They basically take some of his Star juice, put it into their bodies and then walk around proclaiming that they're God.
"Bravo, Nasus, you've ascended yourself into a dog! Now fetch me something more interesting."
"That whole practice of Ascension is adorably amusing, like a kitten trying to walk upright."
Y'all saying Bard can't fight makes me sad. 
They use celestial energy as material, they dont directly use it and become the demigods. I said it in an another thread. Just because you're using frog leg and goat skull in a black magic, it doesn't mean your spell gets the power from frog and skull. It is still your magic. So, that's correct, Ascended Beings use the power of Sun, but there must be much deeper things than the only Sun. Otherwise, Xerath wouldn't be able to have that much power. He is even more powerful than Aurelion Sol in unlimited form.
Aurelion Sol respects Xerath and thinks he is an interesting opportunity to become free. Because as we know, Xerath is able to use all magic types.
Here is my take:
Magic is entirely independant of both Sol and Xerath. It is, by it's nature, rewriting how the world normally functions, it's "laws" as it were.
Sol uses magic to convert energy into mass in a form that creates a chain reaction that isn't controlled, reaching nuclear fission, perpetuity.
Xerath on the other hand, via the ritual of ascendacy has basically become one with magic, however, due in part by his nature, and also in part of his lack of experience, he is essentially unable to use magic in the ingenious way that Sol does. Resulting in what Sol would see as lower tier spells.
However, Xerath's potential, is most likely far greater than Sol's as while Sol is powerful, he is not made of magic, he has no direct access to it, unlike Xerath.
Another possibility:
It's never stated that sol actively learned to make stars, it could very well be an innate gift of his being a cosmic dragon and all. At which point we have to say that sol's power is essentially limited, as he apparently never really cared to learn magic as he could naturally perform miracles.
I love threads like these. [slayer-pantheon-thumbs]
people please lets remember Sol was born during the creation of the universe....
Star Juice?
The more funny part is how much Renekton and Azir shit on him in-game. Such a weakass champion, banworthy if you don't want your mid laner to cost you the game.
And lets not even talk about Targon aspects such as Diana and how much they rape the dragon every single time they see him post lv6 (phanteon post minute 1 and a half)
