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Physics > Option D Astrophysics (SL) > Flashcards

Option D Astrophysics (SL) Flashcards

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1
Q

how many planets are there in our solar system

A

8

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2
Q

name the planets in order

A

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

My Very Enthusiastic Mother Just Served Us Noodles

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3
Q

what is the hottest planet

A

venus

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4
Q

what are asteroids

A

lumps of rock mostly found between Mars and Jupiter in the astroid belt

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5
Q

what are comets

A
  • they are made of dust and ice
  • take hundreds of years to orbit the sun
  • they start to melt when they get close to the sun, this make a tail
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6
Q

What is a planetary system

A

a group of planets orbiting a sun

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7
Q

what are binary stars

A

a pair of stars orbiting their common centre of mass

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8
Q

how can binary stars be detected

A
  • visual - can be seen as two seperate stars
  • eclipsing - detected by periodic variations in brightness as one star obscures the other
  • spectroscopic - detected by changes in the wavelength of light received from each star (due to Doppler effect)
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9
Q

what is a stellar cluster

A

a close group of gravitationally bound stars, gas and dust

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10
Q

what is a globular stellar cluster

A
  • contains about 10,000 – 100,000 stars
  • symmetrically arranged and more closely packed in the centre.
  • contains OLD stars
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11
Q

what is open stellar cluster

A
  • contains several hundred stars
  • irregular in shape
  • contains YOUNG stars
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12
Q

Average distance between starts within a galaxy/average distance between galaxies =

A

10^-6

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13
Q

10^-6 =

A

Average distance between starts within a galaxy/average distance between galaxies

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14
Q

how can stars exist

A

singly or as binary stars

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15
Q

what is a galactic cluster

A

a group of galaxies gravitationally bound together

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16
Q

what is a supercluster

A

a larger group of galactic clusters

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17
Q

what is nebulae

A

an intergalactic cloud of dust and gas

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18
Q

what are constellations

A
  • where certain stars appear to make patterns in the sky.
  • it is important to realise that these stars are not necessarily close to each other.
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19
Q

what measurement do we use in the solar system

A

astronomical unit (AU) = 1.5x10^11m

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20
Q

what measurement do we use outside the solar system

A

light year = 9.46x10^15m

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21
Q

what is stellar parallax used for

A
  • to find the distances to stars we see in the night sky
  • it is used to determine the distance of stars relatively close to Earth (100 parsecs is max distance)
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22
Q

how does stellar parallax work

A
  • the star’s apparent position relative to background stars is noted at 6 month intervals
  • using trig: tan p = earth-sun distance÷sun-star distance
  • as p is a very small angle, tan p = sin p = p (in radians)
  • the angle p is so small it is measured in arc-seconds (1/3600th of a degree)
  • using d (parsec) = 1/p(arcsecond)
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23
Q

definition of parsec

A

the distance at which the angle subtended by the radius of the Earth’s orbit is one arc-second

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24
Q

what is parallax angle

A

half of the observed angular displacement of the star

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25
Q

definition of luminosity

A

the total amount of energy emmitted per second by a star
(unit: W)

this depends on the size and temperature of the star.

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26
Q

definition of apparent brightness

A

the amount of energy received per second per unit area by an observer (unit: Wm^–2)

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27
Q

can you do luminosity ratio questions?

A

Yes, continue
No, pg 15 booklet

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28
Q

what is stella spectra

A

a continuous spectrum of colour, however some wavelengths are missing, leaving an absorption spectrum

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29
Q

how can we use stella spectra

A

it can be used to find what elements make up a star as every element will adsorb a different wavelength of light, therefore the black lines are what elements make up that star.

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30
Q

what can stella spectra tell us

A

what elements make up the star (absorption spectrum)
the temperature of the star (wavelength max)
how the star is moving (doppler effect)

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31
Q

describe where red supergiants, red giants, white dwarfs and main sequence stars are on a hertzsprung-russell diagram

A

red giants: cool + luminous, top right
red giants: cool + luminous, middle-top right
main sequence: through the middle, a slight curvature
white dwarfs: hot + dim, bottom left

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32
Q

what are the axis on a hertzsprung-russell diagram

A

y-axis: logarithmic luminosity scale
x-axis: decreasing temperature

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33
Q

definition of main sequence stars

A

90% of all stars, “normal” stars, they fuse hydrogen into helium, difference between them is their masses, larger stars are more luminous

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34
Q

where does the sun lie in the hertzsprung-russell diagram

A

luminosity of 1
temperature of 5,400 kelvin

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35
Q

mass-luminosity relationship questions

A

see pg 20

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36
Q

describe red giants

A
  • red in colour
  • large mass
  • large surface area
  • are at a late stage in a star’s life
37
Q

describe red supergiants

A
  • red in colour
  • larger mass than red giant
  • larger surface area than red giant
  • are at a late stage in a star’s life
  • fusing Si into Fe
38
Q

describe white dwarfs

A
  • white in colour (relatively hot)
  • small mass
  • small surface area
  • final stage in the life cycle of smaller stars
  • no longer undergoing fusion (cooling down)
39
Q

describe cepheid variables

A
  • unstable
  • regular variations in luminosity (and brightness) due to a periodic variation in size
  • late stage in the life cycle of some stars
40
Q

why do cepheid variables vary in size?

A

helium absorbs radiation
––> helium heats up and expands (radiation pressure > gravitational force)
––> helium cools as it expands
––> grav force > rad pressure so star contracts
––> repeat

41
Q

what is a ‘standard candle’

A

cepheid variable star, because its luminosity can be determined and then used to calculate distance

“shark fin” curve
- rapid expansion
- slower contraction

42
Q

how to find distance using cepheid variables?

A

1) find period: measure variation in brightness of the cepheid variable star over time to determine its period
2) determine luminosity, the period is related to its average luminosity
3) use brightness and L to calculate d, use brightness formula to calculate d

43
Q

how is a protostar formed?

A
  • nebulae come together due to gravity
  • they lose potential energy and gain kinetic energy (temperature increases)
  • the temperature can cause ionisation of the gas
  • light is emitted ––> protostar is formed
44
Q

from protostar, how is a main sequence star formed

A

further contraction increases the temperature of core and fusion of H to He can occur, main sequence star is formed

stable phase as:
inwards gravitational pull = outwards radiation pressure

45
Q

what is nucleosynthesis

A

creation of different elements by fusion

46
Q

how is red giant formed?

A
  • eventually all the H in core of main sequence star has been converted to He via nucleosynthesis.
  • the larger the star the faster this happens.
  • the core contracts but H fusion continues in the outer layers and they expand creating a red giant
  • the contracting core gets hotter so He can now fuse into C and O.
47
Q

how do small stars, (initial mass < 4 solar masses) become their late stage

A
  • layers are ejected, called planetary nebula
  • leaving a very hot core behind
  • produces a white dwarf which cools over time
48
Q

how do medium stars, (initial mass 4–8 solar masses) become their late stage

A
  • further fusion with neon, sodium and magnesium
  • before ejecting outer layers (planetary nebula) leaving white dwarf
49
Q

define the Chandrasekhar limit

A

maximum mass of white dwarf is 1.4 solar masses. Due to electron degeneracy pressure (the electrons cannot be packed any closer by the gravitational force), any star above this mass, gravity would overcome electron repulsion and will start to collapse further

50
Q

how do large stars, (initial mass > 8 solar masses) become their late stage

A
  • form more elements through fusion, heavier in core, lighter in outer
  • creating red supergiant
  • silicon ––> iron, once the core is all iron, it contracts rapidly
  • outer layers are ejected as supernova.

remaining core forms neutron star or a black hole (if star was very large)

51
Q

maximum mass of neutron star

A

3 solar masses

52
Q

Oppenheimer-volkoff limit

A

the maximum mass of neutron star which is 3 solar masses

53
Q

rotating neutron stars

A

pulsars:
- neutron stars cool over time as they are no longer producing heat (like white dwarf)
- they will emit energy as radio waves in bursts

54
Q

drawing a star’s life cycle on an H-R diagram

A

for low mass star:

for high mass star:

see pg 27

55
Q

newton’s model of the universe

A
  • infinite in space
  • infinite in time
  • uniform
  • static
56
Q

Olber’s Paradox

A

if newton’s model of the universe was correct, then as there are an infinite number of stars why is the sky dark at night?

no matter how far away a star is, they will all be equally bright

57
Q

redshift definition

A

absorption lines due to the elements found in stars were found at longer wavelengths than expected

58
Q

the Big Bang model

A
  • everything in the universe was located at one point - a singularity
  • this point exploded creating space and time
  • since then the universe has been expanding, creating space as it does
59
Q

singularity definition

A

at some point in time, everything in the universe was located at one point.

60
Q

what is cosmic microwave background radiation (CMB)

A

a microwave radiation was coming towards Earth from every direction in space. CMB was found to have a wavelength that corresponded to that emitted by an object of 2.76K.

shows that the universe would have cooled to 2.76K after almost 15 billion years

61
Q

why is the sky dark due to the Big Bang theory

A
  • light from very distant stars has not yet reached us yet
  • as galaxies are moving away from us and their light is red-shifted, the light from very distant galaxies is shifted into the infra-red which is not visible to our eyes
62
Q

if the redshift ratio (z) is positive, …

if z is negative, …

A

+ve means that there is a redshift (galaxy moving away from Earth)

–ve shows a blueshift (galaxy moving towards Earth)

63
Q

Hubble’s law

A

the speed at which galaxies are moving away from Earth is directly proportional to their distance from Earth

64
Q

explain cosmic scale factor (R)

A

a way of quantifying the expansion that has taken place in the universe:

  • assume a galaxy emitted light of wavelength λº at some time in the past when the cosmic scale factor had a value of Rº
  • due to expansion of the universe the wavelength of this light when detected today would have increased to a new value λ, increasing by Δλ
  • the cosmic scale factor would have changed over this time period from Rº to R
  • this means that space has stretched by a factor of ΔR in the time the wavelength has increased by Δλ

λº––>λ (increase Δλ)
Rº––>R (increase ΔR)

65
Q

rate of expansion of the universe is:
a) increasing
b) decreasing
c) the same

A

a) increasing

evidence was found from Type Ia supernovae

66
Q

why is the universe expansion rate increasing?

A

dark energy

67
Q

what does tells you the universe is expanding?

A

light produced by galaxies is red-shifted. this showed that all galaxies observed were moving away from Earth

68
Q

proton-proton chain

A

stars like the sun undergo fusion from hydrogen to helium
this process is called proton-proton chain

69
Q

CNO cycle

A

stars with mass > 4 solar masses undergo hydrogen to helium
this is a different process called CNO cycle

70
Q

s-process

A

slow neutron capture
- neutrons captured individually
- time for beta-negative decay between captures
- large range of heavier elements formed (up to bismuth)
- occurs in the later stages of a massive star’s lifecycle

71
Q

r-process

A

rapid neutron capture
- large number of neutrons captured
- no time for beta-negative decay between captures
- more limited range of elements formed (beyond bismuth)
- occurs in type II supernovae

72
Q

time-mass relationship

A

t∝M^(-2.5)

73
Q

formation of type Ia supernovae

A

formed when a white dwarf in a binary system gains mass from its companion star and explodes due to fusion restarting
- used as a standard candle due to a known amount of light it emits, allowing distance to be worked out.

74
Q

formation of type II supernovae

A
  • result from the rapid collapse and violent explosion of a massive star (>8 solar masses)
75
Q

distinguishing type Ia supernovae on Earth

A
  • 10^10 times luminosity of sun
  • max luminosity reached rapidly and then gradually falls over 6 months or so
  • no hydrogen lines in spectrum but has a strong silicon line
76
Q

distinguishing type II supernovae on Earth

A
  • 10^9 times luminosity of sun
  • luminosity falls a little before reaching a slight plateau for several days then falls off rapidly
  • hydrogen lines in spectrum
77
Q

isotropic

A

same everywhere

78
Q

homogenous

A

looks the same in all directions

79
Q

critical density (pc)

A

density required for flat universe (gravity slows expansion but takes an infinite time to do so)

80
Q

density parameter (Ωo)

A

actual density / critical density
Ωo=1 flat (gravity slows expansion but takes an infinite time to do so)
Ωo>1 closed (gravity causes the universe to collapse back in on itself - the Big Crunch)
Ωo<1 open (gravity slows expansion but is not strong enough to stop it)

81
Q

derive pc=3Ho^2/8πG

A

Et= Ek+Ep
= 1/2 mv^2 – GMm/r
substitute M=4/3π r^3 p, v=Hor
then simplify
**Ho are in s^(-1) to convert to kms^-1Mpc^-1, ÷ by 3.24 x 10^–20

82
Q

temperature - comic scale factor

A

T∝1/R

83
Q

where is most matter located in a galaxy

A

centre

84
Q

derive v=√(4πGp/3)

A

Fg=Fc
GMm/r = mv^2/r
(M= 4/3π r^3 p)
v^2=4πGp/3

85
Q

for stars close to the centre of galaxy, v and r relation

A

v∝r

86
Q

evidence of dark matter

A

v∝r not v∝1/√r
so matter that is not observable must exist

87
Q

candidates for dark matter

A

MACHOS, WIMPS

88
Q

universe consists of

A

68% dark energy
27% dark matter
5% ordinary matter

89
Q

cosmic microwave background (CMB) contradictions

A

isotropic but can’t be as then there would be no structure
(matter uniformly distributed throughout space)

Physics (13 decks)

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