10. Space

Question 1

What is the study of astronomy?

Answer 1

The study of celestial objects (like stars, planets, and galaxies) and phenomena that occur outside Earth’s atmosphere

Question 2

What’s the study of cosmology?

Answer 2

A branch of astronomy, but it focuses on the universe as a whole
Like the large-scale structure, origin, evolution, and ultimate fate of the universe

Question 3

What’s Luminosity, L?

Answer 3

The amount of energy emitted from a source per second as electromagnetic radiation
- usually stars, or candles
Measured in W (E/s)

Question 4

What’s the Stefan Boltzman’s Law?

Answer 4

Links how the area and temperature of a star effects luminosity
L=σAT^4
Where σ is a constant relating them
σ=5.7x10^-8 Wm^-2k^-4

Question 5

What’s the radiant flux intensity?

Answer 5

radiant flux = luminosity, but can apply to any source of radiation and is more general
Radiant flux intensity is the radiant flux per unit area
As the light spreads out uniformly through a spherical shell, Light sources which are further away appear fainter because the emitted light has been spread over a greater area

Intensity (I) = Luminosity (L) / Area
I=L/4πd^2
where d = distance from centre of star

Question 6

What does the radiant flux intensity assume?

Answer 6

• The power from the star radiates uniformly through space
• No radiation is absorbed between the star and the Earth

Question 7

What’s black body radiation?

Answer 7

A black body absorbs and emits all radiation that falls at all EM wavelengths
Perfect absorber / emitter (radiator)
A star is a black body

Question 8

What’s Wein’s Law?

Answer 8

λ(max)T = 2.9 × 10^-3 mk
The λ(max) is the most common λ that the black body radiates or absorbs (its the same λ)
This means λ(max) ∝ 1/T
So this is why hotter stars are blue compared to red

Question 9

What’s the 4 ways of measuring distances of stars?

Answer 9

Astronomical units (AU)
Light years (LY)
Trigonmetric (stellar) parallax
Parsec (pc)

Question 10

What are astronomical units (AU)?

Answer 10

AU is a measure of distance
Which is the mean radius from the Earth’s orbit to the sun
(measures from centres)
1AU = 1.5x10^11m

Question 11

What are light years (LY)?

Answer 11

The distance light travels in one year
one year = 365x24x60^2 s
light year in m = 3.1536x10^7 s x c
= 9.45x10^15m

Question 12

What are minutes and seconds of arc?

Answer 12

These are fractions of angles
Minute of arc:
1°=60’
Second of arc:
1’=60’’
1°=3600”

The smallest angle we can measure is 0.75”

Question 13

What is trigonometric (stellar) parallax?

Answer 13

By looking at things from another perspective and knowing the angle, we can measure stars
As the Earth orbits, we can see stars from a different perspective
We can see a star behind another one in March, then see the same star with another star behind it in September
If we know the angle difference we can use trig to find the distance

tanθ=1AU/d

Question 14

What is a parsec (pc)?

Answer 14

Using trigonometric parallax, using 1” as the angle, d = 1pc
tan(1”)=1AU/1pc

1pc=3.09x10^16m
A lot further than 1AU

Question 15

What’s a standard candle?

Answer 15

An astronomical object which has a known luminosity due to a characteristic quality possessed by that class of object

Question 16

What’s a cepheid variable?

Answer 16

A type of pulsating star which increases and decreases in brightness over a set time period

This variation has a well defined relationship to the luminosity

Question 17

What’s a Type 1a supernovae

Answer 17

A supernova explosion involving a white dwarf
The luminosity at the time of the explosion is always the same

Question 18

What’s a supernova?

Answer 18

A bright and powerful explosion which happens at the end of a high mass star’s lifetime

Question 19

What’s The Hertzsprung - Russell Diagram?

Answer 19

Plotting star’s luminosity (compared to the sun) and their temperatures
Separates stars into groups:
- Main sequence
- Red Giants (higher luminosity, lower temp)
- Supergiants (higher luminosity, slightly higher temp)
- White Dwarfs (Low luminosity, hight temp)
- Protostars (slightly higher luminosity, lot lower temp
- Nebulae (gas clouds) (low luminosity, very low temp)

only shows stars that are in stable phases
Transitory phases happen quickly in relation to the lifetime of a star
Black holes cannot be seen since they emit no light

Question 20

What decides the exact route a star’s development?

Answer 20

Their initial mass
Low mass: stars with a mass less than about 1.4 times the mass of the Sun (< 1.4 MSun)
High mass: stars with a mass more than about 1.4 times the mass of the Sun (> 1.4 MSun)

Question 21

What’s the first four stages in the life cycle of stars?

Answer 21

The first 4 stages are the same for stars of all masses
1. Nubula
2. Protostar
3. Nuclear Fusion
4. Main sequence star

Question 22

What are the remaining steps for a low mass star?

Answer 22

5. Red Giant
6. Planetary Nebula
7. White Dwarf

Question 23

What are the remaining steps for a high mass star?

Answer 23

5. Red Super Giant
6. Supernova
7. Neutron Star (or Black Hole)

Question 24

How does a nebula form into a protostar?

Answer 24

• All stars form from a giant cloud of hydrogen gas and dust called a nebula
• Gravitational attraction between individual atoms forms denser clumps of matter
• This inward movement of matter is called gravitational collapse
• 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

Question 25

What is nuclear fusion?

Answer 25

Eventually, the temperature from a protostar reaches millions of degrees, and the fusion of hydrogen nuclei to helium nuclei begins

Nuclear fusion is the formation of a heavier nuclei, which releases a tremendous amount of energy in the process
Fusion happens because the mass of the resulting nucleus is slightly less than the combined mass of the original nuclei

Question 26

What’s a main sequence star?

Answer 26

After nuclear fusion, the star reaches a stable state where the inward and outward forces are in equilibrium
- As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases

A star spends 90% of it’s lifetime in the main sequence

Question 27

How does a main sequence star turn into a red giant?

Answer 27

Hydrogen fuelling the star begins to run out
- Most of the hydrogen nuclei in the core of the star have been fused into helium
- Nuclear fusion slows
- Energy released by fusion decreases

Question 28

What’s a Planetary Nebula?

Answer 28

After a red giant, the outer layers of the star are released
A glowing shell of gas and dust
Core helium burning releases massive amounts of energy in the fusion reactions

The ejected material forms the nebula, while the remaining core of the star becomes a white dwarf.

Question 29

What’s a White Dwarf?

Answer 29

After the red giant, the solid core collapses under its own mass, leaving a very hot, dense core called a white dwarf

Question 30

What’s a Red Supergiant?

Answer 30

Hydrogen fuelling the star begins to run out
- Most of the hydrogen nuclei in the core of the star have been fused into helium
- Nuclear fusion slows
- Energy released by fusion decreases

The high mass creates a red supergiant
form from massive stars that burn through their hydrogen fuel much more quickly than smaller stars

Unstable stars

Question 31

How does a red giant form a supernova?

Answer 31

The iron core collapses
The outer shell is blown out in an explosive supernova

Question 32

What’s a neutron star and a black whole?

Answer 32

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

Question 33

What are the 2 forces of a main sequence star?

Answer 33

The inward and outward forces are in equilibrium
Inward force:
- The weight of the gases exerts the inward force due to gravitational pull
Outward force:
- Radiation pressure and gas pressure exerts an outward force, created by the energy produced in the star’s core through nuclear fusion

Question 34

How do cepheid variables fluctuate luminosity?

Answer 34

Their luminosity decreases slowly, then increases quickly
When they decrease in size, it becomes so dense around their core, their luminosity decreases

Question 35

How do cepheid variables luminosity relate to their periods?

Answer 35

The higher the average luminosity of a cepheid variable, the longer periods they have
Luminosity ∝ Period

Question 36

How do we use cepheid variables to measure up to 20 million light years away?

Answer 36

If we know the period of the cepheid variable, we can find it’s luminosity.
We then calculate it’s intensity (brightness) from earth and put it into the radiant flux intensity equation.

Question 37

What’s the Doppler effect?

Answer 37

• If a wave source is stationary, the wavefronts spread out symmetrically
• If the wave source is moving, the waves can become squashed together or stretched out

If the wave source is moving towards an observer the wavefronts will appear squashed (lower wavelength, so high frequency)
If the wavefront is moving away from an observer the wavefronts will appear stretched out (higher wavelength, so lower frequency)

So there is an apparent shift in wavelength occurring when the source of the waves is moving relative to an observer.

Question 38

How can the Doppler effect show on the electromagnetic spectrum?

Answer 38

Comparing a close object, like the sun to a distance object, in a distant galaxy, we can compare the light spectrum produced.

The shifts in wavelengths can tell us if the object is moving and in what direction

Question 39

What’s the difference between a red shift and a blue shift?

Answer 39

Red shift:
When wavelengths get longer, meaning the frequency decreases, meaning the object is moving further away from us
Called a red shift as it moves towards the red side of the spectrum
∆λ=λ-λ(1) is positive
λ = original
λ(1) = new

Blue shift:
When wavelengths get shorter, meaning the frequency increases, meaning the object is moving closer to us
Called a blue shift as it moves towards the blue side of the spectrum
∆λ=λ-λ(1) is negative

Question 40

What are non-relativistic galaxies?

Answer 40

Galaxies that are moving further away from us at a speed way below the speed of light

Question 41

How do we calculate the Doppler redshift speed that galaxies etc move further away from us?

Answer 41

Only works for non-relativistic galaxies
∆λ/λ or ∆f/f = v/c

Question 42

What’s Hubbles Law?

Answer 42

Hubbles Law states the recessional velocity, v, of a galaxy is proportional to its distance from Earth, d

v ≈ H(0)d
Where H(0) is Hubbles constant (kms^-1Mpc^-1)
H(0) = 69.8 kms^-1Mpc^-1

This is evidence of our expanding universe
- he realised this through red shifts of nebula

His Law creates a graph, it has lots of plotted known galaxies on the graph
- there are more in the middle of the graph

Question 43

What is meant by the recession velocity of galaxies

Answer 43

The velocity that they move away from us

Question 44

How do galaxies move away from each other?

Answer 44

They spread out like a balloon being inflated

Question 45

What’s the closed universe theory?

Answer 45

As galaxies move outwards, their gravitational potential energy increases
Leading to their E(k) decreases
The rate of expansion is therefore continually decreasing

The fate of the universe depends on it’s average density
If it exceeds a critical value the gravitational energy will eventually be sufficient to reverse the expansion and the universe will contract, hence closed universe

Question 46

How do scientists measure density parameter of the universe?

Answer 46

Ω=ρ/ρ(c)
where ρ is the average density of the universe
where ρ(c) is the critical density, the density required for the universe to have a flat geometry
leading Ω to be the density parameter

If Ω > 1
Density is higher than critical value
Hence, closed universe

If Ω=1
Critical universe, flat geometry

If Ω<1
Open universe

Question 47

What’s Dark Matter?

Answer 47

Matter which cannot be seen and that does not emit or absorb electromagnetic radiation

Dark matter cannot be detected directly through telescopes
It should make up 26.8% of the mass in the universe

Question 48

What’s Dark energy?

Answer 48

Dark energy is energy we cannot see

It takes up around 68.3% of all mass in the universe

We have no idea what it is
or how to measure it

As Dark matter takes up 26.8%,
Leading ordinary matter to take up 4.9% of all matter