27,000 years ago, Earth was locked in an Ice Age. Glaciers covered much of North America, and sea level was hundreds of feet lower than it is today. People were painting in caves and on canyon walls, and they were developing such weapons as the bow and arrow. Scientists have pieced together that view of the ancient past through centuries of excavations and lab work. But they can see one remnant of that time directly — the light from the heart of the Milky Way galaxy. That illustrates the vastness of our home galaxy. Light travels at 670 million miles per hour — almost six million million miles in a year. Yet it takes light about 27,000 years to cross the great gulf from the galactic center to Earth, which is a little more than half-way out to the galaxy’s edge. So if a star explodes in the galactic center tonight, no one on Earth will see the fireworks for another 27,000 years. Unfortunately, we can’t see the center of the Milky Way with our eyes alone because it’s veiled by vast cloud
