Sun of a…
I think we all have heard things about the lifespan and eventual “death” of our sun. When I asked about it as a kid, it was given an ambiguous “about five billion years” answer and some stuff about it turning red, swelling up and cooking us. There’s a bit more to it than that.
First things first. Our sun has already had a pretty interesting run over the last 4.6 billion years. After forming from its parent cloud of gas and dust, recycling the cast-off from a star who’s life had ended, it began to shine, but not quite as brightly as it does now. The over-simplified point to remember is that the sun’s dimensions are the balance between gravity crushing a lot of hydrogen together and those crushed hydrogen atoms being smashed together and releasing an EXTREME amount of energy as they’re forcefully converted to helium. Every second, 4 million tons of solar mass are lost to these reactions, some of which we get in the form of light, heat and solar wind.
Throughout the time that the earth cooled and things began to get cozy for the possibility of life, the light and heat from the sun gradually increased. Interestingly, even though the sun was less radiant in the past, the temperature of earth’s surface wasn’t any cooler, since we used to have a LOT more CO2 (carbon dioxide) keeping the heat we received local. The first life was all about this early atmosphere.
Then, as time went by, cyanobacteria and eventually plants came around, used up the CO2 and replaced it with O2 (oxygen). This took a while and the sun warmed up a bit. Things worked out.
Of course, the sun’s output is not steady and there are many cycles it goes through that vary the amount of radiance we get out here. I’ll leave the details of this alone for now, but it’s worth understanding how these effect us and our climate. I’ll add that over the eons and millennia, things go up and down predictably, so we know what happened and like a cheesy horror movie, we know what’s around the corner.
In 1.1 billion years the sun will be 10% hotter than is is now. That means our surface water evaporates, the moisture exacerbates the greenhouse effect and our earth’s water begins to be lost into space. Maybe we’ll be able to do something by then, maybe we’ll be gone. The sun will do as it will do, regardless.
In 3.5 billion years the sun will be 40% brighter and make sure no water remains on the surface of earth at all, it’s all leaked out into space. We’ll be something like a dry-hot venus. If anything still manages to be alive, it’ll be underground.
The real fun starts in 6 billion years. That’s when the sun uses up the last of its hydrogen and things start to get CRAZY.
This is when it gets bigger. The sun will swell to extend beyond the orbits of mercury and venus, possibly even us. This is when it becomes what’s called a red giant. Either way, we three inner terrestrial planets are TOAST. While our solar system is enjoying a period with only five planets, the sun will be burning its helium. This only buys it 100 million years and the end of the helium era is a violent time. The sun begins to pulse, blowing off its mass into space.
While the helium was being used, a core formed of oxygen and carbon about the size of earth. This thing is white hot, denser than anything you’ll ever hold and the true beginning of the end. This is all that’s left of our sun, a white dwarf. The size of it is still puffed up slightly from the intense heat that keeps it glowing, but even as it gradually cools and shrinks, there isn’t far for it to go.
After trillions of years, yes TRILLIONS, it will cool into an object that is as cold as the space around it. We call this a black dwarf. We’ve never seen one, aside from the fact that detecting one would be almost impossible from any average stellar distance, because the universe isn’t even close to old enough for one to exist. Still, we know where one will eventually be, long from now, long after us, every morning when we see the sun rise.
I’ll let Dr. Sagan have the last word for this one.