How do you add the star ratings for products in woocommerce?

There's actually a much more terse way to handle this. Just use the built-in functions that Woocommerce has built out:

add_action('woocommerce_after_shop_loop_item', 'get_star_rating' );function get_star_rating(){    global $woocommerce, $product;    $average = $product->get_average_rating();    echo '<div class="star-rating"><span style="width:'.( ( $average / 5 ) * 100 ) . '%"><strong itemprop="ratingValue" class="rating">'.$average.'</strong> '.__( 'out of 5', 'woocommerce' ).'</span></div>';}

I've confirmed that this works for me.

You can put this into your themes functions.php file:

add_action('woocommerce_after_shop_loop_item', 'my_print_stars' );function my_print_stars(){    global $wpdb;    global $post;    $count = $wpdb->get_var("    SELECT COUNT(meta_value) FROM $wpdb->commentmeta    LEFT JOIN $wpdb->comments ON $wpdb->commentmeta.comment_id = $wpdb->comments.comment_ID    WHERE meta_key = 'rating'    AND comment_post_ID = $post->ID    AND comment_approved = '1'    AND meta_value > 0");$rating = $wpdb->get_var("    SELECT SUM(meta_value) FROM $wpdb->commentmeta    LEFT JOIN $wpdb->comments ON $wpdb->commentmeta.comment_id = $wpdb->comments.comment_ID    WHERE meta_key = 'rating'    AND comment_post_ID = $post->ID    AND comment_approved = '1'");if ( $count > 0 ) {    $average = number_format($rating / $count, 2);    echo '<div class="starwrapper" itemprop="aggregateRating" itemscope itemtype="http://schema.org/AggregateRating">';    echo '<span class="star-rating" title="'.sprintf(__('Rated %s out of 5', 'woocommerce'), $average).'"><span style="width:'.($average*16).'px"><span itemprop="ratingValue" class="rating">'.$average.'</span> </span></span>';    echo '</div>';    }}

Note that you may need to change woocommerce_after_shop_loop_item to a different hook depending on your design and where exactly you want the stars to show up.

This page lists WooCommerce action hooks: http://wcdocs.woothemes.com/codex/extending/hooks/ . I don't see any hooks associated with the thumbnail specifically, but you may try woocommerce_after_shop_loop_item_title or woocommerce_after_shop_loop_item.

(This function is essentially copied from WooCommerce's single-product-reviews.php)

As of WooCommerce 3.0+ this is the correct code:

$product = wc_get_product( $id );echo wc_get_rating_html( $product->get_average_rating() );

The get_rating_html() method on the product object has been deprecated.

More information here: https://docs.woocommerce.com/wc-apidocs/function-wc_get_rating_html.html