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a levels > Astrophysics > Flashcards

Astrophysics Flashcards

1
Q

What is normal adjustment of a telescope

A

When the distance between the objective lens and the eyepiece lens is the sum of their focal lengths. Principle focus for these two lenses is in the same place

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2
Q

What is magnifying power

A

larger angle over smaller angle

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3
Q

What is chromatic aberration

A

Focal length of red light is greater than that of blue light so they are focussed at different points. This causes an image with coloured fringing

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4
Q

What is spherical aberration

A

The curvature of a lens or mirror causes rays of light at the edge to be focused in a different position to those near the centre, leading to image blurring and distortion

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5
Q

Where is spherical aberration more pronounced

A

Lenses with larger diameter

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6
Q

How to avoid spherical aberration

A

use parabolic dishes

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7
Q

What is an Achromatic doublet

A

A convex lens made of crown glass and a concave lens made of flint glass cemented together in order to bring all rays of light into focus in the same position

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8
Q

Disadvantages of Refracting telescopes(6)

A

-Glass must be pure and free from defects (hard to achieve for a large diameter lens)
-Large lenses can bend and distort under their own weight due to how heavy they are
-Chromatic and spherical aberration both affect lenses
- Incredibly heavy and therefore can be difficult to manoeuvre
-Large magnifications require very large diameter objective lenses.
-Lenses can only be supported from the edges which can be an issue when they are large and heavy

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9
Q

Advantages of reflecting telescopes(6)

A

-Mirrors that are few nanometres thick can be made which give great image quality
-Mirrors unaffected by chromatic aberration and spherical can be fixed(parabolic)
-Mirrors aren’t as heavy as lenses so easier to manoeuvre to follow objects
-Large composite primary mirrors can be made from lots of smaller mirror segments
-Large primary mirrors are easy to support from behind since you don’t need to be able to see through them

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10
Q

Are reflectors or refractors preferred in modern telescopes

A

Reflectors

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11
Q

How do radio telescopes work

A

They use radio waves to create images of astronomical objects.

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12
Q

Are radio telescopes ground based

A

yes

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12
Q

Why should radio telescopes be in isolated locations

A

To avoid interference from nearby radio sources

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12
Q

Why can radio telescopes be ground based

A

The atmosphere is transparent to a large range of radio wavelengths so it does not absorb them

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13
Q

What are the Similarities of radio telescopes and optical telescopes (4)

A

-Both function in same way. Intercept and focus incoming radiation to detect its intensity
-Both can be moved to focus on different sources of radiation or to track a moving source
-Parabolic dish of a radio telescope is extremely similar to the objective mirror of a reflecting optical telescope
-Both can be build on the ground

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14
Q

What are the differences between radio telescopes and optical telescopes(4)

A

-Radio telescopes have to be much larger in diameter than optical telescopes in order to achieve the same quality image (resolving power)
-Construction of radio is cheaper and simpler because wire mesh is used instead of a mirror
-Radio must move across an area to build up an image, unlike optical telescopes
-Radio experience a lot more man-made interference.

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15
Q

Which has a larger collecting power out of radio and optical telescopes and why

A

Radio because they have a much larger diameter

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16
Q

How do infrared telescopes work

A

Use infrared radiation to create images of astronomical objects. They consist of large concave mirrors which focus radiation onto a detector.

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17
Q

What precaution has to be taken to infrared telescopes because all objects emit infrared radiation as heat

A

They must be cooled using cryogenic fluids to almost absolute zero. They must also be well shielded to avoid thermal contamination from nearby objects.

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18
Q

What are infrared telescopes used for

A

To observe cooler regions in space

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19
Q

Are infrared telescopes ground based

A

No, they must be launched into space

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20
Q

Why aren’t infrared telescopes ground based

A

The atmosphere absorbs most infrared radiation

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21
Q

How do ultraviolet telescopes work

A

Use ultraviolet radiation to create images of astronomical objects. They uses the the cassegrain configuration to bring ultraviolet rays to a focus. The rays are detected by solid state devices which use the photoelectric effect to convert UV photons into electrons and then pass around a circuit

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22
Q

What are UV telescopes used to observe

A

The interstellar medium and star formation regions

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23
Q

Are UV telescopes ground based

A

No

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24
Q

Why are UV telescopes not ground based

A

Ozone layer blocks all ultraviolet rays that have a wavelength of less than 300nm

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25
Q

How do X-ray telescopes work

A

Use X-rays to create images of astronomical objects. They are made from a combination of parabolic and hyperbolic mirrors which are extremely small. rays enter the telescope, skim off the mirrors, and are brought into focus on CCDs which convert
light into electrical pulses.

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26
Q

What are x ray telescopes used for

A

To observe high energy events such as black holes

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27
Q

Are X ray telescopes ground based

A

no

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28
Q

Why are X ray telescopes not ground based

A

All x rays get absorbed by the earths atmosphere

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29
Q

How do Gamma telescopes work

A

They use gamma radiation to create images of astronomical objects. They use a detector made of layers of pixels. As the gamma photons pass through, they cause a signal in each pixel they come into contact with.

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30
Q

Why don’t gamma telescopes use mirrors

A

Gamma rays have so much energy that they would just pass straight through

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31
Q

Describe short lived GRBs (gamma ray bursts)

A

They last anywhere between 0.01 and 1 second. They are thought to be associated with merging neutron stars (forming a black hole), or a neutron star falling into a black hole

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32
Q

Describe Long lived GRBs (gamma ray bursts)

A

They last between 10 and 1000 seconds. They are associated with a Type II supernova (death of massive star)

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33
Q

What is collecting power

A

A measure of the ability of a lens or mirror to collect incident EM radiation

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34
Q

What is collecting power directly proportional to

A

Objective diameter squared

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35
Q

How does a greater collecting power affect the images

A

The images are brighter

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36
Q

What is resolving power

A

the ability of a telescope to produce separate images of close-together objects

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37
Q

What is required for an image to be resolved

A

the angle between the straight lines from the earth to each object must be at least the minimum angular resolution

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38
Q

what is angular resolution measured in

A

Radians

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39
Q

What does the Rayleigh criterion state

A

Two objects will not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other

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40
Q

What is the target like shape that light diffracts into in a telescope

A

airy disc

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41
Q

What is a CCD (charged coupled device

A

Array of light sensitive pixels, which become charged when they exposed to light by the photoelectric effect

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42
Q

What is quantum efficiency

A

the percentage of incident photons which cause an electron to be released

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43
Q

what is spectral range

A

detectable range of wavelengths of light

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44
Q

what is pixel resolution

A

Total number of pixels used to for the image on a screen

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45
Q

what is spatial resolution

A

the minimum distance two objects must be apart in order to be distinguishable

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46
Q

what is the quantum efficiency of a CCD

A

around 80%

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47
Q

what is convenience (with regard to ccds)

A

how easy the images are to form and use

48
Q

what is the spectral range of CCDs

A

Infrared, UV, visible

49
Q

what is the quantum efficiency of the human eye

A

between 4 and 5 percent

50
Q

what is the pixel resolution of a CCD

A

varies, but is around 50 mega pixels

50
Q

what is the spectral range of the human eye

A

Only visible light

51
Q

what is the spatial resolution of a CCD

A

10 micrometers

51
Q

what is the pixel resolution of the human eye

A

around 500 megapixels

52
Q

what is the spatial resolution of the human eye

A

100 micrometers

53
Q

what is the convenience of a CCD

A

Needs to be set up but images produced are digital

54
Q

what is the convenience of the human eye

A

simpler to use as there is no need for extra equipment

55
Q

define luminosity

A

rate of light energy released of a star ( power output of a star)

56
Q

define intensity

A

the power received from a star (luminosity) per unit area

57
Q

What does the intensity of a star follow

A

the inverse square law

58
Q

what is apparent magnitude

A

how bright the object appears in the sky

59
Q

what does the apparent magnitude of a star depend on

A

the stars luminosity and distance from earth

59
Q

what is the hipparcos scale used for

A

to classify astronomical objects by their apparent magnitudes

60
Q

what is the apparent magnitude of the brightest stars on the hipparcos scale

A

1

61
Q

what is the apparent magnitude of the dimmest stars on the hipparcos scale

A

6

62
Q

in the hipparcos scale if the magnitude changes by 1 what does the intensity change by

A

2.51

63
Q

define absolute magnitude

A

what somethings apparent magnitude would be if it were placed 10 parsecs away from earth

64
Q

define parallax

A

The apparent change of position of a nearer star in comparison to distant stars as a result of the orbit of the earth around the sun

65
Q

what is the astronomical unit

A

the average distance between the centre of the earth and the centre of the sun

66
Q

what is a parsec

A

the distance at which 1 au subtends an angle of 1 arcsecond

67
Q

what is a black body radiator

A

perfect emitter and absorber of all possible wavelengths of radiation

68
Q

what are hydrogen balmer lines

A

absorption lines which are caused by the excitation of hydrogen atoms from the n = 2 state to higher energy levels.

69
Q

which spectral classes are balmer lines found

A

O B and A

70
Q

why are hydrogen balmer lines not present when the temperature is too high

A

the majority of atoms will be excited to higher levels than n=2 or electrons might even become ionised

71
Q

why are hydrogen balmer lines not present when the temperature is too low

A

the hydrogen atoms are unlikely to become excited

72
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in O

A

-Blue
-25000-50000k
-He+ He H
- weak

73
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in B

A

-Blue
-11000-25000k
-He H
-Slightly stronger than O

74
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in A

A

-Blue/white
-7500-11000k
-H Ionised metals
-strongest

75
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in F

A

-White
-6000-7500 k
-ionised metals
-weak

76
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in G

A

-Yellow/ white
-5000-6000k
-Ionised and neutral metals
-none

77
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in K

A

-Orange
-3500-5000k
-Neutral metals
-None

78
Q

Colour, temp range, prominent absorption lines and prominence of hydrogen balmer lines in M

A

-Red
- <3500k
-Neutral atoms, Titanium oxide
-None

79
Q

Y axis of HR diagram

A

Absolute magnitude

80
Q

Y axis of HR diagram moving up in 5s

A

+15, +10, +5, 0, -5, -10, -15

81
Q

X axis of HR diagram

A

Temperature /K and spectral class

82
Q

What happens to main sequence star when it uses up all the hydrogen in its core

A

It moves up and to the right on the HR diagram and becomes a red giant

83
Q

How is a red giant different from a main sequence star

A

It is brighter and cooler

84
Q

What happens when a red giant uses up all the helium in its core

A

It will eject its outer layers and will move down and to the lift on the HR diagram to become a white dwarf

85
Q

How is a white dwarf different to a main sequence star

A

it is hotter and dimmer

86
Q

what is one solar mass

A

the mass of the sun

87
Q

stages of stellar evolution for star 1.4<solar mass<3

A

Nebula-Protostar-Main sequence-Red Giant-Supernova-neutron star

88
Q

Stages of stellar evolution for star <1.4 solar masses

A

Nebula-protostar-Main sequence- Red giant- white dwarf- black dwarf

89
Q

Stages of stellar evolution for star >3 solar masses

A

Nebula-protostar-Main sequence-Red supergiant-Supernova-Black hole

90
Q

Describe events of protostar

A

-clouds of gas and dust (nebula)have fragments that clump together under gravity to form denser centre(protostar)
-surrounded by circumstellar disc
-When protostar becomes hot enough it fuses elements producing a strong stellar wind

91
Q

Describe events of a Main sequence star

A

-Inward force of gravity and outward force due to fusion are in equilibrium
-Hydrogen nuclei fused into helium
-greater the mass of the star, the shorter its main sequence period as it uses fuel more quickly

92
Q

Describe the events of a red giant

A

-Once hydrogen runs out the temperature of the core increases and begins fusing helium nuclei to heavier elements
-outer layers of the star expand and cool

93
Q

Describe the events of a white dwarf

A

-When a red giant uses up all of its fuel the fusion stops and the core contracts as gravity is now greater than the outward force
-Outer layers are thrown off
-Core becomes very dense
-eventually cools to a black dwarf

94
Q

Describe the events of a red supergiant

A

-When a high mass star runs out of hydrogen nuclei the same process for red giant occurs on a larger scale
-The collapse of red supergiants in a supernova causes gamma ray bursts
-They can fuse elements up to iron

95
Q

describe the events of a supernova

A

-When all fuel runs out, fusion stops and the core collapses inwards
-outer layers fall inwards, rebound and sent into space in a shockwave
-As shockwave passes through surrounding material, elements heavier than iron are fused and flung out into space
-supernova has rapidly increasing absolute magnitude

96
Q

Describe the events of a neutron star

A

-Gravity is so strong that it forces protons and electrons together to form neutrons
-incredibly dense
-pulsars are spinning neutron stars

97
Q

describe the events of a black hole

A

-Core of giant star collapses, the neutrons are unable to withstand gravity forcing them together
-light cant escape
-event horizon is the point at which the escape velocity becomes greater than the speed of light
-Schwarzchild radius is the radius of the event horizon

98
Q

what is a binary system

A

system where two stars orbit a common mass

99
Q

Describe a type 1 supernova

A

when a star accumulates matter rom its companion star in a binary system and explodes after reaching a critical mass

100
Q

Describe a type 2 supernova

A

The death of a high mass star after it runs out of fuel

101
Q

3 ways that supermassive black holes can form

A

-collapse of massive gas clouds
-normal black hole that has accumulated huge amounts of matter
-several normal black holes merging together

102
Q

why is type 1a supernova dimmer than hubble expected

A

expansion of the universe is accelerating and is older than hubbles law estimates

103
Q

what is dark energy

A

Something that has overall repulsive effect throughout the whole universe causing the expansion of the universe to accelerate

104
Q

What is the doppler effect

A

The compression or spreading out of waves that are emitted or reflected by a moving source

105
Q

how does distance effect red shift

A

greater distance means greater red shift

106
Q

what colour shift is it if something is moving towards the earth

A

blue shift

107
Q

what colour shift is it if something is moving away from earth

A

red shift

108
Q

what is the units of d in the apparent magnitude and absolute magnitude equation

A

parsec

109
Q

how is doppler effect used to identify binary star systems

A

-As the stars eclipse each other, they are travelling perpendicular to the line of sight from the
observer, so there is no Doppler shift in their emitted radiation.
-However, when one star is travelling away from the observer, the other is travelling towards the observer. This causes each spectral line to be split into two, where one is blue-shifted and the other is red-shifted

110
Q

How do we know that quasars are very far away

A

They have very large red shift values

111
Q

what is the big bang theory

A

Universe began from one point (a singularity that was infinitely small and hot). The universe began with a huge explosion from this point

112
Q

How does background radiation provide evidence for the big bang

A

-Background radiation is the radiation coming from all parts of the universe
-The spectrum has a peak in the microwave region corresponding to a temperature of 2.7k
-It can be interpreted as the radiation left over from the big bang

113
Q

How do the relative abundancies of helium and hydrogen provide evidence for the big bang

A

-When universe was very young fusion occurred resulting in the production of helium from fusing hydrogen
-Fusion stopped as the universe then expanded and cooled
-resulting in relative abundance of hydrogen and helium in the ration of 3 to 1

114
Q

what is a quasar

A

supermassive black hole surrounded by a disc of matter

115
Q

3 characteristics of quasars

A

-very large red shift
-very powerful light output
-size not much bigger than a star

116
Q

what is the radial velocity method of detecting an exoplanet

A

-star and planet orbit a common centre of mass which causes the star to wobble slightly
-this causes a doppler shift in the light received from the star ( blue shifted when it moved towards earth and red shifted when it moves away
-The time period of the planets orbit is equal to the time period of the doppler shift

117
Q

what is the transit method of detecting an exoplanet

A

-When a planet crosses in front of a star (transits) the intensity dips slightly
-if the intensity of the star dips regularly then this is a sign that an exoplanet is orbiting it
-size and orbital period of planet can be determined from the amount that the intensity falls and the duration of the dip
-it only works if the line of sight to the star is in the plane of the planets orbit

a levels (5 decks)

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