Making Starlight

In the sky this week…

  • Jupiter is getting lower in the south-western sky in the evenings, but is still easy to spot. Uranus still lies very close to Jupiter, but you will need a telescope to see it.
  • Saturn rises around 11 p.m. and Venus around 5 a.mSaturn rises around 1 a.m., and Venus around 4 a.m.
  • The Moon will be New on January 4 and First Quarter on the 12th

Ken Tapping, January 5, 2011

Some time ago I saw a cartoon of Zeus, King of the Gods, looking down through a hole in the clouds at a farmer toiling in his fields. As he watched, another god gave him a bolt of lightning and said, “Bet you can't hit him!” That cartoon summarized how we regarded the world around us for most of our history – a collection of arbitrary, unpredictable phenomena. Things happened because some god or other willed it. All we could do was grin, bear it and survive as best we could.

Then, a few thousand years ago, a few philosophers proposed something radical. They suggested that what we see happening around us in the universe can be explained in terms of natural laws that consistently apply everywhere. It didn’t matter whether these laws were decreed by a god or not. We could deduce what those laws were and apply them, which is exactly what we have been working on ever since. We use these laws every day, in most aspects of our lives.

Skygazing by Ken Tapping

Starting with that Great Revelation, we can ask very fundamental questions such as, “How do stars shine?” Once our ancestors stepped beyond saying, “They shine forever because the gods will it so,” there arose an interesting problem. How could the Sun and other stars produce prodigious amounts of energy, with only very minor variations in most cases, for millions or billions of years? We now know the Sun has been shining for about 4.5 billion years. How has it managed to do that?

If we ignore the problem that space is a vacuum, and there is no oxygen to support combustion, we can start off with the idea that the Sun is a big ball of some fuel, such as coal. However, it is relatively easy to calculate that a lump of coal the size of the Sun, producing the energy output we measure, would burn out in a few thousand years. Since we know the Earth formed around the same time as the Sun, the “coal Sun” theory does not work.

In the 19th Century, physicists looked at another possibility. If stars form from the collapse of huge cosmic gas clouds, how long could the star be powered by the energy released in the collapse? However, the “gravitational collapse” theory didn't work either; it would only provide energy for 20 million years or so. In the early 20th Century, nuclear science was gaining momentum. The discovery of the possibility of conversion of mass into energy would certainly provide the means for stars to shine for very long periods of time.

Cecile Payne was a British graduate student working at Harvard University. Her research earned her a PhD described by Otto Struve, a leading astronomer, as the most important PhD in that science. She discovered that stars are mainly hydrogen gas. This provided physicists with the key to cracking the problem. They proposed that stars make energy by fusing the atoms of light elements, such as hydrogen, into heavier ones. We now know this to be true and that the by-products of nuclear fusion in stars provide the elements needed to make comets, meteors, planets, and us.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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