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 a highly 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 material in a star due to the gravitational force caused by its own mass, star formation is due to the gradual gravitational collapse of a cloud of gas and dust

Question 9

how does a star form? (1st step)

Answer 9

stars are born when a cloud of interstellar dust and gas (most of which were left when previous stars blew themselves apart in supernovae) slowly clump together due to their gravitational attraction, given enough time these areas will gradually become more dense as more matter is attracted, the inward movement of matter is called gravitational collapse.

Question 10

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

Answer 10

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 11

how are protostars detected?

Answer 11

protostars can only be detected through telescopes designed to observe infrared radiation, as the clouds of gas and dust (nebulae) absorb and scatter most of the visible light

Question 12

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

Answer 12

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 13

what happens as nuclear fusion occurs in a protostar?

Answer 13

enormous amounts of energy are released and the momentum of the photons released by the fusion reactions leads to an outwards force called the radiation pressure

Question 14

define radiation pressure

Answer 14

radiation pressure is due to the momentum of photons released in fusion reactions, and acts outwards (in the direction of energy flow)

Question 15

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

Answer 15

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 16

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

Answer 16

it is stable and is converting hydrogen to helium through nuclear fusion, it will remain in this stage for the majority of its life

Question 17

define gas pressure

Answer 17

• gas pressure is the outwards force caused by the star’s temperature increasing and it’s volume decreasing
• 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 18

define main sequence star

Answer 18

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 19

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

Answer 19

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

Question 20

what stage is the Sun in its life cycle?

Answer 20

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

Question 21

define red giant

Answer 21

a red giant is a star in the later stages of its life that has nearly exhausted the hydrogen in its core and is now fusing helium nuclei, it is bigger than a normal star because its surface layers have cooled and expanded

Question 22

define white dwarf

Answer 22

a white dwarf is the end product of a low-mass star, when the outer layers have dispersed into space, a white dwarf is very dense, with a high surface temperature and low luminosity

Question 23

define planetary nebula

Answer 23

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 24

define electron degeneracy pressure

Answer 24

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

Question 25

define the chandrasekhar limit

Answer 25

is the maximum possible value of mass for a stable white dwarf star and is equal to 1.4 times the mass of our Sun, white dwarfs with masses above this will collapse further to become neutron stars or black holes

Question 26

define red super giant

Answer 26

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 27

define supernova

Answer 27

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

Question 28

define neutron star

Answer 28

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 29

define black hole

Answer 29

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 30

what is the Hertzsprung-Russel diagram (HR diagram)?

Answer 30

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 31

where can you find electrons? how do electrons exist?

Answer 31

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

Question 32

how can electrons gain and loose energy?

Answer 32

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 33

why do energy levels have negative values?

Answer 33

• 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 34

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

Answer 34

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 35

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

Answer 35

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 36

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

Answer 36

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 37

what produces emission line spectra and how?

Answer 37

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 38

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

Answer 38

• 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 39

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

Answer 39

lowest energy level

Question 40

what is a continuous spectrum?

Answer 40

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

Question 41

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

Answer 41

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 42

what are the three main types of spectrum?

Answer 42

• continuous
• emission
• absorption

Question 43

how do you produce a continuous line spectra?

Answer 43

use a transmission diffraction grating

Question 44

how do you produce an absorption line spectra?

Answer 44

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 45

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

Answer 45

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

Question 46

what is Wien’s displacement law?

Answer 46

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 47

what is Stefan’s law?

Answer 47

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 48

what is the luminosity of a star?

Answer 48

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

Question 49

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

Answer 49

an estimate of the radius of a star

Question 50

what is the astronomical unit? (AU)

Answer 50

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

Question 51

what is a parsec? (pc)

Answer 51

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 52

what is a light-year? (ly)

Answer 52

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 53

what is stellar parallax?

Answer 53

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 54

what is a light-year a measure of?

Answer 54

distance, NOT TIME

Question 55

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

Answer 55

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 56

what is the Doppler effect or Doppler shift?

Answer 56

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 57

what is red shift?

Answer 57

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 58

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

Answer 58

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 59

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

Answer 59

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

Question 60

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

Answer 60

Δλ / λ = Δf / f = v / c

Question 61

what is Hubble’s Law?

Answer 61

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

Question 62

what is Hubble’s constant?

Answer 62

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 63

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

Answer 63

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 64

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

Answer 64

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 65

what is the cosmological principle?

Answer 65

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 66

what is the big bang theory?

Answer 66

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 67

how do we know the universe is expanding?

Answer 67

light from distant galaxies is red-shifted

Question 68

how was cosmic background microwave radiation discovered?

Answer 68

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 69

what is cosmic background microwave radiation?

Answer 69

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 70

what was CMBR evidence for?

Answer 70

The Big Bang Theory

Question 71

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

Answer 71

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 72

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

Answer 72

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 73

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 73

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 74

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

Answer 74

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 75

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

Answer 75

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 76

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

Answer 76

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 77

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

Answer 77

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 78

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

Answer 78

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 79

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

Answer 79

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

Question 80

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

Answer 80

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

Question 81

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

Answer 81

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 82

what is dark matter?

Answer 82

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 83

what is dark energy?

Answer 83

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 84

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 84

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 85

is the universe expanding at a constant rate?

Answer 85

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

Question 86

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

Answer 86

supposedly DARK ENERGY

Question 87

wha are the 3 possible outcomes of the universe?

Answer 87

flat universe, open universe and closed universe

Question 88

what is an open universe?

Answer 88

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

Question 89

what is an closed universe?

Answer 89

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 90

what is a flat universe?

Answer 90

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