How Far Away is Vega?
By Bill Pellerin
Houston Astronomical Society
GuideStar Editor
At our annual Astronomy Day event, I’ll often point to Vega and say, “That star is 25 light-years away and this means that the light takes 25 years to reach us.” But, how do we know that Vega is 25 light years away? How did we determine this?
This is an interesting subject. Even the distance to the Sun wasn’t well determined until the 1960’s when radar ranging was used to measure the distance. The process is easy, send a radio pulse to an object, and see how long it takes to get there and bounce back. If you know the speed of the radio pulse, and we do, it’s easy to calculate the distance using the formula we learned in high school — d=r*t. That is, the distance is equal to the rate (speed) multiplied by the time. Since the radio pulse has to travel to the object and back the calculation for this is d=r*t/2.