Galactic Gravitation Appreciate why astronomers think that 90% of the matter in our galaxy is invisible
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Galactic Gravitation Ranking & Summary

Galactic Gravitation Tags

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Galactic Gravitation Description

The Galactic Gravitation application was developed to be a small tool that allows you to appreciate why astronomers think that 90% of the matter in our galaxy is invisible. It is well known that in the solar system the outer planets not only take longer to orbit the Sun than the inner planets, they also travel at a slower speed. This is an immediate consequence of the fact that nearly all the mass of the solar system is concentrated in the Sun itself. The gravitational field strength g at a distance R from the Sun is given by Newton's law of gravity: g = GMO / R2 The disc and halo densities are set to zero but the core mass is set to 90 billion solar masses. This means that the gravitational field strength (shown in the second of the three graphs) falls off as an inverse square law and the at a distance of 8 kPc (the distance of the Sun from the galactic centre) the rotational speed is 220 km s2 In addition to showing the calculated rotational speed, the third graph also shows a number of data points showing the results of actual measurements of rotational speeds of our own and other galaxies. To change the mass of the galactic core, use the numeric input box at the top left hand corner. To change the density distribution of the disc and halo, select one of the pre-set distributions - 'Flat', 'Linear', 'Reciprocal' or 'Exponential' and use the associated numeric input boxes to adjust the curves. Alternatively, you can select the 'Custom' distribution and input values directly on the graph by clicking and/or dragging the mouse. Using the LEFT hand mouse button alters the disc density; using the RIGHT hand mouse button alters the halo density. Basically your task is to adjust the parameters of the core, disc and halo until the calculated rotation curve passes throuigh all the data points. When you have accomplished that you will be able to determine the percentage of dark matter in the galaxy (ie the ratio of the mass in the halo to the total mass).

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