To understand it better, I am answering after keeping your question limited to the Sun.
Sun is called Aditya meaning son of Aditi and Kashyap Rishi. How can a star be born from a living being?
The star was not born from Aditi. It is not true that there was no Sun before the Aditi's son.
The star Sun came out from the Golden Egg in the very begging. From the creation chapters of Matasya and Kurma Puranas:
In the very beginning, inside the golden egg, Brahma created himself. Therefore he is also known as Svayambhuva and Hiranyagarbha.
After Lord Brahma,
From golden egg, the Sun was also born. Since he was the first (adi) being to be born, he is known as Aditya.
As the Sun emerged from the Golden Egg, Hiranyagarba is also name for the Sun same as Lord Brahma.
Due to this very reason Lord Brahma also called as SuryaAgraja.
Markandeya Purana also confirms that the Sun emerged in the very beginning (source 1, source 2).
And after this Sun (star) was emerged, Lord Brahma created Marichi (Kashyap's father) and Daksha (Aditi's father).
Not only the Sun but also,
Everything that there is in the universe was already there, in in embryonic form, inside the egg. There the moon, the stars and the planets etc.
How the Sun became son of Aditi
From Markandeya Purana,
A great battle was fought between the deities and the demons in which the deities were defeated. Aditi, the mother of the deities became very sad. She did a rigorous penance to please the Sun god. She observed fasts and eulogised the Sun god for most of her time. At last, the Sun god appeared before her but she could not bear his radiance which was so powerful that she could not even open her eyes. She requested him to subdue his power so that she could see him.
Markandeya says- 'After being pleased by the eulogy of Aditi, the Sun shed its radiance and became sombre. Now, Aditi could see him. She said- 'O lord! Be pleased upon me. The Daityas and the Danavas have captured all the three worlds from my sons. I request you to take birth as my son and defeat the demons.' The Sun god agreed to take birth as her son.
Now your question boils down to who was this Sun god who appeared before Aditi?
Most of God's names are posts actually they own the responsibility of that thing. For example, if someone says Indra as rain god then Indra is not rain but he owns the responsibility of rain. And when someone owns the responsibility they get some special powers, the one who rules the star Sun should have the powers to bear the heat of the star Sun. He also gets the radiance of the star Sun. So before Aditi's son someone was ruling the Sun or Sun was abode of someone. The same person incarnated as Aditi's son named Vivasvana. And Aditya got one more meaning Aditi's son (other than the first (adi) born).
Karna was son of Sun. How is this possible? How can a star have a son?
Karna was son of Aditi's son Vivasvana, the ruler of the Surya Loka.