Earth and beyond
The Solar system
Gravity is a force that attracts objects towards each other. Any object with mass exerts a force of gravity. The greater the mass, the greater the force. The force of gravity between two objects decreases as the objects move further apart.
Gravity has the following effects:
- Pulls objects on Earth towards the center of the planet with a force of 10 Newtons per kilogram.
- Holds the earth’s atmosphere in place
- Holds all the components of the solar system in orbit (The path taken by one object around another, such as a plant around a star or a satellite around a planet) around the sun.
- Holds all the components of the galaxy together.
The orbits of the planets in the solar system are ellipses- slightly squashed circles- with the sun near the center.
The further a planet is from the sun:
- The weaker the pull of gravity is on it
- The bigger its orbit
- The longer its year- it takes longer to make a complete orbit
Mercury, Venus, earth, mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
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The orbits of comets are elliptical (a shape like a squashed circle). An orbit by a comet takes it very close to the sun and then far away again.
The speed of a comet increases as it gets closer to the sun, and is greatest when it is nearest the sun.
A satellite is an object that orbits a planet. The moon is the earth’s natural satellite .
Artificial satellites orbiting the earth are used to:
- Transmit information between distant parts of the Earth
- Monitor the weather and other conditions on earth
Stars
The Milky Way is just on galaxy (A cluster of billions of stars, held together by gravity) among more than a billion galaxies in the universe.
Stars form when gravitational forces bring enough dust and gas together. Nuclear reactions release energy to keep the star hot. Planets form when gravitational forces bring smaller amounts of dust and gas together.
Stable stars such as the sun change during their lifetime to form other types of star, such as red giants, white dwarfs, neutron stars and black holes. The fate of a star depends on how much matter it contains.
- Stars form from massive clouds of dust and gas in space.
- Gravity pulls the dust and gas together.
- As the gas falls together, it gets hot. A star form s when it is how enough for a nuclear reaction to start. This releases energy and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star’s gravity. Our sun is at this stable phase in its life. Gravity pulls smaller amounts of dust and gas together, which form planets in orbit around the star.
- Our sun is a type of star called a yellow dwarf. It has been shining for nearly five billion years, and has enough hydrogen fuel to last another five billion years. The sun and other stars eventually begin to run out of hydrogen. Gravity makes the core of the star smaller and hotter, which results in the out layers expanding. They eventually expand so much that the star becomes a red giant star.
- After a star becomes a red giant, what happens next depends on how massive the star is. If its mass is relatively small, gravity eventually lead to the star contracting to form a white dwarf. The matter in a white dwarf is millions of times denser than the matter on earth.
- If the red giant star has enough matter, it may collapse inwards under its own force of gravity and then explode outwards, to form a supernova. The explosion throws dust and gas into space; this may eventually form part of a new star. The material left behind after the explosion forms a very dense type of star called a neutron star. (Denser than the white dwarf)
The remnants of a supernova may form a neutron star. However, if enough matter is left behind, it may contract under its own gravity to become extremely dense, with such a strong gravitational field that nothing can escape from it, so it is called a black hole.
Nuclear power stations release energy by nuclear fission, where unstable atomic nuclei decay to form smaller nuclei. Stars release energy by a different process, called nuclear fusion.
The temps and pressures inside a star are so great that small nuclei join together to form larger nuclei, where release energy. During must of a star’s lifetime, hydrogen nuclei fuse together to form helium nuclei. As the star runs out of hydrogen, other fusion reactions take place and form the nuclei of other heavier elements.
The universe
The big bang
- The galaxies appear to be moving away from each other
- The further away a galaxy is from us, the faster it is moving
The universe is expanding.
13.6 billion yrs ago, all the matter in the universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot big bang and it is still expanding.
Astronomers have detected a cosmic background radiation that is thought to be the heat left over from the original explosion.
Big bang evidence.
Red shift
Our sun contains helium. We know this because there are black lines in the spectrum of light from the sun where helium has absorbed light. This lines form the absorption spectrum for helium.
When we look at the spectrum of a distant star, the absorption spectrum is there, but the patterns of lines have moved towards the red end of the spectrum. This is called red-shift.
The universe is expanding and stretches out light waves as they travel outwards from their source. Longer wavelengths are found at red end of the spectrum. So the different colors in a star’s spectrum are red-shifted.