8.1 Solar system Flashcards
Objects in the solar system
The Sun lies at the centre of the Solar System.
The Sun is a star which makes up over 99% of the mass of the solar system
There are 8 planets and an unknown number of dwarf planets which orbit the Sun.
The gravitational field around planets is strong enough to have pulled in all nearby objects with the exception of natural satellites.
The gravitational field around a dwarf planet is not strong enough to have pulled in nearby objects.
There are 4 rocky planets: Mercury, Venus, Earth and Mars.
There are 4 gas planets: Jupiter, Saturn, Uranus and Neptune.
Some planets have moons which orbit them.
Moons are an example of natural satellites.
Artificial satellites are man-made and can orbit any object in space.
The International Space Station (ISS) orbits the Earth and is an example of an artificial satellite.
Asteroids and comets also orbit the sun.
An asteroid is a small rocky object which orbits the Sun.
The asteroid belt lies between Mars and Jupiter.
Comets are made of dust and ice and orbit the Sun in a different orbit to those of planets.
The ice melts when the comet approaches the Sun and forms the comet’s tail.
Our place in space
Our solar system is just one small part of the Milky Way galaxy.
There are billions of stars in the Milky Way galaxy.
Some of these stars also have planets which orbit them.
The Universe is made up of many different galaxies.
Our solar system is just one out of potentially billions in our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire universe.
Distance between planets in the solar system
The planets and moons of the Solar System are visible from Earth when they reflect light from the Sun.
The outer regions of the Solar System are around 5 × 1012 m from the Sun, which means even light takes some time to travel these distances.
The light we receive on Earth from the Sun takes 8 minutes to reach us.
The nearest star to us after the Sun is so far away that light from it takes 4 years to reach us.
The Milky Way galaxy contains billions of stars, huge distances away, with the light taking even longer to be seen from Earth.
The speed of light is a constant 3 × 10^8 m/s
Formation of the sun
The Sun is formed from massive clouds of dust and gas in space.
A cloud of dust and gas in space is called a nebula.
Gravity pulled this cloud together into a giant ball.
As the nebula collapsed the centre of this ball got very dense and hot and began to rotate.
Eventually nuclear fusion was able to begin and a dense protostar was formed – our Sun.
Equilibrium in stars
Stars are held together by a delicate balance of inwards and outwards forces.
One of these forces is the force of gravity.
This is an attractive force which pulls the outer layers inwards.
The other force is the force of pressure.
This is an outward force which is exerted from the expanding hot gases inside the star.
When the inward pull of gravity and the outward pressure acting on the star are equal the star will be in equilibrium.
If the temperature of a star increases, the outward pressure will also increase.
This will cause the star to expand.
If the temperature drops (because, perhaps, the rate of fusion has slowed) the outward pressure will also decrease.
This will cause the star to contract.
Fusion in stars
All the naturally occurring elements, apart from hydrogen, have been formed by nuclear fusion in stars.
Nuclear fusion occurs when two light nuclei collide at high speed and join to create a larger, heavier nucleus.
When the Universe was first formed, 13.8 billion years ago, the only element present was hydrogen.
If two hydrogen nuclei collide with enough energy they will fuse into a helium nucleus.
For example, the nuclei of two different isotopes of hydrogen (protium and tritium) can join to form a helium nucleus by the process of nuclear fusion.
The process of nuclear fusion releases energy.
The energy is released in the form of heat and light.
Formation of new elements
During the majority 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 forming the nuclei of other elements.
For example, two helium nuclei (produced by the fusion of 2 hydrogen nuclei) could fuse together to form a beryllium nucleus.
The beryllium nucleus could then fuse with a helium nucleus to form a carbon nucleus.
Elements lighter than iron are formed in fusion reactions like the ones above.
Formation of elements heavier than iron
Elements heavier than iron are produced in supernova explosions.
A supernova occurs at the end of a massive stars life.
When the star explodes it releases very large amounts of energy and neutrons.
All of the elements which have been produced by the fusion reactions get thrown out and combine with the neutrons to form heavier elements.
The Life Cycle of Solar Mass Stars
Nebula - Protostar - main sequence star - red giant - white dwarf - black dwarf
The Life Cycle of Larger Stars
Nebula - Protostar - main sequence star - Red super giant - supernova - neutron star or black hole.
Nebula
All stars form from a giant cloud of hydrogen gas and dust called a nebula.
Protostar
The force of gravity within a nebula pulls the particles closer together until it forms a hot ball of gas, known as a protostar.
As the particles are pulled closer together the density of the protostar will increase.
This will result in more frequent collisions between the particles which causes the temperature to increase.
Main sequence star
Once the protostar becomes hot enough, nuclear fusion reactions occur within its core.
The hydrogen nuclei will fuse to form helium nuclei.
Every fusion reaction releases heat (and light) energy which keeps the core hot.
Red giant
After several billion years the hydrogen causing the fusion reactions in the star will begin to run out.
Once this happens, the fusion reactions in the core will start to die down.
This causes the core to shrink and heat up.
The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure of the expanding gases as the fusion dies down.
A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form beryllium.
These reactions will cause the outer part of the star to expand.
It will become a red giant.
It is red because the outer surface starts to cool.
White dwarf
The core which is left behind will collapse completely, due to the pull of gravity, and the star will become a white dwarf.
The white dwarf will be cooling down and as a result, the amount of energy it emits will decrease.
Black dwarf
Once the star has lost a significant amount of energy it becomes a black dwarf.
It will continue to cool until it eventually disappears from sight.
Red supergiant
Eventually, the main sequence star will reach a stage when it starts to run out of hydrogen gas in its core.
Once this happens, the fusion reactions in the core will start to die down.
This causes the core to shrink and heat up.
The core will shrink because the inward force due to gravity is greater than the outward force due to the pressure of the expanding gases.
A new series of fusion reactions will then occur around the core, for example helium nuclei will undergo fusion to form beryllium.
These fusion reactions will cause the outer part of the star to expand and it will become a super red giant.
A super red giant is much larger than a red giant.
Supernova
A supernova is a bright and powerful explosion that happens at the end of a massive star’s life.
It occurs when the star is bigger than the Sun.
The explosion releases a large amount of energy.
During a supernova, all of the elements which were produced by the fusion reactions are exploded out along with neutrons.
The neutrons combine with the elements to form even heavier elements
These elements are ejected into the universe by the supernova explosion and form new planets and stars.
Since Earth contains many heavy elements up to Iron, this is proof that it must have once been made from the remains of a Supernova.
Black hole
In the case of the biggest stars, the neutron star that forms at the centre will continue to collapse under the force of gravity until it forms a black hole.
A black hole is an extremely dense point in space that not even light can escape from.
Orbital motion
There are many orbiting objects in our solar system.
They each orbit a different type of planetary body.
A smaller body or object will orbit a larger body.
In order to orbit a body such as a star or a planet, there has to be a force pulling things towards that body.
Gravity provides this force.
The gravitational force exerted by the larger body on the orbiting object is always attractive.
Therefore, the gravitational force always acts towards the centre of the larger body.
The gravitational force is the centripetal force as it will cause the body to move and maintain in a circular path.
Circular motion in an object
Planets travel around the Sun in orbits that are (approximately) circular.
Objects in circular orbit are travelling at a constant speed.
The orbit is a circular path, therefore the direction in which the object is travelling will be constantly changing direction.
A change in direction causes a change in velocity.
Acceleration is the rate of change of velocity, therefore if the object is constantly changing direction then its velocity is constantly changing and so the object in orbit is accelerating.
A resultant force is needed to cause an acceleration.
This resultant force is gravity and it must act at right angles to the instantaneous velocity of the object to create a circular orbit.
This is always towards the centre of the orbit.
The instantaneous velocity of the object is the velocity at a given time.
Circular orbit of planets
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 the same direction around the Sun.
There are also a few differences:
They orbit at different distances from the Sun.
They orbit at different speeds.
They all take different amounts of time to orbit the Sun.
Circular orbit of moons
Moons will orbit planets in a circular path.
Some planets will have more than one moon.
The closer the moon is to the planet:
The shorter the time it will take to orbit.
The greater the speed in the orbit.
Artificial satellites
A satellite needs to travel at a specific speed to maintain a circular orbit at a particular distance from the object.
If the speed of the satellite is too big:
The radius of the orbit will increase and the satellite will spiral into space.
This is because the gravitational attraction cannot provide enough force to keep it in orbit.
If the speed of the satellite is too small:
The radius of the orbit will decrease and the satellite will move towards the object it should be orbiting.
This is because the gravitational attraction is too strong to maintain a constant orbital radius.
If an artificial satellite is to change the radius at which it is orbiting then the speed at which it is travelling must change.
To maintain a stable orbit:
If the speed increases the radius must decrease.
If the speed decreases the radius must increase.
Non circular orbit
The orbits of comets are very different to those of planets:
The orbits are highly elliptical (very stretched circles) or hyperbolic.
This causes the speed of the comets to change significantly as its 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, it loses gravitational potential energy and gains kinetic energy.
This causes the comet to speed up.
This increase in speed causes a slingshot effect, and the body will be flung back out into space again, having passed around the Sun.
As it moves away from the Sun the body will slow down, eventually finishing its orbit and falling back into towards the Sun once more.
In this way, a stable orbit is formed.
