The Beginner's Guide to Making a Star

The Cas A Supernova inset against a picture of the moon to show their comparative sizes.

Chandra carries an instrument called the Advanced CCD Imaging Spectrometer (ACIS) that can measure the energy of incoming x-ray photons and associate them with specific chemical elements. Using the ACIS, astronomers are taking pictures of Cas A that reveal the distribution of heavy atoms like oxygen, silicon and iron in the supernova's rapidly expanding shell that show how those elements are mixing into the ambient interstellar medium of gas and dust.

Recently, NASA released images of Cas A at x-ray wavelengths emitted by ions of silicon (Si), calcium (Ca), and iron (Fe). On the eastern side of the supernova's shell, Ca and Si images reveal a high speed jet erupting into a relatively low-density region of the interstellar medium. Scientists speculate that the jet might signify an asymmetry in the original supernova explosion. On the opposite side, observations at radio and other wavelengths indicate that Cas A is plowing into an interstellar molecular gas cloud that confines the shell's outward flow.

There are intriguing differences between the maps of Ca and Si and the map of Fe, which is clumpier and does not show the jet so clearly. Material rich in iron comes from the inner core of the star where fusion temperatures were highest. Scientists have examined these maps carefully and note that iron-containing knots from deepest in the star seem to be nearest the outer edge of the remnant. This means they were flung the furthest by the explosion that created Cas A.

Cas A's outer envelope is expanding at 800 km/s (about 1.73 million mph). That's rapid enough that images taken by Chandra over the years will show how knots in the shell change and cool. By monitoring these changes, Chandra scientists hope to learn more about how quickly and in what form different elements are deposited into the interstellar medium.