While there are plenty of stars shining on stage at the Williamstown Theatre Festival this summer, even bigger stars are appearing twice weekly in a much more intimate setting. And these stars are millions if not billions of years old. Inside the Hopkins Observatory on the Williams College campus just across Main Street from WTF, the Milham Planetarium and Mehlin Museum of Astronomy hosts a free show every Tuesday and Thursday at 8:30 p.m. through Aug. 7.