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Skywatching

At the core of the Milky Way

Isaac Asimov's Foundation trilogy is one of the iconic works of science fiction.

The book, published in 1953, is based upon our vision of what our galaxy was like at that time, and describes the decline and fall of a galactic empire.

The empire was ruled and administered from the planet Trantor, located at the centre of the galaxy. This location made administrative sense because at the time, there was no known scientific reason not to.

On summer evenings, if we look south you will be looking toward the centre of our galaxy, which lies some 30,000 light years away. However, thanks to great clouds of dust and stars, we can only see a few thousand light years.

Back in the 1950s, large optical telescopes on the ground could not do much better; the centre of the Milky Way was hidden from view.

Radio astronomy was a young science in those days, but radio telescopes had found a strong source of radio waves located at the Galactic Centre.

It was called Sagittarius A because it was the brightest known radio source in the constellation of Sagittarius. However, nobody knew what the source might actually be.

Back then, we believed the universe was on the whole a fairly quiet place, which is the impression we get when lying under a dark, clear sky. Stars grew old and some blew up, but basically things changed only slowly.

Today, that illusion has gone. Now, in addition to optical telescopes on the ground, we have telescopes in space, which can see ultraviolet, X-rays and other high-energy radiation that does not reach the ground.

Radio telescopes have improved immensely. In the past, we could measure the characteristics of radio waves coming from a small patch of sky.

Today, we can make radio images, in some cases better than anything our eyes can give us or we can obtain using an optical telescope.

This has changed the whole picture. One of the big surprises the New Astronomy has given us is that the centre of the Milky Way is not a tranquil place at all.

X-ray and infrared telescopes orbiting the Earth can see through the dust clouds to the core of our galaxy, and modern radio telescopes can image Sagittarius A, not just detect its presence. It turns out that the energy driving that radio source is a black hole, with a mass four million times that of the Sun.

There are stars orbiting close to the hole and in the process of falling into it. There is dust and other material being sucked in and its disappearance from sight is marked by strong X-ray emissions.

It seems that most large spiral galaxies like ours have a large black hole in their cores. There does not seem to have been enough time since the beginning of the universe for the black holes to have been formed through the death of giant stars and the merging together of the resulting small black holes.

At the moment, it looks as though it is more likely these huge black holes were formed when the galaxies formed, and when two or more galaxies merged, their black holes spiralled in on each other and eventually merged too.

Back in the 1950s, the physics of the time predicted the possibility of black holes. However, the overall opinion was that there was more likely a problem with the physics.

It took quite an accumulation of evidence before the scientific community accepted that no matter how bizarre they might seem to us, there are such things.

Today, we are used to the idea that the universe has many more weird things to show us.

The high-radiation environment at the core of our galaxy, together with the general instability, would make Trantor uninhabitable. It might not have even had the time to form.

This is why, decades later, when Asimov wrote sequels to his Foundation books, he relocated Trantor to somewhere safer.

  • Jupiter and Saturn lie low in the southeast before dawn.
  • Mars is low in the sunset glow.
  • The Moon will be New on 10th.

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]



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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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