Note on Cosmic Distances
In astronomy, distances are measured in units of light years, where one light year is the distance that light travels in a year10 trillion kilometers. For historical reasons having to do with measuring distances to nearby stars, professional astronomers use the unit of parsecs, with one parsec being equal to 3.26 light years.
Astronomers compute the distance to remote galaxies (ones that are more than about 20 million light years away) with Hubble's law. According to Hubble's law, the universe is expanding in such a way that distant galaxies are receding from one another with a speed which is proportional to their distance. The recession causes the radiation from a galaxy to shift to longer wavelengthsthe red shift. From a measurement of the red shift and the constant of proportionality, called Hubble's constant, astronomers can determine the distance to a galaxy.
One of the central problems of modern astronomy is to accurately determine Hubble's constant, which is a measure of the rate of expansion of the universe. At present it is known to an accuracy of about 20 percent, so we usually modify distances by saying "about 100 million light years," for example. We assume throughout the Photo Album a value of the Hubble constant that corresponds to a recession velocity of 600 kilometers per second for a source at a distance of 30 million light years or 10 million parsecs (H0 = 60 km/s/Mpc).