P8 Space Physics

milky way

a agalaxy containing billions of stars, including the sun, held together by gravity

the sun

the largest object in the solar system, which planets orbit around

as the distance of planets away from the sun increases…

• their temperature decreases. eg. mercury is 430*C while Neptune is -200*C.

• the time taken to orbit the sun increases. eg. mercury orbits every 88 days while neptune orbits every 165 earth years

for a planet to form…

• its gravity must be strong enough to make it spherical in shape

• its gravitational field must be strong enough to pull smaller nearby objects into its orbit

moons

natural satellites that orbit a planet. some planets have many moons (saturn has 50) while other planets have no moons.

satellites

body that orbits a planet - can be natural (moons) or artificial (communication satellites).

dwarf planet

a strong gravitational field, and so there may be other smaller objects in its orbit around the sun.

asteroid

a rock in space which orbit the sun in highly elliptical orbits. orbit the sun, and may collide with other planets/objects in the process. similar to comets, but are made of metal and rocky material.

comets

a ball of icy rock that follows an elliptical orbit around the sun. similar to asteroids, but are made of dust, ice and rocky material. they begin to vapourise as they approach the sun, producing a distinctive tail.

nebula

a cloud of gas and dust in outer space, which can collapse under gravity to form a protostar if large enough.

how was the solar system formed?

it was a nebula which collapsed under gravity, transferring gravitational potential energy to kinetic energy in its particles. as it collapsed it became denser, and rotated more rapidly. collisions between the particles caused kinetic energy to transfer into thermal energy, meaning the core of the nebula began to form a hot, dense protostar.

when was the solar system formed?

roughly 4.6 billion years ago

what happened in the sun when it was hot and dense enough?

nuclear fusion, in which hydrogen nuclei joined together to form a helium nuclei and released radiation. this radiation causes pressure, however the sun is at equilibrium - gravity pulls it inwards, and the radiation pressure expands it outwards.

_____ provides the force needed to ___________________________________.

gravity, to maintain stable orbit of both planets around a star and of moons around a planet.

what happens if an object is not travelling at the right speed?

it does not maintain a steady, circular orbit.

if a satellite is moving too quickly…

the gravitational attraction between the Earth and satellite is too weak to keep in orbit, so the satellite will move off into space. this occurs at speeds around/above 11,200 m/s.

if a satellite is moving too slowly…

the gravitational attraction between the Earth and satellite is too strong to keep in orbit, so the satellite will fall towards the Earth. this occurs at speeds below 7,600 m/s.

if a satellite is moving at just the right speed…

the satellite will maintain a stable orbit, meaning it will travel around a fixed path.

when an object moves in a circle at a constant speed…

…its direction constantly changes, causing a change in velocity (as it is a vector quantity), meaning acceleration is taking place. this means objects moving in a circle are accelerating even if they are at a constant speed

an object will only accelerate if…

… a resultant force is acting on it. For an object orbiting in a circle, this resultant force is the centripetal force, acting towards the middle of the circle.

gravitational attraction provides…

…the centripetal force needed to keep all planets and satellites in ortbit.

the larger the distance between 2 objects… (in terms of gravitational attraction)

…the weaker the gravitational attraction between them.

the smaller the distance between 2 objects… (in terms of gravitational attraction, acceleration and acceleration)

…the stronger the gravitational attraction between them. this means a greater acceleration will occur as the force is greater. the greater the acceleration, the greater the change in velocity - this means objects in small orbits travel faster than those in larger orbits.

artificial satellites travel in 2 different types of orbits:

• polar orbits

• geostationary orbits

polar orbits

take the satellites over the Earth’s poles (north and south). they travel very close to the earth and so they must travel at very high speeds (nearly 8000 m/s)

geostationary orbits

take 24 hours to orbit the earth, so the satellite appears to remain int he same part of the sky when viewed from the ground. they are much higher than polar orbits, and sot hey travel more slowly (3,000 m/s).

the life cycle of a particular star depends on…

its size.

all stars begin as…

1. a cloud of dust and gas (nebula)

2. turns into a protostar

3. becomes a main sequence star

after this, stars develop differently depending on their size

continuation of life cycle (for stars around the sun’s size)

4. red giant star

5. white dwarf

6. black dwarf

continuation of life cycle (for stars greater than the sun’s mass)

4. red super giant star

5. supernova

6. neutron star or black hole (depending on size)

nebulae are composed mostly of…

…hydrogen.

how does a nebula form?

gravity pulls dust and gas together.

how does a nebula become a protostar?

as the mass falls together (due to gravity), it gets hot (pressure). when it is hot enough, hydrogen nuclei fuses to form helium nuclei, releasing energy which keeps the core of the star hot.

how does a protostar become a main sequence star?

when the force on the star due to gravity and the force it exerts by releasing radiation and expanding are equal, and so it is at equilibrium.

how does a main sequence star become a red giant star?

when all the hydrogen has been used up in the fusion process, larger nuclei begin to form and the star may expand and become a red giant.

how does a red giant star become a white dwarf?

when all the nuclear reactions are over, star begins to contract under the pull of gravity, and so the star fades to white and shrinks as it cools.

how does a red super giant star become a supernova?

a larger star ill go on making nuclear reactions, getting hotter and expanding until it explodes as a supernova. this explosion throws hot gas into space.

what does a supernova turn into and what does this depend on?

either a black hole or neutron star, this depends on the mass of the star at the start of its life.

for most of its lifetime, a star is a…

…main sequence star. this lasts for billions of years.

what is the reaction which occurs in a main fusion star to keep it at equilibrium?

all naturally occurring elements in the universe are produced by…

…nuclear fusion reaction in stars. thuds is because as the stars run out of hydrogen, other fusion reactions take place, forming the nuclei of other elements.

elements heavier than iron are formed in the…

supernova explosions of high mass stars. this is because all elements produced are thrown out into the universe.

emission spectrum of light from the sun

what do the lines on an emission spectrum represent?

the wavelengths which elements in the star absorb

what is the red-shift?

light from different, distant galaxies are shown on an emission spectrum, and the lines are shifted to the left (red), showing an increase in wavelength. astronomers see the red-shift in virtually all other galaxies.

what causes the red-shift?

it is a result of the space between earth and the galaxies expanding. this expansion increases the wavelength of the light from these galaxies. the more red-shifted the light from a galaxy is, the faster it is moving away from Earth.

big bang theory

scientific theory that describes that 13.8 billion years ago, the universe was a small, hot and dense region which expanded outwards into what it is today.

what main evidence is there for the big bang theory?

the more red-shifted a galaxy’s light is, the faster it is moving further away. this means the further away the galaxies are, the faster they are moving outwards. this is similar to an explosion, there the bits moving fastest travel furthest from the explosion. this red-shifted data provides evidence that the Universe is expanding.

what other evidence is there for the big bang theory?

astronomers have discovered a cosmic microwave background radiation (CMBR) coming from all directions and spread thinly across the whole universe, with a temperature of about -270*C. this CMBR is the remains of the thermal energy from the big bang.

what could possibly happen to the universe if it stops expanding?

if the gravitational attraction between all objects eventually slows to a stop, everything will attract back together in a ‘Big Crunch’. scientists do not know if this will happen, but finding out the density of the universe will give them a good idea.

dark energy

an unknown form of energy, put forward as the solution to the problem of why the expansion of the universe is accelerating. to make the universe accelerate as observed, it must make up 68% of our universe.

what is dark matter and why do scientists think it exists?

galaxies seem to rotate too quickly for the mass of their stars. this suggests that there is a mass in the universe invisible to the instruments used by scientists. they only know it is a there because it has gravity that affects the objects nearby. astronomers have calculated that it makes up about 27% of the universe.

only __% of the universe is made up of matter that is currently understood (stars, planets, galaxies, people etc.)

5%