Astrophysics Flashcards
What are the 2 types of lenses
Concave and Convex
What does a concave lens do
(diverging lens) these spread out an incident beam into a diverging emergent beam
What does a convex lens do
(Converging) These focus incident rays at a point
Define Focal Length
Distance between the focus and the centre of the lens
Define Real Image
An image that can be formed on a screen
Define Virtual Image
The image can’t be formed on a screen
When is a real image produced
When an object is further away from the lens that the focal length
When is a virtual image produced
When an object is closer to the lens than the focal length
What is Cassegrain telescope arrangement
A parabolic concave primary mirror and a and a convex secondary mirror
Formula for magnification
M=Angle subtended by image at eye/ Angle subtended by object at unaided eye
In normal adjustment what is the angular magnification given by
The ratio of the focal length of the objective lens to the focal length of the eyepiece lens
Advantages of Refracting Telescope (x2)
- Because of their simple design they are easier to use and more reliable
- The system is more resistant to misalignment
Disadvantages of a Refracting Telescope (x4)
- Chromatic Aberration
- Impurities in the glass absorb some of the light, meaning very faint objects can’t be seen
- Large lenses are very heavy and can only be supported by their edges, leading to a distorted shape
- Large magnifications require large objective lenses and very long focal lengths
What is Chromatic Aberration and why does it happen
-It is the failure of light to focus at a point due to the lens focusing different colours over a range of focal lengths. Violet bends most, while red bends least
What is spherical aberration
It is the light rays being focused at slightly different positions due to the curvature of the lens/mirror
Advantages of a reflecting telescope (x3)
- Mirrors are lighter than lenses and therefore the telescope can easily be supported
-Mirror surfaces can be made very thin, giving excellent image properties
-Doesn’t suffer from chromatic aberration
-
Disadvantages of reflecting telescopes (x2)
- Can suffer from spherical aberration
- If the shape of the mirror isn’t quite parabolic, parallel rays reflecting off don’t all converge at the same point
Characteristics of Radio Telescopes
- Most are manoeuvrable allowing the source of the waves to be tracked
- Wavelength of radio waves is much longer than the wavelength of visible light- so for the same resolving power it must have a much bigger dish
- Easier construction than optical telescopes, as wire mesh can be used since the long wavelength waves won’t notice the gaps
- Located away from artificial sources of radio waves, but can be ground based
Where would UV telescopes be placed and why
In space, as UV is absorbed by ozone
Where would visible light telescopes be located and why
On earth, above cloud level- so that its away from clouds and light pollution
Where would an infrared telescope be located and why
High altitude, dry areas, such as in deserts as infrared is absorbed by water vapour in the atmosphere
Where would a radio wave telescope be located and why
Surface of air in a isolated area, as needs to be away from other radio waves
Define Resolving power
Smallest angle of separation at which 2 points can be distinguished (its the ability to produce separate images of closely spaced objects)
What is resolution limited by
Diffraction
In the diffraction pattern of concentric circles caused by the aperture, what is the central circle called
The airy disc
State the Rayleigh Criterion
2 point objects can be resolved if there angular separation is at least:θ ≈ λ/D. If the airy disc from one star coincides with the first diffraction minimum of the other, they can just about be resolved
What is collecting power proportional to
Collecting power is directly proportional to the square of the diameter
What does CCD stand for
Charge coupled Device
What is a CCD
A semi-conductor device where light is converted into digital information
What is quantum efficiency
The number of photons detected/ Number of photons incident x100
What is the quantum efficiency of a CCD compared to the eye
CCD=80%
Eye=4-5%
What does a high Quantum Efficiency mean
That the time needed to acquire an image of the same intensity relative to other imaging devices is much smaller, so CCDs require shorter exposure times
Compare the Eye to a CCD
- The eye can only detect visible light, whereas CCDs can detect infrared, visible and UV light
- CCDs are better for capturing fine detail as their spatial resolution is much smaller
- CCDs produce digital images which can be stored, copied and shared globally, however the eye doesn’t need any extra equipment
Define Apparent Magntiude
Based on how bright objects appear from earth
Define Absolute Magnitude
The apparent magnitude it would have at a distance of 10 parsecs from an observer
What does the brightness of a star depend on
Luminosity and distance from us
What is luminosity
The total energy emitted per second
What is intensity
The intensity of an object is the power received from it per unit area at Earth
What is the difference in intensity between each magnitude star on the Hipparcos Scale
2.512, the fifth root of 100
On the Hippacros scale which magnitude stars have the greatest intensity
those we a negative apparent magntide
What in the inverse square law formula for apparent brightness
P=L/4πd^2
P=Apparent Magnitude
L=Luminosity
d=dimeter
Why is brightness a subjective scale of measurements
Because we need to know how far away a star is from us in order to know its tree luminosity. Also apparent magnitude refers to the brightness of a star in the visible part of the spectrum
Why may the luminosity of very hot stars be greater than we can detect in the visible spectrum
Because very hot stars radiate much of their power outside the visible spectrum
what is 1AU
1 Astronomical Unit, is the mean distance between the Earth and Sun
What is 1ly
1 light year, is the distance light travels in a year
Define the Parsec
The distance from which 1AU subtends an angle of 1 arc second
IF you have a large parallax angle what does this tell you about the distance to the object
The greater the angle= the nearer the object is to you
State Stefan’s Law
The luminosity of a star is directly proportional to its surface area, and to the fourth power of its temperature
State Wien’s LAw
The hotter the black body, the shorter the peak wavelength of the curve
What is a black-body
A body that absorbs all wavelengths of electromagnetic radiation and can emit all wavelengths of electromagnetic radiation
Points for Drawing a Black-Body Curve
It mustn’t start as zero, because a wave with zero wavelength has infinite frequency therefore it has infinite energy, and this is not possible
- The left hand side must be steeper than the right-hand side
- The end of the right hand-side should be an asymptote
- Mark on the peak wavelength
What is the area under a black body curve approximately equal to
The power. And the area under the curve in the visible spectrum is equal to the brightness
What does the assumption that a star is a black body mean
It produces a continuous spectrum
Colour, Temperature and Prominent Absorption Lines for Spectral Class O
Blue
25,000-50,000
Helium+ Helium and Hydrogen
Colour, Temperature and Prominent Absorption Lines for Spectral Class B
Blue
11000-25000
Helium and Hydrogen
Colour, Temperature and Prominent Absorption Lines for Spectral Class A
Blue-White
7,500-11,000
Hydrogen Strongest
Ionised Metals
Colour, Temperature and Prominent Absorption Lines for Spectral Class F
White
6,000-7,500
Ionised Metals
Colour, Temperature and Prominent Absorption Lines for Spectral Class G
Yellow-White
5,000-6,000
Ionised and Neutral Metals
Colour, Temperature and Prominent Absorption Lines for Spectral Class K
Orange
3,5000-5,000
Neutral Metals
Colour, Temperature and Prominent Absorption Lines for Spectral Class M
Red
<3500
Neutral Atoms
Molecular Bands
How are Hydrogen Balmer Absorption Lines produced
- light from the star passes through the atmosphere of the star (1) which contains hydrogen with electrons in n = 2 state (1)
- electrons in this state absorb certain energies and (hence) frequencies of light (1)
- the light is re-emitted in all directions, so that the intensity of these frequencies is reduced in any given direction, resulting in absorption lines (1)
Describe the general shape of the Hertzsprung Russel Diagram and give the main characteristics of stars in the 3 stages
Long Diagonal Band= MAin Sequence, stars in this band are in their long stable phase where they are fusing hydrogen and Helium
Top Right= Red Giant and Red Super Giants. These have a low absolute magnitude and relatively low temperature and a huge surface area
Bottom Left= White Dwarfs, these have a high absolute magnitude, a high temperature and are small. They are at the end of their lives, where all the fusion reactions have stopped and they are slowly cooling down
On a HR diagram what should be plotted on each axis and what range of values should be marked
Y axis: Absolute Mag 15 to -10
x-Axis: Surface Temperature 50,000 to 2500K
Where is the SUn on the HR diagram
Main Sequence 5800K and an absolute magnitude of approximately 5
Defining Properties of a Supernova
Large and rapid increase in absolute magnitude
Defining Properties of a Neutron Star
Made of neutrons, very dense and rapidly rotating
Defining properties of a black hole
Volume of space within which Escape Velocity>Speed of Light
Why do Gamma rays burst as a supergiant changes stage
Due to the collapse of supergiant stars to form neutron stars or black holes
Compare the energy released by a Supernova to the total energy output of the sun
In a Type 1a Supernova around 10^44J of energy are released, which is the same as the energy output of the sun over its entire lifetime
What is a standard candle
A source with a known luminosity
What is a type 1a Supernova
A standard candle with a known peak absolute magnitude of -19.3
2 defining features of the light curve for a type 1a supernovae
A sharp initial peak
A gradually decreasing curve
What is the event horizon
The surface at which escape velocity=Speed of Light
Properties of the singularity
Finite Mass, Zero Volume and Infinite density
What does the Schwarzschild Radius Calculate
Radius of the event horizon
What is the Doppler Effect
The apparent change in the frequency of a wave caused by relative motion between source of wave and observer
What is a Quasar
They are some of the most distant and luminous objects in the universe, first detected using radio telescopes
They show a large optical red shift
Form from supermassive black holes
What does the Hubble Constant Represent
Gives the ratio of the recessional velocity to distance from Earth
Evidence for the Big Bang
- Cosmological Microwave Background Radiation
- Relative Abundance of Hydrogen and Helium
Why were Quasars discovered
Due to their bright radio source
What is an exoplanet
Any planet not in our solar system
Why are exoplanets hard to find
1) They are orbiting stars which are much brighter than them- the bright light from the stars they’re orbiting drowns out any light from the exoplanet
2) They are too small to distinguish from nearby stars, as the resolving power of our telescopes is too low
Methods of detecting Exoplanets
1) Radial Velocity
2) Transit Method
3) Infrared Direct Imaging
Describe the radial velocity method for detecting exoplanets
-It measures how much the emissions from the stars have been red or blue shifted
Planet and star orbit around common centre of mass that means the star to moves towards/away from Earth as planet orbits Causes shift in wavelength of light received from star
-From this the minimum mass of the exoplanet can be calculated
Describe how the transit method can be used to detect exoplanets
-Measures the change in apparent magnitude as an exoplanet travels in front of a star
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
-From this the radius of the exoplanet can be found
-Issue is that the probability of the 2 paths matching up is low
The life cycle of a star of a similar size to our sun
1) Stars are formed from large clouds of dust
2) Clouds of dust heat up and contract to form a protostar
3) When it reaches critical density- the outward pressure from fusion balances the stars gravity the star is in main sequence
4) When hydrogen runs out the material around the core heats up hydrogen sell burning, helium core burning=Red Giant
5) When the helium runs out and outer layer shed= Planetary Nebula
6) White Dwarf
Some scientists are concerned about the consequences for the Earth of a supernova occurring in a nearby part of the galaxy.
Explain the cause of this concern
Collapsing star can produce gamma ray bursts with energy similar to total output of Sun
Highly collimated – if in direction of Earth, could cause mass extinction event
