Astrophysics Flashcards
1
Q
what are the 2 types of optical telescope and what lens do they use
A
refracting and reflecting telescopes. they both use convex lenses
2
Q
what’s another name for a convex lens
A
a converging lens
3
Q
what are the 5 key features on a ray diagram
A
principle axis
lens axis
principal focus
axial rays
focal length
4
Q
what is an axial ray
A
a ray that is parallel to the principal axis
5
Q
what are the 4 key steps when drawing ray diagrams with 1 lens
A
-draw an axial ray from the top of the object, and is refracted and goes through the focus point
- draw a ray from the top of the object straight through the centre of the lens axis
-draw a ray through the focal point on the left side and parallel from the lens axis
-draw the object
6
Q
what is the scenario for an object more than 2f away
A
image position-between f-2f
orientation- inverted
magnified/diminished-diminished
application- camera
7
Q
what is the scenario for an object at 2f
A
image position- at 2f
orientation- inverted
magnified/diminished- same size
application- inverter
8
Q
what is the scenario for an object between f-2f
A
image position- beyond 2f
orientation-inverted
magnified/diminished- magnified
application-projector
9
Q
what is the scenario for an object closer than f
A
image position- same side as object
orientation-upright
magnified/diminished-magnified
application-magnifying glass
10
Q
what is the difference between real and virtual images
A
real images of inverted and virtual ones are not.
11
Q
in the lens equation, what does it mean when v is negative
A
its a virtual image. if its positive-its real
12
Q
how do you calculate the length of the telescope
A
fo+fe
13
Q
how do you calculate magnification
A
M=fo/fe
14
Q
how do refracting telescopes work
A
they use 2 lenses to form a magnified image. objective (larger/closer to image) and eyepiece (smaller/ further from image)
15
Q
how do you calculate to get the greatest magnification
A
having the objective focal length a high as possible
16
Q
how would you get a clear image
A
make sure the 2 focal lengths meet at the same point
17
Q
what are the steps for drawing a ray diagram with 2 lenses
A
-Draw a non-axial ray through the centre of the
objective lens axis to the eyepiece lens axis
-Draw two parallel rays either side
-These should meet at the focus and then stop on the
eye lens axis
-Draw a construction line that starts from where the rays cross over and passes through the centre of the eyepiece lens
-The three rays emerge parallel to this line
18
Q
how do you calculate magnification with angles
A
M= angle subtended by image/
angle subtended by object
19
Q
how do reflecting telescopes work
A
they use a parabolic mirror to reflect the rays instead of lenses.
a secondary mirror is placed before the focal point which reflects the rays through the hole in the mirror, emerging parallel
this is know as the cassegrain telescope
20
Q
what is a CCD
A
CCD-charged coupled device
used to take digital photos eg. camera
21
Q
what are the physical characteristics of a CCD
A
consists of a series of silicon picture elements (pixels), which are very small (not pixels on a screen)
beneath each one is a potential well, which traps electrons. and above is a filter that only allows certain colour photons through
22
Q
how do CCDs work
A
- filter only allows certain wavelengths of light to hit the pixels
- photons cause electrons to be released into the potential wells. electrons released is proportional to intensity of the photons
- the charge is then collected in the wells. and the amount of charge and the filter tells the computer what colour and brightness is displayed
23
Q
what is quantum efficiency equation
A
number of photons detected/ X100
number of photons incident
24
Q
what are the advantages of CCDs (quantum efficiency)
A
quantum efficiency=
CCDs- 80%
photographic film- 4%
naked eye- 1%
25
what are the advantages of CCDs
exposing film to too much light it gets saturated, which a CCD doesn't
CCDs can detect a larger range of light (invisible spectrum)
the minimum resolvable distance is 10microm compared to 100microm for naked eye (CCDs see finer detail)
CCDs have longer exposure to capture fainter images
26
what is chromatic aberration
light is reflecting different amounts through a lens due to its wavelength. blue light forms to close to the lens whereas red from to far creating a bleeding affect
27
which telescope has a bigger problem with chromatic aberration
refracting due to its larger lens
28
what are the 3 other problems with refracting telescopes
impurities-bubbles and impurities will scatter the light affecting faint objects
lens distortion- lenses are very heavy and can only be supported at the edge creating distortion
length of telescope- for a better magnification the focal length needs to be much larger so large expensive buildings are needed to house them
29
what are the 4 advantages of reflecting telescopes
cost-large mirrors of good quality or cheaper then lenses
support structure- mirrors can be supported from the back decreasing change of distortion
collecting power-the larger the telescope the more light collected and the dimmer the object can be seen
better resolving power-
30
what are the 2 disadvantages of a reflecting telescope
spherical aberration- happens when the mirror isn't a perfect parabola. when the outer rays form too close and the inner rays form to far, causing blurry images.
second mirror-a secondary mirror can also block light and decrease image clarity
31
what happens to light when diffracted through a circular opening
creates a circular diffraction pattern consisting over light (maxima) rings and dark (minima) rings and the central maxima called the airy disc
32
what is the Raleigh Criterion
how to distinguish of stars are resolved or not. two light sources can be distinguished if the centre of one of the airy discs is at least one minima away from the other source
33
how can you calculate resolving power
angle=wavelength/d
a smaller angle means a more more powerful telescope. the smaller the wavelength and larger the diameter the better.
34
what other unit may be used when measuring the angle subtended by an object in space
arc seconds
1 arc second= 1/3600 of a degree
35
why are non optical telescopes built
because of so much radiation in the universe
eg. microwave from cosmic background
- x-ray from blackholes
36
what is the structure like in non optical telescopes
nearly all use a parabolic dish to focus EM radiation to a point. visible/uv/ infrared will use a CCD at this point but radio will use amplifies to boost weak signals and a tuner to focus on specific frequencies
x-ray use grazing mirrors so they are not absorbed making the telescopes very longer. a geiger counter/CCD is used a a detector
37
what is perfection in a non-optical telescope
an imperfection cannot be greater than 1/20th of the wavelength. (bumps and holes)
because UV is the shortest EM wave it has to be the most perfect and radio is this least making this better for radio telescope
38
what is resolving power in a non-optical telescope
a higher resolving power means a smaller minimum angle when resolving 2 stars. directly proportion to wavelength
so shorter wavelengths and bigger telescopes give better resolving power - radio isn't good for this
39
what is collection power in a non-optical telescope
linked to the area- the more area, the more photons can hit it. allowing us to see dimmer stars (less of there photons reach us) radio is the best for this as they are the cheapest to make
collection power is proportional to d^2
40
how does location affect non-optical telescopes
the atmosphere blocks some wavelengths so UV/infrared/x-ray are put in space. radio and visible light telescopes can be put on ground (cheaper)
41
what is a disadvantage of only infrared
they will heat up and need cooling down. coolant will only last a few years
42
what is the parallax method
comparing the view of stars form opposite sides of earth, to get a more accurate placement of them in the sky. distant stars won't appear to move but closer ones will appear in different positions
43
what is an astronomical unit
the centre of earth to the centre of the sun (r)
44
how to get the distance from earth to the star
using r (1Au) and the angle of parallax, using trig can calculate the distance. you can also use small angle approximations
45
what is a parsec
an object is exactly one parsec away when 1Au subtends an angle of 1 arcsecond
46
what is a light year
c=3.00 x10^8
the distance light travels in one year is 1 light year
47
What two things affect a stars brightness
-its power output
-how far away it is from us
48
What is power output/luminosity
The total amount of energy emitted (in EM radiation form)
It’s measured in watts
49
What is the intensity equation
Intensity= power/area
50
what is apparent magnitude
The brightness of a star when viewed from earth
51
What is the hipparcos scale
An apparent magnitude scale going from 1-6
1 being brightest and 6 being dimmest
A star with an apparent magnitude of 1 is 100 times brighter than a star- apparent magnitude 6
It’s a logarithmic scale
52
What is the modern hipparcos scale
It keeps the logarithmic 2.51 increments but extends the scale, brighter objects being negative and dimmer objects exceeding 6
53
What is the brightness ratio equation
I2/I1=2.51^m1-m2
54
What is absolute magnitude
How bright a star is when it’s put 10 parsecs away. This is fairer as it has nothing to do with earth
55
What apparent and absolute magnitude does our sun have
Apparent- -27
Absolute- 4.8
56
What is a type 1a supernova
Also known as standard candles
- unique as when they explode all have the same peak absolute magnitude (-19.3)
-because they are so bright we can see them from very far away, allowing us to measure the distance to distant galaxies
57
Draw a type 1a supernova light curve
(Check on century)
Scale:
Y-axis = 0–20 (Absolute magnitude)
X-axis= 0-300 (Time in days)
58
59
What is a black body
An object that absorbs all types of EM radiation form
60
What is black body radiation
The radiation stars emit, they emit all types of EM radiation but not in equal quantities. Giving it the collective name black body radiation
61
What is a black body radiation curve
It shows the amount of radiation being produced by each wavelength
(Look on century)
62
How to temperature link into a black body radiation curve
Hotter stars produce more radiation and peak at a shorter wavelength so they appear more blue whereas cooler stars appear more red and have a lower intensity
63
What do extremely hot objects look like on black body curves
Extremely hot objects (events horizon in black holes) can peak in the gamma. Therefore it will appear less bright if most of the radiation produces is invisible
This emphases the use of non-optical telescopes
64
What are the 2 main things that affect the power output of a star
The temperature of the star
The surface area of the star
65
What are the 7 spectral classes
O B A F G K M
66
What are the 7 classes based on
Surface temperature
There absorption spectra
67
How are absorption spectra formed
Electrons in a gas are able to absorb very specific energies of photons, unique to that element. Because stars are black bodies they produce a continuous spectra. So the atmosphere around them absorb some of the photons produce creating the absorption spectra. (See which photons were blocked)
68
How are absorption spectra affected by temperature
The hotter the star the more energy each electron has, so they sit in higher energy levels. Therefore there are less photons that can be absorbed so there are less absorption lines.
69
What are hydrogen-balmer lines
The absorption lines created by electrons in hydrogen at level 2 only. There are no electrons in ground state as its too hot
70
How does temperature affect hydrogen- balmer lines
Hotter stars wont have very intense lines as most electrons are in n=3
Cooler stars wont have very intense lines as most electrons are in n=1
A class is where there most intense as the majority of electrons are in n=2, therefore most intense
71
What is a hertz-sprung Russell diagram
A graph that measures absolute magnitude against temperature. All known stars are plotted on these axes and 3 main groups were found
72
What are the 3 groups on a hertzsprung-Russell diagram
The long diagonal section- main sequence stars
Top right corner- red giant
Bottom left corner-white dwarfs
73
Where does our sun lie on a hertzsprung-Russell diagram
Temperature of 5800K—G class
Absolute magnitude of 5
74
How do you draw a hertzsprung-Russell diagram correctly
Y-axis goes from 15 to -15
X-axis goes from 50000K-2500K
The main sequence area is slightly flat in the middle
75
When would you class a star as alive
When fusion is taking place, this depends on
- do you have fuel for fusion (hydrogen and helium)
-is it hot enough in the star for the fuel to fuse
If these conditions aren’t met then the star dies
76
What is the life cycle for a star like our suns (long)
1. Stellar nebula- born in clouds of dust and gas, usually leftover from a supernova, denser clumps contract (gravity)
2. When clumps are dense enough they form a protostar. Continues to contract and heat up. Reaches a few million degrees, hydrogen can fuse into helium
3.main sequence, spends most of its life, pressure from fusion balances out gravitational collapse. (Core hydrogen burn)
4. Hydrogen runs out so fusion stops. Core contracts and heats up. Outer layers expand and cool. Star becomes red giant.
5.material around the core contains hydrogen, as core contracts (heats up) fusion starts again. (Shell hydrogen burn)
6.core continues to contract until its dense and hot enough to fuse helium into carbon/ oxygen. (Core helium burn). Release huge amount of energy pushing away outer layers
7.eventually helium in the core stops, causing forces to unbalance. Heating shell around core, which now starts fusion again (shell helium burn)
8. Low mass stars it wont get hot enough for anymore fusion, contracts to earth size, electrons exert enough pressure to stop it further collapsing
9.core contracts (more unstable). Stars pulsate and throw outer layers into planetary layer. Leaving behind white dwarf. (Dead)
77
What is a supernova
A star whose luminosity increases rapidly and enormously due to exploding, occurs when the core is greater than 1.4 solar masses
78
What happens during a supernova
The outer layers of the star fall and rebound, setting off huge shockwave. Causing a massive increase in absolute magnitude and bursts of gamma rays, leaving behind either a neutron star or a black hole
79
What fusion happens when a star is greater than 1.4 solar masses
They can fuse heavier elements such as carbon and oxygen until they reach iron. (No longer getting energy out of fusion) where the core collapses and explodes.
80
What is a type 1a supernova and why are they useful
Unique because they all have a rapidly brightness, peaking at -19.3 (absolute magnitude)
This means we can calculate the distance to them from earth.
Also known as standard candles.
Because they are so bright we can use them to measure the distance to distant galaxies
81
Why do stars of solar masses greater than 1.4, not turn into white dwarfs
A star is a delicate balance of pressure from fusion against inward gravity.
Without fusion, the core collapses until subatomic forces of electromagnetism and strong nuclear forces takes over.
But the more massive the star, the stronger the force of gravity, so subatomic force may not be enough.
82
What happens to a star less than 1.4 solar masses
Fusion stops, core collapses, gravitational force is less than the force of electromagnetic repulsion between electrons in atoms. (Electron degeneracy pressure). The core only contracts to size of earth= white dwarf
83
What happens to a star that’s between 1.4-3 solar masses
Fusion stops, core collapses, gravitational force is greater than repulsion between electrons. Electrons are forced into protons creating neutrons. Gravitational force isn’t stronger than repulsion by the strong nuclear forces between neutrons. Core contracts to around 20km in diameter = neutron star.
84
What happens to a star greater than 3 solar masses
Fusion stops, core collapses, gravitational force is greater than repulsion between electrons. The gravitational force is greater than the repulsion caused by strong nuclear forces between. No other repulsive forces to stop gravity. Nucleons are forces into other nucleons. Core collapses into an infinitely dense and infinitely small singularity = black hole
85
What are neutron stars
Made up of primarily neutrons. They have the same density as a nucleus despite being 20km across. (4x10^17)
They rotate 600 times a second emitting radio waves when they rotate. Sometimes sweeping past earth like a lighthouse. These pulsing neutron stars are called pulsars
86
What is the accretion disc
A flat disc of matter that is spiraling into the black hole, it consists of pieces of planets and stars that are being shredded. It’s extremely hot and bright
87
What is the Schwarzschild radius (event horizon)
The point where gravity is so strong that the escape velocity equals the speed of light.outside the event horizon the escape velocity is less than c, so things can be seen but inside the event horizon the escape velocity is greater than c so things can’t be seen, therefore being named a black hole. When in the event horizon there’s still matter but light from it will never reach us
88
What is the singularity
This is the remains of the original stars core, which is now an infinitely dense point with a volume of 0, at the center of the event horizon. It is not visible.
89
What is the escape velocity equation
V=(2GM/r)^1/2
Kinetic energy= gravitational potential energy
90
How do you derive the Schwarzschild radius
Start with the escape velocity equation
Change v to c (3.00x10^8)
Rearrange to make r the subject
91
What are quasars
Discovered in the 1950s, thought to be nearby stars as they were so bright. But they had many strange properties.
-produced very intense radio signals
-absorption lines didn’t match any known element
-there spectrums didn’t fit a black body curve
Hence there name quasar (quasi-star) meaning star like
92
How were quasars discovered to be not in our galaxy
Someone noticed that the absorption lines were hydrogen balmer lines that have been massively red-shifted. Because they were shifted so much this suggested they weren’t stars in our galaxy. But were in fact the most distant objects ever seen
93
What are the current theories on what a quasar is
The large red-shift suggests its very far away
They appear as bright as near stars, suggesting they are extremely bright
Theorised they are accretion discs around supermassive black holes
Roughly the size of our solar system
Produce 10 times more power than our galaxy
The black hole must consume 10 suns per year to produce the energy observed
Some eject jets of material (active galactic nuclei), and when aimed at us are called blazers
94
What happens to waves when the source is stationary
All observers around will hear the same frequency of sound
95
What happens to waves when the source is moving
In the direction that the source is moving, the waves getting bunched up and others will spread out. Where the waves are bunched up they will arrive at a higher frequency
96
What is the Doppler effect
The apparent change in the frequency of a wave caused by the relative motion between the source of the wave and the observer
97
What is red shift
The light coming from stars moving away from us is shifted to the red end of the spectrum and the light coming from stars moving towards us is shifted to the blue end of the spectrum.
98
What is red shift (definition)
Red shift is the increase in the wavelength of EM radiation due to the relative recessive velocity between the source and the observer. The greater the stars recessional velocity, the greater the change in the frequency of the wave
99
How do absorption lines red-shift
If a star is moving away from us the absorption lines are shifted towards the red end of the spectrum. The faster they are moving away, the greater the shift
100
What is cosmological red shift
Red shift is usually caused by a star moving away from us, however red shift can also be caused by the fact the universe is expanding. This is known as cosmological red shift and is only noticeable over large distances
101
What is the data used for Hubbles law
He used type 1a supernovae to measure the red shift and distance to galaxies. He noticed that galaxies further away had a greater recessional velocity. The fact they were proportional suggests the universe is expanding
102
What is the cosmological principal
We are not the center of the universe. The principal suggests that no part of the universe is more special than the other
On a large scale:
Homogenous- every part is the same
Isotropic- everything looks the exact same in every direction
103
What is Hubbles constant
Plotting recessional velocity against distance, the gradient gives you Hubbles constant (65-80). If a value isn’t given use the data sheet, otherwise calculate it.
104
How can you find the age of the universe
V=Hd is very similar to St=d you need to find 1/H however you first need to convert units
105
What are 3 pieces of evidence that is evidence for the Big Bang
1. Increasing red-shift of distant galaxies
2. Cosmic background radiation
3. Abundance of hydrogen and helium
106
What is cosmic background radiation
As the universe expanded the EM radiation got stretched into the microwave region, which is why space isn’t at absolute zero but 2.7K
This is found everywhere.
107
Why is there an abundance of hydrogen and helium
At the Big Bang it theorised it was extremely hot and dense, causing hydrogen fusion to occur. This couldnt have lasted long.
Ratio 3H : 1He
108
What evidence is there for dark energy
Observations of the universe expanding show that it’s actually accelerating. The current theory is there is an energy source causing the acceleration but we cannot see it with any part of the spectrum. Hence named dark energy.
This came about when a type 1a supernova was dimmer than it should’ve been. Indicating the universe is expanding faster than when the supernova exploded. (The light had top travel further to reach us than expected with a constant rate of expansion)
109
What is a binary star system
The majority of solar systems observed contain 2 (binary) or 3 (trinary) stars, making our solar system unique with only 1.
110
What are exoplanets
Planets that exists outside of our solar system.
111
Why can’t we see binary stars through direct observation
With our current technology, its impossible to see binary stars as separate stars as no telescope has a high enough resolving power
112
Why can’t we see exoplanets through direct observation
Exoplanets are also hard to directly see because:
-they orbit stars much brighter than the exoplanet
-they dont emit their own light
- too small to distinguish from nearby stars
113
How does the transit method work for binary star systems
-usually one star will be brighter than the other
-as they orbit each other, they will block some light
-transit method measures how much light is received from a star system
-you can use this method to work out the time period of one orbit
114
How does the transit method work for exoplanets
-When an exoplanet passes in front of a star it blocks some of the light
-the drop is much smaller than with binary stars
-you can also measure the time period with this orbit too
- there is only a single drop in light as the planet produces no light
115
What’s is the main disadvantage with the transit method
Only works if the star/exoplanet passes between the star and earth and blocks some light. Meaning we could potentially be missing a lot of exoplanets and binary stars based on the plane of the orbits
116
How does the radial method work with binary stars
-As binary stars orbit around each other, there are points where one star is moving towards earth and the other is moving away
-when moving away, the light from that star is being red-shifted
-when they are moving towards us the light will be blue-shifted
117
How does the radial method work for exoplanets
- as the exoplanet orbits around the star it pulls on it causing the star to wobble
-when it wobbles away from us its red-shifted
- when it wobbles towards us its blue-shifted
- its possible to use this method to find the time period of the orbit and the mass of the planet
- again only works if its orbiting in the same plane
