In the sinus of the aortic arch and arteria carotis exists the nerve terminal, named pressure receptor, which has the capability to feel the change of the pressure. At ordinary times, because the blood of the human body distributes in each part of the body according to the active condition of each organ and the blood pressure stays in a constant level, which will not cause the excitation of the pressure receptor. But the position changes when you do a handstand, due to the increasing quantity of blood in the head, the pressure to the aortic arch and vessel walls of the arteria carotis increases, hence stimulating the pressure receptor, causing stars in your eyes. Those who suffer serious cardiopathy, cerebrovascular disease, nephritis, high blood should not do a handstand.
Well, it is true that by doing handstands, our body will be stronger and it is a good way of building our muscles. However, you should know that it requires good physical conditions, and it would cause an oversupply of blood into your head, which would not be very good, sometimes, stars will arise. So, you should not do it too much. And maybe there are some other causes for that. Just pay more attention to your health.
When you do a handstand, your blood will be upside down which will cause the large amount of blood to store in the brain. That is why your eyes see many stars in the vision. You'd better not keep it too long when you find the symptom of stars. Or else, there may be the bloodshot in the eyes.