Stars & EA 4 Flashcards
What 2 types of properties do stars have and how are they connected?
Stars have physical and observable properties. We must use the observable properties to find the physical properties.
List some physical properties.
Luminosity Absolute magnitude Size Mass Temperature Velocity
List some observable properties.
Position Flux Magnitude Colour Spectral type Lightcurve
Describe 3 characteristics of electromagnetic radiation.
All EMR travels at the speed of light (c = 3x10^8 m/s)
It has wave-like properties; described by wavelength (lambda) or frequency (f), e.g interference.
It has particle-like properties; described in photons as ‘packets’ of energy, e.e photoelectric effect
Write an equation to describe wave-like properties of EMR.
c = f * (lambda)
Write an equation to describe particle-like properties of EMR.
E = hf or E =hc/lambda
h -> Planck’s constant
If a body has a temperature higher than 0K what does this mean?
It is higher than -273C and it emits radiation.
What depends on the temperature of the emitting body?
The wavelength, colour and intensity.
Define blackbodies
An idealised body in thermodynamic equilibrium with surroundings. It absorbs all radiation incident and re-radiates it.
Give 3 examples of blackbodies
- Toaster
- Black tarmac on a hot day
- An oven with a hole
How is Planck’s function and blackbodies related?
A blackbody emits some energy at all wavelengths. This spectrum is described by Planck’s function which is the emitted flux as a function of frequency or wavelength.
At higher temperatures the peak of Planck’s function shifts towards shorter wavelengths.
Describe the difference between hot and cold blackbodies.
At every wavelength a hotter blackbody emits more energy than a cooler one.
Draw a graph showing Planck’s function
X-axis - wavelength
Y-axis - Flux or intensity
higher temperature line, peaks at a shorter wavelength and has a higher peak that lower temperatures.
What is Wien’s law equation?
(lambda)Max . T = 0.0029mK
Describe Wien’s law and what this means?
For a blackbody at temperature T, there is a wavelength (lambda)Max at which it radiates its maximum amount of energy. This relationship is WIen’s law.
Are stars described as blackbodies?
Yes, but
- Energy can only escape from the very outer layer
- Stellar interior is very opaque to virtually all EMR.
What is an effective temperature?
An effective temperature (Teff) is the temperature of a blackbody that wold emit the same amount of radiation as the star, allowing star temperatures to be calculated.
Calculate example (lambda)Max values: Sun = Teff 5800K Hot star = Teff 12000K Cold star = Teff 3000K
Sun 500nm
Hot star 250nm
Cold star 1000nm
Why do stars have different colours?
Stars show different colours depending on their temperatures which affect the appearance of their Planck’s function graph which is the easiest way to visualise this.
What colour is:
Sun
Hot star
Cold star
Sun - yellow
Hot star - blue
Cold star - red
What are the units for Planck’s function?
W m-2 Hz-1 Ster-1
What is the equation for Planck’s function?
B (V, T) = (2hv^3/c^2)/(e^hv/kT -1)
Why do we use the Rayleigh-Jeans approximation and what is the equation?
For long wavelengths and small frequencies this approximation is used e.g it could be used to predict infinite energy towards large frequencies (UV catastrophe)
Describe the Stefan-Boltzmann Law
This law helps to calculate the total energy emitted by a blackbody over all wavelengths, per second, per square metre of the surface of the blackbody.
Basically qualitatively this equates to energy emitted at every wavelength depends strongly on temperature.
Explain the maths that occurs to allow the Stefan-Boltzmann law to occur
-Use Planck’s function
-Intergrate it and substitute hV/kT to X
-This eventually gives
E (T) = (sigma) T^4
(sigma) = 5.67 x10^-8 W m-2 K-4
How does luminosity link to temperature and size?
L = 4(pi)R^2(sigma)Teff^4 L = luminosity R = size (the numbers contribute to the spherical surface area using radius R) T = temperature (Teff is used here)
