A new look at Cassiopeia A

Strong space radio waves

Long, long ago, when I was just a hope-to-be astronomer, the movie "Radio Sky" was shown at a meeting of amateur astronomers. It presented a summary of what was happening in radio astronomy around the world in the 1960s.

Fascinatingly, the movie featured two new radio telescopes under construction at the time. One was a 26-metre dish, at a site near the village of Chilbolton, a small village in Southwest England and the the other was a 46-metre dish being built near Lake Traverse, in Algonquin Provincial Park in Ontario.

During my career, I had the privilege of working with both those radio telescopes. Around the time the movie was made, a technique known as aperture synthesis was developed for making images of cosmic radio sources. That involved a group of small antennas used to collect all the information needed to compute an image made using a really large radio telescope. A highlight of the movie was an image of the radio source Cassiopeia A, made using a radio telescope at Cambridge University.

Cassiopeia A is one of the longest known and strongest radio sources. It can be fairly easily picked up by amateur radio astronomers using backyard radio telescopes. Over the years, improved telescopes have revealed more and more detail about this radio source.

The latest images were obtained using the James Webb Space Telescope. They all show a rapidly expanding spherical shell of hot gas, dust and magnetic fields: the remains of a supernova, the explosion of an ageing giant star.

If we track the expansion back in time to when it began, we estimate that the supernova explosion would have been visible in our skies in the 1690's. The constellation of Cassiopeia, which looks like a "W" formed by five moderately bright stars, lies close enough to the Pole or North Star that it never sets.

It is a "circumpolar" constellation, which means it is always there in our night skies. Therefore, if an object far brighter than everything other than Sun or Moon flashed into view in the night sky, someone should have seen it. However, the only report we have is by John Flamsteed, the Astronomer Royal at the Royal Greenwich Observatory.

He noted a faint star he had not seen before. One suggested explanation was the star that produced Cassiopeia A was far more massive than previously thought, and it “sneezed” off a lot of material, and then exploded. The ejected material could have acted like a smoke screen, hiding the light from the explosion.

After being possibly noted by Flamsteed, the remains of the supernova vanished from history until 1948, when a strong radio source was detected by Martin Ryle and Francis Graham-Smith at Cambridge University.

They measured its position. and optical telescopes were then pointed in that direction to see what sort of object was producing the radio waves. All the telescopes showed were a few wisps of material, which when joined, formed a circular shell.

That led to the conclusion Cassiopeia A happened to be a supernova remnant. Having a bright, closeish (11,000 light years) supernova remnant high in our skies led to it receiving a lot of attention, using a large variety of telescopes, including the Hubble Space Telescope, the Einstein Observatory, the Chandra X-ray Observatory, numerous radio telescopes and most recently, the James Webb Space Telescope.

Modern images show a bubble with a complex envelope of wisps, fibres and blobs of gas. There is source of X-rays in the middle, suggesting very high-energy goings on there, probably a neutron star, formed from the core of the star as it exploded.

Being able to detect the radio emissions from "Cass A,” as it has come to be called, is part of the right-of-passage for backyard amateur radio astronomers.


• Venus and Mercury lie low in the dawn sky.

• Jupiter and Saturn shine in the southern sky after sunset.

• The Moon will reach last quarter on Jan. 3.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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