Cassiopeia A

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Cassiopeia A

An intense radio source 2.8 kiloparsecs distant in the constellation Cassiopeia. It has the largest flux density at low frequencies of all the discrete radio sources (apart from the Sun), the value being about 8200 jansky at 178 megahertz, and a spectral index of –0.77. The flux drops by about 1% per year. It is a supernova remnant – probably of a supernova explosion (unrecorded) of a massive (˜50 MO) star in the late 17th century – and has a ringlike structure about four arc minutes in diameter. The radio radiation is by synchrotron emission and the radio shell is expanding at 2300 km s–1. It is also an extended source of soft X-rays.

Cassiopeia A

[‚kas·ē·ə′pē·ə ′ā]
(astronomy)
One of the strongest discrete radio sources, located in the constellation Cassiopeia, associated with patches of filamentary nebulosity which are probably remnants of a supernova.
References in periodicals archive ?
Until a month earlier, if I thought of Cas A at all, I considered it unobservable because of comments such as this excerpt from its Wikipedia entry: "It is extremely faint optically, and is only visible on long-exposure photographs.
Fortunately, the Fall 2009 issue of Amateur Astronomy Magazine included an article by Arizona amateur William Gates about observing Cas A through a 9.
Regardless of its historical visibility, the Cas A supernova remnant (SNR) is now about 13 light-years across, and it's the brightest radio source in the sky outside our solar system.
Since the 2009 Gates article, Cas A has been gaining increasing notice among visual observers, and growing numbers of observations are found online.
Because it's brighter than some NGC and IC objects, Cas A could have been discovered visually when these catalogs were put together in the 19th century.
The first time you spot Cas A, you're likely to see the northern arc, which is by far the brightest part of the object.
I've become better acquainted with Cas A since then, with my best 28-inch observation so far coming from the 2011 Oregon Star Party.
Best view yet of Cas A, but still no sign of the (southern) arc.
My primary target for this trip was the southern arc of Cas A, and I wasn't disappointed:
Two independent research teams studied the supernova remnant Cassiopeia A, or Cas A for short, the remains of a massive star 11,000 light-years away that would have appeared to explode about 330 years ago as observed from Earth.
Unlike the fuzzy blob that appeared in images from earlier X-ray telescopes, Cas A shone forth as a knotty bubble of hot gas laced with bright filaments.
At the same August 26th press conference where Chandra astronomers unveiled their Cas A image, they also showed off a picture of PKS 0637- 752, a remote quasar in the far-southern constellation Mensa.