astrophysics 8 Flashcards
what’s a universe?
a large collection of billions of galaxies
what’s a galaxy?
a large collection of billions of stars
where is our solar system in?
the milky way galaxy
what does gravitational field strength reply on?
mass of body that’s creating the field ( the larger the stronger the force)
distance (closer to a planet or star the stronger the force)
what’s the differences in orbits between comets, moons and planets?
moons - slightly elliptical, orbit planets
comets - orbital periods are much longer, orbit sun, very elliptical
planets - slightly elliptical, orbit stars
what’s the equation for orbital speed?
(2 x π x orbital radius) / time period
what’s the order of the planets in our solar system?
mercury
venus
earth
mars
jupiter
saturn
uranus
neptune
what’s a satellite?
an object that orbits another body
what are planets?
large objects that orbit a star
in almost circular orbits
what are moons?
moons orbit plants with almost circular orbits
type of natural satellite
what are artificial satellites?
ones that humans have built
usually orbit the earth in a circular orbit
what are asteroids?
lump of rocks and metals that orbit the sun
usually found in an asteroid belt
what are comets?
lumps of ice and dust that orbit the sun
orbits are usually highly elliptical - some travel near the sun to the outskirts of our solar system
how does gravity create orbits?
- if an object is travelling in a circle it is constantly changing direction (therefore constantly accelerating) which means there must be a force acting on it
- the force causing this is a centripetal force, which acts towards the centre of the circle
- this force causes the object to fall towards whatever it was orbiting but as the object is already moving it just causes it to change direction
- the object keeps accelerating towards what it’s orbiting but the instantaneous velocity (which is at a right angle to the acceleration) keeps travelling in a circle
- the force that makes this happen is provided by the gravitational force, the gravitational attraction of the sun keeps the planets and comets in their orbits around it
- satellites are kept in their orbits around plants by the. gravitational attraction of the planet
what does gravitational field strength depend on?
the mass of the body creating the field:
- the larger the mass of the body the stronger the gravitational field
varies with distance:
- the closer you get to a star or planet, the stronger the gravitational force is
how does the magnitude of force affect velocity?
- the stronger the force the larger the instantaneous velocity needed to balance it
- therefore the closer to a star or planet you get, the faster you need to get to remain in orbit
- for an object in a stable orbit if the speed of the object changes, the size (radius) of its orbit must do so too
- faster-moving objects will move in a stable orbit with a smaller radius than slower-moving ones
what are the orbits of the planets and the moon like?
slightly elliptical
what are the orbits of comets like?
very elliptical
orbits the sun at one focus (near one end of the orbit)
what is the comet’s orbital period like in contrast to Earth’s?
- comets orbital periods are much longer than earth’s
- comets travel much faster when it’s near the sun as there is an increased pull of gravity which causes it to speed up the closer it gets to the sun
what are geostationary satellites?
has an orbital period of exactly one day
useful in communications as they are always over the same part of the planet
how do you work out the orbital speed?
orbit speed = (2 x pie x orbital radius) / time period
what are the stellar stages of a star larger than the sun?
- nebula
- protostar
- main sequence star
- red supergiant
- super nova
- neutron star/ black hole
what are the stellar stages of a star smaller than the sun?
- nebula
- protostar
- main sequence star
- red giant
- white dwarf
stellar stages of a star larger/ smaller than the sun:
nebula
stars initially form from a cloud of dust and gas = nebula
stellar stages of a star larger/ smaller than the sun:
protostar
- the force of gravity pulls the dust and gas together forming a protostar
- the temperature rises as the star gets denser and more particles collide with each other
- when the temperature is high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei
- this gives out a huge amount of energy which keeps the core of the star hot
= a star is born
stellar stages of a star larger/ smaller than the sun:
main sequence star
- the star enters a long stable period
- the outward pressure caused by thermal expansion (the energy produced by nuclear fusion tries to expand the star) balances the force of gravity pulling everything inwards
- main sequence star lasts several billion years
- the heavier the star the shorter time on the main sequence
stellar stages of a star larger/ smaller than the sun:
red supergiant/ red giant
- the hydrogen in the core begins to run out and the force due to gravity is larger than the pressure of thermal expansion
- the star is compressed until it’s dense and hot enough that the energy (and so pressure) created makes the outer layers of the star expand
- it becomes red as the surface cools
- large star = red supergiant
- small star = red giant
stellar stages of a star smaller than the sun:
white dwarf
a small to medium star like the sun then becomes unstable and ejects its outer layers of dust and gas
= leaving a hot, dense solid core, white dwarf
stellar stages of a star larger than the sun:
supernova
- start to glow brightly again as they undergo fusion to make heavier elements
- they expand and contract several times as the balance shifts between gravity and thermal expansion
- eventually, they explode in a supernova
stellar stages of a star larger than the sun:
neutron star
- exploding supernova throws the outer layers of dust and gas into space leaving a very dense core
= neutron star
stellar stages of a star larger than the sun:
black hole
- if the star is massive enough it will collapse and become a black hole
- a super dense point in space that not even light can escape from
what does the colour of a star depend on?
the visible light it emits
how much of each frequency depends on its surface temperature
stars of a similar colour will be of…
similar temperature
classifying stars based on their colour:
surface temperature coolest > hottest
red (lowest frequency) > orange > yellow > white > blue (highest frequency)
the hotter the star…
the more light of higher frequencies it will emit
what are the frequencies emitted from white stars?
white stars emit all frequencies of visible light equally
what does the brightness of a star depend on?
its size and temperature
- the bigger and hotter a star, the brighter it is
why is classifying stars dependent on their brightness difficult?
as their brightness from earth depends on the distance from earth
the closer the star the brighter it will appear
we deal with this using absolute magnitude
what is absolute magnitude?
the measure of how bright a given star would appear if it was a fixed distance from earth (around 3.1 x 10^17)
this allows us to compare the brightness of stars without worrying about their relative distances from earth
what is the relationship between the absolute magnitude and the brightness of the star?
the lower the absolute magnitude = the brighter the star
very bright stars have a negative value for their absolute magnitude
what type of diagram can you use to see different types of stars?
hertzsprung-russel diagram
what is a hertzsprung-russel diagram?
a graph of absolute magnitude against temperature
where are red giants and red supergiants on a hertzsprung-russel diagram?
top right
very cool but very large so very bright
where are white dwarfs on a hertzsprung-russel diagram?
bottom left
very hot but small so dim
where are main sequence stars on a hertzsprung-russel diagram?
span the whole range of the graph diagonally from top-left to bottom-right
what is the doppler effect?
when a source of waves is moving relative to the observer, the waves will undergo an apparent change in frequency and wavelength when they are observed, compared to when they are emitted
happens with all waves - including light waves
what is red-shift?
when the light source is moving away from you, the light will appear to be shifted towards the red end of the visible part of EM spectrum
what evidence is there that galaxies will be moving away from each other?
- different elements absorb different frequencies (or wavelengths) of light
- when light is passed through a sample of an element, a pattern of dark lines is produced = with a dark line at each of the frequencies in the visible part of EM spectrum that the element absorbs
- when we look at light from distant galaxies we see the same patterns but at slightly lower frequencies (and so longer wavelengths) than they should be
- the patterns have shifted towards the red end (RED SHIFT)
how do you calculate red shift?
change in wavelength/reference wavelength = velocity of galaxy/speed of light
why does red shift suggest that the universe is expanding?
- measurements of the redshift suggest that all distant galaxies are moving away from us very quickly
- more distant galaxies have greater red shifts than nearer ones, they show bigger observed increase in wavelength
- this means that more distant galaxies are moving away faster than nearer ones
- the inescapable conclusion appears to be that the whole universe is expanding
why is there microwave radiation from all directions?
- scientists can detect low-frequency microwave radiation coming from all directions and all parts of the universe
- known as cosmic microwave background (CMB) radiation
- this background radiation is strong evidence for an initial big bang as the universe expands and cools, the background radiation cools and drops in frequency
why does evidence suggest the universe started with a big bang?
- galaxies are moving away from each other at great speed, suggesting something must have got them going from a single point
- initially, all the matter in the universe occupied a single point
- this tiny space was very dense and very hot
- this single point then exploded = big bang
- space started expanding and the expansion is still happening
