Basic Astrophysics by Dan Maoz

By Dan Maoz

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By Dan Maoz

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Use Kepler’s Law (Eq. 42) to calculate the value of M sin3 i for each basicastro4 October 26, 2006 STARS: BASIC OBSERVATIONS 31 star, where M is the mass and i is the inclination to the observer’s line of sight of the perpendicular to the orbital plane. 18M . c. , the mean value it would have among an ensemble of binaries with random inclinations. Find the masses of the two stars, if sin3 i has its mean value. Hint: In spherical coordinates, (θ, φ), integrate over the solid angle of a sphere where the observer is in the direction of the z axis, with each solid angle element weighted by sin3 θ.

A. Find the mass of each star. Since the binary is of the eclipsing type, one can safely approximate i ≈ 90◦ . Check to what degree the results are affected by small deviations from this angle, to convince yourself that this is a good approximation. 58M . b. Assume again i = 90◦ and find the radius of each star. Is the result still insensitive to the exact value of i? 46r . basicastro4 October 26, 2006 basicastro4 October 26, 2006 Chapter Three Stellar Physics In this chapter, we will obtain a physical understanding of main-sequence stars and of their properties, as outlined in the previous chapter.

This is just the gravitational potential self energy of the star, Egr . On the left side, integration by parts gives [P (r)4πr3 ]r0∗ − 3 r∗ P (r)4πr2 dr. 21) 0 We will define the surface of the star as the radius at which the pressure goes to zero. The first term is therefore zero. The second term is seen to be −3 times the volume-averaged pressure, P¯ , up to division by the volume V of the star. 22) . 3 V In words, the mean pressure in a star equals minus one third of its gravitational energy density.

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