Astrophysics Flashcards
How to calculate the length of a telescope
f0+fe
How to calculate magnification
M=f0/fe
How to calculate angular magnification
M= angle subtended by image / angle subtended by object
How to reflecting/cassegrain telescopes work?
Reflecting telescopes use a parabolic mirror to focus incoming light into a point
A secondary mirror is placed before the focal point to reflect the rays back through a gap in the mirror
The rays cross over then pass into a lens
The rays emerge parallel
What is the structure of a CCD?
A series of silicon picture elements (pixels) which are very small
Beneath each is a potential well which traps electrons
Above each is a filter which only allows certain colour photons through
How do CCDs work?
-The filter allows only certain wavelengths of photon to hit the pixels
-The photons will cause electrons in the pixels to be released into the potential wells
-Amount of electrons released is proportional to the intensity of the light incident (number of photons)
-The charge is then collected from each well
-The amount of charge and colour of filter tells the computer what colour and brightness needs to be displayed
What is quantum efficiency?
When the number of photons incident on the surface causes a response
What percentage of light does a CCD detect compared to the eye and photographic film?
CCD - 80%
Photographic film - 4%
Eye - 1%
What are the benefits of CCDs?
-Film can get saturated with the too much light ruining the image, CCDs don’t get saturated.
-CCDs detect a wider spectrum of light than human eye( infrared, visible and UV)
-Can have long exposures to capture faint images
-Capture finer detail as higher resolvable distance
What is chromatic aberration?
Light reflects by different amounts depending on its wavelength
So different colours focus incoming light into different places after going through lens
short wavelengths like blue focus too close to lens
Red wavelengths focus too far away
This causes colour bleeding
Problems with refracting telescopes
Chromatic aberration
Impurities
Lens distortion
Length of telescope
Explain impurities on refracting telescopes
Any bubbles or impurities in the glass absorb and scatter light so faint objects can’t be seen
Explain lens distortion in refracting telescopes
Large lenses are heavy and can only be supported at the edge so the shape can become distorted
Explain how the length of refracting telescopes is a problem
For large magnification long focal lengths are needed so telescopes are very long requiring big expensive buildings
What are the advantages of reflecting telescopes?
Cost- mirrors are cheaper than large lenses
Support structure - less likely to distort due to light not passing through mirror, meaning they can be supported from the back
Collecting lower- telescope is large so more light can be collected - easier to make large
Resolving power
What are the disadvantages of reflecting telescopes?
Spherical aberration
Second mirror
What is spherical aberration in reflecting telescopes?
Occurs when mirror isn’t shaped as a perfect parabola
Outer rays focus too close
Inner rays focus too far away
Causes blurry images
Why is the second mirror a disadvantage in reflecting telescopes?
The secondary mirror may block and diffract some incoming light
Decreasing image clarity
What are the optical advantages of reflecting and refracting telescopes?
Refracting telescopes:
-No secondary mirror blocking light
Reflecting telescopes:
-Less chromatic aberration
-Better collecting and resolving power
What is the Rayleigh criterion?
Two light sources can be distinguished if the centre of the airy disc from one source is at least as far away as the first minimum of the other source
How is resolving power calculated?
Smaller angle = more powerful
Theta= lambda / D
D = diameter of telescope
What is the structure of visible, UV and infrared telescopes?
-A parabolic dish focuses electromagnetic radiation onto a point
-Visible, UV and infrared telescopes place a CCD at this point
What is the structure of radio telescopes?
A parabolic dish focuses EM radiation onto a point
A combination of amplifiers to boost weak signals and a tuner to focus on specific frequencies
Perfection of non optical telescopes
An imperfection cannot be greater than 1/20 th of the wavelength
UV telescopes has to be the most perfect making them more expensive
Radio can be the least perfect so they are cheaper and much large
Radio dishes often not solid but made of mesh
Resolving power of non optical telescopes
UV and Xray telescopes have a much higher resolving power
Radio have the worst
However resolving pier is inversely proportional to the diameter
A bigger telescope means better resolving power
This is easier with cheap radio telescopes than expensive UV
Collecting power of non optical telescopes
Radio telescopes have the best collecting power
Because large telescope area can collect more photons
So can see the dimmest stars
UV and Xray telescopes tend to be small as expansive to make, so worst collecting power
Where are radio and visible telescopes placed?
Can be on the ground because visible and radio waves can pass through atmosphere
Some visible light is blocked so more effective in space
Where are IR UV and Xray telescopes placed?
Atmosphere blocks most infrared ultraviolet and Xray waves so must be put in space
Although can put infrared telescopes on top of mountains or on planes
How to do the parallax method?
Use trig with the angle of parallax and one astronomic unit for the radius
What is an arc second?
1/3600 of a degree
What is a light year?
The distance that light travels in one year
How to calculate intensity?
I=power/area
I=p/4pi r^2
What is apparent magnitude?
The brightness as viewed from earth
What is the brightness ratio equation?
I2/I1 = (2.51)^(m1-m2)
5th root of 100≈2.51
What is absolute magnitude?
How bright a star appears from 10 parsecs away
What is the distance and magnitude equation?
m-M=5log(d/10)
Remember d is measured in parsecs
Why are type 1A supernovae unique?
When they expose and rapidly increase in brightness they all have the same peak in absolute magnitude
So combined with apparent magnitude from earth we can calculate the distance to them
Peak at -19.3
Called standard candles
What is the definition of a black body?
A body which emits and absorbs all types of EM radiation
What is stefans law?
P= sigma A T^4
What are hydrogen balmer lines?
The absorption lines created by electrons in hydrogen at level 2 or above
What are the stellar spectral classes?
OBAFGKM
What are the temperatures of the stellar spectral classes?
O 50000-25000
B 25000-11000
A 11000-7500
F 7500-6000
G 6000-5000
K 5000-3500
M >3500
What are the colours of the stellar spectral classes?
O Blue
B Blue
A Blue-White
F White
G Yellow-White
K Orange
M Red
What are the absorption lined of the stellar spectral classes?
O Strong He and He+ , weak H
B Strong He and H
A Strongest H lines, ionized metals
F Strong ionized metal lines
G Strong neutrals metal and ionized metal lines
K strong neutral metal lines
M strong neutral atom and molecular compound lines
What is the scale for Hertzsprung Russell diagrams?
Y axis - absolute magnitude +15 at bottom to -15 going down in 5s
X axis - temperature 50000-2500 K
Where does the sun go on a HR diagram?
6000K
+5
Where are red giants and white dwarfs on a HR diagram?
Red giants - top right
White dwarfs - bottom left
How is a star born?
Stars are born in clouds of dust and has known as stellar nebula
Usually left from previous supernovae
The denser clumps contract slowly due to gravity
How does a protostar form?
The clumps from a stellar nebula get dense enough to form protostars
These continue to contract and heat up.
What is core hydrogen burning?
The protostar becomes hot and dense enough to fuse hydrogen into helium.
Pressure produced by this balanced out gravitational collapse
When does a star become a red giant?
The hydrogen from core hydrogen burning runs out so fusion stops
Core contracts and heats up
Outer layers expand and cool
What is shell hydrogen burning?
After core contacts and heats up,
Material around core contains hydrogen
Due to contracting the shell around core is hot enough to fuse hydrogen into helium
What is core helium burning occur?
Core continues to contract until it’s hot and dense enough to fuse helium into carbon and oxygen.
This releases oxygen which pushes outer layers outwards
What is shell helium burning?
Eventually the helium in the core runs out and fusion stops
Forces unbalanced so core contracts again
This heats shell around core so it’s hot enough for helium into carbon it to fuse
How do electrons prevent stars from contracting more?
They exert enough pressure to stop it collapsing further
How is a planetary nebula and white dwarf formed?
After shell helium burning the star pulsates and throws off outer layers into a planetary layer.
This leaves behind a very hot and dense core known as a white dwarf
No more fusion occurs
Overview of stellar evolution
Stellar nebula
Protostar
Core hydrogen burning
Red giant
Shell hydrogen burning
Core helium burning
Shell helium burning
Planetary nebula/white dwarf
What happens when a star is less than 1.4 solar masses?
Fusion stops and core collapses
Gravitational force < electromagnetic repulsion (between electrons in atoms)
This is called electron degeneracy pressure
Results in a white dwarf
What happens when a star is between 1.4 and 3 solar masses?
Gravitational force > electrostatic repulsion between electrons
Electrons are forced into protons creating neutrons
However the gravitational force is not greater than repulsion between the neutrons (SNF)
Results in **neutron star **
What happens if a star is greater than 3 solar masses?
Gravitational force > electrostatic repulsion
Gravitational force > SNF after neutrons formed
Results in black hole
What are the features of neutron stars
-Primarily made up of neutrons
-20km diameter
-Same density as a nucleus: very high
-Rotate 600 times per second
-Emit radio waves as they rotate
What is the accretion disc?
A flat disc of matter that is spiralling into the black hole
Pieces of planets and stars that are being shredded
Hot and bright
What is the schwarzchild radius?
Where gravity is so strong that
Escape velocity = speed of light
What is the singularity?
The original stars code
Infinitely dense point
How to derive shwarzchild radius equation?
Escape velocity = c
Sqrt(2GM/r)=c
Etc
What’s a quasar?
The accretion disc around a supermassive black hole
Produced intense radio signals
Absorption lines did not match known elements
Known as active galactic nuclei
Define the Doppler effect
The apparent change in the frequency of a wave caused by relative motion between the source and the observer
Define red shift
The increase in the wavelength of EM radiation due to a relative recessive velocity between the source and observer
What is cosmological red shift?
Red shift caused by the fact to e universe is expanding
Distant galaxies appear red shifted due to them moving away as the space between expands
When calculating change in lambda over lambda what do you put on the bottom of the fraction?
The lab/normal source
What are the two key principles of the cosmological principle?
The universe is homogenous (every part is the same)
The universe is isotropic ( everything looks the same in every direction)
How to start the calculation of the age of the universe?
v=d/t
v=Hd
d/v=1/H
So t=1/H
How to use Hubbles law?
V=Hd
Velocity is kms^-1
Distance is Mpc
Evidence for the Big Bang
Increasing red shift of distant galaxies
Cosmic background radiation
Abundance of hydrogen and helium
How is large abundance of hydrogen and helium evidence for the Big Bang?
Early universe just have been hot and dense enough for hydrogen fusion to occur
This can’t have lasted long
Hydrogen would have formed first as it is the simplest element
Presence of helium due to fusion
So universe previously much hotter
How is cosmic background radiation evidence for the Big Bang?
The EM radiation produced by the Big Bang cannot have escaped the universe
As the universe expanded the EM radiation got stretched out resulting in microwaves
Why is dark energy controversial?
Cannot be seen
What does dark energy cause?
Causes universe to accelerate
Why can’t we see binary stars through direct observation?
No telescopes have a high enough resolving power
What’s an exoplanet?
Planet outside our solar system
What is the transit method?
Where the light received from a star system as the stars and planets orbit in front of each other
What is the radial velocity method?
An exoplanet orbiting a star causes the star to wobble
This causes red and blue shift
So we can use this to estimate time period and mass
Why is transit method hard?
Only works if the star or exoplanet passes between the star and the Earth
Why is the radial velocity method difficult?
The mass at the planet barely moves the star so red shift must be found
Define the Hubble constant
Gives the ratio of the recessional velocity of galaxies to distance from earth
What is the assumption used when using Hubbes law to estimate age of universe?
The universe is expanding at a constant rate
When comparing apparent magnitude what does a lower apparent magnitude mean?
Brighter
Define a supernova
An object which produced a rapid increase in brightness
Explain what is meant by a parsec
The distance at which 1Au subtends an angle of 1 arc second
