Credit: NASA/JPL-Caltech/WISE Team
Supernovae explode with an energy that can outshine an entire galaxy. But just as quickly the star fades from view leaving an expanding bubble of star guts and in the case of Cassiopeia A, a light echo.
About 11,000 years ago, toward the constellation Cassiopeia, the Queen, a massive supergiant star exploded in the Milky Way, our home galaxy. Supernova happen at the end of these giant stars’ lives. Stars about ten times more massive than our Sun burn through their hydrogen fuel quickly, within only tens of millions of years. They burn super hot and when the fuel runs out they collapse. Collapsing stars become even hotter and as the outer layers of the star heat up a runaway nuclear reaction can occur. The star explodes. The light from the explosion can be brighter than the entire galaxy. The explosion creates and scatters elements necessary for life on Earth, such as carbon and iron. It also spreads heavier elements such as gold, silver and uranium.
Zoom into the bright yellow cloud of dust. This is the bubble of material blown away from the star. As the edge of the bubble moves through interstellar space it heats up gas. The shockwave slams into the calm space at about 18,000 kilometers per second (11,000 miles per second) or about six percent the speed of light. In 11,000 years since the explosion, the expanding ball has expanded about 21 light-years.
Light travels much faster than the expanding bubble of gas and dust left over from the star. WISE, with its infrared eyes, detected infrared echoes of the brilliant flash as the light and heat ripple outward through surrounding star clouds. Look to the upper right for a curtain of orange in the cooler, greenish nebula. These orange echoes are remnants of heat generated when light from the explosion reached this part of the nebula.
Light from this explosion, traveling at more than six trillion miles per year, finally reached Earth in about 1667 AD. We have no record of it. European astronomers didn’t report it, Chinese astronomers didn’t record it. Scientists think that thick dust surrounding the star blocked most of the light, making it a dim supernova. Astronomers discovered the remnant in 1947. When they used new radio telescopes to scan the sky, Cas A was the brightest source they could see.