1
Q
What happens the moment that light leaves the surface of a star?
A
The light begins to spread out uniformly through a spherical shell
2
Q
Why do light sources which are further away appear fainter?
A
Because the emitted light has been spread out over a greater area (as it spreads out uniformly through a spherical shell)
3
Q
What is the equation for the surface area of a sphere?
A
Spherical surface area = 4πr2
4
Q
How can the inverse square law of radiant flux intensity be calculated?
A
F = L/4πd2
Where:
- F = radiant flux intensity, or observed intensity on Earth (W m-2)
- L = luminosity of the source (W)
- d = distance between the star and the Earth (m)
5
Q
What does the equation below assume?
F = L/4πd2
A
- The power from the star radiates uniformly through space
- No radiation is absorbed between the star and the Earth
6
Q
Think luminosity, flux and distance.
What does the equation below show?
F = L/4πd2
A
- For a given star, the luminosity is constant
- The radiant flux follows an inverse square law
- The greater the radiant flux (larger F) measured, the closer the star is to the Earth (smaller d)
7
Q
What is the principle of parallax based on?
A
How the position of an object appears to change as the position of the observer changes
(e.g. when observing the scale on a metre ruler, looking at eye level gets a different reading to viewing from above or below the scale)
8
Q
What is stellar parallax?
A
The apparent shifting in position of a nearby star against a background of distant stars when viewed from different positions of the Earth, during the Earth’s orbit about the sun
9
Q
Why might a nearby star be observed from the Earth in January and again in July?
A
To view the star from two opposite positions in Earth’s orbit (six months apart), which maximises the distance between observations.
This helps make the shift in the star’s position (the stellar parallax) easier to measure.
10
Q
What is the name given to the apparent shift in the position of a nearby star when viewed from different positions of the Earth in its orbit?
A
Stellar parallax
11
Q
What is the general equation for the stellar parallax of a nearby star?
A
p = AU/d
Where:
* p = parallax angle from earth to the nearby star (radians or arcseconds/arcminutes)
* AU = astronomical unit (average Earth to Sun distance)
* d = distance to the nearby star
12
Q
What is the simplified astronomic equation for the stellar parallax of a nearby star?
A
p = 1/d
Where:
* p = parallax angle (arcseconds)
* d = distance (parsecs)
13
Q
What are arcseconds?
A
A unit for measuring very small angles (particularly in astronomy)
1 arcsecond = 1/3600 of a degree
* 60 arcseconds = 1 arcminute
* 60 arcminutes = 1 degree
14
Q
What is the value of 1 arcsecond in degrees?
A
1 arcsecond = 1/3600 of a degree
15
Q
What is the value of 1 degree in arcseconds (“)?
A
1 degree (º) = 3600 arcseconds (“)
(This means that 1 degree (º) = 60 arcminutes (‘))
16
Q
What is the correction for any confusion surrounding arcseconds (“) and arcminutes (‘)?
A
- They use the sexagesimal (base-60) system, which is the same system which was used for time (hours, seconds, minutes)
- This does not mean that arcseconds (“) and arcminutes (‘) are related to time at all, they just have the same names, and are simply used as units for very small angles
17
Q
What is a parsec?
A
A unit of distance that gives a parallax angle of 1 arcsecond (1/3600 of a degree) (“). It uses the radius of the Earth’s orbit (1 AU) as the baseline of a right-angled triangle)
18
Q
Up to what distance is the equation below accurate for?
p = 1/d
A
100 parsecs (pc)
This is because for distances greater than 100 pc, the parallax angles become so small that they are very difficult to be accurately measured
19
Q
What is the value of 1 parsec in metres?
A
1 parsec ≈ 3.1 x 1016 m
20
Q
What is the value of 1 light-year in metres?
A
1 light-year ≈ 9.5 x 1015 m
21
Q
What is the simplified symbol unit for arcseconds?
A
”
22
Q
What is the simplified symbol unit for arcminutes?
A
’
23
Q
What is a standard candle defined as?
A
An astronomical object which has a known luminosity due to a characteristic quality possessed by that class of object
24
Q
What is an example of a standard candle?
A
A type 1a supernovae
(A supernova explosion involving a white dwarf, which always has the same luminosity at the time of explosion)
25
What is a **supernova**?
A bright and powerful explosion which happens at the end of a high mass star's life
26
What is **luminosity**?
The total power output of radiation emitted by a star
27
How can **standard candles** be used to calculate astronomical distances?
* Standard candles are objects with a **known luminosity**
* Astronomers measure how bright they appear from Earth (**radiant flux intensity**)
* They use the **inverse square law of flux**: F = L/4πd2 (where F = flux, L = luminosity, and d = distance)
* Rearranging to find distance: d = √(L/4πF)
* So, knowing L and measuring F lets astronomers calculate d
28
What is the **cosmic distance ladder**?
The name for a series of different methods that astronomers use to determine distances in space:
* Each method is used for objects within a certain distance range (e.g. radar ranging for the Solar System, parallax for nearby stars, standard candles for distant galaxies etc.)
* Combining the results from these methods builds up a “ladder” of measurements, helping us measure distances from nearby stars out to faraway galaxies
29
What is the Hertzsprung-Russell diagram?
A graph that plots the luminosity of stars (relative to the Sun) against their surface temperature.
It is used to classify stars and understand their evolutionary stages. Most stars are found in the main sequence band, while red giants, supergiants, and white dwarfs occupy other distinct regions on the diagram.
30
What can be said about the **y-axis** of the Hertzsprung-Russell diagram?
* The y-axis represents luminosity (brightness) relative to the Sun.
* It increases logarithmically from 10-5 (dimmest) at the bottom to 106 (brightest) at the top.
31
What can be said about the **x-axis** of the Hertzsprung-Russell diagram?
* The x-axis represents surface temperature of stars, measured in Kelvin
* It decreases from left to right, with the hottest stars (around 30,000 K to 50,000 K) on the left and the coolest stars (below 3000 K) on the right
32
On the Hertzsprung-Russell diagram, where are most stars (including the Sun) clustered?
In the **main sequence** band
33
What is the relationship between a **main sequence stars** luminosity and its surface temperature?
L ∝ T
(luminosity is directly proportional to surface temperature)
34
What must be true about stars for them to be shown on the **Hertzsprung-Russell** diagram?
They must be in **stable phases** because transitory phases happen quickly in relation to the lifetime of a star
35
Why can black holes not be seen on the Hertzsprung-Russell diagram?
They don't emit light
36
What are the **first four stages** in the **life cycle of stars** (true for all masses)?
1. A **nebula** is formed
2. A gravitational collapse forms a **protostar**
3. **Nucelar fusion** will occur, fusing **hydrogen** nuclei into **helium** nuclei
4. The forces on and in the star core reach equilibrium, putting the star in its **main sequence**
37
What happens in **stage 2** of the life cycle of stars?
The **gravitational collapse** causes the gas to heat up and glow, forming a **protostar**
* **Work done** on the particles of gas and dust **by collisions between the particles** causes an **increase** in their **kinetic energy**, resulting in an **increase** in **temperature**
* Protostars can be detected by telescopes that can observe **infrared radiation**
38
What happens in **stage 1** of the life cycle of stars?
All stars form from a giant cloud of **hydrogen gas** and **dust** called a **nebula**
* **Gravitational attraction** between individual atoms for denser clumps of matter
* This inward movement of matter is called **gravitational collapse**
39
What happens in **stage 3** of the life cycle of stars?
Eventually the **temperature** will reach millions of **degrees Kelvin** and the **nuclear fusion** of **hydrogen** nuclei to **helium** nuclei begins
* The **protostar's** gravitational field continues to **attract** more gas and dust, **increasing** the **temperature** and **pressure** of the **core**
* With **more frequent collisions**, the **kinetic energy** of the particles **increases**, increasing the probability that fusion will occur
40
What happens in **stage 4** of the life cycle of stars?
The star will eventually reach a **stable state** where the inward and outward forces are in equilibrium, which is a **main sequence star**
* As the **temperature** of the star **increases** and its **volume** **decreases** due to gravitational collapse, the **gas pressure increases**
41
In what stage of the star life cycle with a star spend most of its life?
As a **main sequence star**
42
Roughly what percentage of stars are on the **main sequence**?
90% of stars
43
What mass-range can a **main sequence star** lie within in comparison with the mass of the Sun?
From 0.1x to 200x the mass of the Sun (a highly varied range)
44
What are stages **5 to 7** of the life cycle for a **low mass star** (**less than** 1.4x the mass of the sun)?
5. A **red giant** is formed
6. A **planetary nebula** is formed
7. A **white dwarf** remains
45
For a **low mass star**, what happens in **stage 5** of its life cycle?
The **hydrogen** fuelling the star **begins to run out**
* Most of the hydrogen nuclei in the core of the star have been fused into helium nuclei
* **Nuclear fusion slows** as a result
* **Energy released** by fusion **decreases**
The star initially shrinks and then swells and cools to form a **red giant**
* Fusion continues in the **shell** around the core
46
For a **low mass star**, what happens in **stage 6** of its life cycle?
The **outer layers** of the star are **released**, forming a **planetary nebula**
* The **helium** nuclei **releases** massive amounts of **energy** in as it burns in the core
47
For a **low mass star**, what happens in **stage 7** of its life cycle?
The **solid core collapses** under its own mass, leaving a very hot, dense core called a **white dwarf**
48
What are stages **5 to 7** of the life cycle for a **high mass star** (**more than** 1.4x the mass of the sun)?
5. A **red super giant** is formed
6. A **supernova** occurs (a bright and powerful explosion)
7. A **neutron star** or a **black hole** is formed
49
For a **high mass star**, what happens in **stage 5** of its life cycle?
The star follows the same process as the formation of a red giant
* The **shell burning** and the **core burning** cycle in massive stars goes beyond that of low-mass stars, fusing elements up to **iron**
50
For a **high mass star**, what happens in **stage 6** of its life cycle?
The **iron core collapses**
* The outer shell is blown out in an explosive **supernova**
51
For a **high mass star**, what happens in **stage 7** of its life cycle?
* After the **supernova** explosion, the collapsed **neutron core** can remain intact having formed a **neutron star**
* If the **neutron core mass** is **greater than 3 times the solar mass**, the **pressure** on the core **becomes so great** that the **core collapses** and produces a **black hole**
52
How does a **stationary wave source** affect the **waves it produces**?
The waves spread out **symmetrically** in **all directions** (with no change to their wavelength or frequency)
53
How does the **movement of a wave source** appear to affect the **waves it produces**?
When the **wave source moves**:
* **The wavelength in front** of the source **decreases** and **frequency increases** (waves are squashed together)
* The **wavelength behind** the source **increases** and **frequency decreases** (waves are stretched out)
This **apparent change** is known as the **Doppler effect**
54
What is meant by the **Doppler effect**?
The **apparent** shift in **wavelength** which occurs when the **source of the waves moves towards** or **away** from an observer
55
When a **wave source is moving**, what can be said about the **wavelength** of the **waves produced**?
* The **wavelength received by an observer appears to change** when the source is moving.
* However, the **actual wavelength emitted** by the source **remains the same**
* This apparent change is called the **Doppler effect**
56
Relating to the **Doppler effect**, what is the evidence that the universe is expanding?
* The **light from distant galaxies** is **shifted towards** the **red end** of the spectrum (**redshift**) compared to the **light from close objects** like the **Sun**
* This **redshift** shows that **these galaxies are moving away from us**
* The observation that **most distant galaxies show redshift** is evidence that the universe is expanding
57
What is meant by **redshift**?
The **fractional increase in wavelength** (or **decrease in frequency**) of light when the source and observer are **moving away from each other**
58
What is meant by **blueshift**?
The **fractional decrease in wavelength** (or **increase in frequency**) of light when the source and observer are **moving towards each other**
59
How is the **Doppler effect** most **easily observed** on Earth?
* When a **fast-moving object** that **emits a loud sound** (like an ambulance or a racing car) passes by an **observer**, the **pitch** of the sound **changes**
* The sound is **higher** in pitch as it **approaches** and **lower** as it **moves away**
* This change in pitch is the **Doppler effect**
60
How can the **Doppler effect** of **light** be observed in **space**?
When **spectra** of **distant stars** and **galaxies** are observed. These observations are known as:
* **Redshift** if the object is **moving away** from the Earth (**wavelength increases**)
* **Blueshift** if the object is **moving towards** the Earth (**wavelength decreases**)
61
What does it mean when we say **the universe is expanding**?
Space itself is stretching, causing galaxies to move away from each other over time
62
Are galaxies moving through space away from our galaxy?
No because space itself is expanding between galaxies, so the galaxies appear to move apart.
63
Does the Milky Way galaxy expand as the universe expands?
No because galaxies (and objects like stars or solar systems) are held together by gravity and do not expand along with the universe.
64
What does **non-relativistic** mean for a galaxy?
The galaxy is moving away at a speed much less than the speed of light, so we can ignore relativistic effects
65
What does **relativistic** mean for a galaxy?
The galaxy is moving at a speed close to the speed of light, so equations that include effects from Einstein's relativity theory must be used.
66
Can galaxies move at the speed of light?
No because nothing with mass can reach or exceed the speed of light
67
How can **Doppler redshift** for non-relativistic galaxies be calculated?
∆λ/λ = ∆f/f = v/c
Where:
* ∆λ = shift in wavelength (m)
* λ = wavelength emitted from the source (m)
* ∆f = shift in frequency (Hz)
* f = frequency emitted from the source (Hz)
* v = speed of recession/speed moving away from us due to the universe expanding (ms-1)
* c = speed of light in a vacuum (ms-1)
68
What did Edwin Hubble prove about the universe in 1929?
He proved that the universe was **expanding** by observing the absorption line spectra produced from the light of distant galaxies
69
What did Edwin Hubble conclude about the motion of galaxies and stars?
The galaxies and stars that are **further away** from the Earth are **moving faster** than those which are **closer**
70
What does Hubble's Law state?
The recessional velocity v of a galaxy is proportional to its distance d from Earth
71
What is the equation for Hubble's Law?
v ≈ H0d
Where:
* v = recessional velocity of an object / the velocity of an object moving **away** from an observer (km s-1)
* H0 = Hubbles constant (67.4 km s-1 Mpc-1)
* d = distance between the object and the Earth (Mpc)
72
What is the value of Hubble's constant H0?
| According to the Planck Collaboration VI 2020
H0 = 67.4 (± 0.5) km s-1 Mpc-1
73
What are the units for Hubble's constant H0?
km s-1 Mpc-1
**OR** (if d was measured in km)
s-1
74
What would be **expected** about the **velocity** of an **object** **within a galaxy** as it **moves away** from the galaxy's **centre** and why?
* The velocity of an object within a galaxy would be **expected** to **decrease** as it moves away from the galaxy's centre because of weaking gravitational field strength
* This is because it is what is observed in **smaller mass systems**, like the **solar system** where planets orbiting **furthest from the Sun** have the **slowest orbital velocity**
75
Why does the **velocity of an object** within a galaxy **not decrease** as it **moves away** from the **centre** of the galaxy?
* **Mass** is **not concentrated in the centre** of galaxies, but it is instead **spread out**
* However, all the **observable mass** of a galaxy **is concentrated in its centre**, so there is another type of matter that we can't see (which is known as **dark matter**)
76
What is **dark matter**?
Matter which cannot be seen and that does not emit or absorb electromagnetic radiation
77
Can dark matter be detected directly through telescopes?
No
78
Roughly what percentage of the mass in the universe is made up from **dark matter**?
27% of the mass in the universe is **dark matter**
79
A-Level Physics Edexcel Flashcards
flashcards
Decks in class (15)
# Cards
-
Topic 1 - Working as a Physicist120
-
Topic 2 - Mechanics136
-
Topic 3 - Electricity152
-
Topic 4 - Materials74
-
Topic 5 - Waves & Particle Nature of Light201
-
Topic 6 - Further Mechanics71
-
Topic 7 - Electric & Magnetic Fields217
-
Topic 8 - Nuclear/Particle Physics190
-
Topic 9 - Thermodynamics92
-
Topic 10 - Space79
-
Topic 11 - Nuclear Radiation64
-
Topic 12 - Gravitational Fields60
-
Topic 13 - Oscillations61
-
Command Words2
-
Equations113
