A light-year is a measurement of distance in space, and is defined by how far a beam of light travels in one year – a distance of six trillion (6,000,000,000,000) miles. That’s nothing compared to the size of a galaxy. Our own galaxy, the Milky Way, which is not a large galaxy, is still 600,000,000,000,000,000 miles across, and so takes something even as fast as light 100,000 years to cross. Putting that into perspective, the duration of recorded human history is roughly 5,000 years. So light from a star at one end of our galaxy takes 20 times longer than all of recorded history to get to the other end.