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Astrophysics Notes

Classification of stars

Classification by luminosity

Relation between brightness and apparent magnitude.

Sun luminosity = 3.85 × 10 26 Watts.
Most luminous stars in the galaxy are over a million times more luminous than the sun.
Apparent brightness is different to luminosity.
Total energy in form of…

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The apparent mag, M, is related to intensity, I, by the following

The intensity, I, of an abject that we observe is the power received
from it per unit on earth
o Effective brightness of an object.
Mag difference between stars measure the relative brightness of
the stars.

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o The luminosity of a star is the total energy emited per second and is related to the
temp of the star and its surface area.
o This law tells us that, if two stars have the same black body temp (same spectral
class), the star with the brighter apparent…

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assumes no light is absorbed or scattered by material between star and observer.
Will produce more of light at the blue/violet end of the spectrum.
Shows how much energy a black body radiates at different wavelengths, which wavelengths
it radiates with most intensity and total amount of energy radiated.

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decreases. If the energy has been emitted from a point or sphere then it obeys the inverse
square law.

Use of Stephan's law to estimate area needed for sources to have same power output as the sun.

o The star Sirius B has a surface area of 4.1×1013 m2 and…

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Description of main classes:

Lines in emission and absorption spectra occur because electrons in an atom can only exist in
certain, well defined, energy levels.
o in atomic hydrogen, the electron is usually in ground state (n=1), but there are lots of
energy levels (n=2 to n=infinity. ­ called excitation…

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B Blue 11000 He, H Rigel, `be'
-2500 spica
A Blue-whi 7500- H Sirius, `a'
te 11000 (stongest), Vega
F White 6000- Ionised Conopus, `fine'
7500 metals procyon
G Yellow 5000- Ionised Capella, `girl'
white 6000 and the sun
K Orange 3500- Neutral Arcturus, `kiss'…

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Special types
o R= similar to K stars
o N = spectra similar to M
o S = similar to M stars.

Temp related to absorption spectra limited to hydrogen Balmer absorption lines: need for atoms to
be in n=2 state.

For a hydrogen absorption line to occur in the…

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Reveals presence of 60 chem elements in the sun.

The hertzburg-Russell diagram

General shape: main sequence, drawfs and giants.

Axis scales range from -15 to 10 (absolute mag) and 50000 K to 2500 K (temp) or O B A F G K M (spec

Stellar evolution: path of a…

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Main sequence:
o Stable phase
Hydrogen fusing into helium
o 90% of stars in main sequence.
Other 10% either giants or dwarfs.
Red giants:
o Moved off main sequence
o Fusion reactions occurring including hydrogen to helium
o High luminosity
o Low surface temp
Told this by Stephan's law


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