P16 - Space

• Comet


  Frozen rock that moves around the Sun in elliptical orbits

  
• Meteor


  Small bits of rock that burn up when they enter the Earth's atmosphere
• Objects in the Solar System


  • Comets
  • Meteors
  • Sun
  • Planets
  • Moons
• How the Solar System formed

  1. Clouds of dust and gas pulled together by gravitational attraction

  2. Particles in the clouds speed up

  3. Clouds merge and become more concentrated to form a protostar

  
• Protostar


  A star-to-be, a concentration of gas and dust that becomes hot enough to cause nuclear fusion
• How energy is released inside the Sun

  1. Gravitational potential energy transferred to thermal energy

  2. Hydrogen nuclei fuse to form helium nuclei, releasing energy
• The Sun


  • It is stable because the inward force of gravity is balanced by the outward force of radiation from nuclear fusion in its core
• The heaviest known natural element is uranium, with a half-life of 4500 million years. This indicates the Solar System formed from the remnants of a supernova.
• Supernova


  The cataclysmic explosion that occurs when a massive star collapses

  
• When a star runs out of hydrogen nuclei to fuse

  It reaches the end of its main-sequence stage, its core collapses, and its outer layers swell out

  
• Life cycle of stars about the same size as the Sun

  • Protostar
  • Main-sequence star
  • Red giant
  • White dwarf
  • Black dwarf

  
• Life cycle of stars much bigger than the Sun

  • Protostar
  • Main-sequence star
  • Red supergiant
  • Supernova
  • Neutron star or black hole

  
• Light elements are formed from fusion in stars, while heavy elements are formed in supernova explosions.
• The Sun is about 5000 million years old and will probably continue to shine for another 5000 million years, before turning into a red giant.

  
• Neutron star


  An extremely dense object made up only of neutrons
• Black hole


  An object with such strong gravitational field that nothing can escape from it, not even light
• The presence of uranium on Earth is evidence that the Solar System formed from the remnants of a supernova.

  
• Elements heavier than uranium formed in the debris that formed the Solar System have long since decayed.
• Plutonium-239 is not found naturally because it is formed in nuclear reactors from uranium-238, not during the formation of the Solar System.
• M87 is a galaxy that spins so fast at its centre that it is thought to contain a black hole with a billion times more mass than the Sun

  
• To remind yourself about the heavier elements and half-lives, look back at Topic P73 and Topic P75. Make sure you know how the heavier elements were formed and that they were not formed during the Big Bang.
• Stars become unstable when they have no more hydrogen nuclei that they can fuse together.

  
• Life cycle of stars with about the same mass as the Sun


  • protostar
  • main-sequence star
  • red giant
  • white dwarf
  • black dwarf

  
• Life cycle of stars much more massive than the Sun


  • protostar
  • main-sequence star
  • red supergiant
  • supernova
  • neutron star (or black hole if enough mass)

  
• The Sun will eventually become a black dwarf.
• A supernova is the explosion of a red supergiant after it collapses.

  
• Learning objectives


  • what force keeps planets and satellites moving along their orbits
  • the direction of the force on an orbiting body in a circular orbit
  • how the velocity of a body in a circular orbit changes as the body moves around the orbit
  • why an orbiting body needs to move at a particular speed for it to stay in a circular orbit

  
• The Earth orbits the Sun in an orbit that is almost circular. Most of the other planets orbit the Sun on orbits that are ellipses or slightly squashed circles. The Moon is a natural satellite that orbits the Earth on a circular orbit. Artificial satellites also orbit the Earth. In each case, a body orbits a much bigger body.

  
• Centripetal force


  The force of gravity on the planet from the Sun acts towards the centre of the Sun. This force is the resultant force on the planet because no other forces act on it.
• The direction of the planet's velocity (i.e., its direction of motion) is changed by this force. So it continues to orbit the Sun. The direction of motion of any planet (i.e., the direction of its velocity) in a circular orbit is at right angles to the direction of the force of gravity on it.

  
• A planet in a circular orbit experiences acceleration towards the centre of the circle because the resultant force on it acts towards the centre of the circle. The acceleration is its change of velocity per second, and its change of velocity is directed towards the centre.

  
• The speed of a planet in a circular orbit does not change, even though its velocity changes its direction. This is because the force on it is at right angles to its direction of motion. So no work is done by the force on the planet. So the kinetic energy and the speed of the planet do not change.

  
• Satellites in orbits too close to the Earth gradually lose speed. This happens because of atmospheric drag if a satellite's orbit is in the Earth's upper atmosphere. If a satellite loses speed, it gradually spirals inwards until it hits the Earth's surface.
• A satellite in a circular orbit above the Earth's atmosphere moves around the Earth at a constant height above the surface. The satellite is in a stable orbit. To stay in an orbit of a particular radius, the satellite has to move at a particular speed around the Earth. The same is true for a planet moving in a circular orbit around the Sun.

  
• If the launch speed is too slow, the satellite falls to the surface. If the launch speed is too high, the satellite flies off into space. At the correct speed, the satellite moves around the Earth in a circular orbit at a constant height and a constant speed.

  
• For any small body to stay in a circular orbit around another bigger body, the smaller body must move at a particular speed around the bigger body.

  
• Factors affecting the particular speed needed for a circular orbit

  • The further a satellite is from the Earth or a planet is from the Sun, the less the particular speed needed for it to stay in a circular orbit
  • The further a satellite is from the Earth or a planet is from the Sun, the longer the orbiting body takes to move around the orbit once

  
• If the speed of a satellite in a stable orbit changes, then the radius of the orbit has to change. For example, suppose a space vehicle above the Earth is in a circular orbit and its engines are used briefly to increase its speed. The vehicle moves out of its orbit and gains height. So its speed decreases as it moves into a higher orbit.
• Communications satellites are usually in an orbit at about 36 000 kilometres above the equator with a period of 24 hours. They orbit the Earth in the same direction as the Earth's spin. So they stay above the same place on the Earth's surface as they go around the Earth. These kinds of orbits are described as geostationary.

  
• Monitoring satellites are fitted with TV cameras pointing to the Earth. Their uses include weather forecasting, and monitoring the environment. Monitoring satellites are in much lower orbits than geostationary satellites and they orbit the Earth once every two or three hours.