astrophysics

• the universe


  a large collection of billions of galaxies
• a galaxy


  a large collection of billions of stars
• the sun


  is a star at the centre of our solar system in the Milky Way galaxy
• our solar system is in the


  milky way galaxy
• eight planets


  inner planets:

  1. mercury

  2. venus

  3. earth

  4. mars

  - small and rocky

  - high density

  
  

  outer planets

  5. jupiter

  6. saturn

  7. uranus

  8. neptune

  - large and gaseous

  - low density
• dwarf planets


  - large enough to form a spherical shape

  - not large enough to clear smaller objects from its orbit

  - pluto
• moons


  - a type of natural satellite

  - orbits around a planet
• asteroid belt


  - made from rock

  - similar to density to inner planets

  - located between mars and jupiter
• comets


  - made from ice and dust

  - similar density to outer planets

  - highly eliptical
• planets


  these are large objects that orbit a star
• satellite


  - an object that orbits a planet

  - natural satellite (moons)

  - artificial satellite
• weight


  the force acting on an object due to gravitational attraction
• The force of weight is responsible for:


  -objects staying firmly on the ground

  - objects always falling to the ground

  - satellites being kept in orbit
• The greater the mass of the planet then the greater its gravitational field strength

  A higher gravitational field strength means a larger attractive force towards the centre of that planet or moon
• mass


  stays the same
• gravity


  changes
• the larger the radius of a planet, the smaller the gravitational field strength at its surface
• comets move fastest when they are closer to the sun and slowest when far away from the sun
• when the comets approach the sun, they leave behind a white tail. this white tail is composed of melting ice.

  the tail of a comet faces away from the sun
• artificial satellite


  have been put into orbit around the earth by man to help us:

  - communicate over large distances

  - monitor in detail earth's surface temperature, the world's oceans or progress of forest fires
• gravitational force depends on:


  - the masses of the two objects

  - the distance between the masses
• the gravitational force acts towards the centre of the larger body

  causes the orbiting body to move in a circular path
• - the size of a gravitational force depends on mass and distance

  --> a greater mass produces a greater gravitational force

  --> gravitational force decreases with distance
• orbital motion


  is a result of the gravitational force of attraction acting between two bodies
• in a (near) circular orbit:


  - the sun is at the centre

  - orbital speed is the same everywhere
• in an elliptical orbit:


  - the sun is not at the centre

  - orbital speed increases near the sun

  - orbital speed decreases further from the sun
• as a comet approaches the sun:


  - gravitational increases

  - orbital radius decreases

  - orbital speed increases
• as a comet travels away from the sun:

  - gravitational force decreases

  - orbital radius increases

  - orbital speed decreases
• energy is always conserved

  - this means the total energy is always the same

  as a comet approaches the sun:

  - it gains kinetic energy

  - it loses gravitational potential energy (GPE)

  
  

  as a comet travels away from the sun:

  - it gains gravitational potential energy

  - it loses kinetic energy
• There are several similarities in the way different planets orbit the Sun:

  - Their orbits are all slightly elliptical (stretched circles) with the Sun at one focus (approximately the centre of the orbit)

  - They all orbit in the same plane

  -They all travel in the same direction around the Sun
• There are several differences in the way different planets orbit the Sun:


  - They orbit at different distances from the Sun (different orbital radius)

  - They orbit at different speeds

  - They all take different amounts of time to orbit the Sun
• The further away a planet is from the Sun, the slower it travels and therefore the longer it takes to orbit
• The planets closest to the Sun have higher orbital speeds, whereas the planets furthest from the Sun have lower orbital speeds
• Orbital motion of moons


  - Moons orbit planets in a circular path

  - Some planets have more than one moon

  - The closer the moon is to the planet:

  --> the shorter the time it will take to complete each orbit

  --> the greater the speed of the orbit
• Orbital motion of comets


  -> Their orbits are highly elliptical (very stretched) or hyperbolic

  --> This causes the speed of the comets to change significantly as their distance from the Sun changes

  --> Not all comets orbit in the same plane as the planets and some don't even orbit in the same direction

  -> As the comet approaches the sun, its speed increases

  -> As it moves further away from the sun, its speed decreases
• This orbital period (or time period) is defined as:


  The time taken for an object to complete one orbit
• When planets move around the Sun, or a moon moves around a planet, they orbit in circular motionThis means that in one orbit, a planet travels a distance equal to the circumference of a circle (the shape of the orbit)This is equal to 2πr, where r is the radius a circle

  ->The orbital radius r is always taken from the centre of the object being orbited to the object orbiting
• A star's colour is related to its surface temperature


  - a red star is the coolest (at around 3000K)

  - a blue star is the hottest (at around 30000K)

  
  

  hottest -> coolest

  blue -> white-blue -> white -> yellow-white -> yellow -> orange -> red
• - the colour of a star correlated to its temperature. the bluer the star, the hotter its surface temperature.

  - the redder the star, the cooler its surface temperature
• - astronomical objects cool as they expand and heat up as they contract

  - when a star becomes a red giant it becomes redder as it expands and cools

  - when a star becomes a white dwarf it becomes whiter as it contracts and heats up