Gravity Range of a Galaxy

[Deepak Kapur]

What accounts for the gravitational effect of a galaxy in space.........

 

1. The outermost stars

 

or

 

2. All the stars (matter, black hole etc.) that are present in it taken together

 

 

If the answer is all the stars, how does the gravity of all the stars (entities) accumulate, when there are huge distances among them?

 

 

An example,

 

 

A space probe that has to escape our solar system is concerned with the gravity of the last planet (Neptune or may be Pluto or Sun) rather than the gravity of Earth, Mercury or Venus.

 

When the effect of these 3 planets has a negligible effect on the far away probe, how can stars in the interior of a galaxy (say A) have any significant gravitational effect on the stars of the next galaxy (B), assuming that both the galaxies are a part of a cluster of galaxies? In other words, how do clusters of galaxies stay together?

Edited June 11, 2014 by Deepak Kapur

[pzkpfw]

What is the difference between:

* Distance from stars in the centre of galaxy A to the stars in galaxy B

and

* Distance from stars at the edge of galaxy A to the stars in galaxy B ?

 

And - what is the distribution of stars in the average galaxy?

 

I think you'll see the answer there.

[ajb]

There is Newton's shell theorem or Gauss's law of gravity.

 

We have "The gravitational flux through any closed surface is proportional to the enclosed mass".

 

So in short it is all the mass of the galaxy that matters, not just the outer stars.

[Nicholas Kang]

Dark energy can explain why stars at the outer region of the galaxy won`t fly out of it. Dark matter accounts for the unknown mass of the universe.

[Cosmobrain]

If the answer is all the stars, how does the gravity of all the stars (entities) accumulate, when there are huge distances among them?

 

 

For the same reason the gravity of planets and stars accumulate. They don't have to be touching one another. Each star has a small role in the whole gravity of the galaxy

Edited June 11, 2014 by Cosmobrain

[swansont]

 

An example,

 

 

A space probe that has to escape our solar system is concerned with the gravity of the last planet (Neptune or may be Pluto or Sun) rather than the gravity of Earth, Mercury or Venus.

 

 

The sun is not an outermost body. As ajb has pointed out, it's all the enclosed mass that contributes. In fact, if the mass is evenly distributed, a system will behave as if all of the mass is at the center.

[Delta1212]

 

The sun is not an outermost body. As ajb has pointed out, it's all the enclosed mass that contributes. In fact, if the mass is evenly distributed, a system will behave as if all of the mass is at the center.

Indeed, if you no longer needed to worry about an object's gravity once you'd passed another object we'd be flung out of the solar system every time mercury crossed between Earth and the Sun.

[Deepak Kapur]

 

The sun is not an outermost body. As ajb has pointed out, it's all the enclosed mass that contributes. In fact, if the mass is evenly distributed, a system will behave as if all of the mass is at the center.

 

I mentioned Sun because it is heaviest of all the planets.

 

How to define 'evenly distributed'. I think mass is not evenly distributed in our solar system and it is also not distributed evenly in our galaxy.

Indeed, if you no longer needed to worry about an object's gravity once you'd passed another object we'd be flung out of the solar system every time mercury crossed between Earth and the Sun.

 

This is not what I asked.

 

I take another example.

 

 

There is a galaxy (A). There is another galaxy (B) 8 million light years away from A.

 

Suppose a change takes place in A (say a tilt). As per current understanding, its effect will reach B after 8 million years.

 

1. If such is the scheme of things how do things work out at the cosmic scale? or Is the speed of gravity instantaneous?

 

2. Even if the speed of gravity is instantaneous, we know that the effect of gravity decreases with distance. Seen from this view point, the effect of galaxy A would be extremely weak after travelling a distance of 8 million light years. Just as Mercury, Earth, Venus, Mars, all the asteroids etc. have negligible effect on the space probe that is at the boundaries of solar system.

Edited June 13, 2014 by Deepak Kapur

[swansont]

I mentioned Sun because it is heaviest of all the planets.

It's not a planet, but indeed it is the most massive body. That's an important factor. I was pointing out that it is not consistent with your statement about an object possibly being affected by only the outermost planet.

 

 

How to define 'evenly distributed'. I think mass is not evenly distributed in our solar system and it is also not distributed evenly in our galaxy.

Evenly distributed means you have the same density as a function of radius, everywhere. For a planet, that's not a bad approximation — you can assume that all of a planet's mass is at the center for finding gravity at the surface or outside. If you are below the surface, only the mass inside of the sphere defined by your distance to the center contributes.

 

The solar system obviously does not have a uniform distribution.

 

 

This is not what I asked.

From my reading, it is what you asked. Perhaps it's not what you meant to ask.

 

 

I take another example.

 

 

There is a galaxy (A). There is another galaxy (B) 8 million light years away from A.

 

Suppose a change takes place in A (say a tilt). As per current understanding, its effect will reach B after 8 million years.

 

1. If such is the scheme of things how do things work out at the cosmic scale? or Is the speed of gravity instantaneous?

 

2. Even if the speed of gravity is instantaneous, we know that the effect of gravity decreases with distance. Seen from this view point, the effect of galaxy A would be extremely weak after travelling a distance of 8 million light years. Just as Mercury, Earth, Venus, Mars, all the asteroids etc. have negligible effect on the space probe that is at the boundaries of solar system.

The speed of gravity is that of light; it is not instantaneous.

 

It is true that galactic distances are large, but so are galactic masses. You can see influences of gravity at that scale, and gravity can cause neighboring galaxies to converge and/or distort.