In the Loop, it would be:
$authorname = get_the_author_meta('user_nicename');
Or:
$authorname = get_the_author_meta('displayname');
Or:
$authorname = get_the_author_meta('nickname');
Or any field that get_the_author_meta()
accepts.
$authorname = get_the_author_meta('user_nicename',123);
If you just need to echo
the name just use the_author_meta()
instead:
the_author_meta('user_nicename',123);