> Astronomers have concluded that the universe began some 12 to 15 billion years ago. That means we can only see the part of it that lies within 12 to 15 billion light-years from us. There may be an infinite number of stars beyond that cosmic horizon but we can’t see them because their light has not yet arrived. And the observable part of the universe contains too few stars to fill up the sky with light.
(Emphasis mine because that's where the error is: 13 billion year-old light has traveled over a distance that is now much higher than than 13 billion light-years)
Potential ambiguity (in cases such as this one) is one of the reasons why using light-years for distances may not be a good idea.
(By the way, there are real astronomers arguing about this, too, e.g. http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html . So it's not just this nerd ranting on HN).
GRB 090423 (admittedly, not a star -- it's a gamma ray burst) happened approximately 13 billion years ago. This means that light traveled for 13 billion years to reach us -- but, by the time it got to us, GRB 090423 was no longer 13 billion light-years from us, because all that space got bigger! GRB 090423 "was" about 9.2 Gpc away from us by the time that light reached us, i.e. about 30 billion light-years.
If the light that hits us is 13 billion years old, that object (assuming it still exists) is a lot farther than 13 billion light-years from us now.