what if Earth and Mars switched places?

HOW THE SUN WILL DIE!

 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. 
 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 article, 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 article. 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

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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 article. 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.