Lecture 8 Notes

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7 Mar 2011
Cliff Lau University of Toronto
AST201 Lecture 8 (February 8, 2011)
Of all the light a star emits Earth receives only a small fraction
This small fraction determines the stars apparent brightness or
apparent magnitude, m, as seen from Earth
Inverse square law of light the amount of light received from a star
falls with the square of our distance from it
A stars luminosity and apparent brightness are related
Distance (d) + apparent brightness (I) luminosity (L)
Spectral categories A, B, F, G, K, M, O
They are ordered OBAFGKM (Oh, Be A Fine Girl, Kiss Me)
Each stellar spectrum has absorption lines corresponding to the
chemical elements in the stars atmosphere
The strengths of the lines depend on the temperature of the star. This is
a second, a more precise way to measure a stars temperature
Presence of absorption lines composition
Strength of particular absorption lines temperature
To measure the masses of stars, we rely on binary stars
Binary stars are pairs of stars orbiting their common center of gravity
Most stars are actually in binaries the Sun is an exception
In a visual binary, we can see both members of the system; this only
works for nearby binaries
Usually, we deduce the presence of a binary companion indirectly
Spectroscopic binary we see the Doppler shift in the light from the
system as the two stars orbit one another
Eclipsing binary we see the system dim as the stars eclipse one
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