When there are 30 coauthors and you're in the middle? And when it's a major paper with 112 citations (and increasing)? And how does it compare to writing a paper in something, but one that hardly got any citations at all? And how common is it among astrophysics undergrads?
Of course, being first author of any paper will really help. But in astrophysics, few papers these days get published without having numerous authors (in fact, there's a paper about how [over the last 10 years] the author count has tremendously increased on highly-cited astrophysics papers)
Obviously, it does depend on what you say in your personal statement, and what the professor says in his recommendation. But in the end, most recommendations/personal statements don't contain a huge amount of detail. See http://blogs.discovermagazine.com/cosmi ... ord-cloud/, for example.
And you don't necessarily have to be exceptional to contribute to it. You just have to do something that's important for it (your inclusion could just come from making plots, for example).