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1 Book page Stellar Spectra

2 Emission and absorption Spectra The black lines of the absorption spectrum match up with the bright lines of the emission spectrum

3 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

4 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

5 Observation on Earth

6 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

7 Spectral classes explained

8 Oh be a fine girl kiss me Spectral Class OBAFGKM Temperature Range (K) Colour Example O Blue violet Mintaka B Blue white Riegel A White Sirius A F Yellow white Canopus G Yellow Sun K Orange Aldebaran M Red - orange Betelgeuse

9 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.

10 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

11 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

12 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

13 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

14 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

15 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

16 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

17 Mathematics L M 3.5 L = M L M L L = M M 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

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