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Physics > 3.9 Astrophysics > Flashcards

3.9 Astrophysics Flashcards

1
Q

What are three drawbacks of placing a telescope in orbit?

A
  • It is expensive to send a telescope into space
  • The telescope will need its own power source
  • It is very difficult to service an orbiting telescope if something goes wrong
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2
Q

What is meant by Quantum Efficiency?
Quantum efficiency of CCD?
Quantum efficiency of eye?

A

no. of photons arriving at detector and being detected ÷ total arriving at detector

CCD : QE > 80%
Eye : QE = 1%

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3
Q

What is the defining property of a supernova?

A

The rapid increase in absolute magnitude (become more negative)

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4
Q

What are the defining features of neutron stars?

A
  • Extremely dense
  • Made up of neutrons
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5
Q

Why are scientists worried about the possibility of nearby supernovae?

A
  • In a supernova, a collapsing star can produce gamma ray bursts with energy similar to total output of sun
  • The burst is highly collimated, and if in direction of Earth, they could cause a mass extinction event (due to killing of cells etc.)
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6
Q

2 reasons why it is hard to detect exoplanets

A
  • The star it orbits is much brighter than reflected light from the planet
  • Planet is very small and distant, so subtends a very small angle compared to resolution of telescopes
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7
Q

What number is assigned to the dimmest visible stars on the Hipparcos scale?

A

6

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8
Q

What is the name of the subjective scale where stars are classified according to their apparent magnitude?

A

Hipparcos scale

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9
Q

Difference of 1 on magnitude scale is equal to an intensity ratio of ____?

A

2.51

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10
Q

What is Stefan’s law?

A

P = σ A T^4

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11
Q

What is Wien’s displacement law?

A

λ_max * T = constant

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12
Q

What is a black body?

A
  • An object that absorbs electromagnetic radiation of all wavelengths and can emit electromagnetic radiation of all wavelengths
  • A black body does not reflect any radiation
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13
Q

What is the scale of the absolute magnitude axis on the Hertzsprung-Russell diagram?

A

15 to -10 (going up)

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14
Q

What is the scale of the temperature axis on the Hertzsprung-Russell diagram?

A

50,000K - 2,500K (going right, not linear)

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15
Q

Which events are used as standard candles for determining distances?

A

Type 1a supernovae

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16
Q

Approximately what % of observable stars are on the main sequence?

A

90%

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17
Q

What do scientists believe might be at the centre of galaxies?

A

Supermassive black hole

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18
Q

What is the difference between apparent magnitude and absolute magnitude?

A
  • Apparent magnitude is how bight the object appears from Earth
  • Absolute magnitude is how bright object would appear if viewed from a distance of 10 parsecs
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19
Q

Define the parsec

A

Distance at which 1AU subtends an angle of 1/3600th degree

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20
Q

What is an AU?

A
  • Astronominal unit
  • the mean distance from the centre of the earth to the centre of the sun
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21
Q

What is apparent magnitude?

A

The brightness of star as seen from Earth

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22
Q

What is absolute magnitude?

A

Apparent magnitude (how bright object would appear) if viewed at a distance of 10 parsecs

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23
Q

What are the most distant measurable objects in the universe?

A

Quasars

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24
Q

How are quasars produced?

A

They form around active supermassive black holes, at the centre of active galactic nuclei

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25
Q

What is the Doppler effect?

A

A change in apparent wavelength due to relative motion between source and observer

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26
Q

What is a standard candle?

A

An object with a known absolute magnitude

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27
Q

What is the cosmological microwave background?

A
  • The radiation coming from all parts of the Universe (suggesting universe began in a hot dense state)
  • The radiation has a peak in the microwave region (2.7K), and follows a black-body distribution
  • It can be interpreted as radiation left over from the Big Bang
  • This radiation has been red-shifted into the microwave region as the Universe has expanded
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28
Q

How does the relative abundance of hydrogen and helium supports the Big Bang theory?

A
  • Big Bang theory suggests a very brief fusion period occurred when the Universe was very young, resulting in the production of helium from fusing hydrogen
  • Fusion stopped as the Universe expanded and cooled
  • Resulting in a relative abundance of hydrogen and helium in the ratio of 3:1
  • The observed distribution of matter now follows the predicted ratio
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29
Q

What is meant by the Hubble constant?

A

The constant that gives the ratio of the recessional velocity of galaxies to distance from Earth

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30
Q

What is the assumption made when using the Hubble constant?

A

Universe is expanding at a constant rate

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31
Q

Explain how type 1a supernovae can be used as standard candles to determine distances

A
  • All type 1a supernovae have the same peak absolute magnitude
  • Apparent magnitude can be measured
  • Can then use m-M = 5 log(d/10)
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32
Q

2 advantages of using a telescope with a large diameter objective lens

A
  • Allows fainter objects to be viewed, as collecting power is proportional to d^2
  • Allows for better resolving power as smallest resolvable angle is inversely proportional to d
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33
Q

What is the definition of angular magnification?

A

angle subtended at the eye by the image ÷ angle subtended at the eye by the object

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34
Q

4 Advantages of reflecting telescopes over refracting telescopes

A
  • No chromatic aberration in objective or secondary mirror because mirrors do not refract light
  • No spherical aberration because reflecting telescopes have a parabolic objective mirror
  • No distortion, because the mirror can be supported more strongly
  • Better resolving power and collecting power, as mirrors can be larger
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35
Q

What in the atmosphere in responsible for absorbing infrared radiation?

A

Water vapour

36
Q

How does absorption of light due to atmosphere affect attempts to determine a star’s temperature?

A
  • The spectrum of light from a star can be from a can be used to determine its temperature
  • The longer wavelengths are absorbed by the atmosphere
  • Shifting the peak of graph to shorter wavelengths
  • Making the star appear hotter
37
Q

What is chromatic aberration? What is its effect?

A
  • When different wavelength are refracted by different amounts, due to different speed in glass
  • Results in images with coloured edges
38
Q

Why are Cassegrain telescopes mostly free of chromatic aberration?

A
  • No refraction by the mirrors
  • But there will be some chromatic aberration in eyepiece lens
39
Q

How does chromatic aberration affect the appearance of an image (that should be white)?

A

Edges of image will appear coloured

40
Q

What is Hubble’s law?

A

The recession velocity of galaxies is directly proportional to their distance from Earth (v = H_0 d)

41
Q

Names of two methods for detecting exoplanets

A
  1. Radial velocity method
  2. Transit method
42
Q

What is the radial velocity method for detecting exoplanets?

A
  • A star and a planet will orbit their common centre of mass
  • The star will have small variations in its distance from Earth, shown by tiny periodic red and blue shifts in its spectrum as it moves away from / towards Earth respectively
43
Q

Briefly describe the transit method for detecting exoplanets

A
  • As an exoplanet moves between the star it orbits and the Earth, the star’s brightness appears to decrease slightly
  • This can be observed to happen periodically from observation over a long time period
44
Q

What does the Rayleigh criterion find? What does it state?

A
  • The subtended angle between two objects whose images are just resolvable
  • The angle is when the central maximum of the diffraction pattern of one image coincides with the first minimum of the diffraction pattern of the other
45
Q

Absolute magnitude scale for light cure for Type 1a supernova

A

-16 at dimmest, -19.3 at brightest

46
Q

Time scale for light curve of Type 1a supernova

A
  • 0 days at peak
  • 40 days at end
47
Q

What property of quasars led to their discovery?

A

They are unusually strong radio sources

48
Q

What is the defining feature of a black hole?

A

It’s escape velocity is greater than the speed of light

49
Q

Two reasons why the secondary mirror in the Cassegrain telescope affects the clarity of the image

A
  • mirror blocks light so less light hits objective mirror
  • light diffracted passing secondary mirror affects image

(these are both problems with reflectors if asked)

50
Q

Why is the virtual image formed by the eyepiece at infinity?

A

To reduce eye strain when looking between the object and image

51
Q

Describe how to eliminate chromatic and spherical aberration in lenses

A

Using an achromatic doublet brings all rays of light into focus in the same position by using a convex lens and a concave lens of different types of glass cemented together

52
Q

radio telescopes : structure, positioning, use

A

structure: large parabolic dish that focuses radiation onto a receiver
positioning: can be ground-based but must be in isolated locations to reduce interference with nearby radio sources
uses: galaxies, stars, black holes, quasars

53
Q

infrared telescopes : structure, positioning, use

A

structure: cassegrain mirror system. must be cooled with cryogenic fluid approaching absolute zero
positioning: space (most infrared absorbed by water vapour in the atmosphere)
uses: observing cooler regions in space (from a few tens to 100K)

54
Q

ultraviolet telescopes : structure, positioning, use

A

structure: cassegrain mirror system that focuses radiation onto solid state devices that use the photoelectric effect to convert UV photons to electrons
positioning: space (most UV absorbed by ozone layer)
uses: observing the interstellar medium, star formation regions and the solar corona

55
Q

x-ray telescopes : structure, positioning, use

A

structure: combination of hyperbolic and parabolic mirrors to focus radiation onto a CCD
positioning: space (x-rays absorbed by atmosphere)
uses: observing high-energy events and areas such as active galaxies, black holes and neutron stars

56
Q

Why can’t X-ray telescopes use the same reflecting mirrors as found in optical telescopes?

A

The x-rays have such high energies that they would penetrate the mirror

57
Q

What is meant by parallax? (astro)

A

An ​apparent change of position of a nearer star in comparison to distant stars​ in the background, ​as a result of the orbit of the Earth around the Sun

58
Q

What are Hydrogen Balmer lines? Where are they found?

A
  • Absorption lines caused by the excitation of hydrogen atoms from the n = 2 state to higher energy levels
  • Found in the spectra of O, B and A type stars
59
Q

Describe how a star, like our Sun, is originally formed (spec?)

A
  • A cloud of dust and gas (nebula) has fragments of varying masses, that contract under the force of gravity
  • This becomes a protostar as the temperature increases as the star gets denser
  • When the temperature at the centre of the protostar reaches a few million degrees, hydrogen nuclei start to fuse together to form helium
  • This nuclear fusion give out large amounts of energy and a star is born
60
Q

What would be the main stages of a life cycle of a star of similar size to our Sun?

A
  • Cloud of gas and dust (nebula)
  • Protostar
  • Pre-main sequence star
  • Main sequence star
  • Red Giant
  • White Dwarf
  • Black Dwarf
61
Q

Describe how a main sequence star remains stable

A
  • The outward pressure caused by nuclear fusion is balanced by the force of gravity pulling inwards
  • An equilibrium is reached and the star remains stable for billions of years
  • The greater the mass of the star, the shorter its main sequence period is because is uses its fuel more quickly
62
Q

What is meant by dark energy?

A

The name given to the energy thought to be responsible for the accelerated expansion of the universe.

63
Q

Describe the transition from a red giant through to a black dwarf (spec?)

A
  • The outer layer of the red giant is ejected
  • This outer layer is dust and gas
  • A hot, solid, dense core is left. This is called a white dwarf
  • The White Dwarf cools and emits less and less energy until it becomes a black dwarf
64
Q

What are the four stages in becoming a red giant? (spec?)

A
  1. Core hydrogen burning (main-sequence)
  2. Shell hydrogen buning - hydrogen in core runs out, outward pressure stop, helium core contract, heat from core raises surrounding material enough for hydrogen to fuse
  3. Core helium burning - helium core contracts until hot and dense enough to fuse carbon and oxygen. this releases lots of energy and pushes outer layers further outwards
  4. Shell helium burning - helium in core runs out, carbon-oxygen core contracts and heats surrounding shell so that helium fuses
65
Q

Describe how a red giant becomes a white dwarf (and then black dwarf) (spec?)

A
  1. Fusion eventually stops, and core contracts and gravity greater than outward force
  2. Outer layers are ejects into space as a planetary nebula, leaving behind a hot dense core (white dwarf)
    .3 White dwarf will eventually cool to a black dwarf
66
Q

What is different about the “core burning to shell burning” process of high mass stars? (spec?)

A
  • It can continue beyond helium
  • For really massive stars (red supergiants), this can go all the way up to iron
67
Q

What are pulsars?

A

Spinning neutron stars that emit radio waves in two beams from the magnetic poles as they spin (up to 600 times per second)

68
Q

3 reasons telescopes are placed in space

A
  • the absorption of the electromagnetic waves by the atmosphere
  • the light pollution and other interference at ground level
  • the effect the atmosphere has on the path of the light as it passes through
69
Q

2 reasons why a star could appear brighter

A
  • Could be closer to Earth
  • Could be emitting more power at visible wavelengths
70
Q

2 assumptions for inverse square law

A
  • no light is absorbed or scattered between the source and the observer
  • source can be treated as a point
71
Q

Why are hydrogen Balmer lines weak in O and B class stars?

A
  • The stars atmosphere is too hot
  • Hydrogen is likely to be ionised (i.e. not in n=2 state)
72
Q

How do Type 1a supernovae occur?

A
  • A binary star system where one star is a white dwarf
  • When the companion star runs out of hydrogen it expands
  • The white dwarf accumulates matter from its companion star and explodes after reaching a critical mass
  • After reaching the critical mass, fusion begins and is unstoppable
73
Q

Advantages of CCDs?

A
  • Better resolution due to smaller pixels (so easier to distinguish between objects that are close together)
  • CCDs have a higher quantum efficiency, and can be exposed for longer (so fainter objects can be observed)
  • CCDs can detect a wider range of wavelengths
  • CCDs are more convenient - can capture images when astronomer not present, and image can be stored and analysed on computer
74
Q

What is the main contributor to a star’s luminosity?

A

Temperature (P = σ A T^4)

75
Q

Stellar class of a white dwarf

A

O or B

76
Q

3 ways a supermassive black hole can form

A
  • The collapse of massive gas clouds while the galaxy is forming
  • A normal black hole that has accumulated huge amounts of matter over millions of years
  • Several normal black holes merging together
77
Q

3 limitations when measuring the Hubble constant

A
  • Apparent magnitude may be affected by what the light passes through
  • At large distances, the accelerating expansion of the universe will affect the graph
  • There is lots of random variation in the data
78
Q

What absorbs UV in the atmosphere?

A

Ozone

79
Q

Decreasing spherical aberration leads to…

A

Increased image detail

80
Q

Compare the apparent magnitudes 5.1 and 6

A

5.1 is brighter than 6
(2.29x brighter)

81
Q

Explain the dips in intensity in a star’s spectrum

A
  • Dips are due to absorption
  • Light of particular wavelengths absorbed by gases in outer layers
  • (say all the energy levels stuff)
  • And re-emitted in random directions
  • Leaving dark lines
82
Q

What is meant by an ‘active’ black hole?

A
  • matter is falling into the black hole
  • this leads to energy release
83
Q

What to talk about if asked to describe image’s appearance in ray diagram question

A
  • real/virtual
  • inverted/upright
  • magnified/diminished (bigger/smaller)
84
Q

Chromatic aberration diagram: which colour has shorter focal length?

A
  • Blue shorter
  • Red longer
85
Q

If the difference in apparent magnitude is x, what is the ratio of brightnesses?

A

2.51^x

Physics (11 decks)

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