Astrophysics: Extra Flashcards
Minimum angular resolution
Resolving power
The minimum angular separation which the instrument can resolve/distinguish
Measured in radians
What does minimum angular resolution
How far away they are
How close they are
Why do telescopes not focus stars to a perfect point
Focus as an airy disc/pattern
Seen as the diffraction pattern from a point of light as the light waves diffract through the aperture of a telescope
Formula for minimum angular resolution assuming the limit is due to diffraction by the circular objective
Rayleigh criterion formula on formula sheet
Explain the Rayleigh criterion formula
Theta is the minimum angle that can be resolved in radians
Wavelength of light in metres
D is the diameter of the objective lens or primary mirror in metres
What is collecting power
An important parameter
A measure of its ability to collect incident electromagnetic radiation
Directly proportional to the square of the diameter
Black body
Perfect absorber and emitter of electromagnetic radiation
Absorbs all the EM radiation that falls on it
And a perfect emitter
What is a spectrum
A graph of intensity against wavelength or frequency
What is the area of an intensity wavelength graph
How much energy is emitted for the wavelengths
What can be said about the total amount of radiation from a body when the temperature increases
Total amount of radiation increases
Explain the graph for Wein’s displacement law
Wavelength max on y
Temperature of x
Inversely proportional
Hotter stars will produce more light at the blue/violet end of the spectrum and will appear bluer or blue-white
Cooler stars look red as they produce more of their light at longer wavelengths
Assumptions made for Wein’s displacement law
No radiation is absorbed in space/by earths atmosphere
Assume to be a perfect black body
Why might ground-based observations of stars lead to erroneous conclusions regarding the temperature
Space is better because less impediment to radiation so gives a lower maximum wavelength
Atmosphere not equally transported to all wavelengths
Stefan’s law
Relates the total power output, P, of a star to its black body temperature, T, and its surface area, A
Tells you if two stars have the same black body temperature (are the same spectral class), the star with the brighter absolute magnitude has the larger diameter
Ratio equation for power output against diameter for two black bodies
P1/P2 = T1^4d1^2/T2^4d2^2
Order of spectral classes
O: Oh B: Bugger A: A F: Fucking G: Goat K: Killed M: Me
Hottest temperature is for O (blue) and coolest is for M (red)
Balmer series
Excitation from n=2
Useful since visible light so can be detected
Spectral class O prominence of Balmer lines
Weak
Collisions of atoms at high energy levels ionise hydrogen
Spectral class B prominence of Balmer lines
Slightly stronger
Collisions of atoms at high energies ionise hydrogen
Spectral class A prominence of Balmer lines
Strongest
Collisions cause excitation from n=2
Spectral class F prominence of Balmer lines
Weak
Too cool, collisions not enough energy to cause excitation from n=2
From n=1 instead
Spectral class G prominence of Balmer lines
Very weak
Collisions with not enough energy to separate H2 into H atoms and causes excitation from n=2
Spectral class K prominence of Balmer lines
Very weak
Collisions with not enough energy to separate H2 into H atoms and causes excitation from n=2
Spectral class M prominence of Balmer lines
Very weak
Collisions with not enough energy to separate H2 into H atoms and causes excitation from n=2
Explain the Hertzsprung Russell diagram
Scatter graph
With stars as the point on the graph
y axis is absolute magnitude from +15 to -10
x axis can be spectral class or surface temperature
For the four bubbles on the H-R diagram
Top horizontal = red super giants
Next slanty bubble = red giants
Funky squiggle bubble = main sequence
Bottom = white dwarf
What does Stefan’s law tell us
That hot stars should be relatively bright
Cools stars relatively dim
What does Hertzsprung Russel diagram tell us about white dwarfs
Very dim but very hot
Although T is very high, a really low surface area means a lower lower and hence brightness
Exoplanet/Extra-solar planet
Any planet not in our solar system
Hard to detect don’t emit light
satellite of any star other than the sun
Why are exoplanets hard to find
Orbiting stars much brighter than them
Can’t be seen directly because they mostly only reflect light and are too dim to be seen from Earth
Subtend too small of an angle with their star for resolution in a telescope
Only a few of the largest hottest and nearest exoplanets that are furthest away from their star can be seen directly using specially built telescopes
Two methods to find exoplanets
Doppler shift/radial velocity method
Transit method
Explain the Doppler shift for exoplanets
Measures how much lines from a star has been red and blue shifted from its mean value over a period of time
Due to the star rotating about the centre of mass of the star planet system and wobbles towards us in part of the orbit giving a relative blue shift and away giving a relative red shift
Changes are small since stars have a high mass relative to planet
This method can be used to get the mass of the planet
Stars movement and plane of orbit must be in line with the observers line of sight
Quasar
Galaxy
Particularly large concentration of gas, dust and matter near to the black hole at its centre is draw in and its gravitational potential energy decreases, kinetic and thermal increase
So high energy radiation emit above and below accretion disc due to the black hole and the thermal energy
Which galaxies are believed to have a black hole at the centre
All
Why are Quasars hard to detect/condition to detect quasar
Have to be in line with the jets of high energy radiation (directly above or below)
We observe very large radio wave signals because the gamma wavelengths have been red shifted
So the quasar is moving away from us
Because it is massive (one end of spectrum to the other) they must be the most distant observable things moving very very quickly
Explain the transit method for
Changes in apparent magnitude as an exoplanet travels in front of a star are observed when some of the light is blocked from Earth’s view
Leading to a dip in the apparent magnitude observed
So can find the radius
But the chance of the planets path being perfectly aligned so its crosses the line of sight between the star and the Earth is very low
What radiation is emit/detected and why from a quasar
We observe very large radio wave signals because the gamma wavelengths have been red shifted
So the quasar is moving away from us
Because it is massive (one end of spectrum to the other) they must be the most distant observable things moving very very quickly
What has the greatest redshift
Quasars
Make the graph curve upwards
Was unexpected, so suggests the universes expansion is accelerating
The universe is expanding…
At a constant rate
What provides evidence that the universe is expanding at an increasing rate
Quasars
Why are Quasars different
Have dense mater and dust and gas round their centre
GPE into thermal means they emit jets of high energy radiation
What happens when the hydrogen stars to run out in the core of the sun
Hydrogen is used up Decreases radiation pressure Core contracts rapidly Big increase in temperature Enough so helium can start fusing Expansion of the outer layers which cool
What elements may be produced by fusion reactions in the sun
Helium forms beryllium
Increase in temperature for carbon to form
Increase in pressure further for oxygen to form
Gravitational collapse raises temperature
What is a planetary nebula
Final fusion processes stops due to lack of fuel
Core shrinks to raise temperature and outer layers blown off to lose about 50% of mass
Remainder shrinks to a high density
Lifecycle of a star
Stellar nebula
Average size star = red giant = planetary nebula = white dwarf
Massive star = red super giant = supernova = Neutron star OR if massive a black hole
Explain the universe creation
From gluons pairs of antiquarks and quarks were formed which annihilated and gave off more gluons
Somewhere along this time matter won over antimatter and 1 billion and 1 matter particles were made for 1 billion antiparticles
Universe expanded so leads to a decrease in temperature
Quarks form hadrons (p and n)
Temperature cools to be cold enough for neutrons to decay into protons and form hydrogen
Atoms form out of hadrons and electrons
Pressure increases so stars and galaxies can form and emit radiation
