I didn't participate on the previous Astronomy site, but you can read about the reason it was shut down in When a Site Grows Quiet. The key point is:
But when a site struggles to maintain any semblance of steady progress — when it's struggling to garner an audience, a healthy core of experts, and a steady stream of questions — it becomes increasingly unlikely that the site will find a core audience to sustain it.
If you look at the final Area 51 statistics of the previous Astronomy site, you'll see that it was getting around 350 visits and one question a day after about 11 months of operation. Considering the popularity of the topic in the public imagination (look at the number of images of Pluto people are posting on social media right now for evidence), this seems disproportionately quiet. Our general Physics site appears far stronger.
When I look at the statistics for this iteration of Astronomy at about 11 months from private beta, I find that the traffic averages about 350 visits a day, just like the old site did. But this site got about double the number of questions a day. We've done a lot of research (including an experiment on Parenting) and concluded that the single best predictor of a site's future health is the rate of questions it gets.
Since the previous Astronomy site was shut down, we've changed our philosophy of how to handle quiet sites. Instead of shutting them down and waiting for someone to come along and reboot them (which is a lot of work and no guarantee of success), we now let sites run (crawl?) along as long as they don't descend into spammy backwaters. (This only applies once a site has gone public. We still hold back sites that perform poorly in private beta.) In practice, this has meant we don't shut down beta sites.
Thankfully, the future of this Astronomy site shines brighter than that. Currently, it's averaging three and a half questions per day, which puts it in good company. We aren't really close to talking graduation, but you can rest easy about closure. If you care about the success of this site, make it a habit to ask any Astronomy questions you have here. Remember, there's nothing wrong with answering your own question.