Death of our sun: a beautiful journey will come to an end
Welcome to my blog "Mystery of Galaxy".Today our topic is "how the beautiful journey of our sun will come to an end?" 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.
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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. we know 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 talked about the lowest mass stars, the red dwarfs 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 13.8billion 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 250billion times greater than on Earth and the temperature in the core is 15million 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. 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, 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 100million 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 just10,000 years, the Sun is less than 2% its red giant diameter. The Sun is still 10times (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 breakoff and are blown away in a fast stellar wind. After four or five of the 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.
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| Source: NASA / STScI / Public domain |
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.
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| Source: Baperookamo / CC BY-SA |
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. That's all friends. Thank you and don't forget subscribe our blog for more exciting article
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