Stars Continued

Question 1

What is the hertz
Rung Russel DISGRAM

Answer 1

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

Question 2

What even is luminosity ?
Compare it to brightness.?
What is surface temp?

Answer 2

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

Question 3

How is the luminosity plotted?

Answer 3

Often relative to the luminosity of thr SUN , so make sure to consider that in calculations

Question 4

Relationships we need to know to understand diagram

Answer 4

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

Question 5

All thr postions for stars

Answer 5

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

Question 6

Try explain life of a red giant

1) main sequence to red giant
2) Red giant to whait dwarf

Answer 6

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

Question 7

Main sequence stars

Answer 7

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

Question 8

Super red giant cycle, why does it start top left and end top right!

Answer 8

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

Question 9

And where would a black home go

Answer 9

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

Question 10

How can electrons in an atom exist?

Why are the energy levels negative

Answer 10

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

Question 11

What are ways electrons can be excited?

Answer 11

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

Question 12

When it de excited what happens?

Answer 12

It releases a photon equal to the change in level difference

It can do this in steps releasing multiple frewuencies of photons too

Question 13

So what’s special about energy levels and metals

Answer 13

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…

Question 14

Explain absorption slectrum in the context of stars

Answer 14

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

Question 15

Emission slectrum?

Answer 15

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

Question 16

Tell me rn why the emission and absorbtion aremr necessarily thr polar opposite of each other

Answer 16

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

Question 17

How to idnedity an element in a star using this method of absorption soectrw

Answer 17

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!

Question 18

Why does absonriton soectrw typically start at ground state in general

Answer 18

Because they are cooler gas, most of the electrons start at ground dtate

Question 19

Why do we use a diffraction grating rather than just simple double slit

Answer 19

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

Question 20

Misconception , when doing the diffraction grating from above, we get dots, but when we do it across a table what will we get?

Answer 20

We will get lines! So if it says line in question don’t worry

Question 21

How to calculate largest order n?

Answer 21

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

Question 22

Remember in a practical what aim to do

Answer 22

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

Question 23

Jus explain what happens

Answer 23

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

Question 24

What is a black body emitter

Answer 24

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

Question 25

How does rhe peak wavkgnth for a specific black body change with temp and what law?

Answer 25

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

Question 26

Why is the black body called black

Answer 26

When em radiation any is shone on to it, it absorbs all
- as none is reflected, it appears black

Question 27

So how can we find the temp of most objects then?

Answer 27

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

Question 28

How will a graph change as temp increase / decrease

IMPORTSNT

Answer 28

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!

Question 29

STEFANS law?

Answer 29

Is thst total power , luminosit , is related to different variables such as surface area, radius and temp , with a constsnt

Question 30

How can we then use bith equations to find mass density etc

Answer 30

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