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What Will Happen When the Sun Burns Out?

Our Sun is practically middle-aged. One day, it will die. This is what astronomers believe will happen...

What Will Happen When the Sun Burns Out?
Evolution of the Sun from a main-sequence star, to a red-giant, to a planetary nebula/white dwarf ESO/S. Steinhöfel

Our home star, the Sun, gives life to our planet. We are fortunate in that the Earth is just the right size and at just the right distance from the Sun to sustain life. At the same time, the Earth is shielded from harmful cosmic radiation by its atmosphere and magnetic field. 

The Sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust, called a solar nebula. As the nebula collapsed from the force of its overwhelming gravity, it began to spin faster and eventually flattened, like a pancake, into a disk. Most of the material was pulled toward the center, forming first a protostar, and then our Sun, which accounts for around 99.8% of the mass of the entire solar system.

This process began approximately 4.5 billion years ago, and it's estimated that it took around 50 million years for the pressure and temperatures to increase to the degree that the hydrogen and helium began to fuse together and form heavier elements. What remained of the cosmic debris came together to form the planets in our solar system. The rest, as they say, is history.

The Sun is relatively average, as stars go. It's not so large that it burned extremely bright and exhausted its stellar fuel quickly before exploding into a supernova. It's not dim and tiny like a red dwarf. Though, to be fair, these can remain stable for over a trillion years, which would make them some of the last stars to survive the eventual heat death of the universe. 

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Goodnight Sun, Goodnight Moon: 

It's believed that the Sun is around halfway through its lifespan, and will run out of fuel in another 5 billion years or so. What is happening inside the Sun's core is pretty interesting, and key to understanding how the Sun will inevitably die.

The Sun's core is incredibly dense and extremely hot, about 27,000,000 degrees Fahrenheit (15,000,000 degrees Celsius), hot and dense enough to sustain thermonuclear fusion. It's filled with hydrogen nuclei, which are colliding with other hydrogen nuclei. As they do so, the energy at the core is enough to allow the nuclei to bind together and form helium. In this process, an enormous amount of energy is released. 

The energy produced in the core is what powers the Sun, and produces the heat, light, and radiation the Sun emits. This energy is also an important part of keeping the balance between the Sun and the intense gravitational pull exerted in the Sun's core. One day, there won't be enough energy to counteract the gravitational pull, so the Sun will eventually contract — increasing both the pressure and temperature of the Sun's core. As the amount of helium builds up in the core, the temperature of the fusion reactions will increase in order to counteract the increasing density, which is the beginning of the end. 

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This extra energy will result in the Sun first growing brighter, and then the outer layers will swell up, whereby the Sun's atmosphere will increase to something like 200 times its current size, creating a red giant and putting it right in Earth's path. Say goodbye to Mercury and Venus!

Sun as a Red Giant
A comparison of the Sun now vs the Sun as a red-giant Source: Oona Räisänen/Wikimedia

The hotter sun will kill off life on Earth, but it may allow what is now the coldest reaches of the solar system to become habitable. 

The Sun will also shed its gas off into space, creating a planetary nebula. Even as the outer layers of a red giant star are expanding into a huge cloud, its inner core is contracting. The temperature and pressure in the Sun's core will soar to 10 times their current values. Roughly 1.2 billion years after the Sun leaves the main sequence, the center of the helium core of the Sun will become sufficiently hot and dense that it will ignite and burn, forming a white dwarf. This white dwarf star will be about the same size as Earth, with half the mass of the Sun.

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Electron degeneracy pressure

White dwarfs are extremely interesting objects in and of themselves — they go through a process similar to that of neutron stars, only instead of nuclear degeneracy pressure driving the existence of the object, they are electron-degenerate (a state in which all of the electrons surrounding the atomic nucleus are forced into the lowest energy quantum state). This means that instead of fusion counteracting the pressures of gravity to keep the Sun from collapsing under its own weight (though it's not quite heavy enough to form a neutron star or black hole), electron degeneracy pressure keeps the white dwarf from collapsing further.

The gases that were ejected into space from the Sun's outermost layers will encircle the white dwarf for a few tens of thousands of years. The tiny dense object will release loads of ultraviolet radiation, which will ionize the gases and cause them to glow brilliantly - creating an emission nebula. 

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The Helix Nebula from Blanco and Hubble
A beautiful image of the Helix Nebula taken by Blanco and Hubble Source: C. R. O'Dell, (Vanderbilt) et al. ESA, NOAO, NASA

The nebula left behind might most closely resemble the Helix nebula, which looks like a gigantic eye floating around in space. 

What's Next?

Astronomers aren't sure exactly whether Earth itself will survive the Sun's growing brighter, even before its expansion into a red giant. By some estimates, in around 1 billion years from now, the sun will be 10 percent brighter than it is today. This will also mean an increase in heat energy, triggering a runaway greenhouse effect similar to what Venus experienced.

Scientists aren't sure if the Earth will be consumed once the Sun expands to a red giant. However, even if the Earth were to escape being consumed entirely, the intense heat of the red giant Sun would make it completely impossible for life to survive. 

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While this is obviously not a good thing for Earth, it does open up the possibility that planets like Mars, and moons like Titan and Europa, will thaw out and potentially become habitable. Astronomers believe places in the outer reaches of our solar system, like Pluto or other objects in the Kuiper Belt, may become balmy oases, as the energy from the dying Sun dials the temperatures up to where they may be comparable to Earth's current temperatures. Some of these places are thought to host large quantities of water-ice beneath the surface. They might even contain complex organic molecules.

In a paper published in 2003, astronomers estimated that Pluto, when the Sun meets its end, could possibly develop its own atmosphere, which is crucial for life. Technically, objects within 10 to 50 AU (one AU, or astronomical unit, is the distance between Earth and the Sun) might become habitable for the very first time. 

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Pluto
Pluto, as captured on a 2015 photo from New Horizons Source: NASA/JHUAPL/SwRI

"When the sun is a red giant, the ice worlds of our solar system will melt and become ocean oases for tens to several hundreds of millions of years," says S. Alan Stern, the Director of the Southwest Research Institute’s Department of Space Studies in Boulder, Colorado, and the study's author. "Our solar system will then harbor not one world with surface oceans, as it does now, but hundreds, for all of the icy moons of the giant planets, and the icy dwarf planets of the Kuiper Belt will also bear oceans then. Because Pluto’s temperature will not be very different then, than Miami Beach’s temperature now, I like to call these worlds ‘warm Plutos,’ in analogy to the plethora of hot Jupiters found orbiting sun-like stars in recent years."

The red-giant phase is expected to last a few thousands to a billion years. The Sun will also become much brighter and more unstable as it gears up to end, sometimes pulsating 6,000 times more light and energy than it does now. Pluto's extremely irregular orbit may also adversely affect its potential future habitability. Thankfully, we have a long time to come up with a plan to vacate Earth before the Sun goes out. 

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