Why "une" rather than "un"?
This is also my question. Why "une" ??
Because "la célébrité" is a feminine noun.
In this case the singer is masculine, so wouldnt he become a masculine celebrity?
No, he would become a male celebrity and "célébrité" would still be a feminine noun.
Just as Duo is "une chouette mâle".
'devenir celebrite' wrong ?
I remember 'devenir chanteur/boulanger' without articles.
It works only with occupations, and without adjectives.
→ Il voudrait devenir chanteur.
→ Il voudrait devenir un chanteur célèbre.
But nowadays, being a celebrity has become an occupation! 😁
En France aussi !