Cosmic Compost

In the sky this week…

  • Saturn is low in the west after sunset.
  • Jupiter rises around 11 p.m. and Mars around 3 a.m.
  • The Moon will be New on August 28.

Ken Tapping, August 17, 2011

One of the greatest quests in science is to trace the path between the Big Bang and what we see around us today: a universe with galaxies, stars and planets. A major step in this is to find out exactly what was delivered to the “building site,” from which our Solar System formed.

We are very unlikely to ever find identifiable “left over construction material” lying around anywhere on Earth. Plate tectonics and erosion have erased it. The Earth’s surface is being continually recycled, so there are few really old rocks left. Even though in Canada and Australia we have found rocks dating back almost to the formation of the Earth, some 4.5 billion years ago, it seems some processing had happened even back then. Furthermore they have changed since, so they are not dependable samples of the original material.

Our solar system

Our solar system

If you take a piece of black cardboard and hold it facing the Sun on a really clear day, that cardboard will rapidly get hot, because it is capturing almost 1400 watts of solar energy. Now imagine that piece of cardboard is only one square metre of a huge sphere of black cardboard surrounding the Sun, with a diameter equal to the diameter of the Earth’s orbit. That cardboard sphere will capture all the Sun’s energy. We don’t have to make this sphere because it is easy to calculate the area of that sphere and then multiply by the power captured per square metre, giving the Sun’s total energy output. The result is a huge number, four followed by 26 zeros. That sounds large, but there are stars producing over 100,000 times that amount!

Let’s keep going. The Sun’s diameter is about 1.4 million kilometres. Using basic physics in combination with the science of helioseismology, we have a rough idea of what the Sun is like inside, just as seismology has revealed the internal structure of our world. We have found that the energy is produced by nuclear fusion in the Sun’s core, which has a diameter of about 350,000 km. So its volume is about 2 followed by 25 zeroes. If we divide this number into the total energy output we get the average amount of energy produced in each cubic metre of the Sun’s core. The result is a bit of a surprise – less than 20 watts. A light bulb puts out more than that. A typical human body produces about ten times that amount of heat. So does a good compost heap!

The big difference is that each cubic metre of the Sun’s core can keep this up for millions or billions of years, because there is plenty of hydrogen fuel in that cubic metre. Moreover, from a gardening point of view, we like our compost heaps to complete their work in a much shorter time than that.

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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