Book page 650-652 Stellar Spectra
Emission and absorption Spectra The black lines of the absorption spectrum match up with the bright lines of the emission spectrum
Spectra unique to each element Emission Spectrum Excited electrons move down a level and emit a photon Diffraction grating or prism produces bright line spectra Each line corresponds to a specific wavelength Absorption Spectrum Shining white light through cool gas Photons of correct λ are absorbed by electrons and excite them to higher energy levels Absorbed λ are now missing from continuous spectrum Black lines correspond to absorbed λ
Absorption Spectra of Stars The continuous spectrum of a star forms absorption lines as the light passes through the cooler, less dense upper atmosphere of the star
Observation on Earth
Spectral classes By looking at the absorption lines from a star, the composition of the star s atmosphere can be known The strength of the spectral lines shows the temperature of a star Stars with similar appearing spectra are grouped together into spectral classes, each class related to surface temperature
Spectral classes explained
Oh be a fine girl kiss me Spectral Class OBAFGKM Temperature Range (K) Colour Example O 30000-50000 Blue violet Mintaka B 10000 30000 Blue white Riegel A 7500 10000 White Sirius A F 6000 7500 Yellow white Canopus G 5000 6000 Yellow Sun K 3500 5000 Orange Aldebaran M 2500 3500 Red - orange Betelgeuse
Task Why do green stars not exist? - All stars contain about 74% hydrogen. - Hydrogen shows a green emission line. - Hence the absorption spectrum will show a black line at the wavelength of green light. - Stars do not appear green as all green waves have been absorbed.
The Hertzsprung - Russel diagram Two Astronomers realized in the early 1900 s that a pattern is formed if luminosity of stars is plotted against their surface temperature or colour
Points to note Scale is not linear. It is a log scale to account for small differences Temperature goes from high to low Top left: high luminosity and temperature Top right: large luminosity, but low temperature - these stars must be huge Bottom right: small luminosity and low temperature Bottom left: low luminosity but high temperature - these stars must be small
Brightness, size and the HR diagram Hotter things are brighter Energy radiated per unit area is proportional to T 4 Bigger T means more energy radiated Bigger things are brighter Energy radiated per unit area is proportional to T 4 Bigger surface area means more energy radiated
Radius of stars and in the HR No direct relationship The diameter falls on diagonal lines Coolest MS star are a lot smaller than the Sun The hottest MS Stars are a lot bigger than the Sun
More mass: hotter, more luminous and bigger Stars with same mass have same properties There is a relationship between mass and luminosity The most massive stars are the most luminous
Relationship between luminosity & mass L M 3.5 L = Luminosity (W) or multiples of L M = Mass (kg) or multiples of M This means even a slight difference in mass results in a large difference in luminosity
Mass and Life time of star To be stable, a star needs to be in hydrostatic EQLB Gravitational pressure inward = radiation pressure outward For a higher mass star: - greater gravitational compression - this means higher core temperature - higher T makes fusion more probable - greater rate of nuclear reaction & emission of more energy - hence luminosity increases - this will shorten the life span of the star
Mathematics L M 3.5 L = M L M L L = M M 3.5 3.5 L = L M M 3.5 If you need to find the mass and the luminosity is given: 3.5 L L = M M M = 3.5 L L M