Astrophysics Flashcards
1
Q
Another name for convex lenses
A
Converging lenses
2
Q
What do convex lenses do
A
Focus incident light
3
Q
Another name for concave lenses
A
Diverging lenses
4
Q
What do concave lenses do
A
Spreads out incident light
5
Q
What is the principal axis
A
Line passing through centre of lens at 90 degrees to its surface
6
Q
Principal focus (F) in a converging lens
A
Point where an incident beam passes parallel to principal axis will converge
7
Q
Principal focus (F) in a diverging lens
A
Point where light ray appears to come from, same distance from either side of lens
8
Q
What is the focal length (f)
A
Distance between centre of lens and principal focus
9
Q
How focal length affects strength of lens
A
Shorter focal length = stronger lens
10
Q
What is a real image
A
Formed when light rays cross after refraction, can be formed on screens
11
Q
What is a virtual images
A
Formed on the same side of the lens, light rays do not corss, so can’t be formed on screen
12
Q
Lens formula
A
(1 / distance of object from centre of lens) + (1 / distance of image from centre of lens) = (1 / focal length)
13
Q
What is the power of a lens
A
Measure of how closely a lens can focus a beam that is parallel to principal axis - to do with focal length
14
Q
How focal length affects power of lens
A
Shorter focal length = more powerful
15
Q
Power of lens value for converging and diverging lenses
A
Converging - positive, diverging - negative
16
Q
What is power of a lens measure in
A
Dioptres (D)
17
Q
Power of lens formula
A
P = (1 / u) + (1 / v) = (1 / f)
18
Q
P = (1 / u) + (1 / v) = (1 / f) what is P
A
Power of lense
19
Q
P = (1 / u) + (1 / v) = (1 / f) what is u
A
Distance of object from centre of lens
20
Q
P = (1 / u) + (1 / v) = (1 / f) what is v
A
Distance of image from centre of lens
21
Q
P = (1 / u) + (1 / v) = (1 / f) what is f
A
Focal length of lens
22
Q
What are refracting telescopes comprised of
A
2 converging lenses
23
Q
Converging lenses making up a refracting telescope
A
Objective lens, eyepiece lens
24
Q
What is an objective lens
A
Collects light, makes real image of a very distance object, should have a long focal length and be large to collect as much light as possible
25
What is the collecting power of a telescope directly proportional to
Square of radius of objective lens
26
What is an eyepiece lens
Magnifies image produced by objective lens so that observer can see it, produces virtual image at infinity since light rays are parallel, reduces eye strain as observer doesn't have to refocus every time they look between the telescope image and object in the sky
27
What is the normal adjustment of a refracting telescope
When distance between objective lens and eyepiece lens is sum of focal lengths, (f_o + f_e), so principal focus for the two lenses is in the same place
28
Ray diagram for a refracting telescope in normal adjustment
Photo 1
29
Another name for magnifying power
Angluar magnification
30
Formula for magnifying power of a telescope
M = (angle subtended by eye at image at the eye) / (angle subtended by object at unaided eye) = a / b = larger angle / smaller angle
31
If a and b are both under 10 degrees, what does the magnifying power of a refracting telescope formula become
f_o / f_e
32
When can f_o / f_e be used
When a and b are both under 10 degrees
33
f_o / f_e diagram
Photo 2
34
Most common type of reflecting telescope
Cassegrain reflecting telescope
35
What is a cassegrain reflecting telescope
Concave primary mirror, long focal length, small convex secondary mirror at centre, light is collected and focused on eyepiece lens
36
What does the secondary convex mirror in a cassegrain reflecting telescope do
Allows cassegrain to be shorted than other configurations like Newtonian which utilises plane mirror
37
Cassegrain reflecting mirror diagram
Photo 3
38
Newtonian reflecting mirror diagram
Photo 4
39
Description of mirrors in reflecting telescopes
Very thin (often less than 25nm thick) coating of aluminium or silver atoms stuck on a backing material
40
Benefits of the structure of the mirrors used in a reflecting telescope
Allow the mirrors to be very smooth and minimises distortions in the image
41
What is chromatic aberrations
Focal light of red light is greater than that of blue, so focus on different points (blue is refracted more), can cause a white object to produce an image with a coloured fringing (coloured edges), with effect being more noticeable for light passing throguh the edges of the lens
42
Type of telescope - chromatic aberrations
Caused be refraction so has little effect on reflecting telsecopes as it only occurs in eyepiece lens
43
What is spherical abberation
When curvature of a lens or mirror causes rays of light at edges of lens to focus on a different position to the rays at the centre of te lens, leads to image blurring and distortion
44
When is spherical abbertation most pronounced
In lenses with a large diameter
45
How to avoid spherical abberations
Using parabolic objective mirrors in reflecting telescopes
46
What are achromatic doublets used for
To minimise spherical and chromatic abberations in lenses
47
What is an achromatic doublet made up of
Convex lens made of crown glass and a concave lens made of flint glass that have been cemented together
48
What does an achromatic doublet do
Brings all rays of light into focus in the same position
49
Achromatic doublet diagram
Photo 5
50
Disadvantages of refracting telescopes
Pure, weight, abberations, construction, size, support
51
Pure as a disadvantage of refracting telescopes
Glass must be pure and free from defects, very difficult for such large diameter lenses
52
Weight as a disadvantage of refracting telescopes
Lenses can bend and distort under own weight
53
Abberations as a disadvantage of refracting telescopes
Chromatic and spherical abberations affect lenses
54
Construction as a disadvantage of refracting telescopes
Incredibly heavy so difficult to manoeuvre
55
Size as a disadvantage of refracting telescopes
Large magnification requires large diameter objective lense and long focal lengths
56
Support as a disadvantage of refracting telescopes
Can only be supported from edges - large and heavy
57
Advantages of reflecting telescopes
Thin, abberations, lighter, achromatic doublets, mirror segments, support from behind
58
Thin as an advantage of reflecting telescopes
Only need to be a few nanometers thick and still give excellent image quality
59
Abberations as an advantage of reflecting telescopes
Mirrors are unaffected by chromatic abberations, spherical abberations can be solved by using parabolic mirrors
60
Lighter as an advantage of reflecting telescopes
Not as heavy as lenses so easier to handle and manoeuvre
61
Achromatic doublets as an advantage of reflecting telescopes
Can solve chromatic abberations (from eyepeice lens) bis achromatic doublets
62
Mirror segments as an advantage of reflecting telescopes
Large composite primary mirrors can be made from lots of smaller mirror segments
63
Support from behind as an advantage of reflecting telescopes
Large primary mirrors are easy to support from behind due to not needing to see through them
64
Which sort of telescopes are preferred
Reflecting over refracting
65
66
What do radio telescopes do
Create images of astronomical objects using radio waves
67
Why is possible to build ground based radio telescopes
Atmosphere is transparent to radio waves so it doesn't absorb them
68
Why should radio telescopes be in isolated locations
To avoid interference from nearby radio sources
69
Basic principle of radio telescopes
Use parabolic dish to focus radio waves on receiver
70
Similarities between radio and optical telescopes
Function, movement, parabolic, ground
71
Function as a similarity between radio and optical telescopes
Both intercept and focus incoming radiation to detect its intensity
72
Movement as a similarity between radio and optical telescopes
Can be moved to focus on different sources or to track moving sources of radiation
73
Parabolic as a similarity between radio and optical telescopes
Parabolic dish of radio telescope is similar to objective mirror of a reflecting optical telescope
74
Ground as a similarity between radio and optical telescopes
Can be ground-based as radio waves and optical light can pass through atmosphere easily
75
Differences between radio and optical telescopes
Size, cost, building up an image, interference
76
Size as a difference between radio and optical telescopes
Radio wavelengths are much larger than visible wavelengths so radio telescopes have to have a bigger diameter to achieve the same resolving power as an optical telescope, due to the larger diameter, radio telescopes have a much larger collecting power
77
Cost as a difference between radio and optical telescopes
Radio telescopes are cheaper and simpler to build because a wire mesh is used instead of a mirror, given the mesh size is less than (wavelength) / 20, radio waves will be reflected (not refracted)
78
Building up an image as a difference between radio and optical telescopes
Radio telescopes have to move across an area to build up an image, unlike optical telescopes
79
Interference as a difference between radio and optical telescopes
Radio telescopes experience a large amount of man-made interference from radio transmissions, phone, microwave ovens etc., optical telescopes experience interference from weather conditions, light pollution, stray radiation etc.
80
What do infrared telescopes do
Create images of astronomical objects using infrared radiation
81
Basic principle of infrared telescopes
Large concave mirror which focuses radiation on a detector
82
How is infrared radiation emitted
As heat
83
How to overcome the heat of infrared radiation for infrared telescopes
Cool the telescope using cryogenic fluids (liquid nitrogen or hydrogen) to almost absolute zero
84
How to prevent interference to infrared telescoper
Telescope must also be well shielded to avoid thermal contamination from nearby objects as well as its own infrared emission
85
What are infrared telescopes used for
To observe cooler regions in space
86
Issue with ground based infrared telescopes
Atmosphere absorbs most infrared radiation so telescopes must be launched into space and accessed remotely from the ground
87
What do ultraviolet telescopes do
Create images of astronomical objects using ultraviolet radiation
88
Where do ultraviolet telescopes need to be placed and why
Space, ozone blocks all ultraviolet rays with a wavelengths of less than 300nm
89
Basic principle of ultraviolet telescopes
Utilise cassegrain configuration to bring rays to focus, rays are detected by solid state devices which use photoelectric effect to convert UV photons into electrons which pass around circuit
90
What can ultraviolet telescopes be used for
To observe interstellar medium and star formation regions
91
What do x-ray telescopes do
Create images of astronomical objects using x-rays
92
Where do x-ray telescopes need to be placed and why
Space, atmosphere absorbs all x-rays
93
Why do normal mirrors not work for x-rays
Rays have such high energy that they would pass straight through mirrors in a normal optical telescope
94
X-ray telescope basic principle
Made from a combination of parabolic and hyperbolic mirrors - must all be extremely smooth, rays enter telescope, skim off the mirrors and are brought to focus on CCDs which convert light into electrical pulses
95
What can x-ray telescopes be used for
Can be used to observe high-energy events and areas of space such as active galaxies, black holes and neutron stars
96
What do gamma telescopes do
Create images of astronomical objects using gamma radiation
97
Why don't gamma telescopes use mirrors
Gamma rays have so much energy that they would pass straight through a mirror
98
Gamma telescope basic principles
Used detectors made of layers of pixels, as gamma photons pass through they cause a signal in each pixel that they come into contact with
99
What can gamma telescopes be used for
Gamma ray bursts (GRBs), quasars, black holes, solar flares
100
How many types of gamma ray bursts (GRBs) are there
2
101
What are the 2 types of gamma ray bursts (GRBs)
Short-lived, long-lived
102
Short-lived gamma ray bursts
Last anywhere between 0.01 and 1 second, associated with merging neutron stars (forming a black hole), or a neutron star falling into a black hole
103
Long-lived gamma ray bursts
Can last between 10 and 1000 seconds, associated with a type 2 supernova (death of a massive star)
104
What is collecting power
Measure of ability of lens/mirror to collect incident electromagnetic radiation
105
Collecting power increases with size of lens/mirror elaboration
Collecting power is directly proportional to to the area of the objective lens
106
Area of objective lense formula
((pi)d^2) / 4
107
Collecting power is directly proportional to to the area of the objective lens implies that
Collecting power is directly proportional to to the diameter squared of the objective lens
108
How does the collecting power impact the image produced by a telescope
The greater the collecting power is, the brighter the images are
109
What is resolving power
Ability of telescope to produce separate images of close-together objects
110
What conditions need to be meet for an image to be resolved
Angle betweent the straight lines from Earth to each object must be at least the minimum angular resolution (theta), where theta is in radians
111
Minimum angular resolution formula
theta = wavelength / d
112
theta = wavelength / d what is theta
Minimum angular resolution
113
theta = wavelength / d what is d
Diameter of objective lens or mirror
114
What does the rayleigh criterion state
2 objects will not be resolved if any part of the central maximum of the image falls withing the first minimum diffraction ring of the other
115
What is an airy disc
Circular diffraction pattern, occurs when light enters telescope
116
Central maximum of airy disc
Bright white circle at centre
117
Minimum diffraction rings of airy disc
Dark rings around central maximum
118
Maximum diffraction rings of airy disc
Light rings around central maximum
119
What do CCD's stand for
Charge-coupled devices
120
What are charge-coupled devices
Array of light sensitive pixels, become charged when exposed to light by the photoelectric effect
121
What features of CCDs can be compared to the human eye
Quantum efficiency, Spectral range, pixel resolution, spatial resolution, convenience
122
What is quantum efficiency
Percentage of incident photons which cause an electron to be released
123
What is spectral range
Detectable range of wavelengths of light
124
What is pixel resolution
Total number of pixels used to form an image on a screen (lots of small pixels are better than a few large pixels)
125
What is spatial resolution
Minimum distance between 2 objects to be distinguishable (used to observe small details)
126
What is convenience (comparison of CCDs and human eye)
How easy images are to form and use
127
Quantum efficiency - CCD vs human eye
CCD - 80%, Eye - 4-5%
128
Spectral range - CCD vs human eye
CCD - infrared, UV and visible, Eye - Only visible light
129
Pixel resolution - CCD vs human eye
CCD - varies, about 50 megapixels, Eye - about 500 megapixels
130
Spatial resolution - CCD vs human eye
CCD - 10 micrometers, Eye - 100 micrometers
131
Convenience - CCD vs human eye
CCD - needs to be set up, produces digital images, Eye - simpler to use, no need for extra equipment
132
Advantages of using CCDs
More useful for detecting finer details and producing images which can be shared and stored
133
What is luminosity
Rate of light energy released/power output of a star
134
What is intesity
Power received from a star (luminosity) per unit area
135
Units of intesity
W m^-2
136
Intensity of star and inverse square law
Intensity of star is inversely proportional to the square of the distance from the star
137
What is brightness
Subjective scale of measurement - depends on observer
138
What is the apparent magnitude (m)
How bright the object appears in the sky
139
What does the apparent magnitude (m) depend on
Luminosity and distance from Earth
140
What is the hipparcos scale
Classifies astronomical objects by apparent magnitude (m), brightest is 1, dimmest is 6
141
How much brighter is a magnitude 1 star than a magnitude 6
100 times
142
Hippocarpus scale, type of scale
Logarithmic, intesity changes by 2.51
143
What is the absolute magnitude (M)
What the apparent magnitude would be if the star were 10 parsecs from Earth
144
Difference between apparent and absolute magnitude
Apparent depends on distance from Earth whilst absolute doesn't
145
Relationship between apparent and absolute magnitude
m - M = 5log(d/10)
146
m - M = 5log(d/10) - what is m
Apparent magnitude
147
m - M = 5log(d/10) - what is M
Absolute magnitude
148
m - M = 5log(d/10) - what is d
Distance in parsecs
149
What is parallax
Apparent change of position of a nearer star in comparison to distant stars in background as a result of the orbit of the Earth around the sun
150
What is parallax measured in
Angle of parallax
151
How the parallax angle changes
The greater the angle, the closer the star is to Earth
152
Astrophysics units of distance
Astronomical Unit (AU), Parsec (pc), Light year (ly)
153
What is an astronomical unit (AU)
Average distance between centre of the Earth and centre of the sun
154
What is a parsec (pc)
Distance at which the angle of parallax is 1 arcsecond (1/3600th of a degree), or, the distance at which 1 AU subtends an angle of 1 arcsecond
155
What is a light year (ly)
Distance an EM wave travels in a year in a vacuum
156
How to find distance to star using trig
d = r / theta
157
d = r / theta - what is d
Distance in meters to star
158
d = r / theta - what is r
Radius (sun to earth in orbit) in meters
159
d = r / theta - what is theta
Angle at star between sun and Earth
160
d = r / theta - derived formula
d = 1 / theta (d is in parsecs and theta is in arcseconds)
161
d = 1 / theta - what is d and what units is it
Distance from sun to star - parsecs
162
d = 1 / theta - what is theta and what units is it
Angle at star between sun and Earth - arcseconds
163
What is a black body radiator
Perfect emittor and absorber of all possible wavelengths of radiation
164
Stars in terms of black and white bodies
Aproximately a black body
165
What is Stefan's law
Power output (luminosity) of a black body radiator is directly proportional to surface area and its (absolute temperature)^4
166
P = (sigma)AT^4 - what is P
Power output (luminosity)
167
P = (sigma)AT^4 - what is sigma
Stefan's constant
168
P = (sigma)AT^4 - what is A
Surface area
169
P = (sigma)AT^4 - what is T
Absolute temperature
170
What can stephans law be used to compare
Power output, temperature and size of stars
171
What is Wein's displacement law
Peak wavelength (lambda max) of emitted radiation is inversely proportional to the absolute temperature of the object
172
What is peak wavelength
Wavelength of light released at the maximum intensity
173
What does Wein's law show
Peak wavelength of a black body decreases as it gets hotter - so frequency increases so the energy of the wave increases
174
What can Wein's law be used for
To estimate the temperature of a black body source
175
Black body curve general shape
DIAGRAM
176
What is assued about the direction of emitted light
Light is emitted equally in all directions from a point so it will spread out in the shape of a sphere
177
Formula for P, I and d
I = P / 4(pi)d^2
178
I = P / 4(pi)d^2 - what is I
Intensity
179
I = P / 4(pi)d^2 - what is P
Power output from star
180
I = P / 4(pi)d^2 - what is d
Distance from star
181
What are spectral classes
Ways to classify stars based on strength of absorption lines
182
What are absorption lines dependent on
Temperature of star - energy of particles which make up star is dependent on temperature
183
What are hydrogen balmer lines
Absorption lines that are found in the spectra of O, B and A type stars - caused by excitation og hydrogen atoms from the n=2 state to higher/lower energy levels
184
Why are hydrogen balmer apsorption lines affected by temperature
Too high a temp and majority of hydrogen atoms become excited above n = 2 or electrons become ionised so hydrogen balmer lines won't become present, too low and won't become excited so no hydrogen balmer lines
185
Order of spectral classes
OBAFGKM
186
O spectral class colour
Blue, hottest
187
B spectral class colour
Blue
188
A spectral class colour
Blue/white
189
F spectral class colour
White
190
G spectral class colour
Yellow/white
191
K spectral class colour
Orange
192
M spectral class colour
Red, coldest
193
What is the hertzsprung-russel diagram
Absolute magnitude (y axis) and temperature in kelvin and spectral class (x axis)
194
Temperature scale on HR diagram
Logarithmic, halves at every interval
195
Absolute magnitude scale on HR diagram
Positive at bottom to negative at top because brighest stars have negative absolute magnitudes
196
What sort of star is the sun
Main sequence star
197
What spectral class is the sun
G
198
Absolute magnitude of the sun
4.83
199
HR diagram
Diagram
200
Evolutionary path of a main sequence star on a HR diagram
Move up and right to become a red giant, moves down and left to become a white dwarf
201
When will a main sequence star move up and right on a HR diagram
When it uses up all the hydrogen in its core
202
Red giant vs main sequence star
Red giant is brighter and cooler than a main sequence star
203
When will a rred giant move down and left on a HR diagram
Once it uses up all the helium in its core, it ejects all its outer layer and become a white dwarf
204
White dwarf vs main sequence star
White dwarf is hotter and dimmer than a main sequence star
205
What is a solar mass
Mass of the sun 2 x 10^30 kg
206
How are protostars formed
Clouds of gas and dust (nebulae) have fragments of varying masses that clump together under gravity, irregular clumps rotate and a gravity/conservation of angular momentum spins them inwards to form a denser centre - a protostar
207
Protostar structure
Surrounded by a disc of material (circumstellar disc)
208
How does a protostar become a main sequence star
Begins to fuse elements, produces a strong stellar wind that blows away any surrounding material
209
Why are main sequence stars stable
Inward force of gracity and outward force due to fusion are in equilibrium
210
What is fused in main sequence stars
Hyrdrogen nuclei
211
Relationship between mass and life of main sequence star
Greater mass = shorter main sequence period because it uses its fuel more quickly
212
Red giants
Smaller than 3 solar masses, when hydrogen runs out, temp increases and begins fusing helium nuclei into heavier elements (carbon, oxygen, beryllium), outer layers of star expand and cool
213
White dwarfs
Smaller than 1.4 solar masses, when red giant has used up all fuel, fusion stops and core contracts as gravity is now greater than outward force, outer layers gets thrown off, forming a planetry nebula around the remaining core, core becomes very dense, white dwarf eventually cools to become a black dwarf
214
Red supergiants
Bigger than 3 solar masses, when a high-mass star runs out of hydeogen nuclei it’s the same process as for a red giant but on a larger scale, collapse of a red supergiant in a supernova cause gamma ray bursts, can fuse elements up to iron
215
Supernova
Bigger than 1.4 solar masses, all fuel runs out and fusion stops, core collapses inwards very suddenly and becomes rigid (can't get any closer together), outer layers falls inwards and rebound off the core, launch them into space in a shockwave, as shockwaves pass through surrounding material, elements heavier than iron are fused and thrown off into space, remaining core depends on the mass of the star, has a rapidly increasing absolute magnitude, can release the same amount of energy as the sun in 10 billion years
216
Neutron star
Between 1.4 and 3 solar masses, when the core of a large star collapses,gravity is so strong that it forces protons and electrons together to form neutrons, incredibly dense
217
Pulsars
Spinning neutron stars that emit beams of radiation from the magnetic poles as they spin (up to 600 times per seconds)
218
Black holes
More than 3 solar masses, when core of a giant star collapses, neutrons are unable to withstand gravity forcing them together, gravitational force is so strong that not even light can escape, event horizon is the point at which the escape velocity becomes greater than the speed of light
219
What is the schwarzchild radius
Radius of the event horizon
220
Formula for schwarzchild radius
2GM / c^2
221
What is a binary system
When 2 stars orbit a common mass
