Enjoy Earth Day While You Can, There Are Only 5 Billion Left

"So what do you think happens when we die?" I was asked during dinner at a recent event, after one fellow diner - who I'd only just met - found out I was an atheist.

Seeing the question as more of an accusatory statement and a segue into a religious argument, I decided to reply with a sound scientific answer: "Well, when you die, all the chemicals that make up your body are absorbed into the ground, blown into the atmosphere or biologically recycled. Ultimately, all of your body's elements stay here on Earth. And then, in around 4-5 billion years, the sun will swallow the Earth and your elements will be absorbed by our nearest star."

Silence.

This has been my "go to" response to the "death question" for some time and always ignites an interesting discussion. The fact that all the elements in our bodies heavier than helium and lithium were made by the fusion reactions inside the cores of stars and supernovae - spawning the famous Carl Sagan quote: "We're made of star-stuff" - is an inspirational thought. And by my reckoning, to know that we are ultimately going to end up as star chum is just as inspirational (if a little depressing).

So, as we approach Earth Day on April 22, spare a thought for our planet's mortality.

Ignore all the fake theories of doom, there is only one guaranteed doomsday approaching, and unfortunately (or fortunately, depending on which way you look at it), there will unlikely be anyone to see it happen.

What am I waffling on about? The sun? Eating the Earth? Yes. When the sun runs out of fuel in 4-5 billion years' time, things are going to get a little ... toasty.

For starters, our sun is a "main sequence" star. It burns hydrogen in its core, generating energy and fusing it to make the heavier element helium. It has been doing this for the last 5 billion years or so, providing the Earth with the necessary energy for life to thrive.

Eventually, though, like all main sequence stars, it will run out of its hydrogen fuel. And herein lies the problem.

As the rate of fusion reactions in the core begins to slow through lack of hydrogen - much like your car's engine spluttering as it runs out of petrol - there is less outward radiative pressure countering the sun's gravity. The core will therefore shrink, forcing helium atoms closer together. At a certain point, helium will start to fuse, creating even heavier elements like carbon and oxygen.

This phase of fusion reactions will generate a lot of energy, causing the solar body to heat up radically and expand. This expansion during the sun's "red giant" phase will engulf Mercury, Venus and, possibly, Earth. But before Earth is consumed by the searing plasma, all life, atmosphere and oceans will be vaporized.

Eventually, the helium fuel will also run out and the unstable red giant sun will begin to destroy itself, shedding its outer layers and producing a huge planetary nebula. But shining in the center of the nebula will be a compact white dwarf star - a stellar corpse.

This is all just a natural progression of the life cycle of a star like the sun. Its energy helped life evolve and nurtured our biosphere, but in the end the sun will puff up and destroy the inner solar system.

Astronomers have even looked at debris surrounding ancient white dwarfs to find the dusty refuse of shredded planets and asteroids - the tidal shear produced by white dwarf stars is so strong that the inevitable dynamical chaos causes any planetary system that remains to get ripped to shreds.

By analyzing the spectrum of light emitted by white dwarfs, some idea as to the chemical makeup of the ex-planetary systems can be understood, providing astronomers with information as to whether rocky worlds like Earth used to orbit the ex-sunlike stars. It's a morbid stellar archaeological dig of sorts - an attempt to find out what our dead solar system may look like in 5 billion years' time.

You also have to wonder if the ground-up remains of ancient alien civilizations are out there. Perhaps the chemicals that used to make up their extraterrestrial biology are now sprinkled over the white dwarf's surface, hanging in interplanetary clouds of dust.

So, the big question remains: Will anyone be there to witness the Earth's final days? Well, that depends on whether we can avoid destroying our biosphere, master space travel and avoid all the planet-killing asteroids that will threaten us for the next few billions of years.

In one of the last scenes from the epic "Babylon 5" TV series in the 1990s, the sun's final moments are depicted. Might it go down like this?

Image: It's the end of the world as we know it - artist's impression of the Earth being toasted by an expanding red giant sun. Credit: Fsgregs, at the English language Wikipedia project, Creative Commons.