Astrophysics (Optional) Flashcards
Why is a virtual image formed here?
The light rays do not pass through the point where the image forms (rays don’t converge)
Why do these two object appear the same size?
The angle subtended at the eye is the same
Generally what does a telescope need to do to magnify an object?
Increase the angle subtended at the eye
Copy the diagram and label:
- Angles α and β
- Focal lengths
- Focal points
- Two lens names
Copy the diagram and label:
- Where a real image forms
- Where a virtual image forms
Draw a labelled diagram of the refracting telescope in normal adjustment
Don’t forget the dotted construction line!
What is wrong here?
Rays from eyepiece lens must be parallel to dotted construction line
How is the length of a refracting telescope calculated?
Add the two focal lengths
How is angular magnification of a telescope calculated?
- What is chromatic aberration?
- Sketch a diagram to show it
Different wavelength’s refract different amounts
Focal point different for different colours
- What is spherical aberration?
- Sketch a diagram to show it
Refraction increases towards the edge of the lens
The focal point is different for rays passing through different parts of the lens, the rays aren’t focusing properly
What is the resolution of a telescope?
Ability to distinguish between two points separated by angle
The smaller the resolution the better
How many stars are seen?
2 because the stars are resolved
How many stars are seen?
1 because the stars aren’t resolved
How many times greater is this telescope’s collecting power?
4x
P proportional to SA
What is apparent magnitude (m)?
A measure of how bright a star appears in the sky
What is absolute magnitude (M)?
The apparent magnitude a star would have if it was 10 parsecs away
What is the Hipparcos scale?
Scale of apparent magnitude of 1 → 6
- 6 → naked eye limit (dimmest star)
- Each step = x2.51 (x(100)1/5
How does a 4th magnitude star compare to a 7th magnitude star?
3 magnitudes difference
So apparent magnitude (m) = 2.513 = 15.8 x brighter
(4th magnitude is brighter)
How could star B be bigger than star A?
Star B could be much further away
So appear dimmer from Earth
What does apparent magnitude correspond to?
The intensity of the star from the surface of the Earth
What does absolute magnitude correspond to?
The power output of the star
Define the lightyear
The distance light travels in a year
Define the parsec
Distance to star with parallax angle of 1”
A distant star has a parallax angle of 3”
Calculate how far away it is?
Make a right angled triangle
What information does m-M provide? (magnitude difference)
The smaller m-M → the closer the object
If m-M < 0 …
If m-M < 0 → star less than 10pc
If m-M = 0 …
If m-M = 0 → star at 10pc
If m-M > 0 …
If m-M > 0 → star further than 10pc
What’s wrong with this calculation?
d must be in parsecs!!!
Which star appears brighter?
Betelgeuse → has a smaller apparent magnitude (m)
Which star has a greater power output?
Betelgeuse → has a smaller absolute magnitude (M)
Which star is closer?
Bellatrix → m-M is smaller
What is redshift?
Absorption lines of galaxy moving away are shifted to the left (red end)
Bigger velocity → Bigger redshift
What is blueshift?
Absorption lines of galaxy moving towards are shifted to the right (blue end)
Bigger velocity → Bigger blueshift
How can we potentially tell if a galaxy is rotating?
One side is redshifted
Other side is blueshifted
If a galaxy does not have one side redshifted and one side blueshifted
could it still be rotating?
Yes, if the rotation isn’t along the line between the galaxy and Earth
In the equation for doppler shift what do the different λ represent?
λ0 → Wavelength if observer was stationary
∆λ → observed λ - λ0
How do you calculate the velocity of a galaxy given its redshift?
v = zc
Is a redshift velocity the true velocity of a star?
No, the star could have a v|
For this binary star system
- When is redshift of star A maximum?
- When is redshift = 0?
- When is blueshift maximum?
What is Hubble’s law?
A galaxy’s recessional velocity is proportional to its distance from Earth
v ∝ d
How can you use this graph to calculate the age of the Universe?
(Reciprocal of the gradient)
Why does this graph not mean Earth is at the centre of the Universe?
This graph would be produced if you took measurements from any galaxy
All galaxies are moving away from every other galaxy
What is the radial velocity method?
Small redshift of star caused by exoplanet
Used to identify exoplanets
What is the transit method?
Regular dip in brightness of star due to exoplanet passing in front
What is an exoplanet?
A planet in another solar system
Why are exoplanets so difficult to detect?
Exoplanets don’t emit light, they only reflect
And it is a tiny amount compared to the star
What are the key stages of the Big Bang Theory?
1 Photons produce particles of matter & antimatter from a vacuum
2 An excess of matter over antimatter occurs
3 Nuclear fusion occurs
4 Nuclear fusion stops
5 Atoms form
6 Stars & galaxies form
7 Nuclear Fusion occurs in stars
8 Life starts
What is the main evidence in support of the big bang?
Explain what it is
Cosmic Microwave Background
Radiation leftover from when the Universe formed neutral atoms
(redshifted to microwaves as Universe has expanded)
What is a Quasar?
Very luminous galaxy nucleus where mass is spiraling into the supermassive blackhole at the center
What is required for a quasar to form?
An accretion disk (gas, dust and matter) close to the supermassive black hole at the center
Why can’t most quasars be detected?
Only detected if Earth is in line with radiation jets
Why are radiowaves detected from a Quasar?
High energy emitted gamma radiation is redshifted to radiowaves
What does the large redshift of Quasars (gamma → radio) tell us about Quasars?
They are some of the most distant objects in the Universe
What are the disadvantages of refracting telescopes?
Disadvantages of refracting telescopes:
- Lenses can only be supported at their edges which is tricky
- Glass required must be very pure and perfect so making larger telescopes is difficult
- Larger lenses are very heavy and can bend under their own weight
- Suffer from chromatic abberation and spherical abberation neither of which have effective solutions
- Larger lenses require very long focal lengths which is impractical
What are the advantages of reflecting telescopes?
Advantages of reflecting telescopes:
- Large mirrors are easily made, they are light and easily supportable from behind
- Mirror surfaces can be made just a few nanometers thick giving excellent image properties
- No chromatic abberation or spherical abberation when using parabolic mirrors
- Light mirrors can be easily manoeuvred to respond to astronomical events quickly
- Smaller segmented mirrors can be used to form a large objective mirror
What is the equation for the minimum angular resolution of a telescope (assuming the limit of the relescopes resolution is due to diffraction)
For a circular aperture f the central maxima of one of the diffraction patterns coincides with the first minima of another the image is just resolved
θ=λ/D
How do charged coupled devices (CCDs) work?
- Three electrodes are contained in each pixel on the surface, each one representing a seperate colour (RGB)
- Underneath the layer of electrodes is a layer of trapped electrons and then below that is a potential well (surrounded by p and n type sillicon)
- Light incident on the electrodes releases electrons from them, if these electrons have a greater energy than the potential well then they escape and register a signal
As electrons only register signals if eelectron>Vwell the potential well can be set to specific energies so that only specific wavelengths and frequencies of light are detected
CCDs also detect way more of the incident light than say an eye does
What are some limitations of ground based telescopes?
How can this be combatted?
Many wavelengths of the EM spectrum are blocked by the atmosphere aswell as visible light being distorted
By putting telescopes, high up, in less populated areas or in space
How are gamma ray telescopes used?
Gamma ray telescopes are generally in space and don’t use mirrors or lenses and instad use special detectors which measure the energy and direction of incident gamma rays
Gamma ray emmiters are things such as solar flares, quasars, supernova remenants and pulsars
How do you convert from degrees to arcs or vice versa?
Type the amount of degrees into your calculator and press the button above ENG to go backwards you do the same in reverse
3.1416° = 3°8’29.76’’ (degrees - arc minutes - arc seconds)
What is the definition of a black body?
A perfect emitter and absorber of EM radiation.
Stars are black bodies
What are the stellar spectral classes?
Hottest O: weak balmer lines
B: stronger balmer lines
A: strongest balmer lines
F: weaker balmer lines
G: weaker balmer lines
K: weaker balmer lines
Coldest M: weaker balmer lines
What are balmer lines?
The ionisation of electrons from n=2 state in hydrogen produces visible light. This means that depending on where the absorption lines of a star are in its spectra we can see what state the electrons from the hydrogen are in and therefore determine its temperature
So classes B and A are the perfect temperature for many of the hydrogen atoms to be in the n=2 state, anything hotter will ionise the atoms so and anything a little colder will not have enough energy to keep electrons in the n=2 state and anything colder than that wont have enough energy to split H2 into H atoms
What is the relation of brightness and spectral class (HR diagram)?
Stars can be:
hot and bright - main sequence
cold and bright - red giants
cold and dim - main sequence
hot and dim - white dwarfs
What is the evolution of a star with M <1.4 Msun ?
- Main sequence, star fuses H in its core
- Red Giant, star runs out of hydrogen so the core shrinks until its hot enough to fuse helium. Radiation pressure decreases
- Star goes from having a stable core (Gravitational pressure = radiation pressure) to having an unstable core (Gravitational pressure < Radiation pressure)
- The radiation pressure causes the star to expel its outer layers into a planetary nebula (H and He expelled leaving a C and O core)
- The core cools and you are left with a white dwarf where no fusion has happened since the star nebulised
- If the white dwarf continued to cool theoretically it would become a black dwarf
What is the wavelength of green light?
550 nm
What does a reflecting telescope in Cassegrain Arrangement look like?
What is Spherical Aberration?
Focal length for lens or mirror is affected by distance from centre
(Forms distorted images)
How is Spherical Aberration reduced?
Using reflecting telescopes with parabolic mirrors
What are the Advantages of using Reflecting over Rafracting Telescopes?
Why are Space Based Telescopes used?
- No atmospheric or light pollution
- Can image across entire EM spectrum
What are the only EM regions visible to Ground Based Telescopes?
- Visible
- Radio
- Some IR (dry mountainous regions)
What are the disadvantages of Space Based Telescopes?
- Expensive to build and launch into orbit
- Difficult to repair ad maintain
- Weight limits to size of mirrors and power
- A lot more difficult to control remotely (communicatiuon delays)
- What is Quantum Efficiency?
- What is the Q.E. for the eye?
- What is the Q.E. for a CCD?
Q.E. eye ≈ 5%
Q.E. CCD > 70%
How does a CCD Work?
- Semiconductor chip with millions of pixels
- CCD placed on focal plane of objective lens
- Each pixel made of potential wells
- Potential wells correspond to different photons
- Photons incident on CCD release electrons
- Electrons trapped in potential wells
- Relative amounts of electrons in each well measured to produce image
What is the Rayleigh Criterion Critical Limit (point where 2 objects are only just resolved)
Central Maxima of one diffraction pattern meets the First Minima of the other
What is the Inverse Square Law?
What does the Hertzsprung-Russel Diagram show?
Luminosity (or Absolute Magnitude) of star
vs
Surface Temperature (or Spectral class)
What are the 3 main regions of the Hertzsprung-Russel Diagram?
As the Sun runs out of Hyrdogen in the core how does it progress along the Hertzsprung-Russel Diagram?
- Becomes a Red Giant
- Becomes a White Dwarf (after Planetary Nebula)
How can Red Giants be incredibly luminous but have a Low surface temperature?
They have a massive Surface Area
(Stefan’s Law)
How can White Dwarfs be not very luminous but have a very high surface temperature?
They have a tiny Surface Area
(Stefan’s Law)
What is a perfect Blackbody?
An object that absorbs and re-emits all incident radiation (eg radiators, filament bulbs and stars)
How is the** Blackbody Spectrum** different for a star with a greater surface temperature?
- Peak moves up and to left (shorter wavelength)
- Greater Intensity across all Wavelengths
- Shorter starting wavelength (x-intercept)
What are the 7 Stellar Spectral Classes (in Temperature order)?
What are the surface temperature ranges for the 7 Stellar Spectral Classes?
How does a star produce an absorption spectra?
Colder outer layers of star absorb some wavelengths of radiation produced in the core
Why is the Balmer Series most visible in the Absorption Spectra of A type stars?
Surface temperature hot enough for hydrogen electrons to be in n=2 state
But not too hot that electrons excite to higher states or for hydrogen to be ionised
Describe the strength of the Balmer Series Absorption Lines for each of the Spectral Classes
What is the lifecycle of a regular Star?
(<1.4xMsun)
What is the lifecycle of a bigger Star?
(>1.4xMsun)
When and how does a Red Giant form?
- At end of main sequence hydrogen in core runs out
- Gravitational pressure > Radiation Pressure
- Core shrinks heating up (GPE -> Thermal)
- Outer layers expand
- Core gets hot enough for He to fuse
What is the Schwarzschild Radius for an object?
What it’s radius would have to be shrunk to for it to become a black hole
(where escape velocity > speed of light)
How do Type 1A Supernovae Form?
- In a binary star System
- One star has become a White Dwarf
- But has absorbed enough mass from other so M > 1.4xMSun
What are Type 1A Supernovae useful for?
One of the brightest ‘Standard Candles’ (known Absolute Magnitude)
Used to measure distances to furthest galaxies
What is the Absolute Magnitude curve for a Type 1A Supernovae?
What do the redshifts of the most distant Type 1A Supernovae tell us?
The redshifts are greater than expected
So Universe’s expansion is accelerating
Dark energy causes this acceleration
