Starship Asterisk*

Why do newly formed stars and surrounding dust orbit similar directions around the core of a new spiral galaxy? Why would not the material fall inward due to gravity from different (even opposing directions) and from non-planar directions? What gives the gravity force field directional properties other than an inward direction toward its gravity source? Are we really talking about electromagnetic properties instead of gravitational properties to perform this trick?

dougettinger wrote:Why do newly formed stars and surrounding dust orbit similar directions around the core of a new spiral galaxy? Why would not the material fall inward due to gravity from different (even opposing directions) and from non-planar directions? What gives the gravity force field directional properties other than an inward direction toward its gravity source? Are we really talking about electromagnetic properties instead of gravitational properties to perform this trick?

Gravity doesn't pull things "inward"; it maintains things in orbit. The material making up a spiral galaxy lies on a plane and has a common direction of rotation because of the effects of radiation pressure and the conservation of angular momentum. New stars that form from this material maintain approximately the same orbit because there are no forces to change it. That is, the center of mass of the new star will continue to follow the path of the center of mass of the material from which it condensed.

thanks for asking ,dougettinger. that was my question too.
but i didn't understand your response Chris. how does Gravity maintain things to orbit when the force is just between them?

Amir wrote:thanks for asking ,dougettinger. that was my question too.
but i didn't understand your response Chris. how does Gravity maintain things to orbit when the force is just between them?

In the same way that planets orbit stars without falling into them. The only time gravity will pull things directly together is if they don't have any relative lateral motion, which will be rare. Otherwise, an orbit is established, which is just the radius where the lateral component allows the objects to be falling freely towards each other, but with no change of distance.

Chris Peterson wrote:

dougettinger wrote:Why do newly formed stars and surrounding dust orbit similar directions around the core of a new spiral galaxy? Why would not the material fall inward due to gravity from different (even opposing directions) and from non-planar directions? What gives the gravity force field directional properties other than an inward direction toward its gravity source? Are we really talking about electromagnetic properties instead of gravitational properties to perform this trick?

Gravity doesn't pull things "inward"; it maintains things in orbit. The material making up a spiral galaxy lies on a plane and has a common direction of rotation because of the effects of radiation pressure and the conservation of angular momentum. New stars that form from this material maintain approximately the same orbit because there are no forces to change it. That is, the center of mass of the new star will continue to follow the path of the center of mass of the material from which it condensed.

How does radiation pressure aid in forming disks of dust/gases rotating in a common direction around the center of a galaxy? I thought radiation pressure kept stars from collapsing and evacuated the region around a newly formed star after it had started fusion.

Chris Peterson wrote:

Amir wrote:thanks for asking ,dougettinger. that was my question too.
but i didn't understand your response Chris. how does Gravity maintain things to orbit when the force is just between them?

In the same way that planets orbit stars without falling into them. The only time gravity will pull things directly together is if they don't have any relative lateral motion, which will be rare. Otherwise, an orbit is established, which is just the radius where the lateral component allows the objects to be falling freely towards each other, but with no change of distance.

That is correct. All galaxies should have some lateral motion including the motion involved in the universe expanding.

Chris Peterson wrote:In the same way that planets orbit stars without falling into them. The only time gravity will pull things directly together is if they don't have any relative lateral motion, which will be rare. Otherwise, an orbit is established, which is just the radius where the lateral component allows the objects to be falling freely towards each other, but with no change of distance.

i knew that, but i didnt know "having no lateral motion" is rare! it's obvious though.
thanks Chris.

Everything having lateral motion is not intuitively obvious. Everything under my feet seems to be rather stationary. Hence, I throw out this question for anybody to answer. Why do scientists make a very narrow assumption that the universe is expanding from the Earth's surface outward toward the edge of the universe? Why is not the surface of our Earth, other planets and the Sun expanding, too?

Do atoms inside a crystal structure have lateral motion with respect to each other? I know that electrons with respect to their respective nuclei have lateral motion. Is each and every atom expanding with respect to each other and have lateral motion?

dougettinger wrote:Everything having lateral motion is not intuitively obvious. Everything under my feet seems to be rather stationary.

It's basically just statistics. If you have two "particles" in space (be they atoms or galaxies), having random velocities with respect to one another, it is unlikely that their motion vectors will be collinear. If you decompose their motion vectors into collinear and perpendicular components, the latter is what we're calling "lateral motion".

Why do scientists make a very narrow assumption that the universe is expanding from the Earth's surface outward toward the edge of the universe?

They don't make this assumption. The force of gravity is much stronger than the "force" of universal expansion. Space is expanding only where gravity is too weak to hold it together. So there is no expansion inside of galaxies- galaxies get farther from other galaxies, but they don't get bigger themselves (at least, not because of the expansion of space).

Do atoms inside a crystal structure have lateral motion with respect to each other? I know that electrons with respect to their respective nuclei have lateral motion. Is each and every atom expanding with respect to each other and have lateral motion?

Now you are in the realm of quantum mechanics, where the very concept of "motion" becomes complex (and uncertain <g>). In order to even attempt answering this question, it would be necessary to define just what "lateral motion" even means in a QM regime.

Chris, thank you for posting your very current and honest feelings that I am sure reflect academia. As for myself, I believe humans have a very egotistical manner that gets in the way of true scientific inquiry many times. I cannot say that gravity (whatever it really represents) acts any differently between the core and the surface of the Earth, or the between the Sun's center and Neptune, or between our Milky Way galaxy cluster and other galaxial clusters. The immense scale of things still gets in the way of our thinking.

dougettinger wrote:Chris, thank you for posting your very current and honest feelings that I am sure reflect academia. As for myself, I believe humans have a very egotistical manner that gets in the way of true scientific inquiry many times. I cannot say that gravity (whatever it really represents) acts any differently between the core and the surface of the Earth, or the between the Sun's center and Neptune, or between our Milky Way galaxy cluster and other galaxial clusters. The immense scale of things still gets in the way of our thinking.

I disagree that the scale of things hinders our thinking.

How things in the Universe move is a question that is very accessible to human observation. Given the rate the Universe is observed to be expanding (based on looking at distant objects and their motion relative to us), we can calculate how fast things within our own galaxy ought to be moving away, and that speed is well within our observational capabilities. And when we look, we don't see it. This is all in accord with General Relativity, one of the most solidly supported scientific theories we have. So our understanding of the behavior of gravity and the behavior of the metric expansion of space is extremely well grounded in both theory and observation. I stress behavior to emphasize that we are not necessarily talking about just what gravity or universal expansion actually are. We don't need to know that to describe (in a predictive way) how they operate.

Gravitation obeys the inverse square law. The force of gravity varies inversely with the square of the distance between objects. The vast distances between galactic clusters would imply a very weak gravitational force.

bystander wrote:Gravitation obeys the inverse square law. The force of gravity varies inversely with the square of the distance between objects. The vast distances between galactic clusters would imply a very weak gravitational force.

Actually, this gravitational force does act over a distance proportional to the distance of closest approach so that 1/R is the more appropriate factor.

A star passing by the sun at distance R (in light years) and
with a relative velocity V (in km/s) is deflected by δ degrees:

where δ = 1.607º / [R * V²]

Art or Neufer, I like your equation. I will remember you when I need to find or derive an appropriate equation. In fact, I could use an equation. Assume one smaller spherical body, m1, strikes another spherical body, m2, toward its center of gravity but at a certain angle, A, between the two velocity vectors, v1 and v2, both in the same quadrant. The smaller body is absorbed except for some insignificant collisional debris. What is the equation for the final velocity of this system?

Doug Ettinger
Pittsburgh, PA

dougettinger wrote:Art or Neufer, I like your equation. I will remember you when I need to find or derive an appropriate equation. In fact, I could use an equation. Assume one smaller spherical body, m1, strikes another spherical body, m2, toward its center of gravity but at a certain angle, A, between the two velocity vectors, v1 and v2, both in the same quadrant. The smaller body is absorbed except for some insignificant collisional debris. What is the equation for the final velocity of this system?

You can do this one yourself, Doug, by adding the two momentum vectors
and then dividing the total momentum vector by the total mass.

neufer wrote:

dougettinger wrote:Art or Neufer, I like your equation. I will remember you when I need to find or derive an appropriate equation. In fact, I could use an equation. Assume one smaller spherical body, m1, strikes another spherical body, m2, toward its center of gravity but at a certain angle, A, between the two velocity vectors, v1 and v2, both in the same quadrant. The smaller body is absorbed except for some insignificant collisional debris. What is the equation for the final velocity of this system?

You can do this one yourself, Doug, by adding the two momentum vectors
and then dividing the total momentum vector by the total mass.

The equation is ( [m1v1] + [m2v2] ) / (m1 + m2) = [v3] which is easy assuming that I solve for the vectors correctly. Now I wish to move to a more complicated equation.

Now let's assume that the large spherical body of homogeneous density is spinning at w1 angular velocity, what equation could represent the final w2, if the smaller impactor body struck the larger body on the equator but offset by some longitudinal distance of "a" ?

neufer wrote:

bystander wrote:Gravitation obeys the inverse square law. The force of gravity varies inversely with the square of the distance between objects. The vast distances between galactic clusters would imply a very weak gravitational force.

Actually, this gravitational force does act over a distance proportional to the distance of closest approach so that 1/R is the more appropriate factor.

A star passing by the sun at distance R (in light years) and
with a relative velocity V (in km/s) is deflected by δ degrees:

where δ = 1.607º / [R * V²]

Neufer, how does this equation change if the units for R are kilometers?