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