One of the things that fascinates me the most
about astronomy, is the scale of the Universe. Everything we can see in the night sky, with the
help of some of the most powerful telescopes ever built, is literally billions of years old and
outside of our own galaxy, it's so millions and billions of light years away. The term light
here in and of itself is pretty mindblowing; space is so huge that even light, the fastest
thing we know of, needs an incredibly long time to cross the enormous distances between sources
of light, be that stars or galaxies. In our own stellar neighborhood, the light emitted by the
Sun takes 4 years to reach the nearest star, Proxima Centauri. Collective light from all
the stars that make up our Milky Way takes 2 and a half million years to reach Andromeda,
the closest galaxy to ours within the so-called Local Group of galaxies. If right now there was
an intelligent species looking at our galaxy and they somehow had equipment powerful enough to
find our tiny planet orbiting our small Sun, embedded in one spiral arm of the Milky Way among
millions of other stars, they would look at a green and vibrant planet full of living being,s
but humanity's distant ancestors have just barely begun to straighten their backs and look out
over the tall grass of the African continent. We can't possibly imagine these kinds of
distances. We can work with months and years and decades but anything over a hundred years
old already starts to feel vague and far away from us. Our brains simply didn't evolve to
truly understand anything that is longer than a human lifetime and even a human lifetime is
barely a blip, and even the entire history of our species which goes back some 300,000 years,
merely the blink of an eye compared to cosmic time scales. Everything you see is in the past
because the speed of light is finite. Even the photons bouncing of the mirror take a moment,
albeit extremely short, to cross the distance between the glass and your eyes. Looking up at
the night sky with the naked eye, most photons coming from the stars have been traveling for at
least a few thousand years. If you're lucky and you're in a truly dark area, and you can spot
the very faint little blob that is the visible light of Andromeda for the unaided eye, those
photons have crossed a cold and empty part of space for 2 and half 2 million years. That's for
sure something to consider when stargazing, that this light left Andromeda when not a single human
had ever existed and the oldest human species were most likely not capable of pondering those
tiny lights dotting the curtain of the night. Thanks to powerful machines such as the Hubble
Space Telescope and the James Webb Space Telescope we can capture light from objects that formed
just a few million years after the Big Bang, the origin of the Universe, 13.8 billion years
ago. The Hubble Deep Field images are famous for causing a bit of a revolution in astronomy, since
they were the first to reveal the distant past of the Universe and teach scientists many things
about that time. The telescope itself is already an incredible feat of science and engineering a
few centuries in the making. Galileo Galilei, who was the first to build telescopes for astronomical
observations, could never have imagined people in the 20th century building a telescope the
size of a bus and then putting that thing in an orbit around the Earth by means of enormous
and powerful rockets. Let's also keep in mind that modern digital imaging and computing makes
an image like the Hubble Deep Field possible. The telescope fixed its gaze on a small, seemingly
empty patch in the sky for 10 days in 1995, with a total exposure time of over a 100 hours,
allowing in plenty of light from when Universe was still young. The result is nothing short of
astonishing. Almost all the galaxies you can see in this image are at least 12 billion years old
and so the photons captured by Hubble have been traveling through space for that long. There's no
telling what these galaxies are like right now, we can only see them as they were when their
light left on its journey into the depths of time and space. We can't really wait another
billion years and recapture a Deep Field to see how these galaxies have evolved over time. All
we can really do is take these snapshots of the Universe's past and compare that to what we see
in the Universe today and that's how scientists can try and piece together how the Universe
grew and evolved. Hubble is still a great telescope for observations in the visible and
ultraviolet range of the electromagnetic spectrum, all the wavelengths of light and how much energy
they have, but the much more advanced James web Space Telescope was specifically built to capture
light in the infrared range, low energy light that has been coursing through the Universe since
the time the very first galaxies formed. Light in all its wavelengths, from extremely high energy
gamma rays down to low energy deep infrared waves, is how we can study the Universe and how we can
know how stars and galaxies form and evolve. The kind of Holy Grail of the ancient past of the
Cosmos is finding the oldest starlight that still travels space. Ancient photons forged in the
hearts of collapsing clouds of hydrogen and helium atoms, that were formed in the primordial rage of
the infant Universe, shortly after the Big Bang. At this time, James Webb has not yet confirmed any
light coming from the first stars and scientists suspect not a single one still exists in the
nearby Universe. By studying our Sun and other nearby stars, we have a pretty good idea of
how they form and evolve in the current age of the Universe. Gravity pulls clouds of mainly
hydrogen and helium together until the pressure and temperature at the center become so intensely
high that nuclear fusion of hydrogen into helium begins and so the star begins to shine. Most stars
in the current Universe are on the smaller side, so-call yellow dwarfs like our Sun, but the
majority of all stars are the even smaller and rather dim red dwarfs. Even the biggest red
dwarfs have a luminosity of only about 10% that of the Sun and so not a single one is visible
from Earth with the naked eye. They're too far away and don't shine bright at all. Most stars
visible in the night sky are the much bigger and brighter stars, blue and red giants many times
heavier than the Sun, but all things considered, massive bright stars are quite uncommon even
though they do stand out the most. The mass of a star matters for its longevity: low mass stars
can last for trillions of years, while the most massive stars burn through their fuel in only a
few million years. The higher pressure in their cords caused by their huge mass makes that they
burn their fuel at a higher rate. Live fast, die young, is their motto. In the universe as we
know it today, clouds of gas that birth stars are still mostly made up of hydrogen and helium
but they are laced with all sorts of heavier elements like oxygen, carbon, nitrogen, silicon,
iron, copper, gold, in short heavy elements that are the building blocks of all stars, planets,
and life as we know it. Hydrogen and helium, and tiny amounts of lithium formed right after the
Big Bang because these atoms are the simplest. A hydrogen atom is just one electron circling
one proton, so it's obvious to see why this element is the most abundant in the Universe.
And a helium atom consists of two electrons circling a core of two protons and two neutrons,
and sometimes two protons and one neutron. Every other element that we can find stars today, in
the gas and dust in deep space, in every planet, comet, and asteroid, and every living being on
Earth was forged by nuclear fusion under the immense temperature and pressure in cores of stars
that no longer exist. At the end of their lives, they blew their outer layers away spreading, these
precious elements all around. The heaviest stars exploded in a supernova, leaving behind remnants
that literally warp spacetime itself: black holes and neutron stars. Collisions of neutron stars are
so violent they're the reason elements like gold and platinum exist. Temperature and pressure in
stars never gets to the point gold can be formed. This can only happen on the rare occasion
two neutron stars collide, usually because they're already orbiting each other since they
formed as baby stars. In the entire Milky Way, out of all the 100 billion stars, scientists
estimate there's only about 10 neutron star binaries destined for a collision at some point.
If you happen to have any gold jewelry, know that this shiny metal only exists because extremely
dense and heavy stellar corpses smashed into each other sometimes during the past 12 billion years.
Now this is in the current Universe, in our own galaxy which is easiest to study in detail. Most
stars that exist in Milky Way are younger stars, so-called Population I stars like our Sun, which
contains between 2 and 3% heavier elements or metals, keeping in mind that in astronomy
any element heavier than hydrogen andhelium is considered a metal. So stars with a higher
metallicity are younger because they contain quite a lot more metals than the older Population II
stars which generally have a metallicity of only 0.1%. These stars formed at a time when metals
were still rare because not enough stars had exploded yet to spread these heavy elements across
space. Population II stars are extremely old and were born back when the Milky Way was still
forming, 13 billion years ago, and some of them or even older than that. One of the oldest stars
known and a likely candidate to be the oldest star we've ever found is HD 140283, also known as the
Methuselah Star. It's only about 200 light years away from Earth and that makes it a good subject
to study Population II stars. Older estimates of its age put it at almost 16 billion years old
but that would make it older than Universe and that's not possible. More recent studies put
its age at about 14.46 billion years with an error margin of about 800 million years, so that
could make it as "young" as 13.6 billion years. It must have formed fairly shortly after the
Big Bang but since it does contain some metals, it's not one of the first stars, but it may be one
of the first of the second generation that still roams the Milky Way. Stars don't exactly come
with a birth certificate so one thing scientists can do to determine the age of a star is study the
spectrum of the light that's coming from the star, or any source of light in in the Universe. Dark
lines in a spectrum of a star are all associated with a certain element. These are called
Fraunhofer lines and they're caused by certain elements absorbing some of the stars' radiation
at these specific wavelengths. Certain elements absorbing certain wavelengths of light can be
reproduced in a lab and so that's how scientists know, and that's also how the element helium was
discovered in 1868, when scientists found a line in the spectrum of the Sun that they couldn't
associate with any known element at that time. Astronomers also study the intensity and
brightness of light coming from a star and they use the combined light captured by several
telescopes set up in an array to get more detailed imaging of a star or any object in space that
emits electromagnetic radiation. Now even then you still end up with an error margin, but still,
close enough. As it is, Population III stars, the very first stars to ever exist in the Universe,
remain shrouded in mystery. One of the Webb telescope's main objectives is finding evidence
of these stars and the telescope's sharp gaze can peer so far back into the Universe's past, it
may have already found them. In 2015, the Hubble Space Telescope detected the at that time youngest
and most distant galaxy ever found. Webb has now found galaxies even further away but it has also
been used to study this one dubbed GN- z11 in more detail. Astronomers measure large distances by
determining the red shift of an object. Red shift is a phenomenon caused by the expansion of the
Universe. Every distant object we can see appears to be moving away from us because it's light is
stretched out to longer, redder wavelengths as it travels through expanding space to reach our
telescopes. GN- z11 is a surprisingly bright, infant galaxy and its light reaches us from 13.4
billion years in the past, when Universe was only 430 million years old. Because the Universe
expands, this galaxy is at a proper distance of about 32 billion light years. Studies using
Webb data have shown that this galaxy's unexpected brightness may be caused by the luminous accretion
disc of a supermassive black hole at its center, making it the farthest active supermassive black
hole spotted to date. Another discovery was a gaseous clump of helium in the halo surrounding
GN- z11, and this clump appears to be pristine without any heavier elements polluting it.
And so this cloud may be where clean gas has collapsed and formed population III stars. The
helium in the clump glows because something is producing huge amounts of ultraviolet light.
The amount of high energy radiation required to ionize all that gas is about 600,000 solar
masses worth of stars shining with a combined luminosity of 20 trillion times that of the Sun.
The source of all this light may be coming from some of the first stars that have ever existed and
so this clump of helium may be leftover material of those stars' formation. Still, actually finding
some of the first stars is much like looking for a needle in a cosmic haystack. The Webb telescope
has proved to be vital to find ancient objects and these observations teach us that galaxies
started to form earlier than scientists had expected and they seem to contain fewer metals
than anticipated. Just to be clear this doesn't mean that everything scientists have been saying
about the origin of the Universe is wrong. It's just like with sensitive instruments like the
Webb telescope, they can gather better data and make better estimates of what the early Universe
was like. No other telescope that came before Webb has been powerful enough to capture the ancient
photons coming from the first galaxies to have ever existed, so it's actually not surprising that
there are a few surprises. In fact this is awesome for scientists. When unexpected discoveries
are made that means that they can get to do more science. That being said, the scientific
theory on the origin of the Universe and stars and galaxies is pretty solid so it would take
something completely new and different in order for scientists to have to completely rework
the theory. For now, more data still needs to be gathered and more observations made because
there are still a lot of uncertainties when it comes to those early days of existence. Cosmology
is the study of the observable Universe's origin, the origins of stars, galaxies, and other
large structures, what the ultimate fate of the Universe is, and how it all works, and
while there are still unanswered questions, all the data and evidence collected and studied
so far do point at the Big Bang theory being the most likely explanation for observed phenomena
including the abundance of light elements such as hydrogen and helium in the Universe, the size of
the Universe, and the cosmic microwave background radiation. Now by doing a lot of complicated math,
doing experiments, and running computer models, scientists know that hydrogen and helium were the
only elements formed right after the Big Bang, with such tiny amounts of lithium that you can
pretty much ignore these atoms anyway. And so they can make an educated guess on what the first
stars would have been like, and they were quite different from stars that we can study nearby
in time and space. Aside from traces of lithium, they contained no metals at all, and while
astronomers are quite certain what these stars were made of, they're not so certain of their
size. They may have been extremely massive, bigger than any star we know of today. Some of
the most massive stars in the nearby Universe are found in the Tarantula Nebula in the Large
Magellanic Cloud, and they contain up to 220 times the mass of our Sun. The first stars may
have contained 300 up to a 1000 times the mass of the Sun. One reason scientists believe Population
III stars were so huge is because massive stars live shorter lives and since we haven't found any
metal-free smaller stars so far, it's reasonable to say the first stars were likely absolutely
gigantic. More evidence pointing in this direction comes from simulations of large clouds of hydrogen
and helium, plus the gravitational influence of dark matter and how these clouds could cool and
collapse to form stars just a 100 million years after the Big Bang. They were probably extremely
hot and bright as well. A star a 100 times the mass of the Sun for example would have a surface
temperature of around 100,000 Kelvin and shine with the energy of 1 million Suns, mostly in the
ultraviolet range of the electromagnetic spectrum. If the first stars truly were these extremely hot
and luminous behemoths, not a single one would have existed for very long. Even a star only 60
times the mass of the Sun doesn't last much longer than about a million years. We can imagine beacons
of searing light setting surrounding clouds of gas ablaze for only a few hundred thousand years, to
then burn out, explode, or collapse. Maybe that's exactly what we're looking at in the halo of GN-
z11. Huge metal free stars may see a different end than stars containing metals. We can observe and
study nearby supernova remnants, neutron stars, and black holes, but the unique properties of
extremely massive Population III stars make that they may have exploded in a strange and rare type
of supernova called a pair-instability supernova. Inside a stellar core, the outward pressure of
nuclear fusion pushes against the inward pull of gravity, making it all even out and stable. The
nuclear fusion inside the cores of super massive stars produces a lot of extremely energetic
gamma rays under the enormous pressure of their mass. These gamma rays interacting with atomic
nuclei can form pairs of opposite particles, in this case specifically electron and positron
pairs with opposite charges. This process reduces the amount of gamma radiation because the energy
goes into forming these particle pairs and so this causes a partial collapse, since less energy
in the core makes that the crush of gravity can win momentarily, and increase pressure on the
core. When an electron and a positron meet, they annihilate each other, and this produces energy in
the form of gamma rays again, so the pressure in the core rises even more and energy production
increases even more and this causes a runaway process that results in a supernova that leaves no
stellar remnant behind. No neutron star, no black hole, just the stars' material that gets blown out
into space. And this can only happen in extremely massive stars with virtually no metallicity like
Population III stars. The very biggest metal free stars could even have collapsed directly into
a black hole without exploding at all, with all of their mass contained in the black hole, and so
these stars would not have contributed any matter to form any new stars. These black holes may
have been the seeds of these supermassive ones we find at the centers of galaxies today. This is
what physics and computer models can tell us and if we're very lucky, we might catch a glimpse
of some of the first stars have ever existed, but even with a very sensitive instrument like
the Webb telescope it would be very difficult to see individual stars that far back in time
and space. The ancient galaxies Webb has found so far likely contain first generation stars
but even these baby galaxies contain hundreds of thousands to millions of stars, each had
different points in their life cycle. Today, Population I and II stars exist at the same time
and in the distant past some of the first stars existed alongside the next generation of stars,
formed from metallic dust scattered throughout young galaxies by the first supernova explosions.
Now maybe you've wondered why stars and galaxies exist in first place, why matter in the Universe
has clumped together and formed things like stars and atoms, for that matter. According
to our current understanding of physics, just after the Big Bang the young Universe went
from a quadrillion degrees in that first moment, down to only 10 billion degrees within the first
second, and during this time the established laws of physics may not have applied. Gravity,
the electromagnetic and strong and weak forces emerged at this point and would dictate how
everything in the Universe works and interacts with each other. Physicists suspect the Universe
inflated exponentially for a brief moment between 10 to the minus 36 and 10 to the minus 32 seconds
after the Big Bang, and yes that's over 30 zeros after the point. The inflation theory explains
the origin of the large scale structure of the Universe with all its clusters and superclusters
of galaxies. Quantum fluctuations, tiny ripples in the Universe at this early stage are believed
to be the bases of these structures that would form much later. These smallest seeds would become
overdensities that would in time make that matter could clump together and become the foundations
of the first galaxies, and later clusters and superclusters, and galactic walls. If these
fluctuations didn't happen there would be nothing in the Universe except an thinning, homogeneous
mist of hydrogen and helium atoms. Thanks to gravity however, the fluctuations became gathering
points where huge clouds of gas would collect, contract, and eventually light up. 3 minutes after
the Big Bang, the first hydrogen atoms formed, and the first helium atoms formed through nuclear
fusion in the intense heat of the infant Universe. After 20 minutes, the Universe was no longer hot
enough for nuclear fusion so nothing more than helium and small amounts of lithium were fused out
of hydrogen, but it was still too hot for neutral atoms, so it contained a dense foggy plasma of
negatively charged electrons, neutral neutrinos, and positive atomic nuclei. Essentially, a hot
soup of particles bouncing off each other for the next 377,000 years. When the Universe cooled
down to just a few thousand degrees Kelvin, something amazing happened: it was finally cold
enough for the existing atoms to capture the free electrons. Their charge then became neutral, and
so they could fall into their lowest energy state. By doing that, they released energy in the
form of photons that could now travel freely through a Universe that was transparent for the
first time. These photons can still be detected as the cosmic microwave background radiation and
this is the oldest direct observation we have of anything happening in the entire history
of the Universe. Those released photons would have filled the Universe with a brilliant
pale orange glow at first but as time passed, this glow faded out of the visible spectrum of
light and the Universe would have been truly dark until the first star started to shine. The
ancient light of the cosmic microwave background reaches us from so far away in both space and
time, the wavelengths having been stretched out by the expansion of the Universe, and we're
so far removed from this event that we can only detect these photons using extremely sensitive
instruments. To our eyes, the space between stars and galaxies seems completely dark but by using
machines that can detect long wavelengths in the microwave range of the electromagnetic spectrum,
scientists were able to make a map of the cosmic microwave background. This phenomenon had actually
been predicted as evidence for the Big Bang theory and was discovered in the 1960's. Now after
decades of study, the oldest light in the Universe is an important source of data on the primordial
Universe. It allowed scientists to determine the age of the Universe, and by studying the small
fluctuations in temperature, they learned about the origin of galaxies and large scale structure
that we can observe in the cosmos, and it provides insight into the composition of the Universe as a
whole. Ordinary matter, stars, galaxies, planets, and what all of us are made of, seems to make up
only 5% of all matter that exists. Dark matter, good for about 27% of everything, does not
interact with ordinary matter except through gravity, making it incredibly difficult to
study. The remaining 68% of stuff is for now dubbed dark energy and seems to be responsible
for the accelerated expansion of the Universe, but as it is, scientists are not sure what dark
matter and dark energy truly are. And these are some of the biggest questions still remaining in
modern cosmology. The Hubble and Webb telescopes have shown us glimpses of the early days of the
Universe, when galaxies were still irregular clumps of infant stars containing almost none
of the heavy elements that makes we can exist and study ourselves, and this incredible place
we find ourselves in. We can wonder if planets and even life were at all possible this long ago,
since life as we know it requires a decent amount of specific elements. Still, could an intelligent
civilization survive long enough that they can say their distant ancestors witnessed the last of
these mythical first stars, like how we look at the Methuselah star as a relic of a past we
can just barely make out with our machines. By some inconceivable coincidence, we happen
to be alive in a day and age our technology allows us to know so much about Universe and
its origins. Perhaps this Epoch is the perfect one for life to exist and develop intelligent
beings capable of a deeper understanding of everything that happens. It may not have been
possible before and the future is uncertain. If the Universe continues to expand forever, light
will no longer be able to cross the ever growing distances between galaxies and clusters.
The cosmic microwave background radiation, that first light to travel freely and carries
the echoes of the beginning of everything that exists, will fall out of reach. For anyone
looking back in an attempt to find answers, there would be only darkness as far as any
eye, organic or machine, can see. If maybe some distant descendants of humanity still exist
in a much colder and darker era, they might tell stories of our yellow Sun and our glittering
night sky full of nebula and giant stars and spiraling galaxies, and whoever is listening
could never truly understand, because they would know only their reddish island galaxy full
of aging red dwarfs, adrift in a sea of nothing. For now however, we get to enjoy our time and
place in the Universe as it is, on a small blue World brimming with life, caught in the light of
a star that couldn't have existed without out a few ever so tiny fluctuations in the fabric of
existence when it was less than a fraction of a second old. The past teaches us where we
came from, but the future is in our hands, because the present is where we find the tools,
means, knowledge, and willpower to build it. Thank you for watching, I hope you learned
something new today and I will see you soon. [Music]
