Just a simple question: what's the difference between this and Space Exploration? It's not a doubt, I just want to interest that what is this site about that can't be discussed there?
For one, if you look at the example questions that define the Astronomy proposal, none of them are on topic for the Space Exploration site.
Astronomy is largely an academic site about the observation and scientific study of celestial objects. Space Exploration SE is a much more applied site about the engineering principles and challenges of physical space exploration.
I would not suggest mixing the two.
what's the difference between Astronomy and Space Exploration?
The short answer would be "a couple light years".
Space exploration is practically limited to the Solar System currently. There are a few odd questions leaning on sci-fi, about extrasolar exploration but most are about "getting there", propulsion, orbital maneuvers, technology used in space exploration, discoveries by probes and landers.
Astronomy doesn't have anything "big" to find within the Solar System anymore. It's primarily about the deep space; there is still overlap with Astronomy finding viable targets for Space Exploration, and Space Exploration providing answers to many questions Astronomy is asking, but the overlap is fairly small; these go hand-in-hand like mathematics and physics, but are quite distinct disciplines.
The best phrase I've heard to distinguish them is "Astronomy deals with the science of space while SE deals with science and engineering in space" (Slight modified quote from this question). Basically, Space Exploration is fundamentally an engineering site, Astronomy is fundamentally a science site. There is some overlap, but such overlap is pretty small really, almost always one of the two sites will be better to answer the question. A few examples:
You want to build a space telescope? Ask on Space Exploration. You want to get time on Hubble, and need help? Ask on Astronomy.
Planetary Science is a bit of an overlap, but in general, for solar system planets, will work better on Space Exploration. In addition, per SF.'s answer. Studying weather patterns of distant planets would fall on Astronomy.
The two are closely related. There is an overlap of about 20% comitters, very high for relatively niche sites. But they are different. Enthusiasts are often interested in both, but professionals usually focus on one area or the other. If one or the other site fails, there might be room to consider a merge, but they really are two different subjects.