Astrophysics Flashcards
How to calculate magnification of a refractor telescope ?
M = fo/fe (objective length/eye length) M = angles subtended by image at eye/angle subtended by object at unaided eye
What is Stefan’s Law ?
P = oAT^4
Power output = total energy it emits per second
- P∝A or P∝T^4
What are the defining features of a type 1a Supernova ?
- different supernovas can be represented using LIGHT CURVES - time since peak absolute magnitude
TYPE 1 - no hydrogen lines in spectra
TYPE 1a - formed when white dwarf
What is Hubble’s Law ?
V = Hd
(recessional velocity/Kms^-1 = constant x distance/Mpc)
- universe is expanding by using standard candle to calculate distance
- realised SPEED OF GALAXIES MOVING AWAY DEPENDS ON HOW FAR THEY ARE AWAY
How to observe red shift ?
Can observe red shift using Absorption lines of emitted spectra and red shifted spectra
- compare lines to compare either frequency or wavelength
Z = Δf/f
- red shift means f>fobs
- blue shift means f
What are makes a type 1a Supernova ?
- different supernovas can be represented using LIGHT CURVES - time since peak absolute magnitude
TYPE 1 - no hydrogen lines in spectra
TYPE 1a - formed when white dwarf core absorb matter - identical light curves due to same masses - can use Standard candle (up to 1000Mpc distance)
What is the Cosmological Principle ?
HOMOGENEOUS - every part is same as every other part
ISOTROPIC - everything looks the same in every direction
- shows that earth isn’t the centre of the Universe - is just a illusion
How are Quasars Large distance away ?
As they are red-shifted enormously they have a great velocity - V=Hd so large distance
What is the doppler effect ?
Is the apparent change in wavelength or frequency of a source due to its relative radial velocity with respect to the observer
- Car moving towards you the sound is loud due to wavelength decreasing and frequency increasing
What are the defining features of a type 1a Supernova ?
The light curve has a initial peak followed by a gradually decreasing curve
What are CCD’s ?
are CHARGED-COUPLED DEVICES
- capture images digitally
- divided into pixels
- photoelectric effect on SILICON chip ( depend on amount of photons)
- stored, copied and shared images
How to observe red shift ?
Can observe red shift using Absorption lines of emitted spectra and red shifted spectra
- compare lines to compare either frequency or wavelength
What is involved in the Formation stage of the evolution a star ?
Cloud of dust + gas -> Protostar -> Main sequence
- previous stars blew themselves up so matter close together and heat up
- hydrogen nuclei start to fuse together to form helium
- release energy - create pressure
What are the key words when considering Converging Lenses ?
- Always under NORMAL ADJUSTMENT
- Parallel Axial rays
- Focal Point (F) + Focal Length (f)
- Principal Axis (dotted line through middle)
What is Red Shift ? And How to calculate it ?
Is the increase in wavelength due to relative recessive velocity between observer and source
Z = v/c (v<0 = red shift is positive as moving away
- V<0 = red shift is negative as moving towards
What are Radio telescopes ?
- similar to Optical reflector telescope - has parabolic disk of wire mesh
- Antenna as detector at the principle focus
- High gain amplifier -> tuner (filter unwanted wavelengths) -> computer
BENEFITS - easier + cheaper that optical reflectors and longer wavelength so mirror doesn’t ned to be perfect
How to derive the Schwarzschild Radius ?
Ek = Ep 1/2mv^2 = GMm/r r = 2GM/v^2 = 2GM/c^2
What is meant by Apparent Magnitude (m) ?
-
is the measure of brightness of an object
- brightness depends on power output and distance apart
- lower magnitude means brighter
What is the evidence for the big bang theory ?
1) Red Shift
2) CMBR
3) Dark Energy
4) Relative Abundance of H + He
What is the resolving power of a telescope ?
is just how much detail you can see - it dependents on the MINIMUM ANGULAR RESOLUTION
What is the HERTZSPRUNG-RUSSELL (H-R) Diagram ?
Absolute Magnitude/ temperature graph
- temperature decreasing across
- Absolute Magnitude decreasing up
- White dwarfs - main sequence - red giants
What are X-ray telescopes ?
- different as absorb radiation by a material and pass through it
- nested mirrors can alter direction of x-rays to focus on detector
- GRAZING TELESCOPE
What is Luminosity ?
the total amount of energy emitted in the form of electromagnetic radiation each second
- known as Power output (Watts)
What is Wein’s Law ?
λmax T = 2.9 x 10^-3mK
- higher temperature - shorter the wavelength /λmax
- hotter will emit more radiation that cooler ones
- hotter stars don’t appear as bright as a cooler one because DOESN’T EMIT RADIATION IN VISIBLE REGION
Difference in red shift and blue shift ?
RED SHIFT - moving away - increase wavelength - decrease frequency BLUE SHIFT - loving towards - decrease wavelength - increase frequency
What is meant by Absolute Magnitude (M) ?
is the apparent magnitude would be if it were 10pc away from earth
- more negative = brighter
- Msun = +5
m-M = 5log(d/10)
How to calculate the MINIMUM ANGULAR RESOLUTION ?
- using the Rayleigh criterion
Ѳ = λ/D - D = diameter of aperture
- Ѳ - measures in radians
What is the Spectral classes and what doe they represent ?
Stars are classified into groups called Spectral classes - depends on the strength of certain absorption lines
O-B-A-F-G-K-M (hot to cold)
- Graph is intensity/temperature
How BALMER absorption lines are produced ?
- due to electrons transitions in atomic hydrogen from n=2 state
- energy levels
- light from star through cool gas towards prism so diffraction occurs
- dark lines represents wavelengths absorbed ( darker means more intensity)
How does CMBR give evidence for the big bang theory ?
COSMIC MICROWAVE BACKGROUND RADIATION
- the radiation produced in big bang still observed
- shows universe is expanding because wavelength of CBR stretched and now in microwave section
- perfect black body spectra - temperature of 2.7k
- agrees with Cosmological Principle + support doppler effect
What happens when the two stars rotate ?
1) when the observer in right angle to line of sight - no doppler effect so single absorption line
2) when the observer can see both stars - their is maximum red shift and blue shift so double absorption lines
- GRAPH shows dips in Absolute magnitude
What is involved in the Neutron Stars stage of the evolution a star ?
is very small and dense remnant of a high mass-star and is made from Neutrons
- electrons and protons combine to product neutrons
- core between 1.4-3 solar masses - core collapses to neutron star
- 20Km across and rotates very quickly
- emits radio waves - pulsars
How to estimate the distance and power output of Quasars ?
DISTANCE
- around size of solar system
- distance came from sensitive CCD equipment detected fuzzy cloudy of a galaxy around a quasar
POWER OUTPUT
- is a powerful galactic nucleus centred around a huge black hole
- ACTIVE GALACTIC NUCLEI - super-massive black hole ejecting huge amount of material
What are white dwarfs in bottom left corner of H-R diagram?
- low luminosity
- high temperature
So small surface area
What is involved in the White Dwarf stage of the evolution a star ?
- Low mass stars e.g. sun
- C + O not hot enough for any further fusion
- core continues to shrink till ELECTRON DEGENERACY PRESSURE occurs - stop collapsing anymore
- outer-layer becomes unstable so star ejects the outer-layer into space as PLANETARY NEBULA
- dense core left to cold down and fade away
Difference between CCD and eye detectors ?
CCD’S
1) QUANTUM EFFICACY
- detect up to 80% more light than eyes
2) wider SPECTRUM OF LIGHT than eyes
3) SPATIAL RESOLUTION
- minimum resolvable distance (CCD have 10um - capture more fine detail)
3) Less convenient to use - human eye doesn’t need equipment
What is involved in the Black holes stage of the evolution a star ?
is Produced by the collapse of the core of a very high mass star to an infinitely dense point
- neutron can’t withstand gravitational force
- it has a ESCAPE VELOCITY and a EVENT HORIZON
What is the Spectral classes and what doe they represent ?
Stars are classified into groups called Spectral classes - depends on the strength of certain absorption lines
O-B-A-F-G-K-M (hot to cold)
What does the Black body curve look like ? + What happens if you increase the temperature of a black body ?
- peak then gradually decrease
- range of emitted wavelengths increase
- peak wavelength decrease
- total power output increase
Similarities and Differences between different Non-Optical telescopes ?
1) RADIO POSITIONING - surface of the earth USE - CMB radiation + quasars RESOLVING POWER - very poor (λ large) COLLECTING POWER - large
2) INFRA-RED POSITIONING - high dry places USE - cooler than a star RESOLVING POWER - x10 poor to visible COLLECTING POWER - similar to optical
3) ULTRA-VIOLET POSITIONING - atmosphere USE - hot stars RESOLVING POWER - bette than radio COLLECTING POWER - similar to optical
4) X-RAY
POSITIONING - space only
USE -black holes
- have low collecting power
How type 1a supernovae can be used as STANDARD CANDLES to measure the distance to galaxies ?
use standard candle to calculate absolute magnitude of directly
- type 1a supernovae have same peak in absolute magnitude
- work out distance by looking how bright the supernova appears in comparison to how powerful you know it really is
What are the types of Optical Telescopes we need to Know ?
1) The Refractor
- Objective lens and eye lens
2) Cassegrain Reflector
- Primary (CONCAVE) + Secondary (CONVEX) mirror
How does Dark Energy give evidence for the big bang theory ?
Dark energy is the type that fills the whole of space
- as the age + size of universe is only estimate
- rate of expansion isn’t constant
- mass attached together by gravity which slows down rate of expansion
- Late 90’s - Universe is accelerating - might be explained by dark energy
What are Exoplanets ?
are planets which are not in our solar system
What are SPECTROSCOPIC BINARY STARS ?
- actually observing two stars orbiting each other as too far to be resolved so lines in spectra show a binary star system
- eclipsing system when plane is in line of sight
- use absorption lines that change over time to calculate the orbital period
What is a light year (ly) ?
the distance light travels through a vacuum in one year
- 1 light year = 9.46 x 10^15m
What is the logarithmic scale of apparent magnitude ?
the magnitude of 1 has intensity 100 greater than a magnitude 6 star
- difference of one magnitude = 100^1/5 = 2.51
What is the Big Bang theory ?
” the universe started off very hot and very dense and has been expanding since “
What is a Parsec ?
the distance at which a star will give a parallax angle of ONE ARC-SECOND (1/3600)
- 1pc = 3.08 x 10^16m
- 1Mpc = 1 x 10^6 pc
Problem’s of REFRACTOR telescope ?
1) CHROMATIC ABERRATION
- is a blur caused by different colours refracting different amounts
2) no faint objects seen to bubbles/impurities in glass
3) heavy lenses - shape can become distorted
4) for large magnification - focal length of objective needs to be long = expensive
How does the Relative Abundance of H + He give evidence for the big bang theory ?
- As the early universe was very hot, fusion of hydrogen could occur to form Helium
- Support the big bang theory
What is the Doppler effect detection method ?
- Known as “Radial velocity method”
- Tiny wobble seen in stars orbiting the planet due to the planet and star orbiting the centre of mass and the star is much larger so much closer to centre of mass
- the wobble causes red/blue shift in the star’s emissions which can be detected on Earth
- ## Minimum mass can be calculated
What is the logarithmic scale of apparent magnitude ?
the magnitude of 1 has intensity 100 greater than a magnitude 6 star
- difference of one magnitude = 100^1/5 = 2.51
- sun= -26 -> moon= -11 -> naked eye=5 -> limit of binoculars= 10 -> dimmest object= 30
What is involved in the Supernovae stage of the evolution a star ?
- High mass stars
- continue to fuse beyond helium -> IRON
- too big so EDP can’t stop contacting
- outer-layer falls to core + rebounds causing Shockwaves = SUPERNOVA
- Leaving behind core -> Neutron star or Black hole
What is involved in the Main sequence stage of the evolution a star ?
- star spends most time here
- pressure equals the gravitational force trying to collapse it
- RED GIANT - fusion stops so core contacts and heats up - outer layer expands and cools
- fusion stars to occur in out layer - hydrogen shell burning
- core contacts + heats up till dense enough for helium to fuse into carbon + oxygen
- He -> C + O in both core and shell burning
What are Quasars ?
They are bright radio sources which have large red shifts
- first thought were stars but didn’t show same absorption lines to normal stars
- jet out material which showed emission lines
- the emission lines looked familiar to BALMER series of hydrogen by red-shifted enormously
What is a Parallax ?
the apparent change in position
- imagine in moving car - see a stationary object move faster than object in the distance
- Calculate angle of parallax / d= r/Ѳ
Problem’s of REFLECTOR telescope ?
1) SPHERICAL ABERRATION
- not parabolic mirror causes reflected rays to coverage eat different points
2) secondary mirror can block light and diffract round the mirror
- tarnished and beed re-silvered
What is a black body ?
is a body that absorbs all em radiation of all wavelengths and can emit all wavelengths of em radiation
- black body radiation - hotter that absolute zero emit em radiation due to their temperature (star = black body)
What is the Transit detection method ?
- Measure the change in Apparent Magnitude as an exoplanet travels in front of the star
- Block light so change in apparent magnitude and become more dim
- GRAPH - one drop in graph (light curve)
- amount dimmed depends on size of planet and star = bigger planet means bigger drop in magnitude
- NEGATIVE = planet’t perfectly aligned so passes directly between star and observer is rare + miss the transit due to be very quick
- only use this method to confirm already observed exoplanets
How to calculate the age and size of the Universe ?
AGE
- Age of Universe = 1/H (H in seconds)
- ASSUMPTION - expanding at same rate for whole life
SIZE
- no absolute size so just use observable universe so 13 BILLION LIGHT YEARS radius
Why is it difficult to detect Exoplanets ?
- they are orbiting stars much brighter than them so cannot be seen
- too small to distinguish from nearby stars
- any seen from specially built telescopes - brightest + largest
What is the inverse square law and what do you need to assume ?
I = P/A = P/4πd^2 - P∝d^2 ASSUMPTIONS - star is spherical - fives out even amount of power in every direction - star behaves as a black body
What is a EVENT HORIZON of a black hole ?
is the point at which the escape velocity is equal to the speed of light
- Radius in called the Schwarzschild Radius
What is Collecting Power ?
is the energy collected per second
- Is proportional to collecting area ^2
- bigger disk means more energy so observe fainter objects
What is the Evolution of Sun like stars ?
1) Cloud of dust + gas
2) Protostar
3) Main sequence
4) Red giant
ROUTE 1 - low mass stars
- White Dwarfs + Planetary nebula
ROUTE 2 - high mass stars
- supernova -> neutron star or black hole
What is Cosmological red shift ?
It states that Stars are not moving away but the Universe is expanding so light waves are being stretched
- V must be much lower than C
What is thought to be at the centre of every galaxy ?
A supermassive black hole
What is a ESCAPE VELOCITY of a black hole?
is the minimum velocity of object needed to travel at to leave the gravitational field
- escape velocity > speed of light
What is the MINIMUM ANGULAR RESOLUTION ?
is the smallest angular separation at which the instrument can distinguish two points
- smaller resolution = better resolving power
- resolution is limited by diffraction - centre circle caused AIRY DISC
What are I-R and U-V telescopes ?
- similar to Optical reflector telescope - has parabolic disk to focus
- U-V have shorter wavelength - mirror need to be more precise
- I-R produce own radiation so need to be very low temperature
What is the Doppler effect detection method ?
- Known as “Radial velocity method”
- Tiny wobble seen in stars orbiting the planet due to the planet and star orbiting the centre of mass and the star is much larger so much closer to centre of mass
- the wobble causes red/blue shift in the star’s emissions which can be detected on Earth
- Minimum mass can be calculated
- NEGATIVE = movement need to be aligned with observer’s line of sight
What is the apparent magnitude of brightest and dimmest visible stars ?
1) brightest = 1
2) dimmest = 6
How are images produced for Lenses ?
- use ray diagrams (show how light travels)
- tells you what lens does e.g. magnified
- can be a REAL or VIRTUAL image
- Real - can be captured on screen
- Virtual - can’t be captured on screen - light rays aren’t really where the image appears to be
What are red giants in top right corner of H-R diagram?
- high luminosity
- low temperature
So large surface area
