OCR A-Level Physics - Astrophysics

What is the universe?

Everything that exists.

What is a Nebula?

A large cloud of gas (mainly hydrogen) and dust in space

What is protostar?

A hot dense sphere of gas prior to the ignition of nuclear fusion in the core that creates a star

What provides the energy required for a protostar to begin fusion?

Gravitation energy is transferred to thermal energy.

What is nuclear fusion?

The fusing of light nuclei to produce heavier nuclei releasing energy in the process.

What is required for nuclear fusion to occur?

Extremely high temperatures and pressures.

What prevents gravity from causing stars to collapse?

The radiation and gas pressure pushing outward.

Why do large stars tend to remain stable for less time than smaller starts?

The pressure and temperature in the core is higher so they fuse hydrogen more quickly.

What is radiation pressure in a star?

Pressure from the photons created by fusion in the core of a star.

What is the gas pressure in a stars?

Pressure of the nuclei in the star's core pusing outwards.

What is a main sequence star?

A stable star fusing hydrogen in helium.

What causes a star to leave the main sequence?

It runs low on hydrogen, affecting the stability of the star.

What is a Red Giant?

The stage after the main sequence for a star with insufficient mass to go supernova.

Describe the structure of a red giant.

An inert core surronded by shells fusing elements heavier then Hydrogen.

What is a white dwarf?

The very hot, dense remnant core of a red giant star.

What is the Chandrasekhar limit?

The mass of a star's core below which the star will form a white dwarf.

What is electron degeneracy pressure?

A quantum mechanical effect (due to Pauli exlcusion) which prevents the collapse of a stellar core to create a neutron star in white dwarves.

What is a red supergiant?

A huge star in the last stages of its life prior to undergoing a supernova explosion. Elements up to iron may be produced by fusion in layers around its core

What is a (Type II) supernova?

The process of collapse of a red supergiant (at the end of its life) into a neutron star and the associated ejection of large amounts of stellar material into space.

What is a neutron star?

The extremely dense remnant core of a red supergiant star made almost entirely of neutrons.

What is a planetary nebula?

The outer layers of a red giant star driven off in the shockwave produced by the final collapse of the star.

What is a black hole?

The end stage of a stellar core with sufficient mass and density for its escape velocity to exceed the speed of light.

What is the typical mass of a stellar core that creates a black hole?

3 solar masses.

What is a Hertzsprung-Russell Diagram?

A log-log plot of luminosity (on the y-axis) against average surface temperature (backwards on the x-axis) for a group of stars.

What is the Luminosity of a star?

The total radiant power output of a star (Energy radiated per second).

What is a black body?

An object that absorbs all radiation incident upon it.

What is a black body spectrum?

The characteristic spectrum of radiation emitted by all black bodies in thermal equilibriun with their surroundings.

What is Wien's displacement Law?

The peak emission wavelength of a black body spectrum is inversely proportional to the body's absolute temperature.

What is Stefan's Law

The total power emitted per unit surface area by a black body is proportional to the 4th power of the absolute temperature

What are electron energy levels in an atom?

Discrete energy levels within an atom in which electrons can exist.

What is the ground state (of an atom)?

The lowest allowed electron energy level in an atom.

What is the excited state of an atom?

An atom in which electrons have been raised to an energy level above the ground state.

What is an emission line spectrum

The spectrum of bright lines

What is an absorption line spectrum?

The spectrum containing dark lines in a continuous spectrum produced by light passing through a gas.

What is meant by a discrete quantity?

Occuring only at particular values (as opposed to continous)

What is a diffraction grating?

A large number of closely spaced alternating transparent and opaque 'lines' used as a tool to produce spectra via interference.

What is grating spacing for a diffraction grating?

The spacing between adjacent 'lines' in a grating.

What occurs in excitation of an atom?

The atom gains energy and an electron moves up to a higher energy level.

What processes can lead to the excitation of an atom?

A collision with a charged particle The absorption of a photon by an electron in the atom.

What occurs during de-excitation of an atom?

An excited electron drops down energy levels and emits a photon.

What determines the frequency photons can be absorbed or emitted by an atom?

The size of a gaps between energy levels in the atom.

What determines the frequency of photons that can be absorbed or emitted by an atom?

The energy of the photon must be equal to a gap between energy levels.

What is d in nλ=dsinθ

Distance between slits in a diffraction grating

What is n in nλ=dsinθ?

Order of the maximum

What name is given to the central maximum formed by a diffraction grating?

Zero-order

What is the path length difference between adjacent slits along the nth order maximum?

n wavelengths

How do you find slit spacing of a diffraction grating from the number of lines per m?

1 / number of lines per m

How do you find the highest order maximum for a diffraction grating?

d/λ (rounded down to the nearest whole number)

How do you calculate the number of maxima produced by a diffraction grating?

2n+1 where n is the highest order of a maximum.

What is the effect of reducing the spacing of slits in a diffraction grating?

It increases the angles of each maximum.

What is the effect of increasing the wavelength incident on a diffraction grating?

It increases the angles of each maximum.

What is an astronomical unit (AU)?

The average distance from Earth to the Sun

What is a light year?

The distance that light travels in a vacuum in one year.

What is an arcminute?

An angle equal to 1/60th of 1°

What is an arcsecond?

An angle equal to 1/60th or an arcminute (1/3600th of a degree)

What is stellar parallax?

The apparent shift of 'nearby' stars against the background of 'distant' stars due to the Earth's motion around the Sun

What is a parsec?

The distance of an object with a parallax angle one arcsecond.

What is the maximum distance that can be measured by parallax?

100 parsecs

What is the parallax angle of a star?

Half the angular shift observed in the apparent position of a nearby star from Earth over a period of six months. (using the largest shift in any six month period)

What is d in the equation d = 1/p ?

Distance to a nearby star in parsecs.

What is p in the equation d = 1/p ?

Parallax angle of a nearby star in arcseconds.

What is the Doppler effect?

The apparent change in frequency of waves measured by an observer due to relative motion of the source and the observer.

How is the red/blue shift of a galaxy measured?

The absorption spectrum of the galaxy is compared to those of elements observed on the Earth.

What is blue-shift?

The shift towards shorter wavelengths of absorption lines of a galaxy moving towards the Earth.

What is red shift?

The shift towards longer wavelengths of absorption lines of a galaxy moving away from the Earth.

How is the velocity of a galaxy calculated from its red shift?

v = c x (Δf/f) OR (Δλ/λ)

What observations did Hubble make about distant Galaxies?

Most galaxies are red shifted and that red shift tends to incease with distance.

What is Hubble's Law?

The recessional speed of distant galaxies is approximately proportional to their distance from us

What is the cosmological principle?

The idea that the universe is homogenous and isotropic on a large scale and that the laws of physics are the same everywhere in the Universe.

What is meant by the homogeneous nature of the Universe.

The idea that matter (on a large scale) is distributed uniformly across the universe, which therfore has approximately constant density.

What is meant by the isotropic nature of the Universe?

The idea that (on a large scale) the universe looks the same in all directions to every observer (there is no centre or edge to the Universe).

What is the Big Bang?

The leading theory to describe the origin of the Universe in which space and time expanded from a singularity approximately 13.7 billion years ago.

What is Cosmic Microwave Background Radiation (CMBR)?

The largely uniform microwave radiation with the blackbody spectrum of an object at 2.7K detected in every direction in the sky.

How does the the Big Bang theory explain the Cosmic Microwave Background Radiation (CMBR)?

When it was young the Universe was very hot and filled with short wavelength photons. These photons have been red shifted as space has expanded.

How can Hubble constant be determined?

The gradient of the best-fit line for a plot of recessional speed (y-axis) vs distance (x-axis)

How can the age of the Universe be estimated using the Hubble Constant?

age of the Universe = 1 / Hubble constant

What is (Cosmic) inflation?

A period very early on (10⁻³⁵ - 10⁻³⁰s) in the evolution of the universe during which an unknown mechanism appears to have caused a very rapid expansion of the universe.

What process leads to the conversion of radiation to matter in the early Universe?

The conversion of high energy photons into particle-antiparticle pairs (pair production).

Why was light able to freely propagate in the Universe from around 380,000 years?

Nuclei captured electrons forming neutral atoms.

What is dark matter?

A hypothetical form of matter that doesn't emit or absorb light postulated to explain the difference between the expected and observed orbits of stars around the galactic centres.

What evidence suggests the expansion of the Universe is accelerating?

Light from very distant type 1a supernova are less intense than predicted.

What is dark energy?

A hypothetical form of energy postulated to explain the apparent acceleration in the expansion of the Universe.