3.9 Astrophysics Flashcards
What are three drawbacks of placing a telescope in orbit?
- It is expensive to send a telescope into space
- The telescope will need its own power source
- It is very difficult to service an orbiting telescope if something goes wrong
What is meant by Quantum Efficiency?
Quantum efficiency of CCD?
Quantum efficiency of eye?
no. of photons arriving at detector and being detected ÷ total arriving at detector
CCD : QE > 80%
Eye : QE = 1%
What is the defining property of a supernova?
The rapid increase in absolute magnitude (become more negative)
What are the defining features of neutron stars?
- Extremely dense
- Made up of neutrons
Why are scientists worried about the possibility of nearby supernovae?
- In a supernova, a collapsing star can produce gamma ray bursts with energy similar to total output of sun
- The burst is highly collimated, and if in direction of Earth, they could cause a mass extinction event (due to killing of cells etc.)
2 reasons why it is hard to detect exoplanets
- The star it orbits is much brighter than reflected light from the planet
- Planet is very small and distant, so subtends a very small angle compared to resolution of telescopes
What number is assigned to the dimmest visible stars on the Hipparcos scale?
6
What is the name of the subjective scale where stars are classified according to their apparent magnitude?
Hipparcos scale
Difference of 1 on magnitude scale is equal to an intensity ratio of ____?
2.51
What is Stefan’s law?
P = σ A T^4
What is Wien’s displacement law?
λ_max * T = constant
What is a black body?
- An object that absorbs electromagnetic radiation of all wavelengths and can emit electromagnetic radiation of all wavelengths
- A black body does not reflect any radiation
What is the scale of the absolute magnitude axis on the Hertzsprung-Russell diagram?
15 to -10 (going up)
What is the scale of the temperature axis on the Hertzsprung-Russell diagram?
50,000K - 2,500K (going right, not linear)
Which events are used as standard candles for determining distances?
Type 1a supernovae
Approximately what % of observable stars are on the main sequence?
90%
What do scientists believe might be at the centre of galaxies?
Supermassive black hole
What is the difference between apparent magnitude and absolute magnitude?
- Apparent magnitude is how bight the object appears from Earth
- Absolute magnitude is how bright object would appear if viewed from a distance of 10 parsecs
Define the parsec
Distance at which 1AU subtends an angle of 1/3600th degree
What is an AU?
- Astronominal unit
- the mean distance from the centre of the earth to the centre of the sun
What is apparent magnitude?
The brightness of star as seen from Earth
What is absolute magnitude?
Apparent magnitude (how bright object would appear) if viewed at a distance of 10 parsecs
What are the most distant measurable objects in the universe?
Quasars
How are quasars produced?
They form around active supermassive black holes, at the centre of active galactic nuclei
What is the Doppler effect?
A change in apparent wavelength due to relative motion between source and observer
What is a standard candle?
An object with a known absolute magnitude
What is the cosmological microwave background?
- The radiation coming from all parts of the Universe (suggesting universe began in a hot dense state)
- The radiation has a peak in the microwave region (2.7K), and follows a black-body distribution
- It can be interpreted as radiation left over from the Big Bang
- This radiation has been red-shifted into the microwave region as the Universe has expanded
How does the relative abundance of hydrogen and helium supports the Big Bang theory?
- Big Bang theory suggests a very brief fusion period occurred when the Universe was very young, resulting in the production of helium from fusing hydrogen
- Fusion stopped as the Universe expanded and cooled
- Resulting in a relative abundance of hydrogen and helium in the ratio of 3:1
- The observed distribution of matter now follows the predicted ratio
What is meant by the Hubble constant?
The constant that gives the ratio of the recessional velocity of galaxies to distance from Earth
What is the assumption made when using the Hubble constant?
Universe is expanding at a constant rate
Explain how type 1a supernovae can be used as standard candles to determine distances
- All type 1a supernovae have the same peak absolute magnitude
- Apparent magnitude can be measured
- Can then use m-M = 5 log(d/10)
2 advantages of using a telescope with a large diameter objective lens
- Allows fainter objects to be viewed, as collecting power is proportional to d^2
- Allows for better resolving power as smallest resolvable angle is inversely proportional to d
What is the definition of angular magnification?
angle subtended at the eye by the image ÷ angle subtended at the eye by the object
4 Advantages of reflecting telescopes over refracting telescopes
- No chromatic aberration in objective or secondary mirror because mirrors do not refract light
- No spherical aberration because reflecting telescopes have a parabolic objective mirror
- No distortion, because the mirror can be supported more strongly
- Better resolving power and collecting power, as mirrors can be larger
What in the atmosphere in responsible for absorbing infrared radiation?
Water vapour
How does absorption of light due to atmosphere affect attempts to determine a star’s temperature?
- The spectrum of light from a star can be from a can be used to determine its temperature
- The longer wavelengths are absorbed by the atmosphere
- Shifting the peak of graph to shorter wavelengths
- Making the star appear hotter
What is chromatic aberration? What is its effect?
- When different wavelength are refracted by different amounts, due to different speed in glass
- Results in images with coloured edges
Why are Cassegrain telescopes mostly free of chromatic aberration?
- No refraction by the mirrors
- But there will be some chromatic aberration in eyepiece lens
How does chromatic aberration affect the appearance of an image (that should be white)?
Edges of image will appear coloured
What is Hubble’s law?
The recession velocity of galaxies is directly proportional to their distance from Earth (v = H_0 d)
Names of two methods for detecting exoplanets
- Radial velocity method
- Transit method
What is the radial velocity method for detecting exoplanets?
- A star and a planet will orbit their common centre of mass
- The star will have small variations in its distance from Earth, shown by tiny periodic red and blue shifts in its spectrum as it moves away from / towards Earth respectively
Briefly describe the transit method for detecting exoplanets
- As an exoplanet moves between the star it orbits and the Earth, the star’s brightness appears to decrease slightly
- This can be observed to happen periodically from observation over a long time period
What does the Rayleigh criterion find? What does it state?
- The subtended angle between two objects whose images are just resolvable
- The angle is when the central maximum of the diffraction pattern of one image coincides with the first minimum of the diffraction pattern of the other
Absolute magnitude scale for light cure for Type 1a supernova
-16 at dimmest, -19.3 at brightest
Time scale for light curve of Type 1a supernova
- 0 days at peak
- 40 days at end
What property of quasars led to their discovery?
They are unusually strong radio sources
What is the defining feature of a black hole?
It’s escape velocity is greater than the speed of light
Two reasons why the secondary mirror in the Cassegrain telescope affects the clarity of the image
- mirror blocks light so less light hits objective mirror
- light diffracted passing secondary mirror affects image
(these are both problems with reflectors if asked)
Why is the virtual image formed by the eyepiece at infinity?
To reduce eye strain when looking between the object and image
Describe how to eliminate chromatic and spherical aberration in lenses
Using an achromatic doublet brings all rays of light into focus in the same position by using a convex lens and a concave lens of different types of glass cemented together
radio telescopes : structure, positioning, use
structure: large parabolic dish that focuses radiation onto a receiver
positioning: can be ground-based but must be in isolated locations to reduce interference with nearby radio sources
uses: galaxies, stars, black holes, quasars
infrared telescopes : structure, positioning, use
structure: cassegrain mirror system. must be cooled with cryogenic fluid approaching absolute zero
positioning: space (most infrared absorbed by water vapour in the atmosphere)
uses: observing cooler regions in space (from a few tens to 100K)
ultraviolet telescopes : structure, positioning, use
structure: cassegrain mirror system that focuses radiation onto solid state devices that use the photoelectric effect to convert UV photons to electrons
positioning: space (most UV absorbed by ozone layer)
uses: observing the interstellar medium, star formation regions and the solar corona
x-ray telescopes : structure, positioning, use
structure: combination of hyperbolic and parabolic mirrors to focus radiation onto a CCD
positioning: space (x-rays absorbed by atmosphere)
uses: observing high-energy events and areas such as active galaxies, black holes and neutron stars
Why can’t X-ray telescopes use the same reflecting mirrors as found in optical telescopes?
The x-rays have such high energies that they would penetrate the mirror
What is meant by parallax? (astro)
An apparent change of position of a nearer star in comparison to distant stars in the background, as a result of the orbit of the Earth around the Sun
What are Hydrogen Balmer lines? Where are they found?
- Absorption lines caused by the excitation of hydrogen atoms from the n = 2 state to higher energy levels
- Found in the spectra of O, B and A type stars
Describe how a star, like our Sun, is originally formed (spec?)
- A cloud of dust and gas (nebula) has fragments of varying masses, that contract under the force of gravity
- This becomes a protostar as the temperature increases as the star gets denser
- When the temperature at the centre of the protostar reaches a few million degrees, hydrogen nuclei start to fuse together to form helium
- This nuclear fusion give out large amounts of energy and a star is born
What would be the main stages of a life cycle of a star of similar size to our Sun?
- Cloud of gas and dust (nebula)
- Protostar
- Pre-main sequence star
- Main sequence star
- Red Giant
- White Dwarf
- Black Dwarf
Describe how a main sequence star remains stable
- The outward pressure caused by nuclear fusion is balanced by the force of gravity pulling inwards
- An equilibrium is reached and the star remains stable for billions of years
- The greater the mass of the star, the shorter its main sequence period is because is uses its fuel more quickly
What is meant by dark energy?
The name given to the energy thought to be responsible for the accelerated expansion of the universe.
Describe the transition from a red giant through to a black dwarf (spec?)
- The outer layer of the red giant is ejected
- This outer layer is dust and gas
- A hot, solid, dense core is left. This is called a white dwarf
- The White Dwarf cools and emits less and less energy until it becomes a black dwarf
What are the four stages in becoming a red giant? (spec?)
- Core hydrogen burning (main-sequence)
- Shell hydrogen buning - hydrogen in core runs out, outward pressure stop, helium core contract, heat from core raises surrounding material enough for hydrogen to fuse
- Core helium burning - helium core contracts until hot and dense enough to fuse carbon and oxygen. this releases lots of energy and pushes outer layers further outwards
- Shell helium burning - helium in core runs out, carbon-oxygen core contracts and heats surrounding shell so that helium fuses
Describe how a red giant becomes a white dwarf (and then black dwarf) (spec?)
- Fusion eventually stops, and core contracts and gravity greater than outward force
- Outer layers are ejects into space as a planetary nebula, leaving behind a hot dense core (white dwarf)
.3 White dwarf will eventually cool to a black dwarf
What is different about the “core burning to shell burning” process of high mass stars? (spec?)
- It can continue beyond helium
- For really massive stars (red supergiants), this can go all the way up to iron
What are pulsars?
Spinning neutron stars that emit radio waves in two beams from the magnetic poles as they spin (up to 600 times per second)
3 reasons telescopes are placed in space
- the absorption of the electromagnetic waves by the atmosphere
- the light pollution and other interference at ground level
- the effect the atmosphere has on the path of the light as it passes through
2 reasons why a star could appear brighter
- Could be closer to Earth
- Could be emitting more power at visible wavelengths
2 assumptions for inverse square law
- no light is absorbed or scattered between the source and the observer
- source can be treated as a point
Why are hydrogen Balmer lines weak in O and B class stars?
- The stars atmosphere is too hot
- Hydrogen is likely to be ionised (i.e. not in n=2 state)
How do Type 1a supernovae occur?
- A binary star system where one star is a white dwarf
- When the companion star runs out of hydrogen it expands
- The white dwarf accumulates matter from its companion star and explodes after reaching a critical mass
- After reaching the critical mass, fusion begins and is unstoppable
Advantages of CCDs?
- Better resolution due to smaller pixels (so easier to distinguish between objects that are close together)
- CCDs have a higher quantum efficiency, and can be exposed for longer (so fainter objects can be observed)
- CCDs can detect a wider range of wavelengths
- CCDs are more convenient - can capture images when astronomer not present, and image can be stored and analysed on computer
What is the main contributor to a star’s luminosity?
Temperature (P = σ A T^4)
Stellar class of a white dwarf
O or B
3 ways a supermassive black hole can form
- The collapse of massive gas clouds while the galaxy is forming
- A normal black hole that has accumulated huge amounts of matter over millions of years
- Several normal black holes merging together
3 limitations when measuring the Hubble constant
- Apparent magnitude may be affected by what the light passes through
- At large distances, the accelerating expansion of the universe will affect the graph
- There is lots of random variation in the data
What absorbs UV in the atmosphere?
Ozone
Decreasing spherical aberration leads to…
Increased image detail
Compare the apparent magnitudes 5.1 and 6
5.1 is brighter than 6
(2.29x brighter)
Explain the dips in intensity in a star’s spectrum
- Dips are due to absorption
- Light of particular wavelengths absorbed by gases in outer layers
- (say all the energy levels stuff)
- And re-emitted in random directions
- Leaving dark lines
What is meant by an ‘active’ black hole?
- matter is falling into the black hole
- this leads to energy release
What to talk about if asked to describe image’s appearance in ray diagram question
- real/virtual
- inverted/upright
- magnified/diminished (bigger/smaller)
Chromatic aberration diagram: which colour has shorter focal length?
- Blue shorter
- Red longer
If the difference in apparent magnitude is x, what is the ratio of brightnesses?
2.51^x
