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Skywatching

Sirius has a secret

If you look low into the southwest these evenings, you will see the brightest star in the sky after the Sun, Sirius.

This star is prominent in the winter sky, and now, as we get through spring, it, along with the winter constellations, are vanishing behind the Sun.

Sirius has a little secret. It is a double star. The two partners orbit each other every 50 years. The partner is difficult to see because it is very faint. It is a white dwarf star, the gradually cooling off core of a dead star.

If it were not close to a really bright star, and lost in the glare, we would be able to see it with our unaided eyes.

When an average star, like the Sun, starts to run out of fuel, it swells into a red giant and then sneezes off its outer layers. What's left is the core of the star, about the size of the Earth, and so compressed a teaspoon full of its material would weigh tons.

Although very hot, such a small object loses heat quite slowly, taking billions of years to completely cool off.

This is a white dwarf star.

Having almost no fuel for energy production and insufficient mass to compress that fuel enough to trigger energy production, we would expect white dwarf stars to be among the most stable objects in the universe.

There are certainly lots out there, gradually cooling off. However, on occasion they can go off the rails in spectacular fashion.

We know of one way this can happen. It arises in double stars that are orbiting close to one another. Although the two stars were born together, one will inevitably have a larger mass than the other.

A quirk of the way stars work is that although more massive stars have more fuel, they burn it much faster and run out sooner.

At some point, the more massive star starts running out of fuel, while its partner is still happily shining. It swells into a red giant, sneezes off its outer layers and settles down to retirement as a white dwarf star.

Then, eventually, its partner also runs low on fuel and swells into a red giant. When this happens the star has a much weaker gravitational hold on its material and its white dwarf companion starts to grab it and pull it in.

The result is that an increasing mass of the other star's material accumulates on the white dwarf's surface. This contains a lot of unburned fuel, and eventually enough collects to cause a huge explosion that can destroy both stars.

This type of event is known as a Type 1a supernova.

We have observed many of these, in our galaxy and in others. However, recent research has come up with a really bizarre way white dwarf stars can destroy themselves in spectacular fashion.

It involves uranium.

Stars are powered by nuclear fusion, where small atoms, like hydrogen, combine to form larger atoms, such as carbon, oxygen and so on.

These waste products are useful, in that they make it possible to make planets, and living things. Some stars, especially the more massive ones, can produce much larger atoms, such as uranium.

Such large atoms are not stable; they break into smaller atoms, releasing energy as they do so. The break-up, or fission of a uranium atom can trigger the fission of other uranium atoms.

If there are enough uranium atoms around, then the fission process can run away. We call this a chain reaction. If controlled it can be a source of energy. If not, the result is a nuclear explosion.

As molten material cools, the atoms moving around in it tend to join together making crystals. It has been suggested that in a cooling white dwarf, uranium atoms will do this.

However, a point might be reached where the crystal body reaches the critical mass, and a runaway chain reaction takes place.

This small nuclear explosion can act as a detonator for any unburned fuel in the white dwarf, causing a thermonuclear explosion that can blow the star apart.

It seems that for stars, just like people, ageing does not mean predictability.

  • Mars is high in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast before dawn.
  • The Moon will reach First Quarter on the 19th.

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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