Astrophysics Flashcards
Units:
- Mass
- Distance
- Velocity
- Acceleration
- Force
- Time
- Gravitational field strength
- kg
- m
- m/s
- m/s^2
- N
- s
- N/kg
Milky way galaxy…
Contains billions of stars
The universe…
billions of galaxies
Why gravitational field strength vaires?
An object’s gravitational field strength depends on its MASS. A massive object, like a star, will have a very large g-field. The Moon has less mass than the earth , so its gravitational field is much weaker (aprox 1/6th of Earth’s). This means that we could jump higher on the Moon, and objects would fall more slowly, as they experience a weaker gravitational force.
A planet with a large radius have a weaker gravitational field as its surface is further away from the centre of the planet
Why gravitational force causes orbits?
According to Newton, there is an attractive gravitational force between any two objects- pulling them together
Moons and artificial satellites are attracted to their planets, and so are pulled towards them.
This gravitational force keeps them moving in curved paths called orbits. The Moon does not crash into the earth, and the planets do not crash into the sun because they are moving
Differences in the orbits of comets, moons and planets
Comets have highly elliptical orbits with the Sun at one focus- When they come in close to the Sun they speed up, due to the larger gravitational force on them. They also develop tails that point away from the centre of the Sun. These are caused by tiny ice crystals that melt and break off from the comet and reflect the bright light of the Sun
Moons have circular orbits and planets orbit in slightly squashed circles, called ellipses.
orbital speed =
2 x pi x orbital radius / time period
Classification of stars
- Stars are classified into 7 groups according to their colours (which is due to the surface temperature) blue to red
- Blue are hotter and red cooler
Nebula
Stars form from large clouds of dust and gas particles (nebulae) that are drawn together by gravitational forces over millions of years. As the particles get closer the temperature and pressure becomes so large that nuclear fusion of hydrogen nuclei to helium nuclei occurs. This releases enormous amounts of energy in the form of heat and light.
star (main sequence)
Fusion produces forces that make the star expand outwards, but gravitational force is always pulling the particles within the star inwards. When these two opposing forces become balanced a star is stable and called a main sequence star. It should stay this way for millions of years, at a constant size and temperature.
red giant
Eventually hydrogen fusion stops as the star runs out of fuel. Gravitational force is now bigger than the outward fusion force which causes the star to collapse inwards and compress. This causes it to heat up to even higher temperatures so that fusion of helium nuclei begins. The increased power output causes the star to expand greatly. The surface area is so large that it is cooler than before, so its colour changes to red and the star is called a red giant.
white dwarf
Eventually fusion stops when the star runs out of helium nuclei and the gravitational force causes the star to collapse inwards and compress again. This heats it up so it changes colour to emit white light. The star is squashed so greatly by the gravitational force to become a small and very dense white dwarf. (They are so dense that a teaspoon full would weigh more than a cruise liner). A white dwarf eventually cools down and change colour as it does so, eventually becoming black.
Larger mass than the sun evolution
- After the stable period, a giant star expands into red supergiant. (It produces all the elements up to iron during nuclear fusion). When it finally runs out of nuclei to fuse it collapses due to the gravitational force, and then explodes – an exploding star is called a supernova.
- The explosion throws dust and gas back into space and so another nebula is formed. A dense core remains – called a neutron star, because it is made entirely from neutrons. If its mass is large enough it can compress further to become a black hole. (Their gravity is so strong that not even light can escape!)
