Finance & Accounting
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4. How can we use orbital properties to learn about the mass of the galaxy? What have we learned? If we know a star’s orbital period and orbital radius, we can use Newton’s version of Kepler’s third law to determine the mass of the galaxy, with a minor warning: We get only the mass of the galaxy within the orbit of the star we examine.


The gravitational influence of mass contained within an orbit of a particular size determines the speed (and therefore period) of that orbit. So by measuring the period and size of the orbit, we can determine the mass inside the orbit. This is one method you can use to determine the mass of Jupiter (by looking at the orbits of its moons). Mathematically, the expression for the mass enclosed within an orbit of radius r is M = v2r/G, where G is Newton's gravitational constant and v is the orbital speed of a star at distance r. This concept works equally well for the orbits of stars and gas within spiral galaxies. By looking at the mass inside the orbit of stars or gas at different distances from the center of the galaxy, the mass of a galaxy as a function of radial distance from the center can be obtained from the rotation curve of the galaxy. 7. What are cosmic rays, and where are they thought to come from? Cosmic Rays are extremely high energy charged particles (usually protons) that travel the Universe at nearly the speed of light. Most galactic cosmic rays are probably accelerated in the blast waves of supernova remnants. This doesn't mean that the supernova explosion itself gets the particles up to these speeds. ...
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