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# U08a1 Galaxies Review - Coursework Example

## Extract of sample U08a1 Galaxies Review

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. The remnants of the explosions, expanding clouds of gas and magnetic field, can last for thousands of years, and this is where cosmic rays are accelerated. Bouncing back and forth in the magnetic field of the remnant randomly lets some of the particles gain energy, and become cosmic rays. Eventually they build up enough speed that the remnant can no longer contain them, and they escape into the Galaxy. Because the cosmic rays eventually escape the supernova remnant, they can only be accelerated up to a certain maximum energy, which depends upon the size of the acceleration region and the magnetic field strength. 11. How do we know that spiral arms do not rotate like giant pinwheels? What makes spiral arms bright? Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable. We know that the spiral arms cannot rotate like pinwheels around the center of the galaxy because the inner stars would finish several orbits while the outer stars complete one. This would wind the spiral pattern up, which we do not see. The arms appear bright because of the enhanced star formation in them. The bright, bluer stars in particular make the arms bright since these stars do not live long and so die before spreading far from the arm in which they formed. Therefore spiral arms are bright because they contain many short-lived blue stars that shine for only a few million years. 21. The carbon in my diamond ring was once part of an interstellar dust grain. The carbon in my diamond ring was once part of an interstellar dust grain. This statement is sensible. Much of the carbon in the interstellar medium is in the form of dust grains, so the carbon in the interstellar cloud out of which the Earth formed must also have been largely in the form of dust grains. 36. We measure the mass of the black hole at the galactic center from (a) the orbits of stars in the galactic cente ...Show more

## Summary

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…
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