10. Space Flashcards
What is the study of astronomy?
The study of celestial objects (like stars, planets, and galaxies) and phenomena that occur outside Earth’s atmosphere
What’s the study of cosmology?
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
What’s Luminosity, L?
The amount of energy emitted from a source per second as electromagnetic radiation
- usually stars, or candles
Measured in W (E/s)
What’s the Stefan Boltzman’s Law?
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
What’s the radiant flux intensity?
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
What does the radiant flux intensity assume?
- The power from the star radiates uniformly through space
- No radiation is absorbed between the star and the Earth
What’s black body radiation?
A black body absorbs and emits all radiation that falls at all EM wavelengths
Perfect absorber / emitter (radiator)
A star is a black body
What’s Wein’s Law?
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
What’s the 4 ways of measuring distances of stars?
Astronomical units (AU)
Light years (LY)
Trigonmetric (stellar) parallax
Parsec (pc)
What are astronomical units (AU)?
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
What are light years (LY)?
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
What are minutes and seconds of arc?
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”
What is trigonometric (stellar) parallax?
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
What is a parsec (pc)?
Using trigonometric parallax, using 1” as the angle, d = 1pc
tan(1”)=1AU/1pc
1pc=3.09x10^16m
A lot further than 1AU
What’s a standard candle?
An astronomical object which has a known luminosity due to a characteristic quality possessed by that class of object
What’s a cepheid variable?
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
What’s a Type 1a supernovae
A supernova explosion involving a white dwarf
The luminosity at the time of the explosion is always the same
What’s a supernova?
A bright and powerful explosion which happens at the end of a high mass star’s lifetime
What’s The Hertzsprung - Russell Diagram?
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
What decides the exact route a star’s development?
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)
What’s the first four stages in the life cycle of stars?
The first 4 stages are the same for stars of all masses
1. Nubula
2. Protostar
3. Nuclear Fusion
4. Main sequence star
What are the remaining steps for a low mass star?
- Red Giant
- Planetary Nebula
- White Dwarf
What are the remaining steps for a high mass star?
- Red Super Giant
- Supernova
- Neutron Star (or Black Hole)
How does a nebula form into a protostar?
- 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
What is nuclear fusion?
Eventually, the temperature from a protostar reaches millions of degrees, having extreme pressures
- this causes light nuclei, like hydrogen to form together to form heavier nuclei, like helium
- some of the mass is converted into a tremendous amount of energy compared to mass
Fusion happens because the mass of the resulting nucleus is slightly less than the combined mass of the original nuclei
- due to some being lost to energy
What’s a main sequence star?
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
How does a main sequence star turn into a red giant?
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
What’s a Planetary Nebula?
After a red giant, the outer layers of the star are released
- due to weakness of gravitational binding
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.
What’s a White Dwarf?
After the red giant, the solid core collapses under its own mass, leaving a very hot, dense core called a white dwarf
What’s a Red Supergiant?
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
How does a red giant form a supernova?
The iron core collapses
The outer shell is blown out in an explosive supernova
What’s a neutron star and a black whole?
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
What are the 2 forces of a main sequence star?
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
How do cepheid variables fluctuate luminosity?
Their luminosity decreases slowly, then increases quickly
When they decrease in size, it becomes so dense around their core, their luminosity decreases
How do cepheid variables luminosity relate to their periods?
The higher the average luminosity of a cepheid variable, the longer periods they have
Luminosity ∝ Period
How do we use cepheid variables to measure up to 20 million light years away?
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.
What’s the Doppler effect?
- 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.
How can the Doppler effect show on the electromagnetic spectrum?
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
What’s the difference between a red shift and a blue shift?
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
What are non-relativistic galaxies?
Galaxies that are moving further away from us at a speed way below the speed of light
How do we calculate the Doppler redshift speed that galaxies etc move further away from us?
Only works for non-relativistic galaxies
∆λ/λ or ∆f/f = v/c
What’s Hubbles Law?
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
What does 1/H(0) tell us?
Hubbles law tells us the rate of expansion in s^-1
1/H(0) is in seconds
This value tells us the time the universe has been around for in seconds
What started time?
The Big Bang is likely to started off time
What is meant by the recession velocity of galaxies
The velocity that they move away from us
How do galaxies move away from each other?
They spread out like a balloon being inflated
What’s the closed universe theory?
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
How do scientists measure density parameter of the universe?
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
What’s Dark Matter?
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
What’s Dark energy?
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
