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Astrophysics Flashcards

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1
Q

What are the 2 types of lenses

A

Concave and Convex

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2
Q

What does a concave lens do

A

(diverging lens) these spread out an incident beam into a diverging emergent beam

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3
Q

What does a convex lens do

A

(Converging) These focus incident rays at a point

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4
Q

Define Focal Length

A

Distance between the focus and the centre of the lens

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5
Q

Define Real Image

A

An image that can be formed on a screen

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6
Q

Define Virtual Image

A

The image can’t be formed on a screen

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7
Q

When is a real image produced

A

When an object is further away from the lens that the focal length

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8
Q

When is a virtual image produced

A

When an object is closer to the lens than the focal length

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9
Q

What is Cassegrain telescope arrangement

A

A parabolic concave primary mirror and a and a convex secondary mirror

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10
Q

Formula for magnification

A

M=Angle subtended by image at eye/ Angle subtended by object at unaided eye

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11
Q

In normal adjustment what is the angular magnification given by

A

The ratio of the focal length of the objective lens to the focal length of the eyepiece lens

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12
Q

Advantages of Refracting Telescope (x2)

A
  • Because of their simple design they are easier to use and more reliable
  • The system is more resistant to misalignment
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13
Q

Disadvantages of a Refracting Telescope (x4)

A
  • Chromatic Aberration
  • Impurities in the glass absorb some of the light, meaning very faint objects can’t be seen
  • Large lenses are very heavy and can only be supported by their edges, leading to a distorted shape
  • Large magnifications require large objective lenses and very long focal lengths
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14
Q

What is Chromatic Aberration and why does it happen

A

-It is the failure of light to focus at a point due to the lens focusing different colours over a range of focal lengths. Violet bends most, while red bends least

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15
Q

What is spherical aberration

A

It is the light rays being focused at slightly different positions due to the curvature of the lens/mirror

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16
Q

Advantages of a reflecting telescope (x3)

A
  • Mirrors are lighter than lenses and therefore the telescope can easily be supported
    -Mirror surfaces can be made very thin, giving excellent image properties
    -Doesn’t suffer from chromatic aberration
    -
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17
Q

Disadvantages of reflecting telescopes (x2)

A
  • Can suffer from spherical aberration

- If the shape of the mirror isn’t quite parabolic, parallel rays reflecting off don’t all converge at the same point

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18
Q

Characteristics of Radio Telescopes

A
  • Most are manoeuvrable allowing the source of the waves to be tracked
  • Wavelength of radio waves is much longer than the wavelength of visible light- so for the same resolving power it must have a much bigger dish
  • Easier construction than optical telescopes, as wire mesh can be used since the long wavelength waves won’t notice the gaps
  • Located away from artificial sources of radio waves, but can be ground based
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19
Q

Where would UV telescopes be placed and why

A

In space, as UV is absorbed by ozone

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20
Q

Where would visible light telescopes be located and why

A

On earth, above cloud level- so that its away from clouds and light pollution

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21
Q

Where would an infrared telescope be located and why

A

High altitude, dry areas, such as in deserts as infrared is absorbed by water vapour in the atmosphere

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22
Q

Where would a radio wave telescope be located and why

A

Surface of air in a isolated area, as needs to be away from other radio waves

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23
Q

Define Resolving power

A

Smallest angle of separation at which 2 points can be distinguished (its the ability to produce separate images of closely spaced objects)

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24
Q

What is resolution limited by

A

Diffraction

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25
Q

In the diffraction pattern of concentric circles caused by the aperture, what is the central circle called

A

The airy disc

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26
Q

State the Rayleigh Criterion

A

2 point objects can be resolved if there angular separation is at least:θ ≈ λ/D. If the airy disc from one star coincides with the first diffraction minimum of the other, they can just about be resolved

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27
Q

What is collecting power proportional to

A

Collecting power is directly proportional to the square of the diameter

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28
Q

What does CCD stand for

A

Charge coupled Device

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29
Q

What is a CCD

A

A semi-conductor device where light is converted into digital information

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30
Q

What is quantum efficiency

A

The number of photons detected/ Number of photons incident x100

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31
Q

What is the quantum efficiency of a CCD compared to the eye

A

CCD=80%

Eye=4-5%

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32
Q

What does a high Quantum Efficiency mean

A

That the time needed to acquire an image of the same intensity relative to other imaging devices is much smaller, so CCDs require shorter exposure times

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33
Q

Compare the Eye to a CCD

A
  • The eye can only detect visible light, whereas CCDs can detect infrared, visible and UV light
  • CCDs are better for capturing fine detail as their spatial resolution is much smaller
  • CCDs produce digital images which can be stored, copied and shared globally, however the eye doesn’t need any extra equipment
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34
Q

Define Apparent Magntiude

A

Based on how bright objects appear from earth

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35
Q

Define Absolute Magnitude

A

The apparent magnitude it would have at a distance of 10 parsecs from an observer

36
Q

What does the brightness of a star depend on

A

Luminosity and distance from us

37
Q

What is luminosity

A

The total energy emitted per second

38
Q

What is intensity

A

The intensity of an object is the power received from it per unit area at Earth

39
Q

What is the difference in intensity between each magnitude star on the Hipparcos Scale

A

2.512, the fifth root of 100

40
Q

On the Hippacros scale which magnitude stars have the greatest intensity

A

those we a negative apparent magntide

41
Q

What in the inverse square law formula for apparent brightness

A

P=L/4πd^2
P=Apparent Magnitude
L=Luminosity
d=dimeter

42
Q

Why is brightness a subjective scale of measurements

A

Because we need to know how far away a star is from us in order to know its tree luminosity. Also apparent magnitude refers to the brightness of a star in the visible part of the spectrum

43
Q

Why may the luminosity of very hot stars be greater than we can detect in the visible spectrum

A

Because very hot stars radiate much of their power outside the visible spectrum

44
Q

what is 1AU

A

1 Astronomical Unit, is the mean distance between the Earth and Sun

45
Q

What is 1ly

A

1 light year, is the distance light travels in a year

46
Q

Define the Parsec

A

The distance from which 1AU subtends an angle of 1 arc second

47
Q

IF you have a large parallax angle what does this tell you about the distance to the object

A

The greater the angle= the nearer the object is to you

48
Q

State Stefan’s Law

A

The luminosity of a star is directly proportional to its surface area, and to the fourth power of its temperature

49
Q

State Wien’s LAw

A

The hotter the black body, the shorter the peak wavelength of the curve

50
Q

What is a black-body

A

A body that absorbs all wavelengths of electromagnetic radiation and can emit all wavelengths of electromagnetic radiation

51
Q

Points for Drawing a Black-Body Curve

A

It mustn’t start as zero, because a wave with zero wavelength has infinite frequency therefore it has infinite energy, and this is not possible

  • The left hand side must be steeper than the right-hand side
  • The end of the right hand-side should be an asymptote
  • Mark on the peak wavelength
52
Q

What is the area under a black body curve approximately equal to

A

The power. And the area under the curve in the visible spectrum is equal to the brightness

53
Q

What does the assumption that a star is a black body mean

A

It produces a continuous spectrum

54
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class O

A

Blue
25,000-50,000
Helium+ Helium and Hydrogen

55
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class B

A

Blue
11000-25000
Helium and Hydrogen

56
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class A

A

Blue-White
7,500-11,000
Hydrogen Strongest
Ionised Metals

57
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class F

A

White
6,000-7,500
Ionised Metals

58
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class G

A

Yellow-White
5,000-6,000
Ionised and Neutral Metals

59
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class K

A

Orange
3,5000-5,000
Neutral Metals

60
Q

Colour, Temperature and Prominent Absorption Lines for Spectral Class M

A

Red
<3500
Neutral Atoms
Molecular Bands

61
Q

How are Hydrogen Balmer Absorption Lines produced

A
  • light from the star passes through the atmosphere of the star (1) which contains hydrogen with electrons in n = 2 state (1)
  • electrons in this state absorb certain energies and (hence) frequencies of light (1)
  • the light is re-emitted in all directions, so that the intensity of these frequencies is reduced in any given direction, resulting in absorption lines (1)
62
Q

Describe the general shape of the Hertzsprung Russel Diagram and give the main characteristics of stars in the 3 stages

A

Long Diagonal Band= MAin Sequence, stars in this band are in their long stable phase where they are fusing hydrogen and Helium

Top Right= Red Giant and Red Super Giants. These have a low absolute magnitude and relatively low temperature and a huge surface area

Bottom Left= White Dwarfs, these have a high absolute magnitude, a high temperature and are small. They are at the end of their lives, where all the fusion reactions have stopped and they are slowly cooling down

63
Q

On a HR diagram what should be plotted on each axis and what range of values should be marked

A

Y axis: Absolute Mag 15 to -10

x-Axis: Surface Temperature 50,000 to 2500K

64
Q

Where is the SUn on the HR diagram

A

Main Sequence 5800K and an absolute magnitude of approximately 5

65
Q

Defining Properties of a Supernova

A

Large and rapid increase in absolute magnitude

66
Q

Defining Properties of a Neutron Star

A

Made of neutrons, very dense and rapidly rotating

67
Q

Defining properties of a black hole

A

Volume of space within which Escape Velocity>Speed of Light

68
Q

Why do Gamma rays burst as a supergiant changes stage

A

Due to the collapse of supergiant stars to form neutron stars or black holes

69
Q

Compare the energy released by a Supernova to the total energy output of the sun

A

In a Type 1a Supernova around 10^44J of energy are released, which is the same as the energy output of the sun over its entire lifetime

70
Q

What is a standard candle

A

A source with a known luminosity

71
Q

What is a type 1a Supernova

A

A standard candle with a known peak absolute magnitude of -19.3

72
Q

2 defining features of the light curve for a type 1a supernovae

A

A sharp initial peak

A gradually decreasing curve

73
Q

What is the event horizon

A

The surface at which escape velocity=Speed of Light

74
Q

Properties of the singularity

A

Finite Mass, Zero Volume and Infinite density

75
Q

What does the Schwarzschild Radius Calculate

A

Radius of the event horizon

76
Q

What is the Doppler Effect

A

The apparent change in the frequency of a wave caused by relative motion between source of wave and observer

77
Q

What is a Quasar

A

They are some of the most distant and luminous objects in the universe, first detected using radio telescopes
They show a large optical red shift
Form from supermassive black holes

78
Q

What does the Hubble Constant Represent

A

Gives the ratio of the recessional velocity to distance from Earth

79
Q

Evidence for the Big Bang

A
  • Cosmological Microwave Background Radiation

- Relative Abundance of Hydrogen and Helium

80
Q

Why were Quasars discovered

A

Due to their bright radio source

81
Q

What is an exoplanet

A

Any planet not in our solar system

82
Q

Why are exoplanets hard to find

A

1) They are orbiting stars which are much brighter than them- the bright light from the stars they’re orbiting drowns out any light from the exoplanet
2) They are too small to distinguish from nearby stars, as the resolving power of our telescopes is too low

83
Q

Methods of detecting Exoplanets

A

1) Radial Velocity
2) Transit Method
3) Infrared Direct Imaging

84
Q

Describe the radial velocity method for detecting exoplanets

A

-It measures how much the emissions from the stars have been red or blue shifted
Planet and star orbit around common centre of mass that means the star to moves towards/away from Earth as planet orbits  Causes shift in wavelength of light received from star 
-From this the minimum mass of the exoplanet can be calculated

85
Q

Describe how the transit method can be used to detect exoplanets

A

-Measures the change in apparent magnitude as an exoplanet travels in front of a star
When planet passes in front of star (as seen from Earth), some of the light from star is absorbed and therefore the amount of light reaching Earth reduced 
Apparent magnitude is a measure of the amount of light reaching Earth from the star 
-From this the radius of the exoplanet can be found
-Issue is that the probability of the 2 paths matching up is low

86
Q

The life cycle of a star of a similar size to our sun

A

1) Stars are formed from large clouds of dust
2) Clouds of dust heat up and contract to form a protostar
3) When it reaches critical density- the outward pressure from fusion balances the stars gravity the star is in main sequence
4) When hydrogen runs out the material around the core heats up hydrogen sell burning, helium core burning=Red Giant
5) When the helium runs out and outer layer shed= Planetary Nebula
6) White Dwarf

87
Q

Some scientists are concerned about the consequences for the Earth of a supernova occurring in a nearby part of the galaxy.

Explain the cause of this concern

A

Collapsing star can produce gamma ray bursts with energy similar to total output of Sun 
Highly collimated – if in direction of Earth, could cause mass extinction event 

Physics (9 decks)

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