Gravity twinkling in the sky

13 December, 2007

Gravity is the weakest of the four fundamental forces of nature, and yet it has the biggest effect on the large scale universe. Two of the fundamental forces, gravity and electromagnetism, have an infinite range. But while electromagnetism has positive and negative charges that largely cancel each other out over large distances, gravity is all about attraction. So the more mass there is in an area, that is, the more ‘stuff’ in an area, the bigger the gravitational pull. It is this property that causes small eddies and fluctuations in interstellar gas clouds to form stars.

Most of the universe is made up of Hydrogen and Helium, and large clouds of these gasses still float free (mixed with other elemental remnants of dead stars) within our own galaxy. Some parts of these gas clouds have more molecules in them that other parts, and due to gravity they exert a stronger force on their surroundings. This causes more gas to clump together in this denser area, which in turn causes more gas to be attracted, which makes the area denser, which attracts more gas and so on and on. The gas will naturally take on the shape of a sphere as is gets more massive smoothing out any lumps.

Eventually the pressure at the center of this ball of gas will become incredible. With so much force pushing in on the gas, the electromagnetic force that keeps atoms from getting too close is overcome. This electromagnetic force is what stops us from walking through walls even though both we, and the wall, are made up of ’empty space’. But in the centre of this mass of gas, the nuclei of the atoms are pushed together in spite of electromagnetism and fuse into larger elements. This fusion process releases energy in the form of light. The ball of gas has become a star, and will burn for at least millions, and possibly billions of years.

Gravity may be the weakest of the forces, but on the big scales, it’s the one that dominates the skies.

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