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1

Ironically, it's actually harder to measure the mass of the Milky Way than that of other galaxies. You'd think that with it being RIGHT THERE it would be easy, but alas. Most of the difficulty comes from (1) the galaxy spans a huge part of the sky, so it takes an extremely long time to observe any particular feature in detail across the whole thing (say ...

2

The luminosity of the Galaxy is currently estimated to be around $5\times10^{36}$ W and thus an integrated "mass loss" in the form of radiation of of order $10^{-3} M_{\odot}$/yr. But how much radiation is present in the Galaxy? An order of magnitude estimate could be that the Galaxy (including the dark matter) is of order 100,000 light years in radius and ...

3

It is difficult to estimate the masses of either galaxies or clusters of galaxies, and it depends on what source you consult. You can get different values from different sources, for instance in this reference: the local group is estimated to have $5.27\times 10^{12} M_\odot$ (which does include dark matter). I didn't have access to the wikipedia source.

4

I think the following image sums up why your model, at least for our galaxy, is wrong rather nicely: These are the orbits for 6 stars in the inner region of the galaxy. The orbital period for S2, for instance, is 15 years for an orbit that is roughly twice the size of Sedna's orbit--which takes it 12 thousand years to complete its orbit. Using Kepler's ...

8

There are many problems with this line of reasoning. The most common galaxy types are elliptical galaxies and spiral galaxies, and there might be a parallel with star systems, where the most common types are systems with a single star, and binary systems with two stars in the middle. There is simply no justification for this. The dynamics of stellar ...

-1

The stars in the galactic disk rotates with almost the same orbital velocity 200-230 km/s around the galactic center. Unlike a star system where the planets follow Kepler's third law. To explain the almost constant orbital speed of the stars in the galaxy, we have calculated dark matter, which is distributed in such a way, that it gives stars almost the same ...

22

Why shouldn't the orbits of stars be Keplerian? The answer is simple. Keplerian orbits are predicated on a single central point mass. That assumption fails to some extent even in a solar system. It fails massively in a galaxy. A galaxy is not a point mass.

10

Elliptical orbits are direct consequence of orbiting entirely outside a spherically symmetric mass. Even if you assume that a galaxy has a spherically symmetric mass distribution, the amount of mass at a radial distance less than that of the star would be changing (assuming some eccentricity). Once that happens, the orbit is no longer an ellipse.

5

Not Keplerian, because it is not a conic-section. It is not even explained by Newtonian gravity. In contrast, Kepler's laws are explained by newtonian gravity. The lowest orbital-energy from Keplerian orbit is circular. And the orbits of stars are observed to be approximately circular. Hence:  \frac{mv^2}{r} = \frac{GMm}{r^2} \quad\Longrightarrow\quad v = ...

1

First question Comparing a galaxy (or solar systems) to a merry-go-round is not a good comparison. Acceleration in a merry-go-round is proportional to the distance from the center of the merry-go-round, but acceleration in a central mass system is inversely proportional to the distance from the center. Single star solar systems are very close to central ...

1

At this point in time, your questions are subject to research - we just don't know, yet. We see the effects of dark matter (in rotation curves of galaxies, in gravitational lensing and others), so there is "something". But what exactly that "something" constitutes is not clear. There are several theories, and to think of dark matter as some kind of ...

1

Which scientists have called dark matter and dark energy forces? Dark matter is a form of matter that doesn't feel any force other than gravity (so far as we have observed). It's true that dark matter bends light because it has mass and it makes up about $5/6$ of all the mass in the universe. The gravity from this mass is what bends light through ...

1

Firstly note that cosmologists tend to use the terms matter and energy interchangeably because what matters is normally the energy density and matter is counted as energy using the well known expression $E = mc^2$. But, as Julian says in a comment, it's more common to speak of phantom energy rather than phantom matter. To understand what phantom energy is ...

2

"Only about 10 percent of the total baryonic matter is sufficiently condensed by gravity to form stars and galaxies. More than 90 percent was left between the galaxies." http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2008/searching_for_baryonic_matter_in_intergalactic_space.pdf 6% of baryonic matter is within stars according to the ...

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