By measuring the motions of a large number of stars of a given class, we can estimate their average distance from their average motion. Statistically, stars move at about the same rate therefore, the stars that appear to have larger motions are nearer.

Proper motions: All stars move across the sky, but only for nearby stars are these motions perceivable, and even then it takes decades or centuries to measure. The methods to measure stellar distances greater than a few thousand light years include: Astronomers use more indirect methods beyond a few thousand light years.

Beyond that distance, parallaxes are so small than they cannot be measured with contemporary instruments. Parallaxes give us distances to stars up to perhaps a few thousand light years. How Do We Calculate Distances of This Magnitude It lies about 8 kpc from the center on what is known as the Orion Arm of the Milky Way. The Sun does not lie near the center of our Galaxy. A parsec, equal to 3.26 light years, is defined as the distance at which 1 Astronomical Unit subtends an angle of 1 second of arc (1/3600 of a degree) When we use the parsec for really large distances, we often put a prefix in front of it - like kiloparsecs (kpc), which are equal to 1000 parsecs - or Megaparsecs (Mpc), equal to a million parsecs. Image Credit: The COBE Project, DIRBE, NASAĪlthough the light year is a commonly used unit, astronomers prefer a different unit called the parsec (pc).

A Milky Way Home by Hsinju Chen