Contrary To Popular Thought, The Universe Is Very Empty


The Sunday Nation


20 February 2005


When we look up to the sky on a clear night, see thousands of stars and this gives the impression that universe is densely packed. However, this is not the case. Think about our solar system: It can be pictured as a very thin disc measuring 6 billion kilometres in radius (distance from the sun to Pluto) and 1.4 million km thick (diameter of the sun).

The area of this disc is about 110 billion-billion square kilometres and its volume is about 160 trillion-trillion cubic kilometres. In this huge space, there are about 10,000 bodies (sun, planets, moons, asteroids, etc). If they were evenly distributed we would have one in every 10,000 trillion square kilometres! The average distance between these objects is about 100 million km.

Now, the mass of the sun is about 2,000 billion-billion-billion kilograms, which is more than 99.8 per cent of the whole solar system. Thus the average density of this system of bodies is about 12,500kg per cubic kilometre, or 12.5 milligrams per litre - milligram is one thousandth of a gram! Compare that to water, which weighs one kilogram per litre…

Stars in the universe are grouped in galaxies. Our Milky Way galaxy is a disc measuring about 100,000 light years in diameter and 10,000 light years thick. A light year is the distance that light travels in one year – approximately 10 trillion kilometres. The volume of our galaxy is therefore about 300 trillion cubic light years.

In this galaxy, there are about 200 billion stars; therefore, on average there is one star every 1,500 cubic light years (1,500,000 trillion cubic kilometres). The mean distance between the stars in our galaxy is about 10 light years – 100 trillion km! In this vast interstellar space, there is virtually nothing.

This emptiness is also seen in between the galaxies: Most galaxies measure a few hundred thousand light years across but they are several million light years apart.

Each of the 200 billion stars in our galaxy weighs roughly the same as the sun (2,000 billion-billion-billion kilograms), thus the density of the galaxy is about 13 grams per cubic km. The universe is much less dense than this – it is estimated at about a billionth of a billionth of gram per cubic kilometre!

This pattern is also to be found in matter. The typical atom is about one billionth of a metre in radius – distance from nucleus to the outermost electron. However, the nucleus measures one trillionth of a metre – that is, one thousandth of the size of the atom. If the nucleus were the size of a tennis ball, the electron would be orbiting it at 30 metres away.

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