I've been thinking about setting a better timeline for events occuring before the actual start of my story. In order to do that I need to know, how far ahead can you predict a sun going supernova and how much can it matter that it happens in a fantasy setting? Oracles does exist in my story so perhaps they can see the end of the world coming very early?
In a fantasy story, you can do anything you like. A planet is unlikely to survive a sun going supernova, and even if it did, any life on it wouldn't. However, in fantasy, none of that matters. Jack Vance's Dying Earth (which I also use as a setting) has the sun as basically a dying red dwarf, which in reality, could never happen. But it's fantasy.
I guess I need to clarify a bit. I have this world, Yomi, which is going to die (in how long is what I'm not sure of yet). When this get known the inhabitants start looking for a way for the entire population to migrate somewhere. They manage to open a portal to another dimension and find the world Sekai. Sekai is also inhabited and the conflict between the two people is what my story centers around. The sun going supernova was just the first thought that came into my mind when thinking about the reason for Yomi's pending destruction. So, I guess I can just pick whatever amount of time that suit my need because it's fantasy?
Given that it's a fantasy story, I would try hard not to lean into terms as science-based as supernova in your story. Supernovae come in two types. A Type 1 supernova happens when a binary star system (two stars in partnership orbiting a common point) fall prey to natural imbalance and one star begins to syphon material off the other in a self-reinforcing chain of events. As the consuming star's mass increases, so does its gravity, and it rips more and more material off its partner at an ever-increasing rate. There comes a point where the mass transfer crosses a threshold and the resulting gravity in the consuming star overpowers the forces keeping the star inflated, it collapses and boom! How and when this happens depends on the nature of the two stars in play, one of which is always a white dwarf, but the other can be any kind of star, and there are many kinds, some of which are strange creatures that don't seem like the stars one normally thinks of (neutron stars, magnatars, Wolf-Rayet stars). Regardless, for your story, in a Type 1 situation, your solar system has two stars with a trail of material flowing between them. That's your sky. In a Type 2 supernova, the problem is that these occur in single stars that are much, much larger stars, not yellow dwarfs like our own sun. The likelihood of a viable solar system around such a star is zilch. But again, your story is fantasy. You don't need to (and probably should not) try to lean into all this math and science. I'd go the binary route and create a cultural backstory where the two stars are gods, one chasing the other (make up a suitable narrative about why they chase one another) and have the oracle pronounce that the Great Catching Up is nigh. Something to that tune, along those lines, like that-ish.
In theory, astronomers think the iron flash--the last stage of nucleosynthesis, the last element fused in a sun's core--only burns for like an hour or so. So I suppose if they had a way to detect the iron flash, they could be like, "Holy shit, she's burning iron! We got 90 minutes at best!" But all that is based on estimates and predictions that are so huge that a thousand years is barely considered a rounding error. Supernovae are relatively rare events. So much so, that we only see one visually from Earth every 500 years or so (I want to say 1054 and 1600-something were the last ones, but don't quote me on that). A lot of the observable evidence has come from supernovae in other galaxies, which says a lot about the energy they release, but not what the final days or months (or millennia) are like. So short answer, nobody has any idea when they're gonna blow until they blow. Yeah, I thought of this too. The O and B supergiant stars burn so fast and so young that there isn't time for a rocky planetary system--nevermind, the evolution of life--to develop. And the cooler red giants that have the critical mess required for a supernova event swell and contract by several hundred radii, which would wreck any kind of habitability zone around the star. But if you're doing fantasy with mystics predicting when the sun will explode, I'd say ignore the physics and just riff off the mystical part. Physics almost always ruins everything.
Mind if I use that? In that case one of the two suns will be smaller than the other and the lore will go like this: the god (the bigger sun) is chasing the goddess (the smaller sun) because he wishes to make her his bride. The reason the goddess is running from him is that their union is forbidden due to them being siblings, and the fact that such a forbidden union would bring about the end of the world. Then have oracles proclaim that the prophecised union is coming. In how long I guess I can just pick the time that fits better with the story? The focus will be the conflict the migration causes, not the reason behind the migration after all. Going by the names I chose for my world, yes I guess so., hehehe.
I really like the idea of a prophecy (the forbidden union of the two gods) rooted in a real event (the predicted supernova) to motivate people into a collective exodus. I think it can open a lot of possibilities for the narrative of your book as the story will be riddled with prophecy-encased reality. I would imagine an event like a supernova would take at least tens of thousands of years to happen and it sounds you need something more pressing to motivate people to literally invade another planet, as you talk about conflicts.
Of course, go ahead. All I offered is the bare scaffolding of an idea. It's still for you to flesh out and clothe.
Perhaps just make the event happen sooner rather than later? If the union is set to happen in one hundred years they must first manage to open a portal and what if they have opened a portal more than once before but not been able to migrate because the world on the other side isn't habitable for them? So once they find one that is, they might be down to maybe twenty years? And migrating an entire planet isn't done quickly, especially if there is resistance from the people inhabiting the new world.
the ultimate "deadline" can happen at any time because there's a prophecy. You can make an alignment of stars/planets, when the prophecy is said to occur, and an on-the-way-to-a-supernova scientific event coincide such that their coincidence becomes the catalyst for the migration to occur. If you choose the prophecy option, then I would say that the prophecy becomes a reading key to the story, and what comes before and after can be filtered through its lens. Even without the prophecy, it is realistic to imagine that it will take them more than a few trials to get the portal working, you can choose to talk about it or leave it aside. But do you want your story to focus on the technical trials people will surely have to face in attempting an interplanetary migration? How are those technicalities important to the story? Or do you want your story to focus on the politics of conflict that come after the migration? Politics and mindset of the migrants influenced by how techno-political choices were made during the process of migration?
@ruskaya Thinking about it... Perhaps I am overdoing it. The story is to be from the perspective of the inhabitants of Sekai, who believe they are fighting of an invasion. Only toward the end of the story do they learn that the people of Yomi only want to find a new home because their world is dying. Maybe I just need to state it as such? Their world is dying and they need to move or they’ll die with it.
then it sounds like your story starts during/after the migration and you can work the reason of the migration and its details for that migration (prophecy + Yomi's mindset and motivations) in explaining the behavior of the Yomi as the Sekai get to know their invasor. I think it would be interesting to see how the Yomi's and Sekai's thinking differs, justified by their own motivations (Yomi invading due to a forced escape and Sekai defending their own world), a difference exacerbated by the dramatic setting. What each of them they want is clear, how do their want (and culture) determine how they engage with each other? What makes these two worlds' interaction different such that it is interesting to see how they conflict with each other and makes your story unique?
@ruskaya To be honest I do not think there is something unique about the conflict, it is simply just an allout war between the two. Sure, there are differences between the people of Sekai (refered to as humans) and the people of Yomi (refered to as demons) and some similarities too, but maybe not as many as it should be. I might need to take some time to take this matters into consideration.
