 For the most part, the position of stars is virtually identical from century to century. But a very small number of relatively nearby stars show dramatic motion across the sky. This is called proper motion. This magnitude can be measured using the number of degrees moved and the distance to the star. Motion towards us or away from us is called radial motion. We define it as positive if the star is moving away and negative if it is moving closer. For nearby stars, this motion can be detected by using parallax techniques. With these two numbers, the total star motion with respect to the sun can be calculated using the Pythagorean theorem. For example, here's Barnard's star. It's a dim red star with significant proper and radial motion. It's moving so fast that it's called a runaway star. Here's a look at Barnard's star, photographed in 1985. Its parallax measurement indicates that its distance was 5.98 light years. In this second photograph, taken 20 years later in 2005, we see that the star has moved 200 arc seconds across the sky. That's 10 seconds of arc across the sky each year. This is its proper motion. Its new distance is 5.94 light years, which gives us its radial motion. Combined, we get the full motion of the star with respect to the sun. It moves 19.1 billion kilometers each year. That's 11.9 billion miles. So you can see why it's called a runaway star. In fact, Barnard's star is approaching us so rapidly that around 11,700 AD, it will be 3.8 light years from the sun. And that would make it the closest star to our own.