Title: InkPhysics / Astrophysics
On every note of this deck, unless a note says otherwise:
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Front: DEFINITION
Define: Normal adjustment.
Back: The telescope setting in which the focal planes of the objective and eye lens coincide, so the final image is at infinity and viewed by a relaxed eye.

Front: DEFINITION
Define: Red shift.
Back: The fractional increase in the wavelength of light from a receding source: z = Δλ/λ.

Front: SPOT THE ERROR
Say what is wrong with this claim: The bigger a star's magnitude number, the brighter it shines.
Back: The magnitude scale runs backwards: smaller numbers mean brighter stars, and the brightest objects have negative magnitudes.

Front: DEFINITION
Define: Light year.
Back: The distance light travels through a vacuum in one year, 9.46 × 10^15 m.

Front: DEFINITION
Define: Solar system.
Back: A star together with everything gravitationally bound to it: its planets, their satellites, and the asteroids, comets and dust between them. (OCR A.)

Front: DEFINITION
Define: Angular magnification.
Back: The angle subtended by the final image at the eye divided by the angle subtended by the object at the unaided eye.

Front: EQUATION · NOT ON THE DATA SHEET
Observed frequency from a source moving towards you:
f_o = ?\(f_{o} = \, ?\)f_o = ?
Back: f_o = f_svv - v_s\(f_{o} = \frac{f_{s}v}{v - v_{s}}\)f_o = f_sv/(v - v_s)

Front: RECALL
State what the radial velocity wobble gives.
Back: The period of the wobble is the planet's orbital period; its size reflects the planet's mass.

Front: DEFINITION
Define: Planet.
Back: A body in orbit around a star, massive enough for its own gravity to have pulled it into a nearly spherical shape and to have swept its orbital neighbourhood clear. It does not fuse, so it shines only by reflected light. (OCR A names this set.)

Front: DEFINITION
Define: Quasar.
Back: The active nucleus of a distant galaxy, powered by matter falling into a supermassive black hole.

Front: SPOT THE ERROR
Say what is wrong with this claim: We find planets around other stars by photographing them beside their suns.
Back: Direct photographs are rare because the star vastly outshines the planet: most exoplanets are found indirectly, from the star's periodic Doppler shift or the small dip in brightness during a transit.

Front: RECALL
State what the black-body spectrum depends on.
Back: The radiation spectrum of a black body depends only on its temperature.

Front: RECALL
State the spectral class order.
Back: O B A F G K M, running from the hottest stars to the coolest.

Front: DEFINITION
Define: Quantum efficiency.
Back: The fraction of the photons arriving at a detector that are actually registered.

Front: RECALL
State where the supergiants sit on the HR diagram.
Back: In a band right across the top of the diagram, above the red giants, with the red supergiants at its cool end: tens of thousands of times the Sun's luminosity at a cool surface temperature, which by Stefan's law makes them hundreds of times the Sun's radius.

Front: RECALL
State the quantum efficiency of a CCD and of the eye.
Back: A CCD registers about 80% of incident photons; the eye registers about 1%.

Front: DEFINITION
Define: Parsec.
Back: The distance at which one astronomical unit subtends an angle of one arcsecond.

Front: DEFINITION
Define: Rayleigh criterion.
Back: Two point sources are just resolved when the central maximum of one diffraction pattern falls on the first minimum of the other: θ ≈ λ/D in radians.

Front: DEFINITION
Define: Absolute magnitude.
Back: The apparent magnitude a star would have at a distance of 10 parsecs.

Front: RECALL
State what the Big Bang means for space and time.
Back: It was not an event at some address within space. It is where space-time itself begins, and the expansion measured today is that same space-time still stretching.

Front: RECALL
State the Balmer absorption condition.
Back: Hydrogen can only absorb the visible Balmer lines from atoms already excited to the n = 2 level.

Front: RECALL
Explain the Big Bang evidence from element abundances.
Back: The roughly three-to-one hydrogen-helium mass ratio matches fusion in the first minutes of a hot early universe.

Front: RECALL
Explain why X-ray telescopes use grazing incidence.
Back: X-rays pass into an ordinary mirror, so they are focused by skimming off nested metal surfaces at grazing incidence.

Front: RECALL
State when z = v/c is valid.
Back: The Doppler formulas Δf/f = v/c and z = Δλ/λ hold only for speeds much less than c.

Front: DEFINITION
Define: Chromatic aberration.
Back: Glass refracts blue light more than red, so each colour has its own focal length and images carry coloured fringes.

Front: SPOT THE ERROR
Say what is wrong with this claim: A good telescope is one with the biggest magnification.
Back: Magnification is not what matters: the objective's diameter fixes the light gathered and the detail resolved, and magnifying a blurred image only gives a bigger blur.

Front: DEFINITION
Define: Cosmological principle.
Back: The assumption that, on a large enough scale, the universe is homogeneous (the same composition and density everywhere), isotropic (looking the same in every direction) and governed by the same laws of physics everywhere.

Front: DEFINITION
Define: Astronomical unit.
Back: The mean distance from the Earth to the Sun, 1.50 × 10^11 m.

Front: DEFINITION
Define: Black body.
Back: A body that absorbs all radiation falling on it, and is therefore the best possible emitter.

Front: RECALL
State the origin of the cosmic microwave background.
Back: The red-shifted, cooled radiation left over from the hot early universe.

Front: RECALL
State the parsec in light years.
Back: One parsec is about 3.26 light years.

Front: RECALL
State the brightness factor for a difference of five magnitudes.
Back: A difference of five magnitudes is a brightness factor of exactly 100.

Front: EQUATION · NOT ON THE DATA SHEET
Radiation flux from luminosity and distance:
F = ?\(F = \, ?\)F = ?
Back: F = L4πd^2\(F = \frac{L}{4\pi d^{2}}\)F = L/4πd^2

Front: RECALL
State how to find the age of the universe from H.
Back: Assuming constant recession speeds, the age is about 1/H, with H expressed in s^−1.

Front: RECALL
State the brightness factor for a difference of one magnitude.
Back: A difference of one magnitude is a brightness factor of about 2.51.

Front: RECALL
State the power output of a quasar.
Back: A quasar outshines an entire galaxy from a volume comparable in size to the solar system.

Front: DEFINITION
Define: Hertzsprung-Russell diagram.
Back: A plot of stellar luminosity or absolute magnitude against temperature or spectral class.

Front: DEFINITION
Define: Apparent magnitude.
Back: A star's brightness as seen from Earth, on a scale where smaller numbers mean brighter stars.

Front: SPOT THE ERROR
Say what is wrong with this claim: Observatories sit on mountaintops to be closer to the stars.
Back: The altitude gain is negligible against astronomical distances: mountaintop sites sit above most of the turbulent, absorbing atmosphere, giving steadier images and access to infrared wavelengths.

Front: RECALL
State what the CMB shows.
Back: The cosmic microwave background is strong evidence for the Big Bang.

Front: RECALL
State the peak absolute magnitude of a type Ia supernova.
Back: Type Ia peaks cluster close to absolute magnitude −19.3. They are not identical: the peak is standardised using the rate at which the light curve declines, which is what makes them usable as standard candles.

Front: RECALL
State the parallax equation and the units it needs.
Back: p = 1/d, with p the parallax in seconds of arc and d the distance in parsecs. The units are part of the equation: the parsec was defined to make the constant exactly one, so the relation is false in any other pair of units. (OCR only.)

Front: DEFINITION
Define: Chandrasekhar limit.
Back: About 1.4 solar masses, the greatest core mass electron degeneracy pressure can support as a white dwarf.

Front: DEFINITION
Define: Cosmic microwave background.
Back: Black-body radiation at 2.7 K arriving almost uniformly from every direction.

Front: DEFINITION
Define: Comet.
Back: A small body of ice, dust and rock orbiting a star on a highly elliptical path. Near the star the ice vaporises, and radiation pressure and the stellar wind blow the gas and dust into a tail that always points away from the star. (OCR A.)

Front: SPOT THE ERROR
Say what is wrong with this claim: A red-hot star is hotter than a white one.
Back: Red stars are the coolest, at about 3000 K, not the hottest: Wien's law says a hotter star peaks at a shorter wavelength, so colour moves from red towards blue-white as temperature rises.

Front: RECALL
State how the temperature axis runs on the HR diagram.
Back: Temperature on the Hertzsprung-Russell diagram runs backwards, decreasing from left to right.

Front: DEFINITION
Define: Schwarzschild radius.
Back: The radius of the event horizon: R_s ≈ 2GM/c^2.

Front: SPOT THE ERROR
Say what is wrong with this claim: The Big Bang was an explosion at a point in space, and the galaxies are its shrapnel.
Back: The Big Bang was not an explosion at a point: space itself expands, carrying every galaxy away from every other, so there is no centre and every observer sees recession v = Hd.

Front: DEFINITION
Define: Planetary nebula.
Back: The glowing shell of gas thrown off by a low-mass star at the end of its red giant stage, lit by the hot exposed core it leaves behind. The name is historical and no planets are involved.

Front: RECALL
State the cure for spherical aberration.
Back: A parabolic mirror brings every ray parallel to its axis to a single focus.

Front: DEFINITION
Define: Universe.
Back: Everything that exists: all matter and radiation, together with the space and time they occupy. (OCR A.)

Front: DEFINITION
Define: Planetary satellite.
Back: A body in orbit around a planet rather than around the star directly, covering both natural moons and artificial satellites. (OCR A.)

Front: DEFINITION
Define: Transit method.
Back: Detecting an exoplanet from the periodic dip in a star's brightness as the planet crosses its face.

Front: RECALL
State where Balmer lines are strongest.
Back: In class A stars: hot enough to lift many atoms into n = 2, cool enough not to ionise the hydrogen.

Front: EQUATION · NOT ON THE DATA SHEET
Collecting power and objective diameter:
collecting power ∝ ?\(\text{collecting power} \propto \, ?\)collecting power ∝ ?
Back: collecting power ∝ D^2\(\text{collecting power} \propto D^{2}\)collecting power ∝ D^2

Front: RECALL
State the transit dip depth.
Back: The fractional dip in brightness equals (r_p/r_s)^2, the squared ratio of planet radius to star radius.

Front: SPOT THE ERROR
Say what is wrong with this claim: The Sun will end its life in a supernova.
Back: The Sun is too light to go supernova: it will become a red giant, shed its outer layers, and end as a cooling white dwarf. Only far more massive stars explode.

Front: RECALL
State the unit of the Wien constant.
Back: The metre kelvin (m K), a product: λ_maxT = 2.9 × 10^−3 m K.

Front: DEFINITION
Define: Distance modulus.
Back: The difference m − M between apparent and absolute magnitude, equal to 5 log(d/10) with d in parsecs.

Front: DEFINITION
Define: Galaxy.
Back: A gravitationally bound collection of stars, gas, dust and dark matter, from millions to hundreds of billions of stars, usually with a supermassive black hole at its centre. (OCR A.)

Front: RECALL
State the main stages in the evolution of the universe after the Big Bang.
Back: Quarks bind into protons and neutrons within about a microsecond; those nuclei fuse between roughly one second and twenty minutes, fixing the three-to-one hydrogen to helium ratio; atoms form at about 380 000 years and 3000 K, releasing the microwave background; the first stars and galaxies form over the next few hundred million years; and the expansion has been accelerating for the last few billion, reaching 13.8 billion years today. (OCR only.)

Front: RECALL
State the properties of a neutron star.
Back: One to two solar masses within a radius of roughly 10 km, at a density comparable with an atomic nucleus.

Front: RECALL
Explain the evolution of a massive star after the main sequence.
Back: It swells into a red supergiant while its core fuses on through carbon, oxygen and silicon as far as iron, where fusion stops releasing energy. The core then collapses and the star detonates as a core-collapse supernova, leaving a neutron star, or a black hole if the surviving core is heavy enough.

Front: DEFINITION
Define: Radial velocity method.
Back: Detecting an exoplanet from the periodic Doppler shift of its star's spectral lines as star and planet orbit their common centre of mass.

Front: RECALL
Explain how quasar size is found from variability.
Back: Brightness changes over days limit a quasar to about a light-day across, since nothing can vary faster than light crosses it.

Front: DEFINITION
Define: Parallax angle.
Back: The angle that one astronomical unit subtends at a star, which is half the total sideways shift the star appears to make against the distant background over six months.

Front: DEFINITION
Define: Hubble's law.
Back: The recession speed of a galaxy is proportional to its distance: v = Hd.