Cassiopeia A, the movie

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 09 January, 2009

Using eight year’s worth of data from NASA’s Chandra X-ray Observatory, astronomers have released a movie tracking changes in the dynamic supernova remnant Cassiopeia A.

Chandra first revealed previously unseen structure and detail in Cassiopeia A (Cas A) around ten years ago, and now astronomers have used their observations to piece together a movie that tracks the remnant's expansion and evolution over this time frame. "With Chandra, we have watched Cas A over a relatively small amount of its life, but so far the show has been amazing," says Daniel Patnaude of the Smithsonian Astrophysical Observatory. "And, we can use this to learn more about the aftermath of the star's explosion."

The Cas A movie was created from eight year's worth of X-ray observations from Chandra. It begins with a full-field view of Cas A, cycling through the movie several times. The camera then zooms into three different areas of Cas A where the evolution of different features can been seen. Credit: NASA/CXC/SAO/D.Patnaude et al.

A second visualisation – the first three-dimensional fly-through of a supernova remnant no less – has also been created using data from Chandra, Spitzer and various ground based telescopes by Tracey Delaney of the Massachusetts Institute of Technology and colleagues from the Astronomical Medicine project based at Harvard. The two teams married together the best techniques from astronomy and medical imaging to produce the visualisation.

"We have always wanted to know how the pieces we see in two dimensions fit together with each other in real life," says Delaney. "Now we can see for ourselves with this 'hologram' of supernova debris." Both the movie and the 3-D model will help astronomers with their quest to understand fully the ins and outs of this famous supernova explosion, the death cry of a star that has reached the end of its life.

In a supernova explosion much of the star’s material is thrown out into space, which drives a shock wave into the surrounding interstellar medium. This shock wave sweeps up the gas and dust spewed out by the star to make the supernova remnant, which can be observed long after the original star has faded into the background. Cas A is the remains of a star thought to have exploded about 330 years ago, and is one of the youngest remnants in the Milky Way. Using the movie data, Patnaude and colleagues were able to calculate the expansion speeds of various features observed in the Cas A supernova remnant. Curiously, they found that the expansion is slower than the current theoretical models predict.

This 3-D visualisation of Cas A was made possible by combining astronomical and medical imaging techniques. Credit: NASA/CXC/MIT/T.Delaney et al.

Patnaude puts this mysterious loss of energy down to cosmic ray acceleration. Cosmic rays are energetic particles that are generated, in part, by supernova remnants and which constantly bombard the Earth's atmosphere. By studying the flickering in the movie Patnaude and colleagues could locate where the acceleration of these particles occurs, and estimate that around 30 percent of the energy in this supernova has gone into accelerating these cosmic rays.

The 3-D visualisation also provides a new tool to probe the remnant, and Delaney and colleagues found two components to the explosion: a spherical component from the outer layers of the star and a flattened component from the inner layers of the star. They also observed high-velocity plumes shooting out from the inner layers, composed of silicon in the northeast and southwest and iron in the southeast and north of the stellar remnant. Astronomers had known about the plumes and jets before, but did not know that they all appeared in a broad, disc-like structure.

The implication is that astronomers will have to incorporate this finding into their models of supernova explosions, such that the outer layers of the star are ejected spherically, but the inner layers emanate in a disc-like fashion with high velocity jets emitted in multiple directions.

Studying the explosion mechanisms of supernovae help astronomers learn how the surrounding interstellar space is affected too, for example, by seeding interstellar gas with heavy elements, heating it with the radiative energy and triggering blast waves from which new stars form.