2nd Astrophysics Deck Copy Flashcards
1
Q
What is the focal length of a lens?
A
Distance between focal plane and lens
(Parallel rays converge on the focal plane)
2
Q
How will this be different for a stronger lens?
A
Focal length shorter
3
Q
Why is a real image formed here?
A
The light rays pass through the point where the image forms
4
Q
Why is a virtual image formed here?
A
The light rays do not pass through the point where the image forms
5
Q
Why do these two objects appear the same size?
A
The angle subtended at the eye is the same
6
Q
Generally what does a telescope need to do to magnify an object?
A
Increase the angle subtended at the eye
7
Q
Copy the diagram and label:
- Angles α and β
- Focal lengths
- Focal points
- Two lens names
A
8
Q
Copy the diagram and label:
- Where a real image forms
- Where a virtual image forms
A
9
Q
Draw a labelled diagram of the refracting telescope in normal adjustment
A
Don’t forget the dotted construction line!
10
Q
What is wrong here?
A
Rays from eyepiece lens must be parallel to dotted construction line
11
Q
How is the length of a refracting telescope calculated?
A
Add the two focal lengths
12
Q
How is angular magnification of a telescope calculated?
A
13
Q
- What is chromatic aberration?
- Sketch a diagram to show it
A
Different wavelength’s refract different amounts
Focal point different for different colours
14
Q
- What is spherical aberration?
- Sketch a diagram to show it
A
Refraction increases towards edge of lens
Focal point different for rays passing through different parts of lens
15
Q
What is the resolution of a telescope?
A
Ability to distinguish between two points separated by angle
The smaller the resolution the better
16
Q
How many stars are seen?
A
2 because the stars are resolved
17
Q
How many stars are seen?
A
1 because the stars aren’t resolved
18
Q
How many times greater is this telescope’s collecting power?
A
4x
P proportional to SA
19
Q
What is apparent magnitude (m)?
A
A measure of how bright a star appears in the sky
20
Q
What is absolute magnitude (M)?
A
The apparent magnitude a star would have if it was 10 parsecs away
21
Q
What is the Hipparcos scale?
A
Scale of apparent magnitude of 1 → 6
- 6 → naked eye limit
- Each step = x2.51
22
Q
How does a 4th magnitude star compare to a 7th magnitude star?
A
3 magnitudes difference
So apparent magnitude (m) = 2.513 = 15.8 x brighter
(4th magnitude is brighter)
23
Q
How could star B be bigger than star A?
A
Star B could be much further away
So appear dimmer from Earth
24
Q
What does apparent magnitude correspond to?
A
The intensity of the star from the surface of the Earth
25
What does **absolute magnitude** correspond to?
The **power output of the star**
26
Define the **lightyear**
The **distance light travels in a year**
27
Define the **parsec**
**Distance to star with parallax angle of 1"**
28
A distant star has a **parallax angle of 3"**
Calculate how far away it is?
Make a **right angled triangle**
29
What information does **m-M** provide? (magnitude difference)
The **smaller m-M → the closer** the object
30
If **m-M < 0** …
If **m-M < 0 → star less than 10pc**
31
If **m-M = 0** …
If **m-M = 0 → star at 10pc**
32
If **m-M \> 0** …
If **m-M \> 0 → star further than 10pc**
33
What's wrong with this calculation?
**d must be in parsecs!!!**
34
Which star **appears brighter**?
Betelgeuse → has a **smaller apparent magnitude (m)**
35
Which star has a greater **power output**?
Betelgeuse → has a **smaller absolute magnitude (M)**
36
Which star is **closer?**
Bellatrix → **m-M is smaller**
37
What is **redshift**?
**Absorption lines** of (star or galaxy etc) **moving away** are **shifted to the left** (red end)
Bigger velocity → Bigger redshift
38
What is **blueshift**?
**Absorption lines** of spectrum (star or galaxy etc) **moving towards** are **shifted to the right** (blue end)
Bigger velocity → Bigger blueshift
39
How can we potentially tell if a **galaxy is rotating**?
One side is redshifted
Other side is blueshifted
40
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
41
In the equation for **doppler shift** what do the different **λ** represent?
**λ0** → Wavelength if observer was stationary
**∆λ** → observed **λ** - **λ0**
42
How do you calculate the velocity of a galaxy given its redshift?
**v = zc**
43
Is a redshift velocity the true velocity of a star?
No, the star could have a v_|_
44
For this **binary star system**
1. When is redshift of star A maximum?
2. When is redshift and blue shift = 0?
3. When is blueshift maximum?
45
What is **Hubble's law**?
A **galaxy's** recessional **velocity is proportional to its distance** from Earth
**v** **∝ d**
46
How can you use this graph to **calculate the age of the Universe?**
(Reciprocal of the gradient)
47
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
## Footnote
**All galaxies are moving away from every other galaxy**
48
What is the **radial velocity method**?
Small **redshift of star caused by exoplanet**
Used to identify exoplanets
49
What is the **transit method**?
Regular **dip in brightness of star due to exoplanet** passing in front
50
What is an **exoplanet**?
A **planet in another solar system**
51
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
52
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
53
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)
54
What is a **Quasar?**
Very luminous galaxy nucleus where mass is spiraling into the supermassive blackhole at the center
55
What is required for a **quasar** to form?
An **accretion disk** (gas, dust and matter) close to the supermassive black hole at the center
56
Why can't most **quasars** be detected?
Only detected if Earth is in line with radiation jets
57
Why are radiowaves detected from a **Quasar?**
High energy emitted **gamma radiation is redshifted to radiowaves**
58
What does the **large redshift of Quasars** (gamma → radio) tell us about Quasars?
They are some of the **most distant objects** in the Universe
59
What does a **reflecting telescope** in Cassegrain Arrangement look like?
60
What is **Spherical Aberration**?
Focal length for lens or mirror is affected by distance from centre
(Forms distorted images)
61
How is **Spherical Aberration** reduced?
Using **reflecting telescopes** with **parabolic mirrors**
62
What are the A**dvantages of using Reflecting** over Rafracting Telescopes?
63
Why are **Space Based Telescopes** used?
1. No atmospheric or light pollution
2. Can image across entire EM spectrum
64
What are the only EM regions visible to **Ground Based Telescopes**?
1. Visible
2. Radio
3. Some IR (dry mountainous regions)
65
What are the disadvantages of **Space Based Telescopes**?
1. Expensive to build and launch into orbit
2. Difficult to repair ad maintain
3. Weight limits to size of mirrors and power
4. A lot more difficult to control remotely (communicatiuon delays)
66
1. What is **Quantum Efficiency**?
2. What is the **Q.E. for the eye**?
3. What is the **Q.E. for a CCD**?
Q.E. eye ≈ 5%
Q.E. CCD > 70%
67
How does a **CCD** Work?
1. Semiconductor chip with millions of **pixels**
2. CCD placed on focal plane of objective lens
3. Each pixel made of potential wells
4. Potential wells correspond to different photons
5. Photons incident on CCD release electrons
6. Electrons trapped in potential wells
7. Relative amounts of electrons in each well measured to produce image
68
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
69
What is the **Inverse Square Law**?
70
What does the **Hertzsprung-Russel Diagram** show?
**Luminosity** (or Absolute Magnitude) of star
**vs**
**Surface Temperature** (or Spectral class)
71
What are the 3 main regions of the **Hertzsprung-Russel Diagram**?
72
As the Sun runs out of Hyrdogen in the core how does it progress along the **Hertzsprung-Russel Diagram**?
1. Becomes a Red Giant
2. Becomes a White Dwarf (after Plaetary Nebula)
73
How can **Red Giants** be incredibly luminous but have a Low surface temperature?
They have a **massive Surface Area**
(Stefan's Law)
74
How can **White Dwarfs** be not very luminous but have a very high surface temperature?
They have a **tiny Surface Area**
(Stefan's Law)
75
What is a perfect **Blackbody**?
An object that **absorbs and re-emits all incident radiation** (eg radiators, filament bulbs and stars)
76
How is the** Blackbody Spectrum** different for a star with a **greater surface temperature**?
1. Peak moves up and to left (shorter wavelength)
2. Greater Intensity across all Wavelengths
3. Shorter starting wavelength (x-intercept)
77
What are the 7 **Stellar Spectral Classes** (in Temperature order)?
78
What are the **surface temperature ranges** for the 7 **Stellar Spectral Classes**?
79
How does a star produce an **absorption spectra**?
Colder outer layers of star absorb some wavelengths of radiation produced in the core
80
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
81
Describe the strength of the **Balmer Series Absorption Lines for each of the Spectral Classes**
82
What is the lifecycle of a regular Star?
(<1.4xMsun)
83
What is the lifecycle of a bigger Star?
(>1.4xMsun)
84
When and how does a **Red Giant** form?
1. At end of main sequence hydrogen in core runs out
2. Gravitational pressure > Radiation Pressure
3. Core shrinks heating up (GPE -> Thermal)
4. Outer layers expand
5. Core gets hot enough for He to fuse
85
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**)
86
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
87
What are **Type 1A Supernovae** useful for?
One of the brightest **'Standard Candles'** (known Absolute Magnitude)
Used to **measure distances to furthest galaxies**
88
What is the Absolute Magnitude curve for a **Type 1A Supernovae**?
89
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
90
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)
91
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
