Show author name in index loop

Hello.

I want to show the author’s name in the main page (index page) in the loop, after the posted on <date> (or instead of it), but I’m not sure where the “Posted on <date>” code is generated. I know how to display the author, I just don’t know where (which file to edit).

I’ve checked in the index.php page and the code that is generating the posts in the loop is:
<?php get_template_part( 'content', get_post_format() ); ?>

but both the get_template_part and get_post_format functions are defined in the wordpress core, which I don’t want to edit unless really necessary.

Could anyone help me with this? Thank you.