Most of the stars are so far away that they appear as no more than pinpoints of light to even the largest telescopes. But a few are close enough or big enough to see them as tiny disks. That allows astronomers to measure how big they are — measurements that help them understand how stars work. One of the stars that has been measured is Arcturus, in Bootes, the herdsman. It’s far to the left of the Moon as they climb skyward in mid-evening. It’s one of the brightest stars in the night sky, so you shouldn’t have any trouble spotting it. Careful measurements reveal the star’s apparent diameter — how big it looks in the sky. That’s then combined with measurements of its distance — about 37 light-years. The combination reveals that Arcturus is about 25 times the diameter of the Sun. What’s particularly important is that the measurement matches nicely with predictions of how big the star should be. Those predictions are based on models of how stars behave — how they produce energy, how they
