Fhapter 19 Stars Flashcards
What is a planet
Object that orbits a stars
- has cleared its atmosphere of most things in orbit like asteroids
- has high enough grab field to give it a round shape
- has NO FUSION REACTIONS LEFT ( unlike stars)
Planetarybsatlritesn
These are anythign that orbits a planrt like moons or satellited
Comets
These are collection of dust and ice and rock , which orbit thr SUN
Solar system
Includes the sun and all the planets and objectd in jtd irbit ( comets, moomens etc)
Galaxies
Include all the stars and objects, interstellar dust and hsd
Universe
All the matter, energy and space time available conssits of our universe
These stars will have their own galaxies and solar systems
Nebulaloe
This is collection of dust that attracts itself to each other over time. The tiny gravational force of attraction from the particles cause them to clump up together, to form a collection of dust and GAS. HYDROGEN GAS
How is a peotodtar made
As the nebulae becomes more and more massive, grvaitnal energy from psrticles is transferred to ke as the particles become closer. In a more dense region, this starts to form a hot core , and thus a protostar is msde
As soon as the core is hot enough for fusion to take place, the star is born and enters its main sewuence
Why must their be high temlergsured and pressures for fusion to occur?
Fusion occurs when the nuclei get close enough for thr strong nuclear force attrvstice rsnge . At these small distances the electrostatic force of repulsion from protons in the nuckeud is so strong, and thus high temperatures and pressures are needed so nuclei have enough energy to overcome this force of attraction, and get close enough to fuse
What happens when the star is in its main sequence?
As hydrogen fused, this releases energy outwards. The photons preoduce create a RADIAITON PRESSURE, and also gas pressure . This works to counter to gavrwrionsl force of atrevstion due to the mass of the star on the outer layers .
While there is enough hydrogen and hydrogen is fusing, this is sustained, and the Starr is in a STEADY STATE FOR A LONG TIME
so the hrvaronal force acts to compress the star. It this is countered by gas and radiation pressured produced due to fusion
Why do bigger masses have a lower period as mains rauende compared to lower?
Bigger mass stars have higher temperature and pressure cores, due to more gpe being converted into ke as particles accelerate to its centre . Higher tempegured and pressure means fusion of hydrogen happens fsdter, and so it runs out of its store quicker. Therefore it has a lower time as main sewuende
How do low mass stars thrn become red giants after fusion of hydrogen runs out
- fusion of hydrogen runs out, so radiation and gas pressure drops
- therefore gravational pull is now bigger force than the pressure
- so the star begins to compress
- this makes the core of the star much hotter as it compresses
- which allows for fusion of hydrogen to take place in the shells of the core, due to right conditions
- the pressure produced by this expands the layers of the star
- eventually they cool down and become red in colour as a result
Thus red giant
Then what happens once hydrogen in shells run out?
Once hydrogen in shells run out , there is no longer any radisjton pressures
- by this time most of the outer layers have been eejected as planetary nebulae
- but now thr rgvatwitnal force of attraction is greater then outward oressure, the core of the Starr which is what’s left begins to compress
- if it has mass of less than 1.44M, it remains stable as a WHITE DWARF
- which then cools and becomes a black dwarf
-
How can the core end up sd a stable liece of mass, when there is no pressure?
There is .
- as the core compressed, a certain ELECTRON DEGENERACY pressure is extended outwards
- this is produced due to do Pauli exclusion principe. Which stated that electrons can’t exist at the same energy state
- therefore as the core compressed, the electrons exert a pressure otjwards , which counters the force from gravational attraction
- for masses less than 1.44M , the electron degeneracy pressures Is enough to counter the grab pull, and thus remain as a stable white dwarf
However for masses ABOVE 1.44M, the grav force is TOO strong and the electron degeneracy pressure is not enough to counter. Other things happen, but the core is only 1.4d m and abive if it’s a big mass tar that becomes a super red giant anyways
This limit is called the CHANDRESHAKR limit
Features of a white dwarf
Extremely hot due to small size and, photons still emitting from before
However no more fusion, therefore luminosity is minimum
Small and high density m high mass, but masses are generally less than 1.44M obviously
What happens for a high amss star 10M plus afternintiwllyruns out of hydrogen
Runs out if hydirgen, the gravational pull then is bigger than radiajton pressured.
This causes the star to compress , which increase the temperature of th encore
Yet because the star is so massive, this means the core becomes hot enough for fusion of heavier stars to occur , with the hottest part being the middle.
As a result , pressure generated from this caused expansion of the outer layers, creating a super red giant
Here, the core keeps fussing until the whole core becomes iron, as this is the most stable element , with the highest binding energy.
What happens when all the hydrogen is finally used up, as well as ENITRE CORE iron?
Once entire core iron, no more fusion can take place
- no more ourtwar pressure, so the force due to gravity rapidly compressed the outer layers
- these rebound of the core and are ejected into the soace as part of a SUPERNOVA ! An IMPOLOSION , then Supernova that is a shockwave ejection alm layers
Describe the superoniva again
What about fusion
Once entire core is fused to iron, the gavtisotnal force pulls on layers leading to implosion which REBOUND of the core and into space as a supernova, ejecting all the layers
At this point, fusion of heavier elemetsoccur … things more than iron
What can happen to the super red gian core after supernova
1) if thr mass is between 1.44 and 3, As this exceeds the chandeskha r limit, the core continued to compress, until it forms a neutron star
- this is extreme,y low volume but high density such that it is massive.
2) if Mass if core event bigger than 3M, it compressed to become a black hole
X black hole is so massive that the grvatjnal pull means photons can’t even escape, and thus an escape velocity greater than the speed of light is needed.
Neutron star
High density
Small volume
Thus high mass
Remember why cantnothing escosea black nowl
Gravational o field strength is so massive an escape velocity greater than the speed of light is needed, which is not possible …
What is luminosity
Luminosity is the total outward POWER from a star
Can be thought of as brightness from star, even tho it is energy per unit time
What is hertprung russle diagram used to shown
Show how different stars and lifecycle of stars , their luminosities and surface temperatures, and shows the progression of a star
We have luminosity on y axis ( take care it’s not logged) and temperature surfsce in x in REVERSE ORDER, therefore hotter on left and coldest on right
l/ L0 what does this mean?
Luminosity is always plotted relative to SUNS luminosity, so make sure to multiply by L0 and then reverse lognfrocalculation
Some basic relationships to help understand hertz
Rung
Bigger size, generally means LESS TEMLERTAURE, as same energy spread over a larger volume means less temperature
However bigger size, means HIGHER luminosity, this si beavuse for higher size, means more outward pressure is needed which is due to fusion and a measure for luminosity to counter grav pull
More fusion = More luminosity
Describes whole hertsprung diagram
So bottom left = white dwarf
Bottom right = main sequence stars
Curve up = red giants
All the way top left = super red taunts
And higher is the chain for suler red giants , and they end on top right
Life of a red giant, from main sequence
As fusion of hydrogen from shells happen, rste of fusion incresses, so luminosity increases?
However we the star expands , it gets bigger , so temperature decreases , so goes up snd rigjt
When it becomes a white dwarf, no more fusion so luminosity compelled drips, thus all the way down
- however as very small and still some photons emitted, surface temlertaure is thus extremely hot
= bottom left
Main sequence stars? Do they move!
No, as the rwte of fusion same, and volume temp snd lumniosity dtyss constant
Going up the line you get more luminous stars which mesns MORE HEAVIER ONES, it’s likely the line exceeding the red Gina line becomes SUPER RED GIANTS
Super red giant lifecycle
Very masidve so high lumniosity tonstaet off with, as well as being hot from cored.
As the super red giant desists, itslayers are ever edmapming, and thus temperature continued to DECREASES. lumniosity stays the same duentoits high rste of fusion of heavier elements, which release a lot of enerhu
Black hole?
As the temperature is unknown , csntplace it on the graph?
Certisntlynnot lumnikus
Electrons and atoms
Electrons in atoms exist at specfic energy levels, where they have discrete amounts of energy
These levels are negative, and they relresent the energy needed to leave the atom and ilnsie it
They must exist within this energy level, can’t exist between
- potential values for attraction are megwrife
- and indicates electrons are bounded to nucleus
Electron excited
When a lhtonnof exact energy is given such that it is equal to the different in enrgry levels, the electron can excite
- if this ohtoj is more energy than what is needed to leave the atom, then the photoelectric effect occurs
- if it colldies with another electron fo ke, then it csn trnasfer however much it wants for it to escote, and keep the rest. As long as the ke hsd somewhere to go, it can be tesnsfered if not, then eked tin with excite
Electrons descifign produces a Photojournalistinnen of same energy due to DIFFERENC ein energy levels . As a an electron csn de ss IGE in muktilenstepsdifferent photons can be emitted from what was absirbedtonescited in thrifter roamed
,,
Special about energy levels and Metalls
Is that each metal will have its OWN energy levels, and rherefoewill emit charvsteristictype don ohtojd
Analysing this as an emission spectrum csn tell us what the element is in the thing that emitted it, when comparing the same spectrstona lab
RMEMEBR minimum wvakeenfh t
Means MAXIMUM DIFFERENCE , as difference safe proteins to the FREQUENCY, not wavelength
Explain all the spectra
Emission spectra
- when hotter gases have excited electrons due to gain in energy, they emit photons when they de excite
- this spectra can be shown against a black background to show the exact frequencies of light emitted
Absorption spectra
- when cooler gases absorb photons of different frequencies, if ti matches their difference in energy level electron will escited
- observing this from the other side, you will get all the frequencies minus the ones absorbed
- you might say won’t it re emit? It will but never in the same direction rarely with lower intensity, so we don’t see them
So absorbtion slectra I’d the light we see when cooler gases absorb light
Now why aren’t these the reversed of Esch other?
- because hotter gases csn de excited from DIFFERENT LEVELS, so ti shows you all the intermediary photons
- whereas cooler gases ar ethnically from the ground state, and thus the absirbtion of photons from in engeren states is never seen, and his thr omission of this frewuency either
In the context of Stars?
In the context of Stars, gases made from differen elements inmlsyers around Stars absorb the wavelength sof light which is emitted a skcnitnous soectrum from the star itself
- they re mit but with lower intensity. Therefore the light we see is an absorbtion spectra
- however, this will be red shifted, as the star had recessional velcoty
- so to idneifty this element, we will need to predict if it could be hydrogen for example, and find the red shift for this, and then reverse on each soectwal line, as they will have the same elrcsnfage difference in shift
X if this niw cimapred with the spectral lines of absorbtion we see on earth, we can now begin to decipher the other elements. If it isn’t, we need to keep going until we find one correct elect so we are certain the red shift change is the same!
Why donwe use a diffraction grating rather than a double slit to observe interference pattern
Many diffraction gratings means the pattern produced is much cleared than if ti was nust two slits. , making jrnwasier ti determine the centre of Esch maximum
Light passes through, diffracts as slits are comparable to wvakentgh than of light, superpose and produce an intervener elatreen of maxima and minis, and this can be explained due to the path length differnces
When shone across a table,how does this look.
Why does white light Solti
It looks line
White light is made up of all the different wvakenghtdkfnlight. The ineswhich are mconalrsbel ( so bigger) than the gal , will diff f more. Therefore white light sorted out
Max order can be what how to find
What about to number of DOTS
Max order is when sin theta is 90 because we can’t go higher than that
So re range to fidn order
And MUST ROUND DOWN, because you’ll never get the order abi, but will below
In terms of dits, double the fits and ADD ONE MORE FOR THE CENTRAL MAXIMA, WHERE N = 0
Explain eacalty how the pattern is produce d
Light passed though slits and is diffracted. The light normally a laser such that it’s a mocha atif coherent source of light. This will then diffract and superpose. Where meet in ohase / anti ohase will proeuced cinsturbcuve interfecne destructive and snow as maxima or minus if cimoeltley in ohase
Can explain in PLD, where the DIFFERENC in oaths is a whole number multipke of lambda, a maixma will occur, and when is odd multiple of 1/2 then minima
What is a black body emitter?
A perfect emitter and sibriber of light at a CONTINOUS SPECTRUM
Absorbs every wavelength of light and doesn’t reflect, therefore appears nlack
How do balck bodies emit evalengths based on temeprwyiree
Emote them all, however based on the temperature it’s at it will emit some at HIGHER INTENSITIES then others
Change the temp snd different wavelengths will be seen more, as others emitted at such low intensity we don’t even see them
Wiens law
Is that the surfsce temlertureof a black body emitted is inversely portion Wl to itd PEAK intensity wavelength
Therefore if something gets hotter, lower wavelengths are emitted (which makes sense as higher frequencies)
Therefor hotter something gets it becomes closer to blue, blue the hottest,
So how can we find the temperature of most objects then?
We basically model EVERYBTING AS A BLAKC BODY EMITTER, and in doing so , we can chekc the wvakentgh to using a diffraction grsting, snd usign Wiens CONSTSNT can relate this to its surface t,wlefture. But remember it must be the LRIGNAL wavelength snd not the red shifted one
Can model the whole universe as a black body emitte, and thus for the uniform wvakeenfht we see ( microwaves), we relate this a tmoefgure of 2.7k which is the temelrgsure fo the universe
How does rhe intensity Absinthes wvakentgh graoh change for increased tmelrgure
Obviously wavkenght will shift to the left
However the intensity also increases, for reason unknown
We can predict that this is because the energy has increased sonitnneidty too?
Therefor cooking csuses shift to right, and downwards aswel
Stefans law
Relates LUMINOSITY ( so power too dint lack ) to radius of star, surfsce area, surfsce Romeo and stefans CONSTSNT
How to use both equations
We know the wvalenght ifnlifjt using a diffraction grating and red shift
Thus we kneonsurfsce temp
We know the temp , we also know the luminosity , and thus we know now the rsfius
Thus the volume , and then if we know mass density etc…
How do they find masses ?
They use Newton gravitation law with sensitive sensors to dinf out
