AQA a level astrophysics

describe table of Spectral Class, Intrinsic Colour, Temperature, and Prominent Absorption lines

remember: Oh Be A Fine Girl, Kiss Me

based on the strength of absorption lines

describe the T1A supernova light curve and how one is formed and how is used

peak at -19.3 peak absolute magnitude.

(about 20 days from the beginning of collapse)

When a white dwarf accumulates matter from its companion star in a binary system and explodes after reaching a critical mass

all occur at same critical mass and same peak mag so used as standard candle to calc distance of far away galaxies

equation for brightness

=L/(4nR²)

what is 1 astronomical unit

mean distance between earth and sun

what is 1 parsec

distance that creater angle of 1 arcsecond when measured with an opposite side of 1 Au

describe the normal adjustment refractor diagram

when the distance between the teo lenses is the sum of their focal lengths. AKA they share a principal focus

what is a newtionian reflecting telescope

what is a reflecting telescope

use of a concave primary mirror.

very thin (less than 25nm thick) aluminium/silver coating so that smooth as possible and less distortion

light is collected and focused on eyepiece lense

what is a cassegrain telescope

long focal length

convex mirror allows cassegrain to be shorter than the newtonian

what is a refracting telescope

two converging/convex lenses

objective lense: collects light to create a real image of a very distant object. must have long focal length and large radius to collect as much light as possible

eyepiece lense: magnifies image produced by objective lense so observer sees it. produces virtual image at infinity since light rays are parallel. reduces eye strain as dont need to refocus every time they look at image and object in sky.

power of a lense

measure of how closely a lense can focus a beam that is parallel to the principal axes

the shorter the focal length, the more powerful a lense

convering: positive

diverging : negative

measured it Dioptres (D)

what can be said when both angles in Normal refractor diagram are less than 10 degrees

angle subtended at eyepiece/ angle subtended at objective lense = objective focal length/ eyepiece focal length

what is chromatic abberation

when white light is shone thru a convex lense. blue is refracted more the red so red has a longer focal length than blue. they are focused at different points which cause images w/ coloured fringing .

it has very little effect on reflecting telescopes as it only occurs in eyepiece lense(newtonian) / small secondary convex lense (cassegrain)

spherical aberration in lenses

the curvature of a lense causes rays at edge of lense to be focused in a different position → image blurring and distortion. most pronounced in lenses with large diameter

minimised by achromatic doublet: (also minimises chromatic) convex lense made of crown glass and a concave lense made of flint glass cemented together. brings all rays of light into focus at the same position

spherical aberration in mirrors

the curvature of a mirror causes rays at edge of lense to be focused in a different position → image blurring and distortion. most pronounced in mirrors with large diameter

avoided by using parabolic objective mirrors in reflecting telescopes

disadvantages of refracting telescopes

can only be supported by the edges

glass must be pure and free from defects. hard to do this for large diameter lenses

affected by chromatic and spherical aberration

incredible heavy, hard to move

advantages of reflecting telescopes

excellent image quality from jsut a few nanometers of Al/ Ag

mirrors unaffected by chromatic, spherical can be fixed by parabolic mirrors

not as heavy, easier to handle

mirrors can easily be supported from the back as u dont need to see thru them

large composite mirrors can be made from mirror segments

what is a radiotelescope

uses radio waves to create images

atmosphere does not absorb large range of radio waves so these can be built ground-based

in isolated locations to avoid interference with radio sources

infrared telescope

uses infrared radiation to create images

large concave mirror: focuses light on detector

cryogenic fluids to almost absolute 0: cool down the telescope as all objects emit IR as heat

well shielded: avoid thermal contamination from nearby objects and itself

used to observe cooler regions of space but atmosphere absorbs so launched into space and accessed remotely

UV telescope

uses UV radiation to create images

ozone layer blocks out all waves under 300 nm so must be positioned in space

Cassegrain. rays detected by solid state devices which use photoelectric effect to turn UV into electrons which go in circuit

used to observe interstellar medium and star formation regions

X-ray telescopes

use x-rays to create images

all xrays absorbed by atmosphere so need to be in space

such high energy taht using normal mirrors would pass right through

made from parabolic and hyperbolic mirrors- must be extremely smooth

rays skim off mirrors and come in contact with CCDs which convert light into electrical pulses

observe high energy events such as active galaxies, black holes, neutron stars

gamma telescopes

use gamma radiation to create images

do not use mirrors cuz gamma so high energy it would pass through

uses detectors made of layers of pixels

when gamma photons come in contact with pixel, they cause a signal in pixel.

observe gamma ray bursts, quasars, black holes, solar flares

2 types of gamma ray bursts

shortlived- 0.01-1 second. from merging neutron starts to form a black hole or a neutron star falling into a black hole

long lived- 10-1000 seconds type2 supernova (death of a massive star)

collecting power

a measure of ability of a lens/mirror to collect incident EM radiation

directly proportional to area of objective lense

the greater the CP the brighter the image formed

collecting power ∝ (objective diameter)²

resolving power

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

for an image to be resolved, the angle from earth between the two objects must be atleast the minimum angular resolution (rayleigh criterion)

what are CCDs

charge coupled devices. array of light sensitive pixels which become charged when exposed to light by the photoelectric effect

compare CCDs with human eye

+images can be shared and stored in CCDs

what are exoplanets

planets that are not in our solar system. they orbit stars but hard to detect cuz they are obscured by the light from their host stars

names of 2 methods to observe exoplanets

transit method

radial velocity method

what is transit method

observing intensity of light ouput of star. if a planet crosses infront of star, the intensity dips. if dips regularly, shows there may be planet orbiting it. if variation in regularity, may have multiple planets orbiting

size and period can be determined form intensity and duration of dips

only works if line of sigh is in plane of planets orbit, which is more likely for planets with small orbits as larger ones tend to be inclined

what is the radial velocity method

star and planet orbit a common centre of mass which causes a ‘wobble’ this causes a doppler shift in light received from star

most noticeable with high mass planets as higher grav pull on starr

blue shifted when moves towards earth, red shifted when moves away from earth

shows that something else near star that is exerting grav force on it

time period of doppler shift = time period of planets orbit

what is luminosity

rage of light energy released / power output

what is intensity

power received from star (luminosity) per unit area. Wm^-2

therefore intensity of star follows inverse square law

the brightness

I = P/ 4πd² where d is distance from star

what is apparent magnitude

how bright an object appears from earth.

what is the hippacros scale

logarithmic scale of apparent magnitude.

brightest stars = 1. faintest visible stars = 6

when magnitude -1, brightness x2.51

what is absolute magnitude

what the apparent magnitude would be if object was placed 10 parsecs away from earth.

brightest stars have negative absolute mag

what is a black body radiator

a body that is a perfect emitter and absorber of all possible wavelengths of radiation (stars)

what is stefans law

the luminosity (power output) is directly proportional to its surface area and absolute temperature⁴

used to compare luminosity, temp, and size of stars

P = σAT⁴

whats weins displacement law

he peak wavelength (λmax) of emitted radiation is inversely proportional to the absolute temperature (T) of the object.

this shows peak λ decreases as temp increases meaning frequency increases and energy of wave increases

used to estimate temp of black body sources

describe how the black body curve

hydrogen balmer lines

caused by excitation of hydrogen from the n=2 state to higher/lower levels. found in O B A.

if temp too high, they wont show well because hydrogen excited to higher levels than n =2 / get ionised

if temp too low the H atoms are unlikely to become excited

the hertzsprung-russel diagram

hertzsprung-russel diagram sequence

when main seq star uses up all hydrogen in core → red giant which is brighter and cooler

when red giant uses up all helium in core → ejects outer layers and becomes white dwarf which is hotter and dimmer

pathway of lifecycle of a star

how is a protostar formed and what are its features

nebulae (clouds of gas and dust) have bits of varying masses which clump tgt under gravity

the clumps rotate and a gravity spins them inwards to form a denser centre (protostar)

has a circumstellar disc

when it gets hot enough it begins to fuse elements producing a stellar wind that blows away surrounding material

how is main sequence star formed and what are its features

when inward force of gravity = outward force of fusion, star is in equilibrium and is stable therefore main sequence

hydrogen nuclei fused into helium

the greater the mass, the shorted its main sequence period because it uses its fuel more quickly

how is a red giant formed and what are its features

when hydrogen runs out, the temp of core increases and begins fusing helium nuclei into heavier nuclei (C, O)

the outer layers expand and cool

happens to star < 3 solar masses

how is a white dwarf formed and what are its properties

when red giant uses up all fuel, fusion stops and core contracts as gravity > outward force of fusion

core becomes very dense

outer layers are thrown off, forming planetary nebula around core

will eventually cool to a black dwarf

for a star < 1.4 solar masses

how is a red supergiant formed and what are its properties

when a high mass star runs out of fuel, temperature increases and starts fusing helium nuclei together into much heavier elements, up to Fe

the collapse of these in a supernova cause gamma ray bursts

for a star > 3 solar masses

how is a type 2 supernova formed and what are its features

when all fuel runs out, fusion stops and core collapses inwards very suddenly and becomes rigid

outer layers fall inwards and rebound, launching them into space as a shockwave

elements heavier than iron are fused and flung out into space

rapidly increasing absolute magnitude

releases 10⁴⁴ J of energy

happens to a giant > 1.4 solar masses

how is a neutron star formed and what are its features

when core of large star collapses, gravity is so strong it forces protons and electrons to fuse and form neutrons

incredibly dense- same density as nuclear matter

pulsars are spinning neutron stars that emit beams of radiation from its magnetic poles as they spin

for stars of solar mass between 1.4 and 3 solar masses

how is a black hole formed and what are its features

when core of giant star collapses, neutrons are unable to withstand gravity forcing them together

the grav pull is so strong not even light can escape

event horizon is point at which escape velocity ios greater than speed of light

happens to giants > 3 solar masses

what is a binary system

where two stars orbit a common mass

how r supermassive black holes formed and why are they thought to exist

the collapse of massive gas clouds while the galaxy was forming

several normal black holes merging tgt

a normal black hole that has accumulated huge amounts of matter over millions of years

stars and gas orbiting very fast near centre of galaxy so must have very strong grav field attracting them. therefore thought ot be supermassive black hole at centre of every galaxy

how do we know universe expansion is accelerating

type 1a supernovae have been seen to be dimmer than expected. therefore more distant than hubbles law predicted. suggests that its accelerating

what do we think is causing the universe expansion to be accelerating

Dark energy- has an overall repulsive effect throughout whole universe

gravity follows inverse square law but dark energy remains constant throughout whole universe therefore causes expansion to accelerate as it has greater effect than gravity

controversial cuz noone knows whats causing it