Stars Continued Flashcards
What is the hertz
Rung Russel DISGRAM
It’s a diagram to show the relationship between stars TEMPERTAURE and their luminosity
Luminosity on y axis and TEMPERTAURE in k on x axis, but with axis DECREASING
Can be log so be careful
What even is luminosity ?
Compare it to brightness.?
What is surface temp?
Luminosity is the TOTAL power radiated out. He can think of it as power
- although this is energy output per second, can think of it as brightness too
Surface temp is the temp on surface
How is the luminosity plotted?
Often relative to the luminosity of thr SUN , so make sure to consider that in calculations
Relationships we need to know to understand diagram
Rate of fusion = power = more luminous
More mass= more rate of fusion needed to counter its grav pull because of more mass = more luminous . Therefor anythign that’s highly luminous must be massive
TEMPERTAURE inv prop to VOLUME. As volume increases (during expansion), TEMPERTAURE will decrease. Why? Because the power is spread out over large volume, so TEMPERTAURE is less .
Or during collapse like white dwarf, even if it emits barely any energy bevause volume so small surfsce temp is crazy
All thr postions for stars
Line going bottom right to top elft is main sequence stars
Half way though line that breaks off is red giants
Small lime bottom left is white dwarf
Line too is super red giant
Try explain life of a red giant
1) main sequence to red giant
2) Red giant to whait dwarf
Main sequence star, gains lumniosity as fusion of hydrogen in shells takes place, causing it to be a red giant
Temp will also decrease as the red giant expands
1) increase lumniosity because rate of fusion as fusion of shells happens increases
2) becomes colder thus moves to the right due to volume increasing too
White dwarf
1) no no fusion takes place, thus no lumniosity. This postions it in the lower band
2) however as the volume has decreased a lot, the TEMPERTAURE has increased , thus highest temp
Therefore ends bototm left
Main sequence stars
During their lifecycle , their luminosity and their temp will stay the same , as rate if fusion and volume is constsnt
Going up the line, you get heavier stars (and thus more luminous). It’s likely that the heavier stars are going to convert to super red giants after lifecycle
Super red giant cycle, why does it start top left and end top right!
1) lumniosity musr be high for two reasons
- very high mass
- fusion of heavy elements going on
But because this fusion of heavy elemtnd is constsnt, it’s not like the lumniosity WILL INCREASE OR DECREASE, thus stays in top band
2) however TEMPERTAURE decreases and it goes right, as the more the super red giant expands and expands
And where would a black home go
Black hole = high mass but no fusion, so would expect it to be very high temp
However don’t know temp for sure so can’t place it properly on the diagram
How can electrons in an atom exist?
Why are the energy levels negative
They MUST exist within an energy level that’s it , can’t exist between them etc
2) negative because they rpresent the amount of energy needed to remove the electron from the atom. Remember that potential values for attraction is negative, because it represent the energy needed to take it to infinity . When electron reaches highest energy level the ionisation level it’s free, this is a 0 energy. Also indicated the electrons are BOUNDED TO THE NUCLEUS
What are ways electrons can be excited?
Excited is when an electron moves up an energy level , from the ground state or otherwise
- if a photon is given to the electron, the energy of the photon MUST BE EXACTLY the same as the difference between the gap, as photon energy is discrete and can’t dissapead
- however if the energy of the photom is greater than the ionisation gap, then this is lit the photoelectric effect. The remaining energy is transfered as ke
- an electron thst bumps into electron to make it excited such as in x ray can give it as much energy as needed to go to next level. As a result it can keep remaining energy and dispersed with reduced ke too, np
When it de excited what happens?
It releases a photon equal to the change in level difference
It can do this in steps releasing multiple frewuencies of photons too
So what’s special about energy levels and metals
Each element has its OWN set of energy levels, like a fingerprint and thus will emit its own different frewuencies, we can use this later to analyse and work out elements…
Explain absorption slectrum in the context of stars
Okay stars ligjt they produce is of continous slectrum straight away but…
Near the star are COOLER GAS
- these absorb the wavelengths from Comtinous slectrum
- based on the elements in this gas, specific wavelengths coresPonding to exact electron levels will be absorbed, and everything else transmitted
- thud from earth, we will see all the wavelengths of light coming from thst star EXCEPT the few frewuencies the elements ABSROBED when exciting
- even though the element will eventually remit the photon, or in a series, this will be in all sorts of directions, so the orignal intensofu will be far less
From earth, this produces a continous slectrum with missing black limes. These lines correspond to specific frewuencies the elements absorbed, snd these will be UNIQUE for every single element
Emission slectrum?
When you have a hot gas with electrons in excited state, when they de excite, they will release all sorts of frewuencies of ligjt
It’s these frewuencies that buil up sn emission spectrum, which will be specifically unique to an element
Tell me rn why the emission and absorbtion aremr necessarily thr polar opposite of each other
Because the emission spectrum can emit in series or in one go
But the absorbtion slecturm tends to only absorb from GROUND STATE, as most of electrons have started from this place already. This is because they are of cool gas, so most exist here, whereas hot gas are in all sorts of places do emit all of the steps in between
So not necessity polar oppsite, but berg similarly
How to idnedity an element in a star using this method of absorption soectrw
Light analysed from a star on earth will produce an absorption spectra , which will correspond to a specific element due to them having the same sbsonrtion soectrw
However, on earth, this spectra will have been RED SHIFTED, so we can’t tell straight away by composing it to elements
Instead we need to reverse the red shift, but to do that we need to know the original frequency of the element (assuming we know speed of planet ) in order to work out the percentage shift and apply it to all
To do this, we gonna have to guess. Pick sn element, likely hydrogen , snd find the percstnge shift for thst. Now apply it to the rest of the thing COMPLETLEY and see if it matched. If it does, now can compare all the other frewuencies missing
Because rhe wnskrntion spectra will never just show one thing will it, it will shoe all the elements superimposed on top. As long as we have one match, we can safe,y ckmalred it with abdkrbion spectra of all elements found on earth!
Why does absonriton soectrw typically start at ground state in general
Because they are cooler gas, most of the electrons start at ground dtate
Why do we use a diffraction grating rather than just simple double slit
Double slit only 2, so fringes produced aren’t very sharp = hard to identify the postion of the cenree of each maxima (as that’s what we measure)
However diffraction grstifm can have hundreds of slits. All the diffraction from here happens like a slit and it produces a much clearer and brighter interference patterns that makes us easier to measure the distance
Misconception , when doing the diffraction grating from above, we get dots, but when we do it across a table what will we get?
We will get lines! So if it says line in question don’t worry
How to calculate largest order n?
Thr orders spread out just like PLD for normal interference
Largest order n assumes largest angle which isn90, so sin 90 is 1
Here remember to ROUND DOWN as CANT round up as that’s never possible but rounding down is possible
Remember in a practical what aim to do
Find anglenusign distance and tan theta
To find the wvanegleng of light. Lrosbly shoukd find it for different orders and overage if if you can? As the angles might be off a bit
Aim it at the floor and try get the biggest distance ti emaudre ti reduce the undercertsintiy
Jus explain what happens
As ligjt passed through the slit esch gets diffracted, and the resulting sources of light all superpose to produce interference pattern visible to us as a series of fringes / lines Due to diffraction
Now where they meet in phase / antiphase constutive destructive agsin. This is agsin relative to PLD , at 0 order PLD is 0, then 2st order minima 2/2 etc
These relate to angles too instead of function of distance
What is a black body emitter
A black body emitter is a perfect absorber of ligjt and a perfect emitter, emitting all the wavlenghts at a CONTINOUS SPECTRUM
However, modelling intensity agsisnt eavkenfth if light , we can see there’s always a PEAK wavelength, this is the one we see the MOST
How does rhe peak wavkgnth for a specific black body change with temp and what law?
Wiens law = temp of surfsce of black body is inversely prop to wavelength
So higher temp = lower wavelength emit peak (high frewuency makes sense) and so becomes more blue the higher temp it is
That’s why the big red giants are acc blue because of how high temo they are
Why is the black body called black
When em radiation any is shone on to it, it absorbs all
- as none is reflected, it appears black
So how can we find the temp of most objects then?
By looking at the peak wavelength they emit and corresponding to temp usign Wien constsnt
This all by ASSUMING THINGS TO BE BLACK BODY EMITTERS, if we do this, then we can apply this to anythign from whole universe to a mammal
How will a graph change as temp increase / decrease
IMPORTSNT
Well if temp increases, the wavelength decreases, so graoh shifts to the LEFTA
BIT ALSO the intensity of every part of the curve increases. This is probably to do with energy related to amplitude and intensity
So 2 things 1) graph shift, and intensity increase / decrease!
STEFANS law?
Is thst total power , luminosit , is related to different variables such as surface area, radius and temp , with a constsnt
How can we then use bith equations to find mass density etc
We know the wvalenght leak usign diffraction grating, and thus the surface temp
We know the lumniosity by measuring it
Now we can find the raidus, and thus mass density etc
