Changes

Hartnett also turned his attention to another vexing problem: the rotation curves of large spiral galaxies. A rotation curve plots rotational speed of any part of a galaxy (or orbital speed of any star in it) as a function of radial distance from that galaxy's core. Again, every astronomer has known for some time that galactic rotational speed does not vary inversely as the square of radial distance. In fact, at the fringes of the galaxies, the objects within them rotate much faster—too fast for the amount of luminous mass detected. Hence we observe a discrepancy between luminous mass and ''dynamical mass'', computed as:

Hartnett also turned his attention to another vexing problem: the rotation curves of large spiral galaxies. A rotation curve plots rotational speed of any part of a galaxy (or orbital speed of any star in it) as a function of radial distance from that galaxy's core. Again, every astronomer has known for some time that galactic rotational speed does not vary inversely as the square of radial distance. In fact, at the fringes of the galaxies, the objects within them rotate much faster—too fast for the amount of luminous mass detected. Hence we observe a discrepancy between luminous mass and ''dynamical mass'', computed as:

<math>M = \frac {v^2 \times R}{G}</math>

<math>M = \frac {v^2 \times R}{G}</math>

where M is the dynamical mass of all objects within radius R, v is the speed of a test "particle" at radius R, and G is the [[Isaac Newton|Newtonian]] gravitational constant, first measured by [[Henry Cavendish]].

where M is the dynamical mass of all objects within radius R, v is the speed of a test "particle" at radius R, and G is the [[Isaac Newton|Newtonian]] gravitational constant, first measured by [[Henry Cavendish]].