Stars and Their Spectra Flashcards
we model stars as…
self gravitating spheres of gas with the inward pull of gravity balanced by the outward gas pressure
the star’s core must be hot enough to
allow nuclear fusion to take place
ie around 10 million kelvin
the internal structure of a star and the way that energy flows through it depends
primarily on mass but also chemical composition
what does mass of star determine
internal structure of star and way energy flows through it
surface temperature
luminosity
convection dominates in
lower mass stars
radiation dominates in
larger mass stars
where do we learn about flux variations with energy or wavelength
spectra
where do we learn about flux variations with time
light curve
we learn everything we know about stars from
spectra
light curves
EM theory
primary source of information about physical nature of stars
their spectra
planck function
everything constant except frequency
wien’s displacement law derived from it
cooler stars peak wavelength
towards infrared
stellar photospheres are good approximations to a blackbody function but in reality….
their spectra are complicated by the presence of spectral lines and edges
edge
jumps in intensity on spectra
spectral sequence
OBAFGKM
stars in our neighbourhood have roughly
the same chemical composition
stars in our neighbourhood have different spectral lines due to
different temperatures
the stellar surface or atmospheric temperature determines
which ions or excited states are present
‘early’ group in the spectral sequence
OBA stars
7500<T<25000+
He, H lines
A stars have weaker H, stronger metals
‘solar type’ group in the spectral sequence
FG stars
5000<T<7500
very weak He, some metals
G stars have strong metal ions and neutrals
‘late’ group in the spectral sequence
KM stars
coolest with 3500-<T<5000
metal ions, neutrals , molecules at low enough temps
metal definition for astronomy
anything other than H or He
why do H and He lines only occur in hot stars
helium and hydrogen have high ionisation/ excitation energies (to move e- to upper level) so can only occur in hot stars
why are metal lines formed over a large temperature range?
many species with a range of ionisation energies depending on atomic mass