Physics Notes

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P1 - The Earth in the Universe

Planets don't produce their own light like stars do. They reflect it from the Sun. That is why they look like stars. Comets are balls of rock, dust and ice that orbit the Sun in very elongated elliptical orbits. We only see comets for a very short part of the orbit because they travel in very elongated elliptical orbits with the Sun near the end. So they are only close enough to the Sun for us to see their reflected light for a short time. Comets have much more elliptical orbits than asteroids and have a tail. Asteroids are lumps of rock with a more regular orbit, mostly between Mars and Jupiter. Comets are smaller than moons, which are smaller than planets, which are smaller than the Sun. The Solar System started as a cloud of dust and gas. Over millions of years, this cloud contracted under the force of gravity. At its centre, the cloud became hot enough for nuclear fusion to start, hydrogen nuclei fuse together to form helium nuclei. The process gives out lots of energy in the form of heat and light. This is how the Sun formed. Other parts of the cloud slowly clumped together to form other objects e.g., planets, moons or asteroids in the Solar System. The Sun's diameter is about 100 times bigger than the Earth. The diameter of the milky way is about 600 billion times the diameter of the Sun. The distance between galaxies is usually millions of times more than the distance between stars.

The big bang theory states that all the matter and energy in the Universe must have been compressed into a very small space. Then it exploded and started expanding and the expansion is still going on now. Scientists believe the big bang happened about 14 billion years ago. According to the big bang theory, space itself is still expanding. When a galaxy is moving away from us, the wavelength of light from it changes. It becomes redder (red-shift). The greater the red-shift, the faster it is moving away from us . The further away a galaxy is, the faster is is moving away from us. This provides evidence that the universe is expanding. The mass of the universe determines how quickly the expansion slows down.