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A-Level Physics > Astro > Flashcards

Astro Flashcards

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1
Q

What is the Rayliegh Criterion?

A

The minimum angular separation between two light sources such that they can be resolved into 2 distinct objects

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2
Q

What is the centre circle of a diffraction pattern called?

A

Airy disk

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3
Q

What is a light year?

A

The distance that light will travel in one year in a vacuum

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4
Q

What is an AU?

A

The mean distance between the Earth and the sun

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5
Q

What is parallax?

A

The apparent change in the perceived position of an object due to the position of the observer

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6
Q

What is a parsec?

A

The distance when 1 AU subtends an angle of 1 arcsecond

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7
Q

What is luminosity?

A

The amount of energy emitted by an object in the form of electromagnetic radiation each second (power output)

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8
Q

What is apparent magnitude?

A

A measure of how bright a star appears to be

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9
Q

What is absolute magnitude?

A

The apparent magnitude of a star when it is at a distance of 10 parsecs from Earth

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10
Q

Brightness ratio equation?

A

LEARN

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11
Q

How much brighter is a magnitude 1 star than a magnitude 2 star?

A

2.51 times

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12
Q

What is a standard candle?

A

an object with a known absolute magnitude

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13
Q

What is a black body?

A

An object that absorbs all electromagnetic radiation of all wavelengths and emits all electromagnetic radiation

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14
Q

What happens to the peak on power v wavelength graph as temp increases?

A

moves to the left

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15
Q

Why might hotter stars not appear as bright?

A

power output has moved out of the visible light range

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16
Q

What do you need to assume for the inverse square law to be true for stars?

A

they are spherical and emit em radiation in all directions

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17
Q

What do you need to assume for wien constant equation?

A

stars are black bodies

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18
Q

Why do you get absorption lines with stars?

A

light from star passes through cooler gas in its atmosphere, exciting its electrons. photons remitted in all directions (making certain wavelengths less intense)

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19
Q

where is the sun on a H-R diagram?

A

G5

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20
Q

What are balmer lines due to? When are they most prominent?

A

Hydrogen balmer lines are due to hydrogen in stars being in the n = 2 state (wavelengths correspond to hydrogen visible light part). When light is shone through them they are excited and dexcite emitting photons in the visible light range.
They are most prominent in stars that are 7500-11000k (lower = not excited, higher = too excited) - think goldielocks

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21
Q

What are the different spectral classes?

A 
O 25000-50000 (He/He+) Blue
B 11000-25000 (H/He) Blue
A 7500-11000 (H-B) White/blue
F 6000-7500 (metal ions) white
G 5000-6000 (neutral/ metal ions) yellow/white
K 3500-5000 (neutral metals) yellow
M below 3500 (neutral metals) orange
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22
Q

How do stars form?

A
  • cloud of dust/gas dense enough = protostar

- protostar starts doing fusion = main sequence

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23
Q

What happens after main sequence?

A
  • stop fusing hydrogen and outer layers expand (core compresses and heats up)
  • core hot enough to fuse hydrogen in outer layers
  • core gets hot enough to fuse helium to C/O
  • runs out of helium
  • core compresses and heats up again (not hot enough to fuse C/O)
  • compresses to the point at which the pressure from electrons makes it so it cant compress anymore
  • ejects layers as planetary nebula leaving white dwarf
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24
Q

What is a supernovae?

A

an object that exhibits a rapid and enormous increase in absolute magnitude

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25
When will an object explode in a supernovae?
when it has a mass 1.4x solar mass (1.4x less = white dwarf)
26
What is the peak value in supernovae light curve?
day 20 = -19.3
27
What can supernovae be used for?
to measure distances to galaxies
28
Why are type 1a used as standard candles?
They reach the same peak value of absolute magnitude
29
When do gamma ray bursts occur?
When a star collapses in a supernovae to form a neutron star or black hole
30
how powerful are gamma ray bursts?
same power as the suns lifetime
31
What are the properties of gamma ray bursts?
They are collimated and so don't spread out much
32
When will a black hole form rather than a neutron star?
more than 3 times solar mass
33
neutron star or black hole which is more dense?
neutron star
34
What is the event horizon of a black hole
Where the escape velocity equals the speed of light
35
What are quasars?
The furthest measurable objects
36
properties of quasars?
they are radio sources and show very large optical red shifts
37
What is red shift?
The increase in wavelength of electromagnetic radiation due to the relative recessive velocity between the observer and the source
38
When is velocity positive/ negative?
``` positive = towards observer Negative = away from observer? ```
39
Red shift or blue shift which is positive?
red shift is positive | blue shift is negative
40
What increases red shift?
If recessive velocity increases
41
Why is red shift called that?
Wavelength shifted towards red or even further
42
How do you find delta wavelength/frequency?
observed - actual
43
Covert hubbles constant
do it
44
how does increasing distance affect recessional velocity? What does this mean?
increases recessional velocity. Further away galaxies are moving away quicker
45
How o you find the age of the universe?
time = distance/speed (have to assume the rate of expansion has been constant)
46
What suggests rate of expansion is increasing?
Supernovae were dimmer than expected (were further away than thought) due to dark energy
47
What is the big bang theory?
The universe has expanded from a single extremely hot and dense point
48
What is CMBR?
radiation detected from all parts of the universe (heat left from the big bang - universe is 2.7 k hotter than it it was in a steady state)
49
What is the ratio of helium and hydrogen in the universe?
1:3 (helium to Hydrogen)
50
What does the big bang theory suggest?
When the universe was young there was a brief period in which it was hot enough for hydrogen to fuse to helium but as it expanded it cooled preventing the further fusion of helium. This meant that the relative abundances of helium and hydrogen were spread uniformly throughout the universe (heavy elements on in star)
51
what is a binary star system?
a star system in which 2 stars orbit a common center of mass
52
when is there a single absorption line with binary star systems?
when they are at right angles to the line of sight of the observer (no shift), at a max when they are moving along the line of sight ( one shows max blue and other shows max red)
53
what is an exoplanet?
a planet outside our solar system?
54
What are the two ways of detecting exoplanets?
doppler shift way and transit method?
55
What is the doopler shift method?
A star and a planet orbit a common center of mass (closer to the star) the gravitational pull causes tiny wobbles in the stars orbit resulting in small red and blue shifts (amount is related to the stars velocity towards and away from Earth). Can be used to determine mass
56
What is the transit method?
Light curve showing constant value with dip  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 
57
limitations of the transit method?
Can only be used to confirm an exoplanet as something else could cause the dip in the light curve
58
Limitations of the doppler shift method?
If plane of planets orbit is perpendicular to line of site = no red or blue shift
59
Why are exoplanets hard to detect?
- there light is obscured by their star - If they are too far away they wont reflect enough light to be detected - planet and star tend to be too close for optics to resolve them.
60
defining properties of a neutron star?
- density of nuclear matter and are relatively small - spin rapidly (due to conservation of angular momentum as the core collapses) - v powerful radio sources due to their v strong magnetic fields
61
What is a CCD
Charged coupled devices, they are sensitive light detectors used to capture images
62
Describe how CCD works?
- photons hit silicon in a pixel creating free electrons confined to the pixel = charge accumulates
63
How can the charge in CCDs be used?
- can be measured and used to create a digital signal showing where the light hit and its intensity allowing a digital image to be created
64
What is the quantum efficiency of CCDs?
Around 80%
65
What is quantum efficiency?
The proportion of incident photons that are detected (proportion that release an electron)
66
What is the quantum efficiency of the eye?
1%
67
Why are CCDS better than the eye?
They detect far more light and have a broader spectrum (infrared, UV and visible)
68
How much better are CCDs at resolving than the eye?
10x better spacial resolution (finer detail)
69
Disadvantages of CCDs
often less convenient to use but produce digital images that can be stored and shared
70
What is chromatic aberration and what suffers it?
- refracting telescopes | - caused by different wavelengths refracting by different amounts (blurs image)
71
Why are telescopes beyond a certain collecting power implausible?
Making big enough lenses is difficult and costly
72
advantages of mirrors over lenses?
They are cheaper to produce
73
What is spherical aberration and what suffers it?
- large mirrors (aren't perfectly parabolic) | - caused by light not coming to a clear focal point (blurry)
74
What are infrared and UV telescopes?
like optical refracting telescopes that use CCDs (have parabolic mirror to focus radiation onto a detector)
75
Why do mirrors in uv have to be more precise?
- loner wavelength = less affected by mirror imperfections | - UV has a short wavelength and so is affected by spherical aberration if telescope isn't more precise
76
Why do you have to have a different structure for Xray telescopes?
They don't reflect like other EM waves
77
What do you have to use to focus an X Ray
Grazing mirrors used to direct waves to a focal point
78
Where is best to locate UV/infrared/X ray telescopes?
best located in space to overcome atmospheric absorption - UV/IR can be put on high and dry places (not for x rays as they are absorbed higher in the atmosphere and with some IR)
79
What does resolving power depend on?
The rayliegh criterion and the quality of the detector (how many pixels on the CCD or how fine the wire mesh is - in x ray ones)
80
if dish size was the same, would a UV or radio telescope be better and why?
UV because it has a better resolving power as UV is shorter wavelength
81
What is collecting power proportional to?
Dish diameter squared ( for X ray = size of telescope opening)
82
Why are a bigger dish/mirror better?
collects more energy from an object in a given time = more intense image (can observe more faint images)
83
What can collecting power also be known as?
energy collected per second
84
What are the features of a radio telescope?
have a large parabolic dish with an aerial at the focal point of the dish
85
What is the range of radio waves that the atmosphere transmits?
0.001m to 10m
86
How to radio telescopes work?
dish is directed at an astronomical source that emits radio waves in the range, the waves reflect from the dish onto the aerial producing a signal
87
How do radio telescopes compensate for the earths rotation?
Dish is turned by motors to enable it to scan sources and to compensate for earths rotation
88
What is the amplitude of the signal in R telescopes?
Measure of the intensity of the radio waves received by the dish.
89
How do radio telescopes produce an image?
Dish scans source, signal maps intensity of radio waves across source to give radio image
90
What do radio dishes consist of?
Wire mesh which is lighter than metal sheets and just as effective at reflection
91
Why can mesh be used?
long wavelength so not diffracted?
92
equation for collecting power?
1/4piD^2
93
advantages of radio telescopes?
lighter and cheaper than optical
94
optical for radio better at resolving
lots are linked together
95
List of parts in radio telescopes?
Dish - amplifier - tuner - pc

A-Level Physics (10 decks)

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