Stars and Radiation Flashcards
What is meant by a black body?
A black body is a conceptialised body that absorbs all incident electromagnetic radiation, while being able to emit every wavelength of electromagnetic radiation.
Hence it will have a continous spectrum
Stars are modelled as black bodies
Use the graph of a black body curve and appropriate laws to state and explain the relationship between intensity (power output/energy) of a star and temperature
The relationship between intensity and relationship is as followed:
- Intensity emitted from a star will be greater for higher temperatures of said star
- This is because peak wavelength will be shorter as temperature increases due to Wiens Law
- Since wavelength and frequency are inversely proportional, a increase in frequency must accomidate this decrease in wavelength for hotter tempertures
- Hence for hotter temperatures there will be increased frequency
- Thus more frequency means more energy (intensity) using E = hf
State what can be known from Wiens law, and the accompanying equation
For a higher surface temperture of a star, the peak wavelength will be shorter
(Peak wavlength is when peak intensity occurs (peak of graph))
Why may a star with higher power output radiation appear dimmer than a star with lower powerout put radiation?
The radiation emitted by the star with more intensity may not be in the visible part of the spectrum of electromagnetic radaition
State what is known by stefans law and the accompanying equation
The power output radiation of a star is dependant on surface Area and Temperature at surface
Note: Stefans constant shows this proportionality and is given in formula book
Equation of inverse square law of intensity
Radiation spreads out and becomes diluted hence intensity decreases using inverse square law
In what situation could you expect to achieve an absorption spectra for stars
Since stars are approximated as black bodies, they emit continous radiation (Kind of act as white light from a prism looking back at paper 1)
Hence you will absorption lines when radiation of star passes through a cool gas (such as a nebula)
Explain what Hydrogen Balmer lines are?
We look at hydrogen specifically because hydrogen fusion in stars is the primary process that powers stars during main sequence phase of their lives
For fusion to take place, this hydrogen must be excited, in order to overcome strong nuclear force and fuse to make helium
Hence atomic electrons in hydrogen will need to be at n = 2 energy level
Balmer lines are transitions to or from n = 2 energy levels
Explain the relationship between temperature and intensity of Balmer lines
Hydrogens electrons must be at n=2 state for Balmer lines
This happens at high temperatures, where collisions between atoms give electrons enery
If temperture is too high then majority of electrons will be in n=3 states
If temperature too low then majoiruty of electrons will be in n = 1 state
Thus intensity is dependant on temperture, relation shown in this graph:
What are the orders of the spectral classes
Stars are classified into spectral classes, dependant on relative strength of certain absorption lines
O B A F G K M
In order of decreasing temperture
O is the hottest (blue)
M is the coolest (red)
The sun is a G class star
For spectral class O, state:
- The colour
- The temperature range
- The strongest spectral lines and why
Spectral class O
- Blue
- 25,000 - 50,000
- Strongest spectra lines are He + and HE
They have very high temperatures therefore have weak intensity Balmer lines
For spectral class B, state the:
- Colour
- Temperature range
- Spectral lines
For spectral class B:
- Blue
- 11,000 to 25,000
- Spectra show strong helium and hydrogen absorptions
For spectral class A, state the;
- Colour
- Temperature range
- Spectral lines
For spectral class A:
- Blue white
- 7,500 to 11,000
- Strongest hydrogen Balmer lines, but also some metal ion absorption
(if confused look at graph)
For spectral class F, state the:
- Colour
- Temperature range
- Spectral lines
For spectral class F:
- White
- 6000 to 7500
- Strong metal ion absorptions
For spectral class G, state the:
- Colour
- Temperture
- Spectral lines
For spectral class G:
- Yellow white
- 5000 to 6000
- Both metal ion and metal atom absoprtions
For spectral class K, state:
- Colour
- Temperature range
- Spectral lines
For spectral class K:
- Orange
- 3500 to 5000
- Spectral lines are mostly from neutral metal atoms
For spectral class M, state:
- Colour
- Temperature
- Spectral lines
For spectral class M:
- Red
- less than 3500
- Spectral lines are from neutral atoms, as well as molecular band absorptions from compounds such as titanium oxide TiO (since cool enough for molecules to form)
State the scale and patterns seen on the Hertzsprung Russel Diagram (you may be asked to draw this in the exam)
The scale (y axis) is absolute magnitude:
From 15 at origin all the way up to -10 (with a scale of interval 5)
The scale (x axis) is temperature:
From 50,000 at origin, to 2500 (non linear interval)
Explain how stefans law applies to red giants and white dwarfs
Red giants - Low surface temp and high luminosity, therefore must have a large surface area due to stefans law.
White dwars - Low luminosity but high temperature must be very small due to stefans law
Explain, in detail, the formation of a protostar
- Cloud of hydrogen gas in space = nebula
- Hydrogen has mass therefore hydrogen atoms are drawn together by gravitational attraction
- Irregular clumps of mass rotate and due to conservation of angular momentum spin inwards to form dense and hot centre
- Gravitiational potential enery is transferred to kinetic energy, therefore centre is high temperture (proportional to kinetic energy)
Once a protostar has been formed, explain the formation of a main sequence star
- As temperature of core rises, hydrogen electrons gain energy to fully escape from hydrogen (become plasma hence ionised nuclei)
- Kinetic energy of hydrogen nuclei is sufficient to overcome electrostatic repulsion due to strong nuclear force, hence fuses together to form helium
- As fusion occurs an outward force (radiation pressure) is exerted
- When this force is balanced with the inward force (gravitational attraction) the star is in equilliubrium, hence is in main sequence phase
Describe in detail the fusion process during a stars main sequence phase
- Two hydrogen nuclei fuse to form hydrogen-2 (deuterium)
- Deuterium fuses with another hydrogen atom to form helium-3
- This process releases a gamma photon
- Gamma photons are either absorbed or reflected by surrounding nuclei, thus nuclei experience an outward force to oppose gravitational attraction called radiation pressure
Thus when sufficient fusion occurs, outward force = inward force therefore is main sequence
Explain in detail how a red giant is formed
- All stars generate outward radiation pressue by fusing hydrogen to form helium
- Once there is a high abundance of helium (most of hydrogen has been used)
- Hence fusion stops so radiation pressue decreases
- Stars core begins to collpase
- Beings to spiral inwards due to gravitiational attracton to atoms
- GPE converted to KE therefore temperature increases
- Hydrogen shell surrounding core is high enough temperature to fuse at high rate
-This creates massive radiation pressure - Shell rapidy expands at same time cools due expansion
- Due to Wiens law a hotter temp = shorter wavelength
- Therefore since cool (low temp) will emit radiation of a longer wavelength
- Hence will have red pigment
Explain in detail how a white dwarf is formed
-In lower mass stars the temperature doesnt reach a point where the helium can be fused
- (Not enough GPE to transfer to KE since low mass)
- This happens when mass is lower than 1.4 solar masses
- However as more helium is produced the mass of the core increases
- Therefore GPE transfers to KE
- Temp now reaches a point where helium can fuse to oxygen and carbon
-Releases massive amount of energy known as helium flash
- Process creates massive radiation pressue ejecting outer layers as a planetary nebula (leaving just the core behind)
- The core remains initally very hot, no further fusion therefore luminsoity is low
- Known as a white dwarf
- Since no fusion there is no radiation pressure
- So star is kept in equillibrium by electron degeneracy pressure (electron cannot occupy same quantum state)
Note: white dwarf is smaller than sequence star but has much higher density due to being made of carbon and oxygen
What is meant by shwartzchild radius? and equation
The gravitational field of the blackhole is so strong that at a certain distance even light doesnt travel fast enough to escape
This distance is known as schwartzchild radius
Occurs when escape velocity is greater than the speed of light
Why can type 1a supernova be used as a standard candle?
A standard candle = has a known luminosity hence can be used to measure distance
All types of supernova at same critcal mass
Therefore all have a similar peak absolute magitude (-19.3)
Have identical light curves
NOTE:
Type I has no hydrogen line
Type II has a hydrogen line
Therefore there should be no hydrogen line for type Ia
What is meant by doppler effect?
- Apparent change in frequency/wavelength of a wave due to the relative motion of the source and observer
(Compressions or spreading out of the waves that are emitted or reflected by a moving source)
Equation for red shift (same for blue) involving velocity
Amount of redshit depends on velocity, observed wavelength of light from source with higher velocity has a longer wavelength hence redder
source moving away V<0 therefore redshift is positive
source moving towards v>0 therefore blue shift negative
Equations for redshift involving frequency and wavelength
if star is moving away f emitted is GREATER than f observed
if star is moving toward f emitted is LESS than f observe
Explain how the doppler effect can be used to indenfitify spectroscopic binaries
Binary nature is revealed through periodic changes in spectral lines (due to doppler effect)
When spectra is red shifted (lines move closer to red end)
When spectra is blue shifted (lines move closer to blue end)
- As stars eclipse each other they are travelling perpendiuclar to the line of sight of the observe, so there is no doppler shift
- As one star travels away and the other travels towards, the spectral line is split into two, one is blue shifted and one is red shifted
These can be identified from their charactersitics light curves
What is the typical light curve of eclipsing binaries (which can be used for identification)
What is stated by hubbles law?
Hubbles law states that a galaxies recessional velocity is directly proportional to its distance from the Earth
(hence the universe is expanding from a common starting point)
How can hubbles law estimate the age of the universe
What is the concept of dark energy?
Can only estimate age and size of the universe
- since rate of expansion hasn’t been constant
- All mass in the universe is attracted by gravitational force - this slows down rate of expansion
- It was thought expansion was decelerting until about 5 billion years ago
- in late 90’s astronomers found evidence expansion is now accelerating
- Dark enery is thought to be reason behind this
- It is described as having an overall repulsive effect throughout the whole universe
What is meant by a quasar?
A quasar is a active galactic nucleus,
A supermassive black hole surrounded by a disc of matter
As this disc falls into blackhole jets of radiation are emitted from the poles
Explain how the transmit method works for detecting exoplanets
- If a planet crosses in front of a star (transits) the intensity dips slightly.
- If the intensity of a star dips regularly it could be a sign that there is an exoplanet orbiting it
LARGER exoplanet = Greater dip
Further away = longer dip (takes longer to transit star)
- If there are variations in the regularity of the dips this could mean that there may be serveral planets orbiting the same star which have a gravitiatinal effect on the transmititing planet
How can we confirm its an exoplanet using transmit method from a light curve?
We need to see periodic dips in apparent magnitude to confirm existance of exo planet
