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Parallax Measurement
When the Earth revolves around the Sun during its regular cycle, nearby stars as we view them show an apparent shift in relation to stars farther out. This is called a parallax shift. By using the diameter of Earth's completed orbit and knowing the magnitude of the shift, astronomers can determine the parallax angle over the sky and calculate the distance of objects.
Parallax Calculation Limits
If the star exhibits a small or limited shift when observed and recorded, it means it is farther away than a star that has a long shift. The method of calculation only works for stars that reside within 200 light-years of Earth. The parallax shift becomes too slight to accurately measure once the distance of objects exceeds 200 light-years.
Cepheid Variable Measurement
When the distance for measuring stars passes the capability of the parallax technique, the Cepheid variable measurement must be used. Cepheid stars change in luminosity over long periods of time. Astronomers can calculate the distance by comparing the difference in the apparent brightness to the true brightness of the star.The difference in luminosity determines the distance by using a graph that corresponds to the distance in light-years. The distance of globular clusters and distant galaxies can be found using the Cepheid variable measuring technique.
Hubble's Law
During the 1920s, Edwin Hubble discovered that he could use the period luminosity of different variable stars to determine the extreme distances of galaxies, and even farther celestial objects. Hubble's Law dictates that a relation exists between a galaxy's distance and red shift -- the red shift is the spectral lines near the end of the rainbow. By passing the light of a galaxy through a spectrogram, the red shift can be determined, which provides a fairly accurate distance. This measuring method demonstrated that the universe is expanding, and the method has been used to calculate distances for the most remote celestial objects.
Supernovae Observation
Another method of calculating distances involves the observation of supernovae, or exploding stars. Exploding stars have a very regular maximum luminosity, which can be depended upon for more accurate calculations than the Cepheid measuring technique. Since the supernovae are billions of times brighter than Cepheid stars, they can be picked up in telescopes at much greater distances. Supernovae observations and calculations have provided their own "standard candles" measuring graphs.