After researching white dwarf stars to determine their life span, which I did because of one of your earlier utsuhou comics, I've realized that you may need to start employing handwavium for the 'internals like a white dwarf' explanation of lifespan.
FYI:
Handwavium: It flat out violates laws of physics. We're waving our hands and saying pay no attention to the man behind the curtain.
Why would you need to employ handwavium, in excess of that needed for normal yukkuris anyways, you might ask? White dwarfs are thought to be formed of 'electron degenerate matter' which happens to be VERY VERY Dense. Electron-degenerate matter has a density of 1,000,000,000 kg/m^3, so even at the size of a basket/soccer ball, an Utsuhou would probably weigh on the order of tens of millions to hundreds of millions of kilograms. ......So yeah, that would mean an utsuhou(that obeyed physics XP) would probably cause small, localized, earthquakes every time it hops.
I can see it now.... *hop* *BOOM* *hop* *BOOM* *hop* *BOOM* "Why does everyone always run away from Unyu?...T-T"
BTW, the answer to the lifespan question, for those interested, is: BILLIONS of years. There are no Black dwarfs(what a white dwarf 'cools off' too) yet, and the universe is approximately 13.7 billion years old. Therefor any white dwarfs present in the universe are less then 13.7 billion years old, but still old enough to have become a white dwarf, which is a process that takes billions of years.
Therefor any white dwarfs present in the universe are less then 13.7 billion years old, but still old enough to have become a white dwarf, which is a process that takes billions of years.
Its when I make mistakes like the above that I REALLY wish we could edit/delete our comments. Let me rephrase: Even white dwarfs that formed only say, 3 or 4 billion years after the big bang(I'm too tired to go research if any would have formed by then) are STILL in the white dwarf stage, and will probably still last at least a few billion more years.
"I've realized that you may need to start employing handwavium for the 'internals like a white dwarf'"
Keep in mind that I compared the age of an utsuho to that of a white dwarf star, and attributed this to its internal structure - I didn't actually compare its internal structure to that of a star. Subtle difference.