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AQA A level Physics > Astrophysics > Flashcards

Astrophysics Flashcards

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1
Q

Principle Axis of a lense

A

An imaginary line that passes through the centre of a lens and through the centres of curvature of the faces of the lens

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2
Q

Focal Point of a lense

A

The point at which rays parallel to the principal axis of the lens are brought to a focus

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3
Q

Focal Length

A

The focal length of a lens is the point at which rays parallel to the principal axis of the lens are brought to a focus

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4
Q

How does an astronomical telescope ray diagram look

A
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5
Q

Spherical Abberation ray diagram

A

Rays from a distant object are not brought to a focus at a single point

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6
Q

Chromatic abberation ray diagram

A

Different colours of light are refracted by different amounts

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7
Q

What are the advantages of a refracting telescope over a reflecting telescope

A
  • The lenses in a refracting telescope are held in place by a metal tube. So little maintenance is required. The mirror in a reflecting telescope s exposed to the air and might need recoating
  • The mirrors in a small reflector can get out of alignment if the telescope gets knocked. So sometimes the mirrors just need adjustment. The strong constructuon of the refracting telescope makes such misalignment less likely

The second mirror in a relecting telescope has the disadvantage of blocking some of the light from entering the primary mirror

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8
Q

What are the advantages of a reflecting telescope over a refracting telescope

A
  • A good astronomical telescope requires a diameter of about 15cm or more, so that sufficient light is gathered. It is difficult to make a high quality lens of diamater 15cm, easier to make a concave mirror of that size
  • Reflecting mirror has no chromatic abberation, because light is reflected over a metal surface without passing through glass
  • Sphericak aberration can be reduced more easily in a relecting telescope by making the concave mirror parabolic shape
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9
Q

Explain whether sources can be resolved when two sources, emitting wavelength λ, have angular seperation θ are viewed through an aperture of diameter D

A
  1. If θ > λ/D - the sources can be resolved
  2. If θ = λ/D​ - the sources can just be resolved
  3. If θ < λ/D​ - the sources cannot be resolved
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10
Q

What is a CCD

A
  • charge coupled devices
  • a CCD is a slice of silicon that stors electrons freed by the energy of incoming photons
  • Charge on the electrons builds up an image as a pattern of pixels
  • CCDs have a very high quantum effciiency.
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11
Q

What is quantum efficiency

A
  • Means a high percentage of photons that strike the CCD produce charge carriers, that are then detected

Quantum Efficiency = number of electrons produced per second/ number of photons absorbed per second

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12
Q

Give the relative quantum efficiencies

A

Eye - 1-4%

Film - 4-10%

CCD - 70-90%

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13
Q

Brightness

A

The brightness of a star is a measure of how much visible light from the star reaches our eyes

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14
Q

Luminosity

A

The luminosity of a star is the energy it emits per second

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15
Q

Apparent Magnitude

A

A star’s apparent magnitude is a measure of its brightness as it appears in the sky

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16
Q

Parralax

A

Nearby objects apear to move relative to far-away objects when viewed from a different angle

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17
Q

Aborption spectrum

A

This spectrum is seen as a series of dark lines in a continous spectrum, when some elements absorb speicfic wavelenths of light.

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18
Q

Name the absorption spectrum spectral classes

A
  1. O
  2. B
  3. A
  4. F
  5. G
  6. K
  7. M
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19
Q

O stars

A

Colour: blue

Temperature: 25,000-50,000K

size: very large and massive

Spectra: helium and ultraviolet light

Example: Zeta

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20
Q

B stars

A

Colour: blue

Temperature: 11,000 - 25,000

size: large and massive

Spectra: helium and hydrogen

Example: Rigal

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21
Q

A stars

A

Colour: blue-white

Temperature: 7500-11000K

size: moderate size and very luminous

Spectra: Strong hydrogen lines, ionized metals

Example: Sirius

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22
Q

F stars

A

Colour: white

Temperature: 6000-7500K

size: 1.2-1.6 times bigger than the sun

Spectra: weak hydrogen lines, strong calcium + other ionised metals

Example: Canopus

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23
Q

G stars

A

Colour: yellow-white

Temperature: 5000-6000K

size: 0.8-1.1 times bigger than the sun

Spectra: weak hydrogen lines, neutral ionized metals

Example: Capella

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24
Q

K stars

A

Colour: Orange

Temperature: 3500-5000K

size: smaller and cooler than the sun

Spectra: faint hydrogen lines, strong neutral metallic lines

Example: Alpha Tauri

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25
Q

M stars

A

Colour: red

Temperature: <3500K

size: half the size of sun

Spectra: Neutral atoms, titanium oxide

Example: Antares + Betelgeuse

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26
Q

Explain the Hertzsprung-Russel diagram

A
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27
Q

Lifecycle of a star

A
28
Q

Nebula

A
  • stars are formed in a Nebula
  • A Nebula is a very large cloud of gas and dust in space
29
Q

Protostars

A
  • Gravity makes dense region of gas more compact
  • soon take on definite shape and are called proto stars
  • Once the core of the protostar reaches a certain temperature, nunclear fusion begins and the protostar ignites to become a star
  • A star is a ball of plasma undergoing nuclear fusion and gives off large amounts of energy in the form of electromagnetic radiation
  • As long as there is a nuclear reaction taking place, the internal forces will balance the external forces
30
Q

Main sequence stars

A
  • A star in which hydrogen burning takes place
  • This thermonuclear fusion of hydrogen nuclei into helium nuclei
31
Q

Why are more massive stars more luminous than smaller stars

A

Gravitational forces that thend to collapse a star increase with mass

So for the star to be in equilibirum, it means the outward pressure from the core must be larger

Therefore, the nuclear reactions must run at a higher rate generating more power, which leads to the star having a higher luminosity

32
Q

Red Giant

A

When the hydrogen in a main sequence star is consumed, fusion stops and the forces suddenly become unbalanced

Mass and gravity cause the remaining gas to collapse on the core

The collapsing outer-layers cause the core to heat up and the fusion of helium into carbon begins

The forces regain balance and the outer shell expands rapidly

A planetary nebula is created when this Red Giant blows off its outer layers after it has run out of carbon to burn. These outer layers of gas expand into space, forming a nebula which is often the shape of a ring or bubble.

33
Q

Red Supergiant

A

If the mass of a star is 3 times that of our sun or greater, than the Red Giant will become a Red Supergiant

When the massive Red Giant fuses all of the helium into carbon, fusion stops and the outer layers collapse on the core

34
Q

Supernova

A

The Core of the supergiant will then collapse in less than a second, combine protons and electrons into neutrons (creating a neutron star core)

Other matter rebounds off the core and fusion again starts in the outer star layers causing a massive explosion called a supernova

In a supernova, a massive shockwave is produced that blows away the outer galaxies of the star

35
Q

White Dwarf

A

Planetary Nebula breaks down to its core

A white dwarf is very hot when it forms, but because it has no source of energy, it will gradually cool as it radiates its energy.

This means that its radiation, which initially has a high color temperature, will lessen and redden with time.

As it cools, the light it gives off will fade and form a Black Dwarf

36
Q

Neutron star

A

Under the intense gravitational forces in a Supernova, the core collapses under the pull of gravity

The core rises to temperatures as high as 100 billion Kelvin

In such high temperatures and pressures, proyons and electrons can combine, in a reverse beta decay, to form neutrons and neutrinos

In this way the centre of the core turns into a ball of neutrons wich will become a neutron star

37
Q

Black Holes

A

If the mass of the surviving star is greater than 3 solar masses than a black hole forms

A black hole is a core so dense and massive that it wil generate so much gravity not even light can escape it

38
Q

What are the three ways of detecting exo-planets

A

Variation in Doppler shift

Planetary transits

Direct imaging

39
Q

Variation in Doppler Shift

A

A giant planet and a star orbit around a common centre of gravity

There will be times when the star will be moving towards the Earth, and times when the star will be moving away from the earth

So there will be small changes to the spectrum of the star, which will be seen as small redshift and small blueshift

40
Q

Planetary transits

A

If an exoplanet crosses in front of a star’s surface, then the brightness we see will drop by a small amount

Therefore a light intensity vs time graph would show a suddent drop in intensity

It allows rough estimations to be made of the planet’s radius and thus mass

41
Q

Direct Imaging

A

Light from the star has been digitally removed to enhance view of planets

42
Q

Hubble Space telescope

A

Can observe visible, ultra-violet and infared rays

43
Q

Chandra x ray telescope

A

Used to observe high energy objects such as neutron stars and supernovae

44
Q

Spitzer

A

Infared telescope in orbit around the sun at the same distance as the earth but a long way away - originally cooled by liquid helium

45
Q

What can magnification also equate to

A

hi/ho

(Height of image/height of object)

46
Q

What is a standard candle

A

A class of objects whose distances can be computed by comparing their observed brightness with their known luminosity.

47
Q

What is a black body

A

Emmitters and absorbers of radiation

48
Q

How does temperature effect wavelength

A

When yo heat something up from absolute zero it will give off shorter wavelengths as the temperature increases

49
Q

What is the evidence for the Big Bang theory

A
  • In the early universe protons and neutrons produced
  • protons are stable and neutrons decay into protons
  • Neutrons combine with protons to form Helium and Deutrium
  • These are stable, so no further decay occurs
  • Hence by comparing the number of free protons to the number of helium atoms we can test models for the formation of the universe
  • Big Bang fits this very well - the universe is made up of H and He
50
Q

CMBR

A
  • Cosmic Microwave Background Radiation
  • Red shifted background radiation left behind after the big bang - radiation stretched that it is observed in the microwave region - hence cosmic microwave background radiation
51
Q

State two advantages of a larger diameter telescope

A
  • larger resolving power - resolving power proportional to D
  • Power is proportional D^2, more power detected by larger diameter telescope
52
Q

What is a Quasar

A

A massive and extremely remote celestial object, emitting exceptionally large amounts of energy

They are distant objects powered by black holes a billion times as massive as our sun.

53
Q

Define parsec

A

parsec : distance to an object subtending 1 sec of arc to Earth’s orbit

To prove a parsec convert it into radians then covert it into light years

54
Q

Define Hubbles Constant

A

Gives the ratio of the (recessional) velocity (of galaxies) to distance from Earth

55
Q

Define red shift

A

increase in wavelength (of em radiation) due to relative recessive velocity between observer and source

56
Q

Give another equation for magnitude

A

Angle subtented by image at eye/ angle subtended by object at unaided eye

57
Q

Radio telescopes

A

situated on the ground as it detects radio waves which are not absorbed by the atmosphere

Large mirrors so collecting power very high

58
Q

Ultraviolet telescopes

A

Careful consideration required for the position of an ultraviolet telescope because the majority of ultraviolet is absorbed by the atmosphere

Better resolving power than radio wave telescope as ultraviolet has a shorter wavlength than radio waves

59
Q

X ray telescopes

A

X-ray telescopes are usually situated in space because the atmophere prevents the majority of X-radiation reaching the Earth’s surface.

X rays are very penetrating and not easily reflected off metal surfaces

60
Q

Cepheid variable

A

A bright star whose intensity caries over a matter of days. The period of the variation of intensity is linked directly to the absolute magnitude of the star

61
Q

Explain how a CCD operates

A

The image formed on the CCD is created by incident photons. These photons cause electrons to be released.

 The electrons are trapped in (“potential wells” in the CCD)

 The number of electrons liberated (in each pixel) is proportional to the intensity of the light/number of photons falling (on each pixel). or so that the pattern of the charge built up is related to the image.

62
Q

State was is meant by the Rayleigh criterion

A

Two objects will just be resolved when the first minimum/edg of the airy disc in the diffraction pattern of one image

 Coincides with central maximum/centre of the airy disc of the other.

63
Q

How long does a supernova last

A

10-400 days

64
Q

wavelength x axis for light curve

A

100-2500 nm

65
Q

Y axis for hertspirng diagram

A

+15 to -10

66
Q

Conterversies from measurements of typa 1a

A

Measurements of supernovae do not agree with predictions (from Hubble’s Law)

 So Universe must be expanding at increasing rate/accelerating

 (Controversial as) no known energy source for expansion or reference to dark energy

AQA A level Physics (17 decks)

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