*Astrophysics

(Those viewing past 8th June 2026, all my flashcards made using mix of pmt detailed notes, pmt definitions and mark schemes from past papers (for niche parts of spec)

Absolute Magnitude (M)

The apparent magnitude that an object would have if it were placed at a distance of 10 parsecs away from Earth

Achromatic Doublet

A convex lens made of crown glass and a concave lens made of flint glass cemented together in order to focus all light rays in the same position

An achromatic doublet is a solution for spherical and chromatic aberration

Apparent Magnitude (m)

How bright an object appears in the sky

This depends on the object’s brightness and its distance from Earth

Arcsecond

A unit used to measure small angles

An arcsecond = 1/3600th of a degree

Astronomical Unit (AU)

The average distance between the centre of the Earth and the centre of the Sun

Big Bang Theory

The theory that the universe originated as a small, dense and hot region that expanded and cooled forming the structures in the universe we see today

Binary Star System

Two stars orbiting a common centre of mass

Black Body Radiator

A perfect emitter and absorber of all possible wavelengths of radiation

Black Hole

A region which has an escape velocity greater than the speed of light. Black holes are formed when the core of a giant star collapses

Cassegrain Reflecting Telescope

A reflecting telescope with a concave primary mirror and a small convex secondary mirror in the centre, with the eyepiece lens just behind the centre of the primary mirror

The mirrors in a reflecting telescope are actually a very thin (often less than 25nm thick) coating of aluminium or silver atoms that are deposited onto a backing material. This allows the mirrors to be as smooth as possible and minimises distortions in the image.

Charge-Coupled Device (CCD)

An array of light-sensitive pixels which become charged when they are exposed to light (by the photoelectric effect)

Chromatic Aberration

An effect caused by the different focal lengths of different wavelengths of light that leads to different colours being focused at different points

This can cause a white object to appear as if it has coloured edges

Collecting Power

A measure of the ability of a lens or mirror to collect incident EM radiation

Collecting power is directly proportional to the area of the objective lens/primary mirror

Concave/Diverging Lens

A lens which spreads out incident light – the light rays diverge

Convex/Converging Lens

A lens which focuses incident light – the light rays converge

Cosmological Microwave Background Radiation (CMBR)

After the big bang the hot dense state of the universe was full of photons which interacted with the matter in the universe

At a certain time this interaction stopped due to the lower temperature of the universe and these photons were allowed to propagate freely, at this point these photons were gamma rays

At present the universe has expanded, red shifting these photons so that they are microwaves

Doppler Effect

The apparent change in the wavelength of a wave as the source moves relative to an observer

For a source moving away the wavelength increases (red shift), for a source moving towards the observer the wavelength decreases (blue shift)

Eclipsing Binaries

A binary star system in which the stars’ plane of orbit is in the line of sight of the Earth

This means that the stars will appear to cross over each other as they orbit

Event Horizon

The boundary of a black hole, along which the escape velocity is equal to the speed of light

Exoplanet

Planets that are not part of our solar system and orbit other stars. They are often difficult to detect due to the light of their host star obscuring them

Eyepiece Lens

The lens in a telescope that magnifies the image produced by the objective lens

It produces a virtual image at infinity (since the light rays are parallel) in order to reduce eye strain for the user as they do not have to refocus every time they look between the telescope image and the object in the sky

Focal Length (f)

The distance between the centre of the lens and the principal focus

Hipparcos Scale

A way of classifying astronomical objects by their apparent magnitude

The brightest stars have an apparent magnitude of 1 and the faintest visible stars have an apparent magnitude of 6

The intensity of a magnitude 1 star is 100 times greater than a magnitude 6 star so the scale is logarithmic

Hubble’s Law

The speed of a galaxy moving away from ours is proportional to its distance away from us

The constant of proportionality is Hubble’s constant

Hydrogen Balmer Spectrum

A spectrum formed from the excitation of hydrogen atoms from the n=2 level

The prominence of the Balmer lines from a star can give an indication of the star’s temperature and state of the hydrogen within it

Intensity

The power received from a star per unit area

Lens Power

A measure of how closely a lens can focus a beam that is parallel to the principal axis (i.e. how long the focal length is)

The shorter the focal length, the more powerful the lens

Light Year (ly)

The distance that an electromagnetic wave travels in a year in a vacuum

Long-Lived Gamma Ray Burst

Bursts of gamma radiation that last anywhere between 10 and 1000 seconds

These are thought to be associated with a type II supernova (the death of a massive star)

Luminosity

The rate of light energy released by a star

This is the same as the power output of a star

Magnifying Power/Angular Magnification (M)

The ratio of the angle made by the image from the eyepiece to the angle made by the object with the unaided eye

Main Sequence Star

The equilibrium stage of a star’s life cycle, where the inward gravitational forces balance the outward forces caused by fusion

In this stage, hydrogen nuclei fuse to form helium

Neutron Star

An incredibly dense star that is formed when the core of a large star collapses

Protons and electrons are forced together under gravity to form neutrons

Normal Adjustment

When the distance between the objective and eyepiece lenses in a refracting telescope is equal to the sum of their focal lengths (fo + fe)

This means the principal focus (F) for these two lenses is in the same place

Objective Lens

The lens in a telescope that collects light and creates a real image of a very distant object. Objective lenses should have long focal lengths and be large in order to collect as much light as possible

The collecting power of a telescope is directly proportional to the square of the radius of the objective lens

Parallax

The apparent change of position of a nearer star in comparison to distant stars in the background

This happens as a result of the Earth’s orbit around the sun

Parsec (pc)

The distance at which the angle of parallax is 1 arcsecond

Primary Mirror

The mirror equivalent of an objective lens

The primary mirror collects light and focuses it onto a secondary mirror in a reflecting telescope

Principal Axis

The line passing through the centre of the lens, perpendicular to its surface

Principal Focus (F)

In a converging lens: the point where incident rays travelling parallel to the principal axis will converge

In a diverging lens: the point from which the light rays appear to come

Protostar

A young star formed when clouds of gas and dust are pulled together under gravity

Protostars are surrounded by a circumstellar disc, and when the centre becomes hot enough, the star will begin to fuse elements

Quantum Efficiency

The percentage of photons incident on a CCD which causes an electron to be released

Quasar

Active galactic nuclei – supermassive black holes surrounded by a disc of matter, which, as it falls into the black hole, causes jets of radiation to be emitted from the poles

Quasars display very large red shift, indicating they are very far away

The power output of a quasar is around that of an entire galaxy

Radial Velocity Method

A method of detecting exoplanets

It involves the observation of the host star’s line spectrum

If it is blue-shifted when it moves towards the Earth and red-shifted when it moves towards the Earth, it suggests that the star is ‘wobbling’ and so there must be a mass (exoplanet) exerting a gravitational force on it

Rayleigh Criterion

This states that two objects will not be resolved if any part of the central maximum of either of the images falls within the first minimum diffraction ring of the other

Real Image

Formed when light rays cross after being refracted by a lens

Real images can be projected onto a screen

Red Giant

A stage in the life cycle of a star less than 3 solar masses, in which the hydrogen has run out and the temperature of the star increases

Helium nuclei fuse to form heavier elements

Red Shift (z)

The shifting of an object’s wavelength towards the red end of the spectrum due to the object moving away from the Earth (Doppler effect)

Red shift is evidence for the expansion of the universe

The more distant the object, the greater its red shift

Red Supergiant

For massive stars that are greater than 3 solar masses, when the hydrogen runs out, instead of transitioning into a red giant, the star will transition into a red supergiant (same processes as a red giant but on a larger scale)

The temperature increases and fusion up to iron can occur

Reflecting Telescope

A telescope which uses mirrors to focus incident light onto an eyepiece lens

Refracting Telescope

A telescope which uses lenses to focus incident light

Resolving Power

The ability of a telescope to produce separate images of close-together objects

Schwarzschild Radius (R_S)

The name given to the radius of a black hole’s Event Horizon

Short-Lived Gamma Ray Burst

Short bursts of gamma radiation that last anywhere between 0.01 and 1 second

These are thought to be associated with merging neutron stars (forming a black hole) or a neutron star falling into a black hole

Spectroscopic Binaries

A binary star system in which the stars are too close together to be resolved by a telescope

The Doppler shifts of the light from the stars as they move towards or away from the Earth are used to detect these binary systems

Spherical Aberration

An effect caused by the curvature of a lens or mirror that can lead to light rays at the edges to be focused in different places to those from the centre

This can cause an image to be blurry or distorted

Stefan’s Law

A law stating that the power output (luminosity) of a star is directly proportional to its surface area and its absolute temperature to the 4th power

Supernova

When a star greater than 1.4 solar masses dies, the core collapses rapidly inward and becomes rigid

The outer layers then fall inward and rebound off of the core in a shockwave, causing heavy elements to be fused and distributed into space

Transit Method

A method of detecting exoplanets

It involves the monitoring of the host star’s light intensity

If a planet crosses in front of it, the intensity will dip

Timings and other data can be combined to determine information about the size and speed of the exoplanet

Type I Supernova

The consequence of a star in a binary system accumulating matter from its companion star

When the star reaches critical mass, it will explode

Type I_a Supernova

A consequence of a white dwarf star exploding

They can be used as a standard candle since they always reach the same maximum absolute magnitude (-19.3)

Type II Supernova

The consequence of a high mass star dying when it runs out of fuel

Virtual Image

Formed on the same side of the lens as the object

The light rays do not cross after refraction, so the image cannot be projected onto a screen

White Dwarf

A stage in the life cycle of a small star (less than 1.4 solar masses) that occurs when all the fuel has been used up

The star contracts since there is no longer an outwards force caused by fusion

Wien’s Displacement Law

A law stating that the peak wavelength of emitted radiation is inversely proportional to its absolute temperature

Lens formula

1/u + 1/v = 1/f

u - distance of object from centre of lens

v - distance of image from centre of lens

f - focal length of the lens

Power of lens equation

P = 1/u + 1/v = 1/f

Power is measured in Dioptres (D)

The shorter the focal length, the more powerful the lens

In converging lenses this value is positive and in diverging lenses this value is negative.

Ray diagram for a refracting telescope in normal adjustment

When α and β are < 10⁰, then

M = α/β = f_o/f_e