A star becoming a black hole happens in the same way as Ernest Hemingway described going bankrupt: “First it happens gradually, then all at once.” The gradual part of the process can take a lot longer than the time it takes to drain a bank account—billions of years in some cases. That’s why astronomers were puzzled when they found evidence that supermassive black holes existed less than a billion years after the Big Bang.
In order for the timeframe to make sense, the stars that collapsed to form these black holes would’ve had to have been gargantuan—1,000 to 10,000 more massive than our sun.