5.5 Astrophysics & Cosmology

Question 1

define the term planet

Answer 1

• bodies that move in circular or elliptical orbits around a star to form a solar system
• an object around a star with a mass large enough for its own gravity to give it a round shape, that undergoes no fusion and that has cleaned its orbit of most other objects

Question 2

define the term planetary satellite

Answer 2

a body in orbit around a planet (artificial or manmade)

Question 3

define the term comet

Answer 3

a small, irregular body made of dust, ice and rock that orbits a star with in highly eccentric elliptical orbit around the sun

Question 4

define the term solar system

Answer 4

a planetary system consisting of a star and at least one planet in orbit around it

Question 5

define the term galaxy

Answer 5

a collection of stars, interstellar dust, and gas bound together by their mutual gravitational attractions

Question 6

define the term universe

Answer 6

everything that exists within space and time

Question 7

what is the order of the planets?

Answer 7

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune

Question 8

what is gravitational collapse?

Answer 8

gravitational collapse is the inward movement of particles of dust and gas in a star due to the gravitational force caused by its own mass.

Question 9

How does a star form

Answer 9

Nebulae’s are formed when particles of dust and gas come together due to their gravitational attraction - process known as gravitational collapse. Now due to tiny variations in the nebula, denser regions begin to form, and so mass is gained.

as the gravitational force pulls more and more matter together, work is done on the particles of dust and gas which leads to an increase in KE and therefore temp also, some of the denser areas will begin to glow, the large core of material is called a PROTOSTAR

the protostar’s gravitational force field strength will continue to attract more and more matter until the temperature at the core of the star will reach millions of degrees kelvin and this will mean nuclear fusion can start to occur

Question 10

how does a star form? (1st step)

Answer 10

Nebulae’s are formed when particles of dust and gas come together due to their gravitational attraction - process known as gravitational collapse. Now due to tiny variations in the nebula, denser regions begin to form, and so mass is gained.

Question 11

how does a star form? (step after cloud of dust and gas)

Answer 11

as the gravitational force pulls more and more matter together, work is done on the particles of dust and gas which leads to an increase in KE and therefore temp also, some of the denser areas will begin to glow, the large core of material is called a PROTOSTAR

Question 12

Why is very high temperature required for fusion reaction in a star

Answer 12

To overcome the electrostatic repulsion between hydrogen nuclei in order to fuse them together to form helium nuclei

Question 13

how are protostars detected?

Answer 13

protostars can only be detected through telescopes designed to observe infrared radiation

Question 14

what does a star form? (step after protostar forming)

Answer 14

the protostar’s gravitational force field strength will continue to attract more and more matter until the temperature at the core of the star will reach millions of degrees kelvin and this will mean nuclear fusion can start to occur

Question 15

what happens as nuclear fusion occurs in a protostar?

Answer 15

there is a radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei which all act together in the core to push outwards, in order to balance the force from the gravitational attraction

Question 16

define radiation pressure

Answer 16

radiation pressure is due to the emission of photons during fusion reactions

Question 17

what happens at the point when a protostar of stable size becomes a main sequence star in terms of forces?

Answer 17

the radiation pressure (from the nuclear fusion reactions) and the gas pressure (force outwards caused by the star’s temperature increasing and it’s volume decreasing) is in equilibrium with the gravitational force acting inwards

Question 18

what is happening when a star is a main sequence star?

Answer 18

it is stable and is converting hydrogen to helium through nuclear fusion, it will remain in this stage for the majority of its life where it will be be radiating electromagnetic energy into space

Question 19

define gas pressure

Answer 19

• gas pressure is the outwards force caused by the star’s increasing temperature and it’s decreasing volume
• gas pressure, p, is related to the temperature, T, and volume, V, of the gas using pV = nRT, and also to the mean square speed of the gas atoms using pV = 1/3 Nmc^2(bar), gas pressure acts in all directions at a point inside a gas such as inside a star (book definition)

Question 20

A stars life expectancy is dependant on

Answer 20

depends on the size and mass of it’s core.

A larger core is much more hotter, releasing more lower and converting its available hydrogen into helium in a much more shorter time. So life expectancy is shorter than that of a smaller core

Question 21

define main sequence star

Answer 21

a main sequence star is a star in the main part of its life cycle, where it is fusing hydrogen to form helium in the core, the main sequence star are shown as a curved band on a plot of a star’s luminosity against temperature

Question 22

how much of the stars in the universe are main sequence stars?

Answer 22

they make up 90% of the stars of the stars in the universe

Question 23

what stage is the Sun in its life cycle?

Answer 23

the sun is about half way through its life as a main sequence star

Question 24

Show the life cycle of a star with a mass between 0.5M and 10M

Answer 24

Main sequence&raquo_space;> red giant&raquo_space;» planetary nebula and white dwarf

Question 25

define red giant

Answer 25

a star between 0.5M and 10M will evolve into a red giant in the later stages of its life where it has nearly used up all its available hydrogen in its core and is now fusing helium nuclei

At this stage the core of the start begins to collapse due to the gravitational force being greater than reduced force from radiation and gas pressure.

Question 26

Why is red giant bigger than that of a normal star

Answer 26

it is bigger than a normal star because its surface layers have cooled and expanded.

Question 27

Explain the core of red giant in terms of forces

Answer 27

At this stage the core of the start begins to collapse due to the gravitational force being greater than reduced force from radiation and gas pressure.

As the core shrinks, the pressure increases to start fusion in a shell around the core. (This fusion reaction does not happen at the core.)

Question 28

why does the core of red giant begin to collapse

Answer 28

At this stage the core of the start begins to collapse due to the gravitational force being greater than reduced force from radiation and gas pressure. As the core shrinks, the pressure increases to start fusion in a shell around the core. (This fusion reaction does not happen at the core.)

Question 29

define white dwarf

Answer 29

a white dwarf is the end product of a low-mass star, when the outer layers have dispersed into space, known as a planetary nebula, leaving behind a hot core known as a white dwarf

Question 30

Does nuclear fusion take place at the core of a white dwarf

Answer 30

Also no fusion reaction takes place. It emits energy because it leaks photons created in earlier evolution.

Question 31

Describe a white dwarfs mass and size

Answer 31

a white dwarf is very dense, with a mass that of a sun and a volume that of planet earth

Question 32

Solar mass M =

Answer 32

1.99 x 10^30 kg

Question 33

define planetary nebula

Answer 33

a planetary nebulae is an expanding, glowing shell of ionised hydrogen and helium ejected from a red giant star at the end of its life

Question 34

State the Pauli exclusion principle

Answer 34

two e-‘s cannot exist in the same energy state. when the core of a star begins to collapse under the force of gravity, the e-‘s are squeezed together, and this creates a pressure that prevents the core from further gravitational collapse. Pressure created by e-‘s is known as electron degeneracy pressure.

Question 35

define electron degeneracy pressure

Answer 35

this is the pressure that prevents the further gravitational collapse of a low-mass star (providing that the chandrasekhar limit is below 1.4 solar masses), this is the pressure that prevents a white dwarf from collapsing

Question 36

What is the pressure called that prevents a white dwarf from collapsing

Answer 36

Electron degeneracy pressure

Question 37

define the chandrasekhar limit

Answer 37

it is the maximum possible value of mass for a stable white dwarf star which is 1.44M, white dwarfs with masses above this will collapse further to become neutron stars or black holes

Question 38

What will happen if a white dwarf has a mass below and above the Chandrasekhar limit

Answer 38

white dwarfs with masses above this will collapse further to become neutron stars or black holes

And white dwarfs below this will remain a white dwarf

Question 39

Show the life cycle of a start with a mass greater than 10M

Answer 39

Main sequence&raquo_space; red super giant&raquo_space; supernova&raquo_space;>

neutron star/ or black hole

Question 40

define red super giant

Answer 40

a red super giant is a star that has exhausted all of the hydrogen in its core and has a mass much higher than the sun

Question 41

Why in high mass star there is greater rate of fusion

Answer 41

Greater fusion equals greater outward force to counteract the enormous amount of gravitation force acting inwards

Question 42

Why does white dwarf emit energy

Answer 42

It emits energy because it leaks photons created in earlier evolution.

Question 43

define supernova

Answer 43

a supernova is a huge explosion when the core of a red super giant collapses

Question 44

define neutron star

Answer 44

a neutron star is the remains of the core of a red super giant after it has undergone a supernova explosion, it is incredibly dense and composed mainly of neutrons

Question 45

define black hole

Answer 45

a black hole is the core of a massive star that has collapsed almost to a point, black holes are very dense and very small with a gravitational field so strong that light cannot escape (escape velocity is greater than the speed of light)

Question 46

what is the Hertzsprung-Russel diagram (HR diagram)? And draw one

Answer 46

a luminosity-temperature graph, it can also be used to visualise the life cycle of a star as it moves from the Main Sequence to a Red Giant then a White Dwarf, or perhaps even a Red Supergiant.

Question 47

where can you find electrons? how do electrons exist?

Answer 47

-they orbit the nucleus, they can only occupy certain energy levels with quantised energies

Question 48

how can electrons gain and loose energy?

Answer 48

by moving from one allowed energy level to another, absorbing or emitting electromagnetic radiation with a frequency f, determined by the energy difference of the levels according to:
hf = ΔE or hf = E1 - E2
the energy difference ΔE can also be written as ΔE = hc / λ

Question 49

why do energy levels have negative values?

Answer 49

• because when the electron is is infinitely away from the nucleus it has an energy of ZERO, as you move the electron closer to the nucleus because it is attracted to it, the lowest energy level has the highest magnitude of energy e.g -13.6eV because at this energy level it will require a large amount of energy to remove it from this level because of the high attraction
• an electron’s energy is taken to be zero when the electron is a very long away from the atom’s nucleus (similar to how we define gravitational potential to be zero at infinity), as the electron moves towards the nucleus from very far away, its energy decreases below zero so energy levels inside an atom have negative values

Question 50

what equation should you use to work out the energy difference between energy levels?

Answer 50

ΔE = hc / λ or hf = ΔE and ΔE= E1 - E2
where λ = wavelength of the photon that is absorbed or emitted by the atom
h = planck’s constant
c = speed of light

Question 51

what is an emission line spectrum? what does it look like?

Answer 51

the emission line spectrum of an element is the spectrum of frequencies of electromagnetic radiation emitted due to electron transitions from a higher energy level to a lower one within an atoms of that element

Question 52

what is an emission line spectrum? what does it look like?

Answer 52

the emission line spectrum of an element is the spectrum of frequencies of electromagnetic radiation emitted due to electron transitions from a higher energy level to a lower one within an atoms of that element, since there are many possible electron transitions for each atom, there are many different radiated wavelengths - a line spectrum consists of a series of bright rainbow lines against a dark background

Question 53

what produces emission line spectra and how?

Answer 53

an emission line spectrum is produced when an excited electron in an atom in a gas moves from a higher to a lower energy level and emits a photon with an energy corresponding to the difference between these energy levels

Question 54

how can an emission line spectrum be used to identify elements within stars?

Answer 54

• because each element’s emission line spectrum is unique and acts as a fingerprint for the element
• fusion reactions in a star’s core produces photons of electromagnetic radiation which move upwards through the layers of gas surrounding the core, the photons are constantly absorbed by atoms in these gases which then re-emit photons of many different frequencies, in random directions

Question 55

what is the ground state (in terms of electrons)?

Answer 55

lowest energy level

Question 56

what is a continuous spectrum?

Answer 56

a continuous spectrum is a spectrum that appears to contain all wavelengths over comparatively wide range (full rainbow)

Question 57

what is an absorption line spectrum? what does it look like?

Answer 57

an absorption line spectrum is the pattern of dark lines in a continuous spectrum from a light source and is caused by light passing through an absorbing medium such as a gas -looks like a rainbow with dark lines (the dark lines represent the wavelengths that are absorbed)

Question 58

what are the three main types of spectrum?

Answer 58

• continuous
• emission
• absorption

Question 59

how do you produce a continuous line spectra?

Answer 59

use a transmission diffraction grating

Question 60

how do you produce an absorption line spectra?

Answer 60

pass white light (with a continuous spectrum) through a cold gas where an atom is the gas absorbs a photon of the correct energy and an electron moves from a lower energy level to a higher one which emits a photon but in all directions so we don’t see as absorbed (dark lines)

Question 61

how can you use a diffraction grating to determine the wavelength of light? what equation do you use?

Answer 61

(same as AS experiment)
nλ = dsinϴ
see page 105 or flashcards in module 4 ‘waves’

Question 62

what is Wien’s displacement law?

Answer 62

Wien’s displacement law states that λmax is directly proportional to 1 / T or λmax x T = constant, where T is peak surface temp. of a star in kelvin

Question 63

what is Stefan’s law?

Answer 63

Stefan’s law relates the luminosity, L, of a star (the radiation flux emitted from the surface of a star) with its absolute temperature
T, L = 4πr^2σT^4
where the constant σ is Stefan’s constant and is equal to 5.67x10^-8 and r = star’s radius

Question 64

what is the luminosity of a star?

Answer 64

the luminosity of a star is the total energy that the star emits per second, or the POWER

Question 65

what can Stefan’s and Wien’s law be used to calculate an estimate of?

Answer 65

an estimate of the radius of a star

Question 66

what is the astronomical unit? (AU)

Answer 66

the astronomical unit AU is the mean distance from the centre of the Earth to the centre of the Sun

Question 67

what is a parsec? (pc)

Answer 67

the parsec is a unit of distance that gives a parallax angle of 1 second of arc (1/3600 of a degree), using the radius of the Earth’s orbit (1 AU) as the baseline of a right angled triangle (see page 109 for good diagram)

Question 68

what is a light-year? (ly)

Answer 68

a light-year is the distance travelled by light in one year, one light year is approximately equal to 9.5 x 10^15m

Question 69

what is stellar parallax?

Answer 69

stellar parallax is the apparent shifting in position of a star viewed against a background of distant stars when viewed from different positions of the Earth, such as at different positions of the Earth’s orbit around the Sun (see page 109 for good diagram)

Question 70

what is a light-year a measure of?

Answer 70

distance, NOT TIME

Question 71

what is the equation for parallax? and how do you derive it?

Answer 71

p = 1 / d
where p = stellar parallax measured in seconds of arc
d = distance in parsecs
first draw the right angles triangle with base of 1AU or (r of the Earth’s orbit around the sun) and with d being the distance from the Sun to a nearby star, we can say that tanp = r / d, for small angles expressed in radians tanp = p, using this small angle approximation and the definition of the parsec in terms of AU, it can be shown that the relationship between the distance to a star from Earth and the angle of stellar parallax is given by p = 1 / d

Question 72

what is the Doppler effect or Doppler shift?

Answer 72

the change in wavelength caused by the relative motion between the wave source and an observer, for electromagnetic radiation of frequency f and wavelength λ, the Doppler equation is Δλ / λ = Δf / f = v / c

Question 73

what is red shift?

Answer 73

red shift is the apparent increase in wavelength of electromagnetic radiation caused when the source (e.g a star) is moving away, relative to the observer

Question 74

if you observe red shift is the object moving away or towards you? what is happening to the wavelength?

Answer 74

away from you, wavelength getting larger, in terms of spectral lines, the pattern of lines are shifted towards the red end of the spectrum

Question 75

if you observe blue shift is the object moving away or towards you? what is happening to the wavelength?

Answer 75

towards you, wavelength getting smaller, in terms of spectral lines, the pattern of lines are shifted towards the blue end of the spectrum

Question 76

what is the Doppler equation for a source of electromagnetic radiation moving relative to an observer?

Answer 76

Δλ / λ = Δf / f = v / c

Question 77

what is Hubble’s Law?

Answer 77

Hubble’s law states that the recessional velocity, v, of a galaxy is directly proportional to its distance, d, from the Earth

Question 78

what is Hubble’s constant?

Answer 78

the Hubble constant Ho is the constant of proportionality in the equation v = Ho x d, the SI unit for Ho is s^-1, but it can also be quoted in km per second per megaparsecs

Question 79

how did Hubble come up with his law? what did he discover?

Answer 79

in 1929, the astronomer showed that the universe was expanding from his evidence from red shift (light from distant galaxies was being shifted), not only did he find out the universe was expanding, it found that the furthest galaxies were moving away faster than those nearest to us

Question 80

how can you use Hubble’s law and constant to estimate the age of the universe?

Answer 80

the time since the Big Bang for a galaxy to reach its measured distance from Earth is found by dividing this distance by the speed of recession, if we assume the speed of recession has been constant over the history of the universe, we can find the time since the expansion began, use t = 1 / Ho or t = Ho^-1

Question 81

what is the cosmological principle?

Answer 81

the cosmological principle states that on a large scale the universe is isotropic (the same in all directions) and homogenous (of uniform density as long as a large enough volume is considered) and that the laws of physics are universal

Question 82

what is the big bang theory?

Answer 82

the big bang theory states that the universe was created from a single ‘point’ where all of the universe’s current mass was situated, at the time of its creation, the universe was much smaller, hotter and denser than it is now, time and space were both created at the instant of the Big Bang

Question 83

how do we know the universe is expanding?

Answer 83

light from distant galaxies is red-shifted

Question 84

how was cosmic background microwave radiation discovered?

Answer 84

two astronomers found unexpected radiation that they believed did not come from the Milky Way they seemed to detect unwanted signal or noise in all directions, they first believed it can be pigeons that were roosting in the radio antenna, they then removed the pigeons and pointed the radio telescope away from New York, they expected the noise to vanish but it didn’t, no matter what direction they focused their radio telescope, the same level of noise was always detected, this is now known as cosmic background microwave radiation (CMBR) - the ‘echo’ of the big bang

Question 85

what is cosmic background microwave radiation?

Answer 85

cosmic microwave background radiation is microwave radiation from all over the sky originating from after the Big Bang, when the universe had cooled to a temperature near 3000K, as the universe has expanded this radiation is now just a faint microwave glow with a peak wavelength corresponding to a temperature of 2.7K (the same temperature of the universe)

Question 86

what was CMBR evidence for?

Answer 86

The Big Bang Theory

Question 87

how were astronomers able to calculate the temperature of the universe?

Answer 87

the measured the wavelength of the cosmic microwave background radiation’s peak intensity as 0.11m which they could estimate the temp which they found to be 2.7K (just 2.7 degrees above absolute zero) - this microwave intensity and wavelength and hence the temperature was the same in all directions

Question 88

what is the idea behind the Big Bang Theory in terms of expansion of the universe?

Answer 88

if the universe is expanding (as shown by red-shift), then there must have been a point at which it started to expand, in other words there must be a point in time at which the universe was smaller, hotter and more dense, taking this argument further, there must have been a time at which this expansion started, which we call the Big Bang

Question 89

how can the expansion of the universe with the three space dimensions and one time dimension be thought as? what analogy can be used?

Answer 89

an expanding balloon with dots on it - regardless of what galaxy you look at, the other galaxies will all appear to e moving from one another by the same amount as the universe expands (which is in agreement with the cosmological principle)

Question 90

what do we know about the universe at a time of 10^-43 seconds? (temp > 10^22K)

Answer 90

scientists cannot determine what the universe was like at this point, the tools of science will never allow us to find out, it is possible that the universe was infinitely dense, infinitely hot and infinitely small but we simply do not know and we never shall

Question 91

what do we know about the universe at a time of 10^-43 - 10^-4 seconds? (temp = 10^22K)

Answer 91

this was a period of expansion known as inflation, after the end of inflation matter and antimatter are formed, particles that are the building blocks of the universe come into existence - quarks, leptons photons and their antiparticles, at this point these particles cannot form heavier particles such as protons, neutrons and other hadrons or baryons because of the enormous temperatures, there is slightly more matter than antimatter in the universe, as matter and antimatter annihilate, they leave a matter-dominated universe made from particles and not antiparticles

Question 92

what do we know about the universe at a time of 10^-4 seconds? (temp = 10^12K)

Answer 92

the universe has now cooled enough for the quarks to join together to form protons and neutrons, but there are no atoms, matter and antimatter continue to collide and annihilate, resulting in the production of enormous quantities of high-energy photons and these photons constantly interact with charged particles, so that photons are continually absorbed and re-emitted

Question 93

what do we know about the universe at a time of 100 seconds? (temp = 10^9K)

Answer 93

the temp of the universe has cooled and it now behaves in the same way as the core of a star, helium and lithium nuclei start to be formed, but the temp is becoming too low for further fusion to occur, the matter in the universe is in plasma form (which is when protons and electrons are not bound to one another because the temp is too high)

Question 94

what do we know about the universe at a time of 250,000 years? (temp = 10^4K)

Answer 94

the temp is now low enough for hydrogen and helium atoms to be formed, as electrons can combine with nuclei, generally as the temp falls, more of the electrons will become attached to the protons present - this process is called decoupling as it is the moment at which radiation and matter decouple from each other, photons can now travel freely though space and the universe becomes transparent, since neutral atoms do not interact so readily with photons, the photons that existed at this time become the CMBR that we detect today

Question 95

what do we know about the universe at a time of 1 million years? (temp = 6000K)

Answer 95

tiny density fluctuations result in the first large-scale structure of the universe - the seeds for galaxies to form

Question 96

what do we know about the universe at a time of 1 billion years? (temp = 17K

Answer 96

more structures form, heavy elements from as a result of the gravitational collapse of stars

Question 97

what do we know about the universe at a time of 13.7 billion years? (temp = 2.7K)

Answer 97

the present time (life on Earth), the ratio of protons and neutrons is about 4 : 1, the slight changes in density throughout the universe mean that clusters of galaxies, and individual stars, have formed from the clumps of matter in space

Question 98

what is dark matter?

Answer 98

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

Question 99

what is dark energy?

Answer 99

dark energy is a type of energy that permeates the whole universe and opposes the attractive force of gravitation between galaxies via the exertion of a negative pressure, it is not detected directly, but we know it exists because we now know the universe is accelerating as it expands

Question 100

with Stefan and Wien’s law what values are we able to obtain from observing the star and what can we therefore work out?

Answer 100

looking at the spectrum of light we can get a value for λmax, and we can measure a star’s luminosity L, then use both their laws to work our the peak surface temperature and the star’s radius

Question 101

is the universe expanding at a constant rate?

Answer 101

no - the expansion of the universe is not constant, in fact it is accelerating

Question 102

what is causing the non-constant expansion of our universe?

Answer 102

supposedly DARK ENERGY

Question 103

wha are the 3 possible outcomes of the universe?

Answer 103

flat universe, open universe and closed universe

Question 104

what is an open universe?

Answer 104

an open universe - the universe expands for all time, galaxies continue to separate from each other

Question 105

what is an closed universe?

Answer 105

a closed universe - the universe will eventually collapse and contract because the gravitational forces will prevent galaxies going on out into space for ever, eventually galaxies will slow down and there will be a Big Crunch

Question 106

what is a flat universe?

Answer 106

a flat universe - the universe will expand to a limit because the density of the universe is equal to a critical value, this implies that the galaxies will gradually slow down over a very long time but never quite stop

Question 107

why doesn’t nuclear fusion take place at the core of red giant

Answer 107

There is very little hydrogen that remains and the temperature is not high enough for the helium nuclei to overcome the electrostatic force of repulsion between them. However fusion of hydrogen into helium continues in the shell around the core which causes the periphery of the start to expand

Question 108

Materials that do not form into a protostar, what else could they be

Answer 108

They could form into planets

Question 109

Explain why heavier Elemis are formed in the core of a red giant star

Answer 109

Temperature is not high enough, therefore the kinetic energy of larger nuclei is not large enough to overcome the electrostatic force of repulsion and fuse together

Question 110

Describe the process of the creation of heavier elements (nucleosynthesis) and how these elements come to be found distributed throughout the universe

Answer 110

Elements formed in red supergiants
Core is hot enough to fuse large nuclei together
Fusion takes place in shells around the core
Eventually when the core becomes iron the star will implode and then explode in a supernova
Elements will then be ejected into space

Question 111

Calculate the minimum and maximum values of the mass of a star that will form a red giant

Answer 111

0.5 x 1.99 x 10^30 min

10 x 1.99 x 10^ 30 max

Question 112

Suggest where a black hole might appear on an HR diagram

Answer 112

Luminosity is zero but temperature is unknown would not appear on the diagram