Cassiopeia A is a relatively young supernova remnant, or SNR, in our Milky Way Galaxy. Apart from our Solar System, it is also the strongest radio source in the sky. In the early days of radio astronomy (1940s and 1950s), astronomers named their radio sources by letter (A, B, C, ), with A designating the strongest source in a given constellation. By noting the expansion rate of the gases in the SNR, astronomers have dated the progenitor supernova explosion to the 17th century. The initial explosion must not have been very noticeable because historical records fail to provide a definitive record of the event. [Contrast this with the Crab Nebula, the result of a supernova explosion that was clearly recorded and can be traced to the year 1054.] Astronomers now believe that the original Cassiopeia A explosion occurred in either 1667 or 1680.
Visible: DSS and Visible: Color (Robert Fesen)
Unlike many astronomical phenomena, supernova remnants are often unimpressive at visible-light wavelengths. These images (above) barely reveal the expanding spherical gas shell, a remnant of the original supernova explosion, with the most prominent emission seen to the north. In the color image, you should be able to trace the faint southern fringes of the SNR.
At the short near-infrared wavelengths (above left), emission from Cassiopeia A is still rather weak. However, it is possible to trace the wispy shell of gas around most of its circumference. The colors are a result of the scheme used in combining the three 2MASS photos into a single mosaic: blue corresponds to emission at 1.2 microns, green to 1.6 microns, and red to 2.2 microns.
Mid-Infrared: ISO and Far-Infrared: IRAS
At longer infrared wavelengths (above left), the SNR blossoms into view. This image was obtained with a camera aboard the Earth-orbiting Infrared Space Observatory at a wavelength of 11 to 12 microns. The pixelization is due to the relatively low spatial resolution, a feature noted in many of the IRAS mid- and far-infrared images in other galleries. Nonetheless, this photograph vividly shows a ring of bright knots of thermal IR emission produced by dust mixed throughout the expanding shell of gas. Supernova explosions are an important source in the creation of dust and heavy elements in the interstellar medium. It is this dust that eventually serves as the seeds of creation for the next generation of stars!
The IRAS far-infrared image (above right) was obtained at a wavelength of 60 microns. Since this satellite flew more than a decade before ISO, its detectors provided even lower spatial resolution. The bright emission features in the northern ring of the ISO photo have blended into a single bright peak at longer wavelengths. Another way to look at this is that the improved resolution of ISO is able to resolve the single emission peak observed by IRAS. The IRAS emission is stretched along a long running from upper left to lower right. This is an artifact resulting from the rectangular detectors used aboard IRAS.
This image, from the Spitzer Space Telescope, provides better spectral resolution. In this image 3.6-microns is shown as blue, 4.5-microns as green and 8.0 microns as red. As the star's layers whiz outward, they are ramming, one by one, into a shock wave from the explosion and heating up. Material that hit the shock wave sooner has had more time to heat up to temperatures that radiate X-ray and visible light. Material that is just now hitting the shock wave is cooler and glowing with infrared light. Consequently, previous X-ray and visible-light observations identified hot, deep-layer material that had been flung out quickly, but not the cooler missing chunks that lagged behind. Spitzer's infrared detectors were able to find the missing chunks -- gas and dust consisting of the middle-layer elements neon, oxygen and aluminum.
Radio: © 1992 NRAO
This impressive radio view of Cassiopeia A was obtained at a wavelength of about 21 cm with the Very Large Array interferometer in New Mexico. This image reveals a complex structure in the emission shell, due in part to the variable density of the surrounding interstellar medium. The radio emission is primarily associated with synchrotron radiation, the result of fast-moving electrons immersed in a magnetic field.
The x-ray image (above) is historic: it was the first science image obtained and released by the Chandra X-ray Observatory (CXO) This NASA Great Observatory was launched in 1999 and provides a high-energy (short wavelength) complement to the Hubble Space Telescope. CXO offers the best spatial resolution of any x-ray observatory to date.
The colors denote different x-ray energies (or wavelengths), with red
corresponding to the lowest energies and blue corresponding to the
highest. The clarity provided by CXO allows astronomers to ascertain
the chemical composition of the individual knots within the gas shell.
The small greenish-white clumps, most notably to the southeast (lower
left), are enhanced in silicon and sulfur. The reddish filaments in the
outer shell to the southeast are enhanced in iron. These elements are
remnants from deep within the progenitor star that exploded and produced
this supernova remnant.