Astrophysics and Cosmology (DONE)

Question 1

What is a planet?

Answer 1

• Around some stars we have planets, a planet is an object in orbit around a star with a mass large enough for its own gravity to give it a round shape.
• it undergoes no fusion and that has cleared its orbit of most other objects.

Question 2

What is a planetary satellite?

Answer 2

• Other things we find in space are planetary satellites: a body in orbit around a planet (artificial or man-made).
• this can include satellites or moons, the earth only has one moon however other planets such as jupiter have many moons.

Question 3

What are comets?

Answer 3

• We also have comets in space: a small irregular body made of dust, ice and rock that orbits a star (with a highly eccentric elliptical orbit).

Question 4

What is a galaxy?

Answer 4

• If you could zoom out what we would see is a huge number of different stars, if you have a lot of stars together we have a galaxy: a collection of stars and interstellar dust and gas bound together by their mutual gravitational attraction.
• Galaxies have on average around 100 billion stars and there are about 100 billion galaxies.

Question 5

What are the 2 main shapes of galaxies?

Answer 5

• There are 2 main shapes of galaxies, we have a spherical shaped galaxy and we also have a spiral galaxy which is the shape of our milkyway galaxy.
• A spiral galaxy from top down view will have a central mass and often it is orbiting a massive black hole and then you have long spiralling arms.
• From a side on view there will be a bulge in the centre with the distance from one side of the galaxy to the other being 100 thousand light years.

Question 6

What is the universe?

Answer 6

• everything that exists within space and time.

Question 7

What is a nebula and how does it form?

Answer 7

• A nebula is a star forming region where all of the gas and dust starts to concentrate due to the force of gravity.
• Around the universe there are very large clouds of gas mainly made from hydrogen gas along with some helium and a few heavier elements.
• When this cloud is concentrated it becomes a nebula, this is where stars are formed.
• this is the first stage in the formation of a star.

Question 8

How does a protostar form from a nebula?

Answer 8

• As gravity causes the gas in the nebula to get closer, the nebula loses some of its potential energy as it turns into kinetic energy.
• Gravity pulls everything in the cloud together so there is a massive force inwards due to gravitational attraction.
• We find that this cloud of dust and gas comes together to create a protostar which is the second stage in the formation of a star.

Question 9

What is happening inside a protostar and what equation can be used to show this?

Answer 9

• In a protostar no fusion is happening.
• In the protostar the density in the middle starts to get larger meaning that the pressure and temperature increases.
• Using the equation E = 3/2 kT we can say that kinetic energy is proportional to the temperature.
• As the gas gets closer to the centre it speeds up therefore its temperature increases.

Question 10

How does a protostar become a main sequence star?

Answer 10

• We get to a point where gravity pulls the gas in the protostar so tightly towards the centre of the region that the temperature and pressure is high enough for fusion to occur.
• The protons (hydrogen) are able to overcome their electrostatic repulsion and react with each other to cause fusion.
• It is when fusion starts to happen in the protostar that we then call it a main sequence star.

Question 11

What forces act in a main sequence star and what happens to it over time?

Answer 11

• Inside the star there are 2 forces acting, the force of gravity towards the centre holding the star together and also the radiation/gas pressure which is a force caused by fusion which acts outwards and counteracts the gravity.
• If the star is in a stable position we know it is in equilibrium.
• This can occur for billions of years.
• As time goes on the hydrogen may start to get used up as we start to make more helium, our sun loses about a million tonnes of mass per second as the mass is being converted to energy which is released by the star.

Question 12

What is a solar mass and how does a solar mass compare to stars?

Answer 12

• An M beside a small circle with a hole in the middle stand for 1 solar mass, which is unit a unit used to compare things in the solar system.
• If we look at stars which have 0.5 – 10 solar masses they are fairly lightweight stars.

Question 13

How does a main sequence low mass star become a red giant?

Answer 13

• Inside the main sequence star we have 2 forces acting, force of gravity inwards and radiation/gas force outwards.
• However as hydrogen fuses to make helium the number of hydrogen atoms decrease meaning the fuel will run out.
• When there is no fuel the force of gravity will stay the same but the radiation pressure decreases and when this happens the core will collapse creating a red giant with an inert core as it is not hot enough for helium to fuse. become a red giant.

Question 14

How does a red giant become a white dwarf (low mass star)?

Answer 14

• As the star collapses into a red giant some of the potential energy turns into kinetic energy and the atmosphere around it heats up.
• There is still some fusion occurring in the atmosphere around it so the atmosphere swells and gets bigger.
• Because we still have the same amount of energy being given off around a larger area it means the temperature cools down.
• The temperature of the star will therefore cool down and it changes from a white/yellow colour to a red colour.
• in summary there is a large outer layer atmosphere where fusion is still occurring but in the helium core there is no more fusion as the helium cannot overcome the electrostatic repulsion.
• However we do not have enough hydrogen to fuse forever and eventually we will run out of fuel again, this is when we get to the third stage of the star which is the white dwarf.
• Towards the end of the life of a red giant the outer layer will start to drift off into space leaving the dense core behind.

Question 15

What is a white dwarf?

Answer 15

• the dense core left behind after the outer layer of the red giant has drifted away.
• Most of the mass of the star is in the core so it is very hot, no fusion is occurring in the white dwarf.
• The heat from the white dwarf does disappear over time as it is radiated however this is a slow process.
• This is how our sun will end up.

Question 16

What happens to white dwarfs when they are too massive?

Answer 16

• The force acting outwards in a white dwarf is the electron degeneracy pressure caused by electrons being pushed together.
• electrons cannot exist at the same point so as the electrons get closer together there is a force of repulsion between them which stops the stars collapsing any more.
• In high mass stars there is a point where within the white dwarf the force of gravity inwards is bigger than the electron degeneracy pressure so when you have a star greater than 1.44 solar masses the white dwarf cannot exist anymore.

Question 17

What is Chandrasekhar’s limit?

Answer 17

• 1.44 solar masses is called the Chandrasekhar limit

Question 18

How does a high mass main sequence star form a red super giant?

Answer 18

• High mass stars which tend to be greater than 10 solar masses have a large force of gravity acting inwards, this means the temperature inside the star is high and a huge amount of fusion occurs.
• Therefore the force outwards is very high.
• However because there is so much fusion occurring the star burns the fuel very quickly and so it has a short lifetime.
• When the fuel gets used more and more the outwards force decreases causing it to collapse and the atmosphere to swell and the main sequence star forms a red supergiant.

Question 19

What is in the core of the red giant/supergiant after it collapses?

Answer 19

• As the core of the main sequence star collapses the temperature rises massively which allows the fusion of helium particles.
• Meaning we then get a lot of other elements such as carbon, neon, oxygen, magnesium, silicon.
• Inside of the core we have different layers and as we have heavier elements being fused together we get deeper into the centre of the core.
• in the middle of the core there are elements that fuse together to make iron which is the most stable element and therefore will not undergo fusion.
• Therefore inside the centre of the core of the red supergiant is heavier and heavier elements.

Question 20

What is a supernova and how is it formed?

Answer 20

• A supernova is when the outer part of the red supergiant collapse, bounce off the core and are ejected out into space.
• At the end of the red supergiants life it runs out of elements to fuse, at this point the force of gravity pulls everything inwards so all of the elements in the star come together, the outer layers bounce off the central part and we have a supernova.
• At this point we start to make all of the heavier elements as anything heavier than iron can only be made in a supernova.
• This is categorised as a type 2 supernova.
• A lot of the elements in our bodies could only have been made in a supernova.

Question 21

How does a neutron star form from a supernova?

Answer 21

• After the outer part of the red supergiant collapses, bounces off the core and is ejected out into space, what is left behind is the core which If it is greater than 1.44 solar masses we get something where all of the electrons get pushed into other particles in the centre of the star.
• If you have a proton +1 and an electron -1 and you push them together you get a neutron.
• This means what we then make is a neutron star

Question 22

How is a black hole formed?

Answer 22

• If the mass of the core left behind after the outer layer of the red supergiant is ejected is greater than 3 solar masses the force of gravity is so strong that neutrons are pushed inside other neutrons.
• Eventually all of the mass then tends towards a singularity which is a black hole.

Question 23

What axis’ are used on the Hertzsprung-Russell diagram?

Answer 23

• If we look at all the stars that we have seen in our galaxy and in the universe what we can do is plot them on a graph with the luminosity on the y axis and temperature on the x axis.
• We usually compare everything to our sun which has a luminosity of 1 so anything above is brighter and anything lower is dimmer but because there is such a spread of luminosity we can use a logarithmic scale so we have a value of 1 then something 10 times brighter, 100 times brighter etc.
• With temperature it measures the surface temperature of stars and it does not go from left to right, it goes from right to left meaning coldest is right and hottest is left.
• The temperature on the x axis is also a logarithmic scale usually ranging from 3000K to 4000K.

Question 24

How do the positions of stars vary on the Hertzsprung-Russell diagram?

Answer 24

• If we look at our sun it is somewhere around the middle point on both axis but when we look at other stars we find that some are a lot brighter and hotter, and some are a lot colder and less bright.
• Most stars tend to fit onto a sequence which is sort of like an S shape, however some stars can have cold surface temperatures but high luminosities and some stars can have low luminosity while being very hot.

Question 25

How would you track the life cycle of a star on the Hertzsprung-Russell diagram?

Answer 25

• We can use the graph to track the life cycle of a star.
• The S shaped part of the graph is called the main sequence, the stars above the line are the red giants and the stars below the line are white dwarfs.
• If we were to track a star we would see a low mass star moves from main sequence up to a point where it is a red giant and then it moves down to a white dwarf.
• If you have a high mass star these burn very brightly giving a high luminosity so it also moves towards red supergiant which has a higher luminosity than a red giant and then it disappears from the diagram as it turns into a neutron star or black hole.

Question 26

What do energy levels show?

Answer 26

• we can refine the model of the atom by showing how electrons exist in different energy levels.

Question 27

What are each energy level called?

Answer 27

• The first energy level is what we call the ground level n = 1, the second level will be n = 2 and so on.
• The number of levels will depend on the atom.

Question 28

Where are electrons most stable and how can they move energy levels?

Answer 28

• When the electrons are at their ground state this is when they are most stable, we can excite the atom and cause electrons to move from ground state to above energy levels.
• The electrons can only exist at certain quantisised energy levels, an electron will not exist between energy levels it will drop to the lowest level possible.

Question 29

What can be used to find the change in energy when an electron moves between energy levels?

Answer 29

• We can say that the change of energy E = hf where h is plancks constant.
• We give the energy levels a value of energy in electron volts, all of the values are negative numbers.
• We can look at the energy between these bands and when you excite an atom and the electron drops back down to a lower energy level it gives out a quantum of energy in the form of a photon.
• If we had an energy change from closer shells the energy given off is less.
• So now there will be a smaller energy and therefore a smaller frequency of photon.
• we can also say that the change in energy E = hc/lambda

Question 30

What did Hubble find when looking at star light from different stars?

Answer 30

• In the early part of the 20th century hubble was looking at star light from different stars, he found that there was a shift in absorption lines in the spectra, most of this was red shift.
• All the stars in the universe appeared to be red shifted meaning they are moving away from us, and also the further away the stars were the greater amount of red shift was shown.

Question 31

How did Hubble prove that speed of recession of galaxies are proportional to the distance away from the earth?

Answer 31

• Hubble took data and plotted the recessional velocity in kms^-1 against the distance to the stars in Mpc.
• The data he collected showed a scattered graph but because they could not calculate the distance to far away galaxies the graph only measured up to a small value for distance relative to the size of the universe.
• Therefore because of this hubble found the general trend and extrapolated it to fill a larger range of distances.
• Hubble said the speed of recession of the galaxy is proportional to the distance away from the earth.

Question 32

What did new data prove about Hubbles theory on speed of recession of galaxies?

Answer 32

• Since Hubble, scientists have found better ways of looking at things very far away, using luminosities of stars we were able to fill in some data for larger distances from the earth.
• This means we can look at stars without using a stellar parallax method.
• The new data confirmed the link between recessional velocity and distance from the earth.

Question 33

What is Hubbles constant and why is there a lot of uncertainty over it?

Answer 33

• The gradient of the line on Hubbles graph of recessional velocity against distance from earth is called hubbles constant with symbol H0.
• It has units kms^-1Mpc^-1
• However we don’t exactly know what hubbles constant is, the reason for this is because there is a lot of variation.
• Firstly it is hard to look at the light emitted from very distant stars but it is also hard to measure their distance which can be hundreds of light years.
• Therefore we have uncertainty for both measurements and it will take some time before we get an accurate idea of what hubbles constant should be.
• Our best estimate at this time is H0 = 67.80 +- 0.77 kms^-1Mpc^-1

Question 34

What other units can Hubbles constant be shown in?

Answer 34

• Hubbles constant has units kms^-1Mpc^-1.
• Effectively with hubbles constant we have a distance per second per distance and therefore we can express hubbles constant in seconds s^-1.

Question 35

How can Hubbles constant be used as evidence for the Big Bang theory?

Answer 35

• galaxies are all moving away from the earth and the further away they are the faster they are moving, so this means the universe is getting bigger.
• If the universe is expanding at a constant rate then if we go back far enough in time the universe must’ve been at one point incredibly small.
• This gives one piece of evidence for the big bang model.

Question 36

How can Hubble’s constant be used to calculate the age of the universe?

Answer 36

• We can say that Hubbles constant H0 = v/d
• Therefore velocity v = H0*d
• If we have a uniform expansion of the universe and we have 2 galaxies with one at point A and B, which are a distance d apart and the galaxy is moving away with constant speed v, we can calculate how long it has taken for the galaxy to get from point A to B.
• The equation used for this is t = d/v, if we assume we have a finite aged universe and had a big bang we can say that the time taken for the galaxies to get from the same point where they were to now is going to be equal to the age of the universe.
• We know that v/d = H0 but here we have d/v for the equation for time, therefore we can say time t = 1/H0 which is the age of the universe.

Question 37

How can you change Hubble’s constant into SI units and why does this need to happen?

Answer 37

• Hubbles constant is given in units distance per second per distance which means we need to convert this into the SI units of s^-1.
• Firstly we need to multiply the value of H0 by 1000 giving ms^-1Mpc^-1
• Then we need to convert from mega parsecs into metres so we need to divide by 3.1 x 10^16 x 10^6 which means H0 = 2.2 x 10^-18 s^-1
• Therefore to find the age of the universe t = 1/H0 = 1/(2.2 x 10^-18) = 4.6 x 10^17 s.
• In order to make this more meaningful we would convert it into years to give t = 14 x 10^9 years

Question 38

How can you show the emission spectra of an element in gas form using equipment in a lab?

Answer 38

• You can use a high voltage EHT power supply to excite the elements inside which are in gas form.
• When you excite the elements inside you find that only certain frequencies of light are emitted and these correspond to the energies of certain photons.
• Sodium and other elements will only emit a certain colour of light, the reason for this is due to the energy levels inside the atoms of the element.

Question 39

What are the 3 types of spectra?

Answer 39

We have 3 types of spectra:

• continuous
• emission
• absorption

Question 40

What is a continuous spectrum and when is this spectra emitted?

Answer 40

• A continuous spectrum is an array a colours going all the way from red through to violet. (Richard of York gave battle in vein).
• This has every frequency of light you can have, it is something you get when you heat up a metal filament bulb, it gives out a continuous spectrum of every frequency of light.

Question 41

When is an emission spectra formed?

Answer 41

• When you heat up certain gases made from 1 element you then have an emission spectra.
• For something like sodium when you excite it, it will only give out orange and yellow light.
• If you excite a gas the electrons inside the energy levels will drop down and will only emit certain frequency of light.

Question 42

How does an absorption spectra form and why do elements only absorb certain frequencies?

Answer 42

• The absorption spectra is the negative image of the emission spectra.
• If you have a cool gas perhaps sodium, and passed white light/ a continuous spectrum of colours through it, the gas will only absorb certain frequencies of light.
• This is because the electrons in the atom can only absorb certain quantities of energy which is the exact amount of energy which will allow it to move to an above energy level.
• Any photons which don’t have the exact amount of energy needed will pass straight through the atom.
• this creates a spectrum where the frequencies absorbed shown as a black lines.

Question 43

What calculation can be used to show energy given off when an electron drops to a lower energy level?

Answer 43

• When a gas is excited it causes electrons to move up to a higher energy level but the electrons will then lose that energy and fall back to a lower level.
• The size of the energy gap between the energy levels tells us how much energy is given off.
• The amount of energy given off E = hf where f is the frequency of light emitted.
• There will only be certain frequencies of light given off as the electron falls, for example with sodium there are orange and yellow photons.

Question 44

Why do people wear masks when they are wielding?

Answer 44

• when something gets hot it will emit red light which if it gets hot enough will turn into orange and eventually white.
• the masks protect the person from ultra violet light emitted which could damage their eyes.

Question 45

What graph did Wien create and what did it show ?

Answer 45

• Wien drew a curve with relative intensity on the y axis against wavelength on the x axis.
• He found that if you have an object at a certain temperature we get a disproportionate curve where there is a steep gradient leading to a peak followed by a shallower gradient which gets increasingly shallow.
• The peak of each curve is a certain wavelength where most of the radiation is emitted, this is called the wavelength of maximum intensity (lambda max) which varies for different temperatures.
• The area underneath the curve is = total intensity of all light emitted, which is smaller for colder temperatures.

Question 46

What is Wien’s displacement law and how can it be used for stars?

Answer 46

• Wien’s displacement law states that the wavelength of maximum intensity is proportional to 1/temperature of the black body emitting the radiation.
• We can say that wavelength of max intensity, lambdamax*T = constant.
• This constant is wiens constant which = 2.90 x 10^-3 mK^-1
• If you know the temperature of a body then this allows you to calculate the wavelength of max intensity of the light emitted.
• Assuming stars are black bodies we can find the wavelength of max intensity of the light they emit.
• the hotter the star the smaller the wavelength of max intensity meaning the light emitted will move towards the ultraviolet part of the spectrum and away from the red part of the spectrum.
• Even when things are quite cool they can emit certain long wavelengths in the infra red part of the spectrum and this is how infrared cameras work.

Question 47

What is the luminosity?

Answer 47

• The luminosity is the total power output of a body which tells us the total amount of energy emitted per second, we can give it the symbol L.

Question 48

How can you derive an equation for luminosity?

Answer 48

• In the case of stars the luminosity L will be proportional to 4pier^2 where r is the radius of the star.
• We can therefore say that luminosity is proportional to the surface area of the star.
• The luminosity is also proportional to T^4 where T is the surface temperature of the star.
• You can combine these 2 relationships together to say the luminosity L is proportional to 4pier^2 x T^4
• The constant of proportionality which needs to be put in the equation is stefans constant which is a sigma letter.
• The resulting equation is therefore luminosity L = 4pier^2sigma*T^4

Question 49

What is Stefan’s law and constant?

Answer 49

• Stefans law relates the luminosity of a black body with its absolute temperature, L = 4pier^2sigma*T^4.
• Stefan constant = 5.67 x 10^-8 W m^-2 K^-4

Question 50

How can Stefan’s law be useful when investigating stars?

Answer 50

• If you observe the colour a star appears to be you can work out the wavelength of max intensity.
• the wavelength of max intensity is proportional to 1/T meaning if you know what colour the star is you know the temperature.
• If you also know how bright the star is you can calculate the luminosity.
• If you know both the luminosity and temperature you can calculate the radius of the star.
• Once you know the radius of the star you can calculate the mass of the star.

Question 51

What is lambda max/the wavelength of max intensity?

Answer 51

• lambda max is the wavelength in the absorption spectrum where the absorbance is maximum.
• the lambda max will determine the colour that a star appears, this is because when the certain wavelength is absorbed the electrons are excited and then release the energy in the form of photons of same wavelength in all directions.

Question 52

What are the 3 units to measure distance in space?

Answer 52

• astronomical unit AU
• light year ly
• parsec pc

Question 53

What is the astronomical unit and what is it useful for?

Answer 53

• The astronomical unit AU depends on our sun and the earth.
• 1 AU is the mean distance between the sun and the earth.
• This distance 1 AU = 1.50 x 10^11m
• This is useful when talking about things in our solar system so if a distance between a planet and a sun is less than 1 AU then it is closer to the sun than the earth, if we are looking at Jupiter it is well over 1 AU so we can see how our solar system is set up.

Question 54

What is the light year and how can we calculate 1 ly?

Answer 54

• The light year (ly) is the distance light will travel in a vacuum in 1 year.
• We know that displacement x = vt.
• We also know the velocity of light = c and we know the time is 1 year (31536000 seconds), therefore we can calculate 1 ly = 9.46 x 10^15 m.

Question 55

What is the parsec?

Answer 55

• The parsec (pc) is a unit of distance that gives a parallax angle of 1 second of arc, using the radius of the earths orbit as the baseline of a right angled triangle.
• Perhaps there is a star a certain distance away from the earth, we can look at the distance to the star from the sun in terms of the parsec and the angle it makes when we observe it from earth.
• If you draw a diagram of this you will have the earth perpendicular to the sun at a distance 1 AU.
• The star being observed will also be perpendicular with the sun at a certain distance.
• If you connect the sun, earth and star you will get a right angled triangle with a parallax angle of theta.
• When the value of theta is equal to 1 second of arc, the distance between the sun and star is 1 parsec.

Question 56

What is 1 second of arc?

Answer 56

• Inside a complete circle we have 360 degrees.
• Each 1 degree can be thought of as 60 minutes so 1 degree = 60 mins.
• We can then convert minutes into seconds so 1 degree = 3,600 seconds.
• If we want to calculate 1 arc second in degrees it would be 1/3600 = 0.00027778 degrees showing that 1 arc second is a very small value.

Question 57

How can you calculate the value of 1 parsec?

Answer 57

• Using trigonometry you can find the value of 1 parsec.
• By using SOHCAHTOA you can say tantheta = O/A.
• This can be rearranged to A = O/tantheta.
• If you substitute values for the opposite side (1AU) and theta (1/3600) you get:
  A = (1.5x10^11)/[tan(1/3600)] = 3.1 x 10^16 m.
• Therefore 1 pc = 3.1 x 10^16 m

Question 58

What is stellar parallax used to find?

Answer 58

• Stellar parallax is a way that we can measure the distance of fairly nearby stars.

Question 59

How can you demonstrate parallax?

Answer 59

• If you hold your thumb out in front of you and cover your left eye then cover your right eye your thumb will appear to be in different positions.
• When we look at objects close to us they appear to change position but things far away appear to stay in the same position.

Question 60

How was stellar parallax discovered?

Answer 60

• Scientists found that when they took pictures of stars outside our solar system at different times of the year in summer and winter the stars would have a different arrangement.
• In summer there would be a certain arrangement but then in winter it appeared that 1 star would change position and then return the next summer.
• This wasn’t because the star was moving from one place to another, but its due to the fact the star is nearby and the position of the earth has changed by 6 months.
• if we imagine the earth above and below the sun we can see that the position of the nearby star will change.
• This is similar to us as the earths act as our eyes and the sun our nose.
• This is a way we can work out the distance to fairly close stars in our galaxy.

Question 61

How can you calculate the distance to stars in our galaxy using stellar parallax?

Answer 61

• You need to create a diagram with the sun and the earth above it and also below it (positioned 6 months apart).
• You then have stars scattered a large distance away from the sun.
• We then have the star that the distance is being measured which is closer to the sun than the rest of the stars.
• If you look at the nearby star when the earth is below the sun and draw a line from the earth to the star you will see the star in a certain position among the distant stars.
• When the earth changes position every 6 months if you then draw another line from the earth to the star you will see its position among the distant stars changes.
• you then need to add a baseline through the earths and sun.
• We know the distance between the earth and sun = 1AU.
• We need to work out the distance between the sun and the star therefore we need to know angle theta to work this out.
• We can call theta, p which is the parallax angle.
• If we measure angle p in seconds, we can say the distance in parsecs, d = 1/p.

Question 62

What is a parallax error?

Answer 62

• The parallax error is if you are looking at an object and if you don’t look at it head on it appears to be in a different position.

Question 63

Why can stellar parallax only be used for stars within our solar system?

Answer 63

• As the parallax angle p gets smaller and smaller it is going to get harder to work out the distance as it requires more precision.
• We can get values of p only to about 1/100 arc seconds, this limits our distance d to be 100pc.

Question 64

What is isotropic?

Answer 64

• Isotropic means that something looks the same in all directions.
• we say that space is isotropic as it looks the same in all directions.

Question 65

What is homogenous?

Answer 65

• homogenous is where there is a uniform distribution of matter.
• we say that space is homogenous as stars for example are distributed uniformly.

Question 66

What is the cosmological principle?

Answer 66

• The cosmological principle states that the universe is isotropic and homogenous, and the laws of physics are universal.

Question 67

What is an example of the Doppler effect in real life?

Answer 67

• We can think of the doppler effect using a duck swimming across water, if you have a duck and you throw some bread in a pond the ducks will start swimming towards it.
• We see a pattern from the circular ripples of the duck in the water.
• We find that in front of the duck there is a short wavelength (the ripples are closer together) and behind the duck the wavelength gets longer (the ripples are further apart).
• This is due to the Doppler effect which is the change in wavelength and frequency of waves emitted by an object caused by the object moving.

Question 68

How is star light analysed by scientists for proving the Doppler effect?

Answer 68

• With stars all we can do is observe them and analyse the light we see.
• If we take the white light we see from a star and put it through a prism or a transmission diffraction grating you get a spectrum of colours.
• Scientists found that when you look at star light there are certain lines which are missing.
• This is effectively an absorption spectra as it shows which wavelengths of light are absorbed by the atmosphere of the star.

Question 69

How did scientists use star light to prove the Doppler effect and red shift?

Answer 69

• Scientists found that when you look at star light there are certain lines which are missing in the absorption spectra.
• This is because certain wavelengths of light are absorbed by the atmosphere of stars.
• Scientists looked at other stars and used diffraction gratings to view the spectra and they found the absorption lines were of the same pattern but in a different position on the spectra.
• One explanation for this was that they could be different elements, however as they looked at more star light they found the same pattern was repeated but shifted towards the red end of the spectrum.
• This is proof of the Doppler effect as the absorption lines are being shifted towards the red end of the spectrum.

Question 70

How can the Doppler effect be used to calculate the recessional velocity of a star?

Answer 70

• If the wavelength of a star is getting longer it is being red shifted.
• We call the velocity which it is moving the recessional velocity.
• You can measure the change in wavelength in light emitted by the star.
• If you know the change in wavelength and the original wavelength, you can use the equation:
  (change in lambda)/(lambda) = v/c to find the recessional velocity, v.
• Most things in our galaxy are red shifted which means everything is moving away from us.
• We can also say that (change in lambda)/(lambda) = v/c = (change in frequency)/(frequency).

Question 71

Can blue shift exist in the universe?

Answer 71

• Some things in the universe are blue shifted, this could be nearby stars moving towards us but scientists have found that every galaxy is moving away from us and galaxies farthest away are moving faster.

Question 72

What are the 2 pieces of evidence for the Big Bang?

Answer 72

• Evidence for the big bang is the red shift of starlight and also background cosmic radiation.

Question 73

How was cosmic background radiation created?

Answer 73

• After several hundred years of the universe’s existence as a small dense point we had transparency.
• At that point we had a large amount of gamma radiation which has a short wavelength.
• As the universe got bigger and bigger the gamma radiation was stretched.
• As the universe got close to its current size we found that the wavelength of what was originally the gamma radiation was now in the microwave region.
• These microwaves are now known as background cosmic background radiation.

Question 74

How did the universe reach a temperature of 2.7K?

Answer 74

• Microwaves have their own temperature which is around 2.7K.
• The energy which was in the early universe was very hot but as the universe increased in size the energy has spread out and the temperature has gone down to 2.7K.

Question 75

How was cosmic background radiation discovered?

Answer 75

• Cosmic background radiation was discovered in 1965 by a couple of scientists with a radio telescope.
• They tried to do radio astronomy but they kept finding that there was a signal getting in the way, there were stories about them thinking it was pigeon droppings so they went to their radar dish and cleaned all of the droppings but they still found interference.
• They realised that this interference had a wavelength of about 1.1cm which had a corresponding temperature of 2.7K.
• Penzius and Wilson discovered by accident that radiation in the microwave wavelength region was cosmic background radiation.

Question 76

What are the main stages in the evolution of the universe?

Answer 76

• At time t = 0 an infinitely dense and hot singularity caused the Big Bang which created all of space and time.
• a short time later the 4 forces unified during a period of inflation and expansion.
• at t = 10^-6s the temperature started to cool, we had a sea of quarks and leptons but no atoms yet.
• then quarks started to join making protons and neutrons with more protons being formed at a ration of 4:1.
• when this matter formed there was similar amounts of matter and antimatter, this is because of pair production where gamma rays cause a matter and antimatter pair which can combine to form energy.
• there was slightly more matter than antimatter hence why the universe is made from matter.
• at t = 100s temperature dropped and neutrons and protons joined to form deuterium and helium, heavier elements such as lithium also formed.
• 25% of the universe was helium and over 70% was hydrogen.
• at t = 100,000 years electrons orbited protons getting rid of ions and forming atoms.
• at this point gamma photons were passing around the transparent universe which is the origin of CBR.
• at t = 1,000,000 years we had structure in the universe with early stars forming, rapid hydrogen burning made heavier elements.
• at this point time t = 14,000,000,000 years and the temperature is 2.7K.

Question 77

What is the universe composed of in terms of percentages?

Answer 77

• 5% of the universe is made from matter which we know about consisting of protons, neutrons and electrons.
• 25% of the universe is dark matter which could be made from matter that we can’t see or particles which are yet to be discovered.
• 70% of the universe is dark energy which is responsible for the expansion of the size of the universe.

Question 78

What is dark matter and how do we know that it exists?

Answer 78

• Dark matter is matter which cannot be seen and that does not emit or absorb em radiation.
• it is not detected directly, but indirectly based on it gravitational effects relating to either the rotation of galaxies or by gravitational lensing of starlight.

Question 79

How do we know that dark energy exists?

Answer 79

• Observations of the universe such as those by Hubble still stand but when scientists looked at the rate of expansion of the universe they found it was expanding at an increasingly fast rate meaning it is accelerating.
• The acceleration of the size of the universe must be caused by an energy pushing everything apart but it is an energy that we don’t know about which is why we call it dark energy.
• Therefore dark energy is responsible for the expansion of the size of the universe.