Astronomy Summary Notes

Astronomy

Cosmic Scale

• We know the universe consists of trillions of galaxies.
  • These galaxies are collections of billions of stars.
  • Within galaxies there are Solar systems, ours is the Milky Way, and we revolve around the sun.
    • Stars are massive spheres of gas large enough and hot enough to release energy by the process of nuclear fusion.
    • Major) planets are objects that orbit a star and have a mass above a certain value.
    • Minor planets (which includes asteroids) are objects that orbit a star and have a mass below that certain value.
    • Comets are collections of ice and dust that have very elliptical orbits.
    • Planets may be large enough to have smaller objects orbiting them.  These are called satellites, or moons.  The Earth’s Moon is an example of a natural satellite.

Gravity

• All objects with mass create a gravitational field. (If this mass is larger, the field will be larger.)
• Gravitational fields become weaker with distance. The closer an object is to the centre of the gravitational field the stronger the pull on it will be and vice versa.
• Therefore, the gravitational field on the Moon is weaker than the Earth because the Moon has a smaller mass than the Earth. The Sun has the strongest gravitational field in our solar system because it is the object with the highest mass.
• Gravitational forces cause moons, artificial satellites, planets and comets to orbit the earth and sun, respectively.
• Newton’s first law of motion described the fact that any object will continue moving at a steady speed in a straight line unless acted on by a resultant force.  Therefore, an object is moving in a circle, a force must be acting on the object
• Gravitational forces are what act on planets and comets to keep them in orbit around a star, and what act on satellites to maintain their orbit around a planet. 

Orbits

• We describe most orbits as circular, and this is often true for planets and satellites (as they have a constant speed.
• However, the orbits of…