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Author Topic: Weight of a Galaxy?  (Read 1289 times)

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Weight of a Galaxy?
« on: 13/08/2012 00:37:56 »
When scientists observe the rotation of a galaxy and calculate the number of stars within the disc they reach a figure that does not allow the galaxy to hold together under its own gravity so dark matter is included in the sum to hold it all together.But do these calculations include the increase in mass of the outer stars due to there momentum? Or are they calculating the mass just on the number of stars that inhabit the Galaxy as equal?
I would have thought the faster the galaxy rotates the less likely it would fly appart as the outer stars mass increases due to their momentum on a Galactic scale.
I can't help but think dark matter is going nowwhere .

CliffordK

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Re: Weight of a Galaxy?
« Reply #1 on: 14/08/2012 00:15:26 »
A few months ago I tried to create a simplified galaxy with a spreadsheet.

http://www.thenakedscientists.com/forum/index.php?topic=43936.0

There are several issues and parameters.

One has the orbital velocity of the stars.
Orbital period of the stars.
Light emitted by the stars (which is used as an estimate for mass).
Number of stars per unit area.
Stability of spiral arms.

When looking at our solar system, the outer planets not only have a further distance to travel per orbit, but they also travel at a slower speed.  However, there is only minimal gravitational interaction from one planet to another.

A large galaxy has masses both inside and outside of the different stars.

By playing around with the masses of the stars (as an estimate for the stellar density), I could create simplified galaxies that had a number of properties such as uniform orbital periods for the outer stars (thus it would be consistent with spiral galaxies), or uniform orbital velocities for the outer stars (what is generally observed here).

I then found a mass distribution for the Milky Way, and tried to put it into my model.

The mass estimates that I found put about a 100 fold difference between the inner mass and the outer mass of the Milky Way.  Using the "real" data, I lost the ability to have a uniform orbital velocity of the stars.  And, thus I needed to have proportionally less mass in the middle and more mass at the edges of the galaxy.

I believe the mass estimates that I was finding were based on the number of stars and the light emitted.

I don't know if relativistic mass calculations are included in the original estimates.  They could be added easily enough, with the outer stars being observed with essentially uniform velocities.  However, I don't believe it would make more than a couple of percent difference, where the big problem was the hundred fold difference in mass (luminosity) between the inner and outer regions of the galaxy.

I think I referenced a couple of papers in the earlier discussion (above) that showed the mass distribution of the Galaxy.

I thought it was a direct luminosity/mass estimate.  I don't know how they accounted for factors such as metallicity, or whether there could be more interstellar stuff in the outer part of the galaxy which might get mopped up in the central bulge of the galaxy.  If I remember right, I tried to estimate the hydrogen ion density from cosmic rays, and that was insufficient to make the difference.

Emc2

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Re: Weight of a Galaxy?
« Reply #2 on: 24/08/2012 08:22:53 »
"ANY' mathematical equation that attempts to calculate the weight of "ANY" galaxy can not be accurate.

the best you can do, is get it to a "possibility", or "probability"
you can NEVER get an accurate number to be used accurately in any equation.

The Naked Scientists Forum

Re: Weight of a Galaxy?
« Reply #2 on: 24/08/2012 08:22:53 »