Let's try to keep the Topic of the week alive, this week no topic was chosen.
Due to lack of traffic and active members in our meta (I think it's only more severe with the new top bar and it used to be easier to switch between main and meta with the previous one, but it's hard to judge in late December when number of visits plummet everywhere on Stack Exchange), we've decided to keep the current TOTW running until the next one is sufficiently voted on. For the better of me, I can't remember who was involved in deciding this or where we noted it, but it was likely in our main chat. I'll add a quote here once I find it tho.
The problem is two-fold, one already mentioned reason is low traffic in our meta, and the other is that the TOTW thread isn't featured any more to display in the Community Bulletin box, which should offset that a bit. Those featured are set to expire automatically (not sure what's the time they're valid for), and moderators need to refresh them. Since there's less activity from moderators too during these festive days (understandable, I'm not judging), it happened. Not a big deal tho, and we could cycle to the next TOTW ourselves too, by deciding which one should be next by votes and then simply editing the TOTW thread to reflect that. Most users active in meta will have ability to do that (reputation privileges), and if they don't, then stopping by in our chat room and mentioning it should remind someone that could.
Now, I've upvoted your own suggestion for milky-way and added it to the list of topics for the week on December 29, 2013 - January 4, 2014. If this seems as a bit one-sided decision, yes, it is. But with the lack of activity, I guess it's the best we've got and keep the TOTW scheme running. I'll also remind Astronomy moderators to feature the meta thread, and I can pin the notice for currently running TOTW in the chat wall myself. TOTW do recycle on Sundays though, so please remind someone either in chat, by adding a custom flag to the thread, or anything like that if we forget. This does happen, and perhaps it wouldn't be wrong if we discussed moving TOTW start dates on some other day of the week too, to avoid such complications in the future. Perhaps on the day when Astronomy.SE Newsletter goes out, which is Wednesdays at around 5 p.m. UTC?