19 - Astrophysics Flashcards
units for measuring light
lux (lx), lumen (lm), candela (cd)
- using a light metre
hottest to coolest colours of stars
blue - white - yellow - orange - red - (black)
1 lux is equal to
1lm/m^2
= 1cd sr/m^2
sr is a spherical radian
flux
how many vectors go through an area per second
why can’t we visibly see which stars are brighter
unknown distances - the more distant, the dimmer due to inverse square law
inverse square law
I = P/4πr^2
I - intensity, flux, power over area
P - luminosity
inverse square law - as distance doubles, intensity…
quarters
what is apparent magnitude
apparent brightness of star when viewed from earth, using a logarithmic scale to give order of magnitude.
it is compared to the brightness of Vega
absolute magnitude
logarithmic scale describing luminosity
m-M - 5log(d/10)
m - apparent
M - absolute
what is the HR diagram a chart of
star’s luminosity (energy output) or absolute magnitude against temperature or spectral type
axes on HR diagram are what scale
logarithmic scale
spectral classification
Depending on the relative proportions of hydrogen, helium and the rest, a star can be classified according to a letter scheme: OBAFGKM
these also correspond to the temperatures of the stars and also their colour
why is it more likely for life to evolve with cooler stars
cooler stars burn slower and so longer lifetime and more time for life to evolve
range of luminosity axis
(Lsun)
10-4 10-2 1 102 104 106
range of temp axis
(K)
40000 20000 10000 5000 2500
what happens when electrons get excited
when an electron is given enough energy, it gets excited as moves up towards an upper energy level. these distinct energy levels are different for each element. the electron gains energy, absorbing a photon of light with a frequency corresponding to the energy change (can be related using E=hf). only photons with the exact energy of the energy change will be absorbed.
for a particular element, this will produce an ABSORPTION spectra with dark lines against a continuous spectra with the specific wavelengths of the photons absorbed
what happens when electrons relax
when an electron relaxes, it loses energy and moves down to lower energy level, emitting a photon of light. the change in energy corresponds to a particular frequency of light for one photon that is emitted. only photons with the exact energy of the energy change will be emitted.
this produces a unique EMISSION line spectrum with each spectral line correspnding to the specific wavelenghts of photons emitted.
spectra - brighter lines show
more prominence
what is deflection of light
change in objects velocity after contact or collision with a surface
what is diffraction
the spreading out of a wave as it passes through a gap/aperture or when it encounters an obstacle
change in wave properties during diffraction
speed, wavelength and frequency remain unchanged
conditions for diffraction
diffraction will only occur if the wavelength of the wave is similar size to the size of the aperture. effects are stronger when the wavelength is similar size to the gap.
diffraction and specific colours
in diffraction, red is diffracted the least and violet is diffracted the most
diffraction grating pag equation
dsinϑ=nλ
d - distance between two slits
ϑ - angle from zero order maximum
n - number of maximum
λ - wavelength of light
life cycle of stars - beginning to main
nebula is a cloud of dust and hydrogen gas. the force of gravity pull the dust and gas together and friction causes heat and forms a protostar after more mass and pressure
protostar - no fusion occurs. eventually hot enough so hydrogen is able to fuse in helium atoms causing a pressure outwards
main sequence stage - pressure caused by fusion equals the gravitational force inwards - equilibrium, star is stable and obeys Ideal Gas Laws (hot, bigger)
life cycle of stars - ending for below C. limit
when the hydrogen fuel runs out, the star becomes a red giant. the size of the star does not surpass the Chandrasekhar limit and so is unable to overcome electron degeneracy pressure. the star gets unstable after running out of helium fuel,expelling outer layers and becomes a white dwarf held by electron degeneracy pressure (no fusion)
life cycle of stars - ending for above C.limt
star goes from main sequence star to red super giant when fuel runs out. star surpasses Chandrasekhar limit therefore can overcome electron degeneracy pressure. fuel runs out and star collapses causing supernova. star then either becomes neutron star or black hole. neutron stars are made of neutrons and spin very quickly. black holes have infinite density and gravitational force so high light cant escape.
what is the Chandrasekhar limit
the theoretical limit to the size of a star that will become a White dward (1.44Ms) - 1.44 solar masses
what is the event horizon
the boundary around a black hole in which nothing affects the universe outside it - anything in the boundary can’t escape the black hole’s vicinity due to large grav field
blach hole also known as
singularity with infinite density
what is Schwarzschild radius
the event horizon for light around a black hole. any light within this boundary can’t escape the vicinity of the black hole
difference between asteroid, meteorite, meteor, comet
asteroid - rock
meteorite - rock in earth’s atmosphere
meteor - shooting star
comet - ice and gas
examples of dwarf planets
pluto, ceres
stars, plasma
stars are composed of plasma - no electrons
nuclear fusion process
small hydrogen atoms fuse together to create larger helium nuclei, causes pressure outwards
what is the cosmological principle
the idea that the universe has the same large scale structure when observed from any point within it
three assumptions of cosmological principle
universe is homogenous (same density everywhere)
universe is isotropic (same in all directions)
laws of physics are universal and can be applied anywhere in the universe
what is 1 AU
average distance from earth to sun in m
parallax distance equation
distance (pc) = 1/parallax(arcseconds)
diffraction grating equation
dsinϑ=n𝝀
d - distance between two slits (calculate from diffraction grating reading)
ϑ - angle from zero order maximum
n - number of maximum used
why is a laser used for diffraction grating PAG
coherent - same frequency and equal phase difference
why are interference patterns created on screen
due to waves superimposing - constructive and destructive interference cause max and mins
sunspots
sunspots are cooler and fire atomic dust. magnetic field directs towards poles creates auroras such as Northern lights
for something to appear a certain colour it…
absorbs everything else but reflects that certain colour
white..
reflects all light
what is a black body
a theoretical object that absorbs all light that hits it, appearing black when cold. when heated above absolute 0, emits light across whole EM spectrum
black…
absorbs all light
Wiens displacement law
the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object
the hotter the black body, the shorter the peak wavelength
𝝀max ∝ 1/T
𝝀maxT=constant
constant = 2.9x10^-3 (mK)
𝝀max is light of max wavelength and max intensity and T is the absolute surface temperature of object
luminosity is equal to
energy star radiates per second (power output)
Stefans Law
luminosity of a star is directly proportional to its surface area and the 4th power of its temperature
L=AσT^4=4πr^2σT^4
temp to kelvin
0C = 273K
power equals
luminosity
what is a continuous spectra?
all visible wavelengths/ frequencies are visible. can be produced by heating of solid metal like a lamp filament.
what is a light year
the distance light travels in one year in m
define stellar parallax
the apparent shift in a stars position against a backdrop of other stars
define one parsec
the distance at which a radius of 1AU subtends an angle of one arcsecond
in one degree there are…
60 arcminutes and 3600 arcseconds
therefore 1arcsecond = 1/3600 of a degree
electrons bounded to atoms can only
exist in discrete energy levels.
an electron with zero energy
is free from the atom
the energy level with the most negative value is referred to as
the ground state
why are energy levels negative in an electron
external energy is required to remove an electron from the atom. (also indicate electrons are bound to the positive nuclei.)
emission spectra
each element produces a unique emission line spectrum because of its unique set of energy levels
absorption spectra and light from stars created by
light from a source moving through a cooler gas - photons are absorbed by atoms in the gas. in stars they are absorbed by atoms of cooler gas in the outer layers of a star so light from stars produce absorption spectra
why does white line being shone on disc split up
the disc acts as a diffraction grating to diffract the light ,spliting it into beams of different colours and causing an interference pattern
how are maxima formed using diffraction grating
superposition of diffracted waves, dependent on path difference and phase difference waves from all slits.
dark energy
energy that fills up space and accelerates expansion (around 68% pf total energy in universe)
dark matter
assumed that as distance from centre of galaxy increased, velocity would decrease due to lower grav field but observations showed the velocity to not decrease, leading to the idea that mass is not concentrated in the centre of the galaxy - another type of matter called dark matter exists around 27%of mass in the universe. - particles suggested include axions and gravitinos.
future of universe
open - continue to expand for all time p<p0
closed - collapse in on itself in big crunch p>p0
flat - universe expands to a limit but never reaches it p=p0
what is intensity
total power output of star per unit area
using hubbles law for our galaxy
cant use hubbles law as it only applied to receding galaxies not stars in our own galaxy
graph for Wien’s displacement law
y axis - intensity
x axis - wavelength
as temp increases, peak wavelength decreases an becomes sharper
