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Albireo: The Boy Scout star

Chapter 5: Binary stars and stellar masses

Kepler’s Three Laws of Planetary Motion

1.Planetary orbits are elliptical2.A planet sweeps out equal areas in equal times3.P2 = a3

M1

M2

r1

r2

CM

Two stars orbit their center of mass

M1

M2

a

Make one star the focus of an ellipse

Visual binary: not viewed face on

Centauri system (visual binary)

Spectroscopic binary

MASS-LUMINOSITY RELATION (for MAIN SEQUENCE STARS)

L = M

where [L] = L and [M] = M

Approximately:

M < 0.3 M = 1.8

0.3 M < M < 3 M = 3.5

M > 3 M = 2.8

OBSERVED APPROXIMATE FITS

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a1

a2

M1

M2