So we could replace our sun with a very hot star the size of the star that we see from our viewpoint on earth would be bigger but not by that much however, with a billion times more luminosity, earth would be fried. Next we started talking about some background material that is required before we can discuss the final intrinsic parameter of stars, the mass. Kepler lived around 1600. He had access to a large amount of very nice data on the planets, and from these data he determined that the motion of planets obeyed three laws. We will need to use the third law to weigh stars.
Planetary orbits are ellipses with the sun at one focus.
A line between the planet and the sun sweeps out equal areas in equal times. Let s examine these relationships more closely. Kepler s first law is a law to describe the shape of the orbits of planets and the shape of the orbit is an ellipse . Figure 4-16 shows a good illustration of an ellipse. The eccentricity of the ellipse describes how circular or how long and skinny the ellipse is.