# All Questions

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### Why is the distribution of dark matter in a Galaxy different from the distribution of normal matter?

The dynamics of a galaxy are driven by gravity. And dark matter experiences the same gravitational forces as normal matter. The effects of other forces are very small in comparison. So why is the ...
0answers
46 views

### Gravitational potential energy of an isothermal sphere

I am trying to calculate the gravitational potential energy, W, defined as: $W = -\frac{1}{2}\int\rho(r)\Phi(r)d^{3}r$ for an isothermal sphere. I am given that the density profile varies with r as:...
0answers
65 views

### How to derive the gravitational potential on an infinitely thin disc of finite radius?

I am stuck on a problem that requires me to calculate the gravitational potential on an infinitely thin disc of finite radius $R$ with mass $M$. This infinitely thin disc models a galaxy. Once I have ...
0answers
36 views

### Does dark matter experience drag?

Is dark matter dragged around the galaxy by gravity as stars and groups of stars orbit the galaxy, and, if so, is its shape/distribution deformed? Similarly, would the Oort cloud be deformed by being ...
1answer
69 views

### Do attraction between two galaxies count for dark matter?

We know that the uniform rotation curve of individual spiral galaxies does require presence of some form of dark entity (matter). Does the attraction between two (or more) spiral galaxies also require ...
1answer
52 views

### If a galaxy forms from a spherical stationary cloud, how much of the gas will escape?

Let's ignore the dark matter legend and stay with Keplerian physics. Assuming that there is a cloud with $N$ stationary particles with the same size uniformly distributed in a sphere and they ...
2answers
186 views

### The Sun's Orbit - Is it What We Think?

I was thinking that the sun must orbit something within our spiral arm in the Milky Way, or be affected by other astronomical bodies - surely not just the supermassive black hole centre. I have ...
1answer
2k views

### If our galaxy's dark matter halo is so large and diffuse, why is the ordinary matter in it so much more localised and compactly located?

I just read that our galaxy's dark matter halo is estimated to be 1.5m ly across, compared to the visible galaxy's 100k ly across, needed to explain stellar rotation curves. Why would this be? By ...
2answers
107 views

### Evolution of the virial ratio in $N$-body sim

I have run an 2D $N$-body sim to simulate the cold collapse of a galaxy. Initial conditions: $N=500$, all velocities=0, positions generated randomly within a circle of radius $R$. The system is ...
0answers
150 views

### Why are all galaxies roughly the same size?

what determines the stable mass of galaxies, stars, star clusters or even galaxy clusters? Is there some obvious way to determine the classes of stable mass clusters from the initial conditions of the ...
1answer
71 views

### Orbit of stars in a galaxy

Do all stars orbit around the galactic center? If yes what makes them orbit around the center , what object creates such a massive force which makes stars millions of light years far away from the ...
3answers
3k views

### What causes our Sun to oscillate around its mean Galactic orbit?

According to this answer on Astronomy.SE, The Sun executes oscillations around its mean orbit in the Galaxy, periodically crossing the Galactic plane. I borrowed this illustration (not to scale!) ...
1answer
104 views

### Are there reliable sources regarding the motion of the Solar System through the Milky Way?

So, I've been working on a Newtonian mechanics Solar System simulator. I want to explore what happens when the additional forces due to the other components of the Milky Way are included. My ...
0answers
21 views

### About the use of Newtonian Relations for the movement of stars in the Galaxy [duplicate]

From a General Relativity point of view Gravity is given as the result of spacetime curvature interacting with energy-mass density. To get to the Newtonian limit one needs to take a) Non-relativistic ...
1answer
220 views

### Stars more affected by gravity of stars further away, need help understanding argument why

In the first chapter of Binney and Tremaine's "Galactic Dynamics" there is a statement that I don't yet understand. The first chapter of the book is linked here, straight from the publisher. The ...
1answer
255 views

### Potential of an axisymmetric disc with constant rotation velocity

I am having trouble understanding why the form of the 3D potential for a disc with a constant rotation velocity for circular orbits of stars within the disc \begin{equation} v(R) = v_0, \tag{1} \end{...
0answers
366 views

### Binding energy of a galaxy?

I'm a novice in physics and new to the forms, so please forgive me if this is a dumb question or if this is in the wrong section to be posted, but how would one find the gravitational binding energy ...
2answers
424 views

### Does Newtonian physics work on a galactic scale?

I'm currently working on a simulation that aims to use Newton's Law of Gravitation to simulate how a galaxy behaves gravitationally. While I haven't gotten the simulation finished yet, I have had a ...
1answer
173 views

### Tidal tails of galaxies after collision

When there is a collision of 2 disc shaped galaxies, there is a tail formation created from both the galaxies. I read here that this was due to tidal forces, but I couldn't figure out how this happens....
1answer
281 views

### Galaxy rotation curve [duplicate]

In calculating galaxies' rotation curve, it is assumed that masses outside the orbit has no contribution and masses inside the orbit can be deemed as if they were at the center, just like the case of ...
2answers
1k views

### What is a reasonably accurate but simple model of the Milky Way's gravitational field?

I am putting together a toy program which shows how stars move around in the galaxy. To run the simulation I need to know strength of the Milky Way's gravitational field at any location in it. I'm ...
5answers
18k views

### Why aren't there spherical galaxies? [duplicate]

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies: Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out ...
2answers
804 views

### How long would it take for a galaxy to collapse without dark matter?

I am trying to understand the effects of gravity in the Cosmos without complications of Dark Matter/Dark energy issues. So my question is, assuming that a galaxy (for example, the Milky Way) does not ...
1answer
261 views

### Equation for a galaxy's overall gravitational pull?

Can the Newton's formula be used to express $g$ between two Galaxies in proximity? Gravity on Earth uses two concepts, one is the radius (R) of earth and the other is the distance $h$ from the ...
2answers
175 views

### How Galaxy is formed?

Given the distance among stars (the most massive objective in the space) is so huge, the difference of order of magnitude is about 7. And also, since gravity is such a weak force, how is it likely for ...
2answers
1k views

### How do spiral arms form?

Why aren't all spinning galaxies shaped as discs as my young mind would expect? I understand how the innermost parts of a galaxy spin faster than the outer parts, and that could explain why some ...