Distances to nearby stars are found using the geometric principle of parallax. Parallax is an effect of perspective — as your line of sight changes, an object's position changes relative to its background. You can demonstrate parallax by holding your thumb at arm's length and looking at it alternately with one eye and then the other. By measuring the apparent change in position of your thumb, you can calculate the distance from your eye to your thumb. As Earth travels in its orbit, the same effect can be seen in nearby stars -- the apparent position of a nearby star shifts against the background of more distant stars. The parallax method can accurately determine stellar distances up to about 1600 light-years, but because even nearby stars are far away and their parallax is very small, special instruments are used to make the precise measurements that are needed.

Beyond the limit of the parallax method, distances can be determined with the help of known luminosities. The apparent brightness of an object decreases proportionally with distance, so if the true luminosity of an object is known, the apparent brightness can be used to calculate the distance to that object. For example, within our galaxy, astronomers can figure out what type of star they're observing by examining its spectra -- the light they measure displays particular absorption lines in the spectrum, depending upon which elements are present in the star. And since particular types of stars have particular luminosities, they can then calculate distance.

For objects outside of our galaxy, astronomers rely on "standard candles" — particularly bright objects with known luminosities, such as variable stars and certain supernovae. Standard candles are used to figure distances to galaxies up to a few billion light-years away. Beyond that, galaxy properties, such as how rotation is related to luminosity, can be used to measure distances.

Together, these different techniques comprise a cosmic distance ladder, each rung representing a technique that's applicable to a limited range of distances, and each technique building upon the preceding one. However, this picture is complicated by the fact that the light from stars takes time to travel to Earth. When you look at a specific star, you are actually seeing how it looked at some time in the past. Light travels at a finite speed, and the light you see from a galaxy that is 2 million light-years away has actually taken 2 million years to reach you; it does not represent the galaxy today. And since the universe has only existed for about 14 billion years, the size of the observable universe is limited by the age of the universe.

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