The answer that Urania had always had to keep to herself was that she saw herself as Halley's Comet: predictable and observable, a constant for generations upon generations of people, a view into the vastness of space even for those without access to great machinery. Urania, like Halley's, had existed before everyone who now lived and she would exist long after them, and in that time she would be seen and influence and bring greater understanding with every life she passed through.
But that answer couldn't be given to Rosario - even if Urania was already the comet streaking across her mortal sky. And so she chose her other answer: "A magnetar. A slow moving neutron star with a strong magnetic pull on others, and a very short active life." Yes, she was immortal, but the amount of years she could spend as one person were always so quick cut away.