No, the site doesn't have to focus on a topic, but there should be pages that focus on topics so that they are catnip to searches for on-topic terms.
"The sun" might be an optimistic target given the competitiveness of existing search results, but supposing you were hoping to attract search traffic for the search term "sun", then you would want a page that links to/from all the best content on your site on that topic, incorporates sufficient snippets from them or descriptions of them that search engines can assess its content, and is linkable-to (and actually linked, although that's outside your direct control) as a useful resource from other pages/sites on that topic. If you have this page, then the many topics in the rest of the site's content will not harm your page's relevance to "sun" searches.
The one thing you'll always be missing, and I think this is marginal, is relevance of your domain name itself to topic-specific search terms. Can't have everything.
I know that this is a special case anyway, but consider that Wikipedia is by no means harmed in terms of search traffic by its variety of topics ;-)
It's tempting to think that something like a tag page will fulfil this role provided that your content is well tagged. I'm not sure that's reliably borne out by experience though, perhaps because any auto-generated page is at risk of being judged low quality both by search engines and by potential linkers who want something a bit more accessible and stable than what amounts to search results.
One you have your topic pages, watch their bounce rate carefully, and do everything you can to make it easy for visitors to see something interesting on them at a glance. Your goal, after all, is to get traffic to your authors, not just to get traffic to your index of articles! Of course you also want search traffic going directly to the articles themselves, so you should employ the usual good practice designing those pages to be comprehensible to search engines.
Google has spoken about sites being important in a particular topic, for example here: http://www.seroundtable.com/google-good-guy-algorithm-17848.html. Note that, as you'd probably expect, the implied mechanism to get Google's respect on a topic isn't to jettison all off-topic content, it's to get links from authorities in the topic. This suits Google because the example (a professor of journalism) presumably isn't inherently interested in selling links, so is fairly resistant to questionable SEO.