How Scientists Measure Distances of Stars Light Years from Earth

There are dozens of methods, but the trigonometric Parallax Method is the most popular for stars nearby (100's of light years).

Basically, you take a careful measurement of the star's angular position at a specific time. Then wait about 6 months and repeat the measurement. The Earth's orbit takes us about 180 million miles away from the last measurement point and you can triangulate the star's position.

Another measurement technique is red shift. Everything is moving away from us. The furthest objects are moving away the fastest. If you know the temperature of the light source you can extrapolate its distance based on its apparent color temperature versus its actual color temperature.

Another method is to use a gravitational lensing where a galaxy's gravitational pull actually bends light. Quasars that are located behind the galaxy will be seen as a double image. The degree of the split, the time variability of the quasar, along with estimates of the galaxy's mass, will allow us to calculate the distances of the quasar.

Another popular method is the Cepheid Variable star method. These stars are both bright and vary in intensity over time. Knowing the amount of change in the star's brightness allows us to calculate its distance because all Cepheids have about the same temperature.

Once you have any star's absolute brightness you can calculate the distance by the formula m = M/ d^2, where m is the apparent magnitude brightness and M is the absolute magnitude and d is the distance. Just imagine someone walking away from you with a flashlight. It gets dimmer and dimmer as you get farther away (square of the distance).

There are more exotic methods, too, but they get very complicated. The bottom line is we have been able to do a pretty good job of mapping the universe based on these methods and scores of others.

Just a few minor points of clarification. [Plus a note to keep in mind that once you get beyond simple parallax, each distance scale used is calibrated using the previous (smaller) scale so a small error at the start is compounded as the scales get larger.]

Within our galaxy we use parallax to get the distances to the nearest stars and then 'moving cluster parallax' (also called dynamic parallax) for nearby star clusters. Next would be RR Lyrae-type variables and Cepheid variables. The HR Diagram and mass-luminosity relationships can then be established and applied to the galaxy in general. The key markers here are the Cepheid variables which can also be used to determine the distances to nearby galaxies.

For nearby galaxies Cepheid variables and RR Lyrae variables are used. Once we know the distances to nearby galaxies we can study supernovae. As it turns out, a certain type of supernova (Ia) occurs for stars that are nearly identical in mass and composition; thus these supernovae can be used as 'standard candles' to determine the distances to galaxies at extreme distances. It is at this point that the 'redshift' relationship can be determined. (Stars within our galaxy are, of course, not generally moving away from us. Nor are the galaxies within our 'local group' of galaxies moving away from us; the 'local group' is a small cluster of galaxies of which the Milky Way and Andromeda Galaxy are the major members, and the 'local group' is gravitationally bound together.)

Once the redshift relationship is established, the redshift method, supernovae, and gravitational lensing can be used for the most distant objects seen.

Supporting these more-or-less visual methods are studies by radio astronomers who have used radar within our solar system to determine the distances to Venus and Mercury, which improved our measure of the basic 'astronomical unit', have used radio studies of galactic hydrogen regions to help lay-out the structure of the Milky Way, and have used radio interferometry to help determine stellar and galactic distances and structure.