We live in a vast universe. The photograph above was taken in a portion of the night sky that looks void of stars to the naked eye. What we see through the Hubble telescope is innumerably many galaxies. Astronomical distances like those in the photo are so vast that we define a new unit to describe distances - the light year. The light year is the distance light travels in a year. This distance is easy to calculate given the speed of light and the number of seconds in a year: [br][br][math]1c\cdot yr=2.998\times\frac{10^8m}{s}\left(1yr\right)\left(3.156\times\frac{10^7s}{yr}\right)=9.46\times10^{15}m\approx10^{13}km.[/math][br][br]Some distances measured in light time are:[br][list][*]To moon is 1.25 light seconds[/*][*]To sun is 8.3 light minutes[/*][*]To former planet pluto is [math]6\times10^{-4}c\cdot yr[/math][/*][*]To next nearest star (proxima centauri) is [math]4.3c\cdot yr[/math][/*][*]To next nearest galaxy (Andromeda) is [math]2.5Mc\cdot yr[/math][/*][*]To most distant object visible to naked eye (Triangulum galaxy) is [math]3.14Mc\cdot yr[/math][/*][*]To edge of visible universe is [math]47Gc\cdot yr[/math].[/*][/list]

Above is our galactic neighbor, the Andromeda galaxy.  The photo is the largest composite photo ever assembled by NASA and the ESA.  In this photo are over 100 million discernible stars in a narrow band of the galaxy.  At the distance of 2.5 million light years, the ability to see individual stars is equivalent to taking a landscape photograph of a beach from a sailboat and seeing individual sand grains.