 This supernova star's shredded remains are called Cassiopeia A, or Cass A for short. Cass A is the youngest known supernova remnant in our Milky Way galaxy. The light from this exploding star reached Earth in the late 1600s. This photo shows the upper rim of the supernova remnant's expanding shell. The colors highlight parts of the debris where chemical elements are glowing. The dark blue fragments, for example, are richest in oxygen. The red material is rich in sulfur. These spitzer infrared space telescope images show shifting patterns of glowing dust beyond the remnant itself. These changes are so fast they indicate motion at the speed of light. These are light echoes, just like what we saw with the star Montserritos. The light from a supernova can take hundreds of years to reach surrounding dust clouds. Following the arrows of light, it's clear we'll see the supernova flash first. The light echoing off the dust clouds will arrive later, at various times, delayed by hundreds of years from the original flash. So we're not seeing the dust move near the speed of light. We're seeing the light from the supernova move through the dust. It's spitzer. They can detect the brief boost in the thermal infrared glow.