That "El astro rey" is a poetic name for the sun

Capitalized, alternative proper name for the Sun in English

I haven't heard of the Sun being referred to in English as "the kingly star" or something of the sort, which would be an approximation to "el Astro Rey" (which is very common in Spanish, by the way).

Creo que sería muy rebuscado traducirlo así.

But you could call it something like "the biggest star" (in our galaxy, at least).

You could also get some ideas from this:
http://en.wikipedia.org/wiki/Sun#Solar_cultural_history
Like other natural phenomena, the Sun has been an object of veneration in many cultures throughout human history. Sol (pronounced /sɒl/ in English) is the Latin word for "sun". The Latin name is widely known, but not common in general English language usage, although the adjectival form is the related word solar. 'Sol' is more frequently used in science fiction writing (Star Trek in particular) as a formal name for the specific star, since in many stories the local sun is a different star and thus the generic term "the sun" would be ambiguous. By extension, the Solar System is often referred to in science fiction as the "Sol System". 'Sol' is sometimes used in scientific circles, but 'Sol' is not the "official" name of the Sun, and the word 'Sol' makes no appearances in common reference sources.[64]

Hope that helps!

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Note added at 13 mins (2008-01-23 15:51:03 GMT)
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What I meant is that, in Spanish, "el Astro Rey" is very common and anyone understands it. But you'd have to change the register in English, or otherwise scratch that whole sentence entirely! I don't think it adds any info anyway. Although it leads to the phrase immediately after that: "y es que manda mucho" (as a **king** would do).

Could you eliminate that part or do you really have to keep it?

What about "the ruling star (in our galaxy)"?

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Note added at 27 mins (2008-01-23 16:05:01 GMT)
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Actually, Googling around, I found out that there's another star called Regulus, which has been called, indeed, "the kingly star" or the "Royal Star", which is the brightest star in the constellation Leo. Maybe you could use one of these...

Aparte del uso metafórico, en los estudios esotéricos de la India existe toda una categoría de “Raja Suns”, sobre los que pueden leer en la Carta 23 en The Mahatma Letters, publicado por Adyar.

Jason needs a way out. I agree with psicutrinius, juan jacob and maria teresa, Jason has to find an equivalent expression for Astro Rey or completelly obliterate the frase. It looks like a general level paper and no harm would then be made. However the author may insist in conveying the idea behind Astro Rey. In that case choose between "The biggest star" or "King of all stars" or any other variation in line with "Astro rey".