Scientists have to know how far away a star is before they can begin to analyze details such as its age, size, temperature and mass. The most effective way of measuring a star’s distance from Earth is called the parallax method. If you are travelling in a car and looking out of the window, nearer objects seem to pass by much more quickly than distant ones. In the same way, as Earth orbits the Sun, nearer stars appear to move more quickly through the sky than those further away. The angle through which a certain star moves over a period of six months is called its parallax. This angle is used by astronomers to work out how far away the star is.
Parallax is “the best way to get distance in astronomy,” said Mark Reid, an astronomer at the Harvard Smithsonian Center for Astrophysics. He described parallax as the “gold standard” for measuring stellar distances because it does not involve physics; rather, it relies solely on geometry.
The method is based on measuring two angles and the included side of a triangle formed by the star, Earth on one side of its orbit and Earth six months later on the other side of its orbit, according to Edward L. Wright, a professor at the University of California, Los Angeles.
It works like this: hold out your hand, close your right eye, and place your extended thumb over a distant object. Now, switch eyes, so that your left is closed and your right is open. Your thumb will appear to shift slightly against the background. By measuring this small change and knowing the distance between your eyes, you can calculate the distance to your thumb.
To measure the distance of a star, astronomers use a baseline of 1 astronomical unit (AU), which is the average distance between Earth and the sun, about 93 million miles (150 million kilometers). They also measure small angles in arcseconds, which are tiny fractions of a degree on the night sky.
If we divide the baseline of one AU by the tangent of one arcsecond, it comes out to about 19.2 trillion miles (30.9 trillion kilometers), or about 3.26 light years. This unit of distance is called a parallax second, or parsec (pc). However, even the closest star is more than 1 parsec from our sun. So astronomers have to measure stellar shifts by less than 1 arc second, which was impossible before modern technology, in order to determine the distance to a star.
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