’, you could not, if you wanted life to exist in it.
The arrow of time, the sequence of changes that will slowly but inexorably,
lead the Universe to its death, is the very thing that created the conditions of life, in the first place.
It took time for the Universe to cool sufficiently after the Big Bang and for matter to form.
It took time for gravity to clump the matter together to form galaxies, stars and planets.
It took time for the matter in our planet to form the complex patterns that we call life.
Each of these steps took place in perfect accord with the Second Law of Thermodynamics.
Each is a step on the long road from order to disorder.
The arrow of time has created a bright window in the Universe’s adolescence,
during which life is possible.
But it is a window that won’t stay open for long.
As a fraction of the lifespan of the Universe, as measured from its beginning,
to the evaporation of the last black hole, life as we know it is only possible,
for 10 to the power of 64 billionth of a percent.
The most astonishing wonder of the Universe is not a star or a planet or a galaxy,
it is not a thing at all - it is a moment in time.
And that time is now.
3.8 billion years ago life first emerged on Earth.
200 thousand years ago, the first humans walked the plains of Africa.
2500 years ago humans believed that the Sun was a God.
They measured its orbit with stone towers built on top of the hill.
Today, our curiosity manifests itself not as Sun gods but as science.
We have observatories infinitely more sophisticated than the hill top towers,
that can gaze deep into the Universe.
We have witnessed its past and now understand a significant amount about its present.
Even more remarkably, using the twin disciplines of theoretical physics and mathematics,
we can calculate what the Universe will look like in the distant future,
and make concrete predictions about its end.
In 1977, a space probe called Voyager 1 was launched on a grand tour of the solar system.
It visited the great gas giant planets Jupiter and Saturn,
and made wonderful discoveries before heading off into interstellar space.
13 years later, Voyager turned its cameras around and took one last picture of its home.
This picture is known as the pale blue dot.
The beautiful thing, perhaps the most beautiful thing ever photographed,
is the single pixel of light at its centre.
That pixel, that point, is our planet Earth.
At a distant of over six billion kilometres away, this is the most distant picture of our planet,
that has ever been taken.
As the great astronomer Carl Sagan wrote:
’it has been said that astronomy is a humbling and character building experience’.
There is perhaps no better demonstration of the folly of human conceits,
than this distant image of our tiny world.
It underscores our responsibility to deal more kindly with one another,
and to preserve and cherish the pale blue dot, the only home we have ever known’.
Just as we, and all life on Earth, stand on this tiny speck, adrift in infinite space,
so life in the Universe will only exist for a fleeting, dazzling instant in infinite time,
because life, just like stars, is a temporary structure on the long road from order to disorder.
But that doesn’t make us insignificant.
Life is the means by which the Universe can understand itself, if only for an instant.
That is what we have done in our brief moments on Earth.
We have sent space probes to the edge of our solar system, and beyond.
We have built telescopes that can glimpse the oldest and most distant stars.
We have discovered and understood at least some of the natural laws that govern the cosmos.
This ultimately, is why we believe we are important.
Our true significance lies in our continuing desire to understand and explore this beautiful Universe-
our magnificent, beautiful, fleeting home.