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AQA A-Level Physics > Astrophysics > Flashcards

Astrophysics Flashcards

1
Q

What are convex/converging lenses?

A

Convex/converging lenses are lenses which focus incident light – the light rays converge.

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2
Q

What are concave/diverging lenses?

A

​Concave/diverging lenses are lenses which spread out incident light – the light rays diverge.

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3
Q

Optical Telescopes:

What is the principal axis?

A

The principal axis is the line passing through the centre of the lens, perpendicular to its surface.

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4
Q

Optical Telescopes:

What is the principal focus?

A

Principal Focus (F):​

In a converging lens: the point where incident rays travelling parallel to the principal axis will converge.

In a diverging lens: the point from which the light rays appear to come

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5
Q

Optical Telescopes:

What is the focal length?

A

The focal length is the distance between the centre of the lens and the principal focus.

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6
Q

What do converging lenses do?

A

Converging lenses bring light rays together.

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7
Q

What do concave lenses do?

A

Concave lenses spread out light rays.

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8
Q

Optical Telescopes:

What is meant by ‘real image’?

A

A real image is formed when light rays cross after being refracted by a lens.

Real images can be projected onto a screen.

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9
Q

Optical Telescopes:

What is meant by ‘virtual image’?

A

A virtual image is formed on the same side of the lens as the object.

The light rays do not cross after refraction, so the image cannot be projected onto a screen.

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10
Q

Optical Telescopes:

If the object is _____ than the _____ _____ away from the lens, the image is _____.

If the object’s _____, the image is _____.

A

Optical Telescopes:

If the object is (further) than the (focal length) away from the lens, the image is (real).

If the object’s (closer), the image is (virtual).

Lens equation:

1/f = 1/u + 1/v

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11
Q

Optical Telescopes:

What are refracting telescopes?

A

Refracting telescopes are telescopes which use lenses to focus incident light.

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12
Q

Optical Telescopes:

What are reflecting telescopes?

A

Reflecting telescopes are telescopes which use mirrors to focus incident light onto an eyepiece lens.

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13
Q

Optical Telescopes:

How do refracting telescopes work?

A

Refracting telescopes use two converging lenses:

Objective lens converges the rays from the object to form a real image.

Eye lens acts as a magnifying glass on this real image to form a magnified virtual image.

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14
Q

Optical Telescopes:

How do reflecting telescopes work?

A

Reflecting telescopes use two mirrors and a converging lens:

A parabolic concave mirror (primary mirror) converges parallel rays from an object, forming a real image.

An eye lens magnifies the image, same as in refracting telescopes.

The principal focus is in front of the mirror, so a convex secondary mirror is used so the observer doesn’t block out the light.

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15
Q

Optical Telescopes:

What is the Cassegrain arrangement?

A

Cassegrain Arrangement:

In reflecting telescopes, the principal focus of the mirror is in front of the mirror, so an arrangement needs to be so that the observer doesn’t block out the light.

To overcome this, a secondary convex mirror is used.

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16
Q

Define “absolute magnitude”.

A

Absolute Magnitude: The apparent magnitude that an object would have if it were placed at a distance of 10 parsecs away from Earth.

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17
Q

Define “apparent magnitude”.

A

Apparent Magnitude: How bright an object appears in the sky. This depends on the object’s brightness and its distance from Earth.

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18
Q

What is a parsec?

A

The distance to nearby stars can be measured in parsecs (pc).

Imagine two lines that connect two opposite points of orbit of the Earth around the Sun with a nearby star.

The angle of the lines to the normal of the Sun is the angle of parallax.

A star is exactly 1pc away from Earth if the angle of parallax, θ, is:

θ = 1 arcsecond = (1/3600)°

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19
Q

What is an astronomical unit?

A

An astronomical unit, AU, is the mean distance between the Earth and the Sun.

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20
Q

What is a light-year, (ly)?

A

A light-year is the distance that EM waves travel through a vacuum in one year.

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21
Q

What is a black body radiator?

A

A black body radiator is a perfect emitter and absorber of all possible wavelengths of EM radiation.

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22
Q

Define “luminosity”.

A

Luminosity​ is the rate of light energy released by a star.

Luminosity = Power output of a star

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23
Q

What does the luminosity of a star depend on?

A

Luminosity depends on:

  1. Temperature
  2. Surface Area
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24
Q

What is Stefan’s Law?

A

Stefan’s Law

The power output is proportional to the fourth power of a star’s temperature and is directly proportional to the surface area.

P = σAT^4 given in exam

P = power output in W
σ = stefan's constant
A = surface area in m^2
T = surface temperature in K
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25
Q

What is Wein’s Displacement Law?

A

Wein’s Displacement Law

The peak wavelength of emitted radiation is inversely proportional to its absolute temperature.

λ(max) T = constant = 2.9 x 10^(-3) mK given in exam

metre-kelvin

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26
Q

Why is it hard to get accurate measurements in astrophysics?

A
  • Dust and man-made light pollution - observatories are placed at high altitudes, well away from cities in low-humidity climates to contend this. Best solution is to send up satellites that take measurements above the atmosphere.
  • Measuring devices that astronomers use aren’t perfect since their sensitivity depends on the wavelength. Choose appropriate materials for what you want to measure and carefully calibrate your instruments.
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27
Q

What is the Balmer series?

A

The Balmer series is a series of lines (spectrum) formed from the excitation of hydrogen atoms from the n=2 level, due to the visible light emitted. (Due to wavelength)

28
Q

What does the Balmer series show us?

A
  • Hydrogen atoms release visible light when electrons are excited to n=2.
  • This happens at high temperatures, where collisions give electrons extra energy.
  • If the temperature is too high, the electrons will reach n=3 instead, and not release visible light.

• SO, the itensity of Balmer lines depends on the temperature of the star.

29
Q

List the spectral classes for stars.

A

Hottest

O
B
A
F
G
K
M

Least hot

‘Oh Be A Fine Girl, Kiss Me’

30
Q

What are the characteristics for O stars?

A

O Stars

  • Blue
  • Temperature: 25,000 - 50,000K
  • Strongest spectral lines are helium ion and helium atom absorptions, since they need a really high temperature.
31
Q

What are the characteristics for B stars?

A

B Stars

  • Blue
  • Temperature: 11,000 - 25,000K
  • Spectra show strong helium atom and hydrogen (Balmer) absorptions.
32
Q

What are the characteristics for A stars?

A

A Stars

  • Blue-white
  • Temperature: 7500 - 11,000K
  • Strongest spectral lines are Balmer hydrogen lines.
33
Q

What are the characteristics for F stars?

A

F Stars

  • White
  • Temperature: 6000 - 7500K
  • Spectra has strong metal ion absorptions.
34
Q

What are the characteristics for G stars?

A

G Stars

  • Yellow-white
  • Temperature: 5000 - 6000K
  • Spectra has metal ion and metal atom absorptions.
35
Q

What are the characteristics for K stars?

A

K Stars

  • Orange
  • Temperature: 3500 - 5000K
  • Spectra has mostly neutral metal atom absorptions.
36
Q

What are the characteristics for M stars?

A

M Stars

  • Red
  • Temperature: <3500K
  • Spectra shows absorptions from compounds like TiO, since they’re cool enough for molecules to form.
37
Q

What is the H-R diagram?

A

H-R Diagram

Hertzsprung and Russel noticed that a graph of absolute magnitude against temperature showed distinct areas.

38
Q

What are the 3 main areas in the H-R diagram?

A

H-R Diagram

Main Sequence
Red Giants & Red Supergiants
White Dwarfs

39
Q

Describe the main sequence band in the H-R diagram.

A

Main Sequence:

  • Long diagonal band
  • Stable phase
40
Q

Describe the red giants & red supergiants band in the H-R diagram.

A

Red Giants & Red Supergiants:

  • Top-right corner
  • High luminosity
  • Low surface temperature
41
Q

Describe the white dwarfs band in the H-R diagram.

A

White Dwarfs:

  • Bottom-left corner
  • Low luminosity
  • High temperature
42
Q

Describe the x-axis of a H-R diagram.

A

x-axis:

  • Temperature, K
  • Starts at 50,000K
  • Decreases to 2500K
43
Q

Describe the y-axis of the H-R diagram.

A

y-axis:

  • Absolute magnitude
  • Starts at 15
  • Decreases to -10
  • Brightness is increasing
44
Q

How are stars born?

A
  • Stars are born in a cloud of dust and gas
  • Denser clumps of cloud contract slowly under gravity
  • Cloud fragments into regions called protostars
  • Continue to contract and heat up
  • Temperature reaches a few million degrees
  • Hydrogen nuclei start to fuse together to form Helium
  • Fusion releases an enormous amount of energy
  • Creates enough radiation pressure to stop gravitational collapse
45
Q

What are the 4 main stages of main sequence stars?

A

Stages of Main Sequence Stars:

  1. Core hydrogen burning
  2. Shell hydrogen burning
  3. Core helium burning
  4. Shell helium burning
46
Q

Describe the 1st stage of a main sequence star: core hydrogen burning.

A
  1. Core Hydrogen Burning

• Pressure produced from hydrogen fusion in the core balances the gravitational force trying to compress the star.

47
Q

Describe the 2nd stage of a main sequence star: shell hydrogen burning.

A
  1. Shell Hydrogen Burning
  • All hydrogen in core has fused into helium
  • Nuclear fusion stops - outward pressure stops
  • Helium core contracts and heats up
  • Outer layers expand and cool
  • Star becomes a RED GIANT
  • Material surrounding the core still has hydrogen
  • Heat from contracting helium core raises temperature of this material
  • Hydrogen can fuse again
48
Q

Describe the 3rd stage of a main sequence star: core helium burning.

A
  1. Core Helium Burning
  • Helium core continues to contract
  • Gets hot and dense enough for helium to fuse into carbon and oxygen
  • Energy pushes the outer layers of the star outwards
49
Q

Describe the 4th stage of a main sequence star: shell helium burning.

A
  1. Shell Helium Burning
  • Helium runs out
  • Carbon-oxygen core contracts again
  • Heats a shell around it so helium can fuse
50
Q

A magnitude 1 star has an intensity ____ times ____ than a magnitude 6 star.

A

A magnitude 1 star has an intensity (100) times (greater) than a magnitude 6 star.

51
Q

What does apparent magnitude depend on?

A

Apparent magnitude depends on luminosity and distance.

52
Q

What does absolute magnitude depend on?

A

Absolute magnitude depends on luminosity.

53
Q

How are White Dwarfs formed?

A
  • In low-mass stars, the carbon-oxygen core isn’t hot enough for any further fusion, and so continues to contract
  • The core contracts to about the size of the Earth
  • Helium shells becomes more unstable
  • Star pulsates and ejects outer layers out into space as a planetary nebula, leaving the core behind
  • Star is now a very hot, dense solid called a WHITE DWARF, which will cool down and fade away
54
Q

Where a star is on a Hertzsprung-Russell diagram _____ as it _____.

A

Where a star is on a Hertzsprung-Russell diagram (changes) as it (evolves).

55
Q

What happens when a high mass star reaches the end of its life?

A

High mass stars explode into supernovas when it runs out of hydrogen nuclei.

Collapse of red supergiants into supernova causes gamma ray bursts.

56
Q

What is Rayleigh’s criterion?

A

Rayleigh’s criterion states that 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.

θ ≈ λ / D

57
Q

What is the doppler effect?

A

Doppler Effect

The apparent change in the wavelength of a wave as the source moves relative to an observer.

For a source moving away the wavelength increases (red shift)

For a source moving towards the observer the wavelength decreases (blue shift)

58
Q

Give an example of the doppler effect.

A

Imagine an ambulance is driving past you.

As it moves towards you, its siren sounds high pitched, but as it moves away, the pitch is lower.

This change in frequency and wavelength is called the Doppler shift.

59
Q

Explain the doppler effect.

A

The frequency and the wavelength change because the waves bunch together in front of the source and stretch out behind it.

The amount of stretching or bunching together depends on the velocity of the source.

60
Q

What is the red shift?

A

Red Shift

When a light source moves away from us, the wavelengths of its light become longer and the frequencies become lower.

This shifts the light towards the red end of the spectrum.

61
Q

What is blue shift?

A

Blue Shift

When a light source moves towards us, the wavelengths of its light become shorter and the frequencies become higher.

This shifts the light towards the blue end of the spectrum.

62
Q

What are the equations for doppler shift?

A

For v &laquo_space;c, (v is much less than c)

Δf / f = - Δλ / λ = v / c

f  = emitted frequency
Δf = f(emitted) - f(observed)
λ = emitted wavelength
Δλ = λ(emitted) - λ(observed)
v = how fast source is moving away from observer
c = speed of light
63
Q

What are the units for z, doppler shift?

A

z doesn’t have any units - it’s just a magnitude.

64
Q

What is the HBB?

A

HBB: The Hot Big Bang Theory

The universe started off very hot and very dense and has been expanding ever since.

65
Q

What is a binary star system?

A

Binary Star System:​ Two stars orbiting a common centre of mass.

66
Q

How can the orbital period be calculated by the doppler effect for binary stars?

A

By observing how the absorption lines in the spectrum change with time, the orbital period can be calculated.

As the stars orbit each other, the separation between the lines on the spectra goes from zero (1 line) to a maximum (2 lines) and back to zero again in half a period.

AQA A-Level Physics (14 decks)

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