Image Tour: Cassiopeia A

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Trace the makeup and history of supernova remnant Cassiopeia A. 

Produced by the Space Telescope Science Institute’s Office of Public Outreach in collaboration with NASA’s Universe of Learning partners: Caltech/IPAC, Center for Astrophysics | Harvard & Smithsonian, NASA Jet Propulsion Laboratory, and Sonoma State University.
  • Chandra X-ray Observatory and Hubble Space Telescope image of Cassiopeia A: X-ray: NASA/CXC/RIKEN/T. Sato et al.; optical: NASA/STScI

Written by Claire Blome
Designed by Leah Hustak and Dani Player
Subject matter expertise from Dr. Danny Milisavljevic, Purdue University
Editorial and design input from Dr. Rutuparna Das (Center for Astrophysics | Harvard & Smithsonian), Dr. Quyen Hart, Dr. Varoujan Gorjian (JPL), Yesenia Perez
Music courtesy of Yesh Music (ASCAP)

 A white-outlined box is within a gold-outlined box on top of a multi-colored circular supernova remnant outlined in electric blue on a dark space background. Title, Cassiopeia A, Image Tour. Fast Facts, Constellation, Cassiopeia, Distance From Earth, 11,000 light-years, Size, 20 light-years across, Wavelengths of Light, X-ray and visible 


After a massive star explodes, its debris is excited by shock waves, making it glow brightly. This is known as a supernova remnant. Cassiopeia A is a supernova remnant of particular interest. Its central star exploded, and its light reached Earth around 1680, which in astronomical terms is relatively recent. 

The aftermath of the explosion is largely undisturbed. Tour Stops, Neutron Star. The photo is zoomed into the center to show electric blue lines with a white star in the center. Text, Research suggests that a massive star at the center of Cassiopeia A exploded, blowing off its outer layers and leaving a small, dense core known as a neutron star. A rotating dashed circle highlights the neutron star. 

Text, The neutron star's matter is packed so tightly that a sugar-cube-sized amount of material would weigh more than 1 billion tons. 

The photo zooms back out to show the entire nebula. Text, Tour Stops, Shock Waves. The supernova sent out a forward shock wave that traveled about 40 million kilometers per hour, 25 million miles per hour. A dotted line encircles the outer edge of the nebula and is labeled Forward Shock Wave. 

Text, As the forward shock wave travels, it encounters material and slows down, generating a second shock wave that travels backward. A smaller dotted line encircles the darker center section of the nebula and is labeled Reverse Shock Wave. 

Text, The reverse shock wave heats the gas in the inner regions, causing it to glow again in X-ray light. 

Tour Stops, Bi-polar Jets. In addition to the shock waves sculpting the debris, there are opposing regions of high-velocity ejecta known as jets in Cassiopeia A. 

An irregular-shaped dotted line outlines a longer diagonal gold area that extends across the length of the nebula and outwards past the edges of the shock waves. Text, These jets are rich in silicon, which suggests that they may have originated from deep inside the original stellar explosion. 

Tour Stops, Filamentary Structure. Cassiopeia A has made and expelled most of the elements needed for life. A red filter shows the silicon in the nebula. A yellow filter shows the sulfur. A green filter shows less calcium. A purple filter shows the iron on the outer portions of the nebula. Text, Supernova remnants produce and disseminate most of the elements found on Earth. 

Cassiopeia A will keep expanding for thousands of years, allowing researchers to continue to learn about the supernova that created this remnant.