“Daddy,” my son asked, “what would happen if the sun blew up?”
This was one of those questions he decided to ask out of nowhere. Maybe it was inspired by Star Wars, maybe not. “Well, we’d all die,” I said. Because it’s, you know, the truth.
“Well, what if the Earth had a force field that protected it? Then people could live on and on!”
I shrugged my shoulders at that. “I don’t know.”
Maybe. It’s an interesting question, though.
Sure. So, what would happen if the sun blew up?
A whole lot of astrophysicists would be really confused before they died.
I’ll bite. Why?
Because our sun is, according to everything we know, really, really unlikely to explode. It’s too small.
Yes. See, all stars begin their lives molecular clouds of (mostly) hydrogen that collapse under the influence of gravity to form a protostar. I discussed this in detail way back on 2016 in How Do They Build A Planet?, if you want more details. But, in brief, the molecular cloud becomes denser and denser until gravity causes the hydrogen to begin fusing together. This generates a lot of energy and helium. According to NASA the sun fuses about 600 million tons of hydrogen per second or, put differently, about the Earth’s mass in hydrogen every 70,000 years. Our sun has been doing this for about 5 billion years (meaning it’s fused about 71,000 Earths worth of hydrogen), and will keep doing it for about 5 billion more years.
In about 5 billion years, once the hydrogen has all fused into helium, the sun’s core will contract until pressure and temperature grows great enough to start fusing helium into carbon. This will cause the sun to expand into a red dwarf, becoming large enough to potentially swallow the Earth (and even if it doesn’t, drag from the solar atmosphere will slow our planet down until it no longer can maintain orbital velocity, at which point it will fall into the swollen red star that once gave it life. Unless the expansion sends us sailing off into deep space, that is).
All right. So why can’t our Sun explode?
Again, it’s not big enough. Stars massing eight times the mass of the sun and smaller, as their core collapses after all of the helium has fused into carbon, can’t generate enough temperature and pressure to start fusing carbon. These stars, which obviously include our Sun, end their lives as white dwarf stars.
Bigger stars continue fusing the heavier elements with the residual helium in the core to form other elements. Over about 100,000 years, the carbon fuses with helium to make oxygen. Then, over the course of about 10,000 years, the oxygen fuses with helium to make neon, which fuses with helium to make magnesium, which then fuses with helium to make silicon. From there, over the course of about 24 hours, the silicon fuses with helium to make sulfur, which fuses with helium to make argon, which fuses with helium to make calcium, which fuses with helium to make titanium, which fuses with helium to make chromium, which fuses with helium to make iron, which does not fuse – not at any temperature or pressure a star can generate, at least.
At this point, the rapid (remember, all of this took 24 hours) core collapse stops cold. This causes a shockwave that blasts out through the core of iron and the shells of silicon, oxygen, neon, carbon, helium and hydrogen in a massive explosion called a supernova.
And what happens during a supernova?
There are roughly five stages to a supernova, and we’ve already seen stage one: the 24 hour core collapse as the star races towards making iron. Stage two is the shockwave I mentioned. Over the next few hours the shockwave rebounds outwards through the various shells, compressing and heating them and generating light (which moves faster than the physical shockwave). At stage three the star actually explodes, blasting the surface material out at abut 50 million kilometers per hour. Stage four is the superheated expanding surface of the star, glowing bright enough that (for a short period of time at least) it may actually be brighter in the sky than the galaxy that contains it. And then, at stage five, you have molecular cloud expanding away from the surviving core, which may be a neutron star or even a black hole.
Just for fun, what would happen to us if our sun did supernova? Right now?
That’s an… interesting definition of fun. Still, let’s go with it.
Let’s assume that this all starts at 7:22 am Eastern time on June 11, 2018, and that we’re accelerating the process at a rate of 1,000,000 years equals a day. The Sun would rapidly expand into a red giant, and stay that way until 7:22 am Eastern time on October 9, 2018. Then the core would begin to collapse. At 9:46 am Eastern time, all of the helium would have burned into carbon. Then, 24 seconds after 10:00 am Eastern time, the carbon would have burned into oxygen. At 25.64 esconds after 10 am Estern time, the supernova would begin.
The speed of light is 299,792,458 meters per second, so we’d see the supernova at approximately 44.6 seconds after 10:08 am Eastern time, and it is the very last thing we’d ever see. The neutrinos accompanying the flash of light would kill every human being on earth. Then, at 56.6 seconds after 12:59 am Eastern time, the expanding outer shells of the exploding Sun would smash into our planet. Nobody would be around to see it.
But cheer up! The question assumed we could build a force field to protect ourselves! Next time, we’ll see what would happen if we live!