This video is sponsored by Magellan TV. Stars are beautiful to look at. They
appear as tiny pinpoints of light and seem to be permanently fixed upon the
sky. The exception of course is our Sun, but if anything it's a typical star.
It does what stars do by fusing hydrogen into helium in its core, creating energy
in the process. There's enough hydrogen fuel to keep the Sun burning for about
10 to 11 billion years. But our Sun is already four-and-a-half billion years
old. What happens when that hydrogen is
exhausted in five and a half billion years? Let's find out and see how low
mass stars like our Sun will evolve and eventually die. Welcome back to Launch Pad,
I'm Christian Ready your friendly neighborhood astronomer. In our previous
video on how stars work, we learned that stars exist in a delicate balance
between the energy radiating from its core which exerts a pressure against the
surrounding layers. Meanwhile the surrounding layers attempt to crush the
star inward, exerting pressure on the core which sustains its nuclear fusion.
But sooner or later the star's supply of hydrogen fuel runs out, and when that
happens the star begins to die. How exactly a star dies depends on its mass.
More massive stars burn through their fuel very fast and end their lives in
spectacular supernova explosions, while low mass stars evolve and die in a less
explosive, yet still very dramatic way. The dividing line between high and low
mass stars is roughly around eight times the mass of our Sun. We're going to
talk about the end of high mass stars in an upcoming video, but for now let's
focus on how low mass stars such as our own Sun evolve and die. We talked about
the lowest mass stars, the red dwarfs in a previous video. Red dwarf stars burn
their fuel slowly so they last for up to trillions of years, at which point they
more or less fade away to become black dwarfs. At least that's what we think
will happen; the universe is only 13.8 billion years old so even the oldest red
dwarf is still in its stellar infancy. But starting at around half the Sun's
mass, things get a lot more interesting. Our Sun is four-and-a-half billion years
old so it's about halfway through its normal main sequence lifetime.
It spends his time burning hydrogen into helium in the core. The conditions in the
Sun's core are hellish. Pressures are 250 billion times greater than on Earth and
the temperature in the core is 15 million Kelvin. Yet despite those crazy
numbers the gas basically behaves more or less like a normal, albeit very hot
and highly pressurized gas. The way a gas behaves is defined by the Ideal Gas Law.
This law states that increasing the pressure on the gas raises its
temperature and vice-versa. The more the star squeezes on the core, the higher the
temperature goes and the hotter it gets. 15 million Kelvin is hot enough to get
hydrogen fusing into helium, but not hot enough to fuse helium into anything
heavier so inert helium just builds up in the core like ash in a fireplace. But
helium is four times heavier than hydrogen. As it builds up it squeezes the
core harder, causing the pressures and temperatures to rise. As helium ash
builds up in the core, hydrogen fusion slowly migrates outward, eventually
forming a shell of burning hydrogen surrounding an inert helium core. The Sun
is about halfway through this transition today. But as the temperature increases,
the rate of fusion accelerates and the star burns hotter and faster. In another
four and a half billion years, it'll be 67% brighter.
Less than two billion years after that it'll be more than twice as bright as it
is today. By this time Earth has long since burned
to a searing rock. The oceans and atmosphere have long vaporized and
temperatures hover around 600 degrees Fahrenheit. Meanwhile the rate of fusion
in the Sun's core continues to accelerate. The Sun is evolving so
rapidly now it leaves the main sequence and becomes a subgiant star. Around the
year 7.1 billion, the Sun's core has accumulated so much extra mass it begins
to overcome the pressure exerted by the hot gas and the core contracts. By the
time the core has reached 13 percent of the Sun's total mass, the electrons at
its center are squeezed so tightly together that they run up against the
Pauli Exclusion Principle. This principle states that no two electrons
with the same spin may occupy the same energy level at the same location
normally this isn't a problem. Electrons have plenty of room to move around and
can easily avoid each other. But now the electrons are squeezed so close together
that every possible energy state is filled. The electrons have turned into a
new and ultra dense state of matter called a degenerate gas. In degenerate
matter, the ideal gas law breaks down. Adding heat to a normal gas causes it to
expand and cool, but a degenerate gas cannot expand, so adding heat to it just
causes the temperature to climb higher. Electron degeneracy begins in the center
of the core, but as more helium is added to it it expands and grows and
eventually the entire helium core becomes degenerate. This sets up an ultra
powerful degeneracy pressure and prevents the Sun from collapsing in on
itself but doing so sets the Sun's evolution into overdrive. In just 500
million years after the onset of electron degeneracy, the Sun is 34 times
brighter than it is today. 45 million years after that and the Sun is 105
times brighter and just 40 million years after that the Sun is 2,300 times
brighter than it is today. The Sun expands and grows to Venus's present-day
orbit. As the surface grows larger, it gets farther away from the core and
cools down. The Sun is now a red giant. It seems like a paradox: On the one hand the
Sun is emitting far more energy now than it ever has in its past, but the surface
is so much larger that less energy ends up passing through each square meter of
its surface. There are more square meters now so less energy is passing through
each of them, and the Sun cools down. By the time the Sun reaches its peak
luminosity it burns through more fuel in just six million years than it has in
the eleven billion years leading up to it. This burn rate cannot last.
Something's got to give and when it does the Sun's fate is sealed. We're going to
find out what happens next in just a moment, but first I want to take a moment
to thank MagellanTV for sponsoring today's video. MagellanTV is a new type
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launchpadastronomy will get a one-month free trial. I'll have a link here and in
the description below. As the Sun expands as a red giant,
it's degenerate helium core contracts further as more helium is added to it.
This sends the temperatures up to 100 million Kelvin. At that temperature three
helium nuclei will fused into a carbon nucleus. This leads to a runaway chain
reaction of helium fusion called the Helium Flash. In just a matter of minutes,
the core emits more energy than all of the stars in the galaxy combined! But in
a weird twist of quantum mechanics, the Sun's core absorbs all of the energy of
the helium flash! It rapidly expands and breaks free of the electron degeneracy.
From the outside the Sun doesn't appear to be any different at all. But the
helium flash comes at an enormous cost; the electron degeneracy is broken so the
helium core expands and cools. Less heat is pumped into the hydrogen-burning
shell, so it emits less energy. This sudden loss of energy causes the Sun to
shrink like a deflating balloon. In just 10,000 years, the Sun is less than 2% its
red giant diameter. The Sun is still 10 times (more luminous) than today and its surface
heats up to an orange-yellow temperature, but it seems like it's red giant days
are over. They're not. Its helium-fusing core is surrounded by a hydrogen burning
shell, but the insanely high temperature is needed to keep the helium fusing
means that it must burn through its helium core
a hundred times faster than it burned through its original hydrogen core. Now
it's inert carbon and even some oxygen that's building up in the core. The
oxygen is created when a carbon and helium atom fuse, producing some additional
energy in the process. The relentless increase in density
causes the helium to fuse faster and faster, adding yet more and more carbon
into the core. Eventually the carbon core becomes degenerate and temperatures
skyrocket. The Sun enters into a second red giant phase and ascends the
Asymptotic Giant Branch of the H-R diagram. Its outer layers expand beyond
Jupiter's orbit and its luminosity climbs to more than 3,000 times the
present-day Sun. Despite these temperatures, the core cannot get hot
enough to fuse carbon. The helium shell expands, but in doing so it overtakes the
slower-burning hydrogen shell. The helium shell cuts off its own supply of helium
fuel and the helium shell just sputters out. The Sun contracts, compressing the
helium shell back into electron degeneracy. This sets off a mini-helium
flash in the shell, which in turn dumps more carbon into the core, which in turn drives the
temperatures ever hotter, and the Sun expands once again. The helium shell
overtakes the hydrogen shell once again. and the Sun contracts a second time. This
cycle repeats itself every 100,000 years. Each time it expands the core's
gravitational grip on the surface gets weaker and weaker.
Eventually chunks of the surface break off and are blown away in a fast
stellar wind. After four or five of these convulsions, the Sun coughs out its final
breath. Its outer layers have expanded beyond the solar system. Its exposed core
is a 170,000 Kelvin ball of degenerate carbon and helium. It weighs 55 percent
of the Sun's original mass, yet it is compressed to a volume no larger than
Earth. The Sun is now a white dwarf. It's so hot it radiates mostly at x-rays and
ultraviolet wavelengths. Stars that are slightly more massive than the Sun can
produce white dwarfs that are so hot that they actually can ionize
their surrounding expanding shells causing them to glow. These
objects are called Planetary Nebulae, and we're going to be talking more about
them in an upcoming video. It's not yet clear if a planetary nebula will be the
Sun's ultimate swan song. It's arguably on the lower end of stars that are
capable of producing planetary nebulae. But what is in arguable is that the Sun
is now well and truly dead. With no more pressure bearing down on the core, it
will never re-ignite helium fusion. It will spend the next hundred billion years or
so gradually cooling off, eventually to become a black dwarf. So...what about the
Earth? Well obviously it doesn't fare very well through any of this, but its
ultimate fate is really not yet clear. The Sun will ultimately expand to beyond
Earth's orbit but that doesn't necessarily mean that Earth will be
swallowed up by the red giant Sun. During its red giant phases, the Sun will shed
as much as 45 percent of its mass through its stellar wind. As it loses
mass, its gravitational hold on Earth weakens, and Earth spirals away from the
Sun. It's possible that by the time the Sun's outer layers reaches Earth's
present-day orbit, Earth will have migrated to well beyond Mars's orbit.
However it's also possible that Earth will drag against the thick wind of
material blasting out from the Sun. In that case, Earth would just spiral into
the red giant. Regardless of its fate, Earth will be little more than an
exposed core of nickel and iron. Its surface and mantle would have been
vaporized long ago by the expanding red giant Sun. So what then of us? Or rather,
what then of our evolutionary descendants? Well, they will have to leave
Earth behind and migrate out to the outer solar system long before the Sun
has even entered into the red giant phase.
Fortunately the Sun will give them a hand. As it evolves, its habitable zone
will expand into the outer solar system. Perhaps humanity will make its final
stand from the tropical zones of the moons of Jupiter, Saturn, Uranus, and
Neptune. They'll spend the next few million years basking in the red glow of
the dying Sun after its outer shells expand past them.
All that will be left of the Sun is its white dwarf core. Our descendants will
have to migrate elsewhere into the galaxy, where hopefully they could set up
shop for the next few billion years or so. It seems like a bleak ending for the
Sun, but in fact it's an essential part of the cycle of stellar life. The Sun
effectively returns to the interstellar medium from which it came, this time
enriching the medium with helium, carbon, oxygen, nitrogen, and other elements. Those
elements will then go on to build the next generation of stars, planets, and
possibly even life. I'd like to take a second and thank my Patreon supporters
for helping to make this video and indeed this entire channel possible. If
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arrives until then stay curious, my friends.