The night sky is a field of stars in Scientific American March 2013

This document contains comments about the article The night sky is a field of stars by Steven W Stahler In Scientific American of March 2013.
At the beginning of this document we read:
The vast majority of stars in our own galaxy have no true physical connection to one another.
The meaning of this sentence is not clear. In fact all the stars in one galaxy are in a certain way one physical object, like all the stars in a any galaxy. Later on he writes:
The theory of stellar evolution is one of the triumps of 20th-century astrophysics.
The meaning of this sentence is not clear. The 20th-century gave birth to may discoveries. Later on he writes:
Perhaps the same physical forces had shaped all clusters, regardless of their present ages and sizes.
The concept of physical forces is not defined. IMO he should have written: the same processes. Next:
And perhaps one simple variable could account for the way those forces act on an individual cluster: the mass of the parent cloud from which each cluster is born.
Again here he should have mentioned: processes. However I see a problem: there is a clear relation between size and mass.

Cloudy with a chance of starlight

Next:
Stars coalesce within vast clouds composed chiefly of hydrogen along with other elements and a small admixture of dust.
The concept of dust is not clear. Anyway stars must come from something. The question is how. Next:
These molecular clouds are distributed throughout all galaxies and each exerts a gravitational pull - not only on stars and other objects outside the cloud but also on regions in the cloud itself.
The concept of gravitational pull is not explained nor what that concept does within the cloud itself. Next
Because of the cloud's own gravity regions where gas and dust are especially dense collapse into protostar.
Again what do you mean with its own gravity. Astronomy starts with observations and identical observations result in laws. You can start with laws to predict possible processes but you need observations to prove those predictions.
In this way, clusters of anywhere from a dozen to thousands of stars can arise from a single melecular cloud.
I expect that all stars can from a melecular cloud. How larger the cloud how larger the cluster. On the other hand it is also possible that certain stars in a cluster where not born there. How older the stars how more likely.
A little further on we read:
Scientist have known for some time that the mass of the parent cloud in a T association is far greater than that of its stellar progeny
This seems to me an correct assumption, however difficult to prove in practice.
(Our Sun was a T Tauri star in its youth) I believe this feature accounts for the short life span of these clusters.
The picture emerges that many of the T Tauri only have a very short life and that from the remnants of those much larger will grow. Next:
Mass determines the strength of gravitational force: the greater the mass is, the stronger the gravitational pull.
Again they use a concept that is not properly explained. Of course you can point to Newton's Law, but it is not obvious to what you can use this here.
Next:
So if the mass of the parent cloud in a T association is much greater than that of its member gravity of the cloud - not the gravity exerted by the stars on one another - must be what holds the cluster together.
IMO any combination of physical objects, assuming all those objects can be considered as one entity, will have the tendency to stay together. This can be a cloud without stars or a cloud with stars or a cluster of only stars. The chemical composition of the individual objects involved makes the combination "unstable"
And if the cloud disperses the stars will drift apart.
IMO this is not true. When the cloud disperses the total mass of all the physical objects will stay the same. The problem is that what we are discussing here are objects part of a much larger object (i.e. our Galaxy) and the behaviour of that object will large influence the behaviour of its constituents.
Astronomers think that stellar winds - jets of gas propelled forcefully outward from the stars - eventually strip away the parent cloud, freeing the previously bound stars to head in space.
I doubt that it is as simple as that. More must be involved. Stellar winds must have a cause i.e. other stars and those stars can also influence their surroundings and vice versa.
A little further we read:
The third kind of stellar group in the Milky Way is remarkably stable. Called open clusters, these groups have up to 1000 ordinary stars and persist for hundreds of millions and even billion of years.
I expect the major paramater is here how lonely those clusters are i.e. their position compared to its neighbours.

Push and Pull

In this paragraph we read:
I chose to study two countervailling processes: contraction, caused by the gravity of the parent cloud and expansion promoted by stellar winds and ionizing radiation.
Within the physics of star evolution only one type of force is at stake: gravity as described by newton's law.
Each star-producing cloud is subject to these two opposing influences to a varying degree.
As remarked earlier the only important factor is how lonely each studied cluster is i.e the distance to their neighbours.
And I suspected that the key to this balance might be the original mass of the parent cloud. The mass of a cloud certainly determines its gravity; the cloud gravity in turn governs the rate at which it contracts.
What determines that such a cloud exist in the first place? What happened before?
At the opposite extreme, a cloud with an order of magtitude more mass would undergo a rapid contraction, forming many new stars in close proximity.
Again how is it possible that you have large and small clouds? How do you know?
Even when stellar winds drive away the cloud, the gravitational attraction among these closely packed stars etc.
It would be interesting if the author explained more indetail what the cause and origin is of those stellar winds.
I do not think that stellar winds are the cause that molecular clouds disappear from a cluster of closely packed stars. The cause are those stars themself.

Cloud Contraction

I would have to figure out a way to demonstrate that more mature clusters had undergone contraction long ago.
That means you have to demonstrate that the original molecular cloud and the protostars cluster was much larger than the present day mature cluster.
Next left column page 36:
To test that I needed to figure out how to measure historical star formation rates in clusters.
The theory describes how young stars behave over time etc
but instead of shining they radiate the heat generated by compression as their own gravity causes them to contract
Within time the rate of their compression slows, while their surface temperature climbs
The stars thus get both dimmer and hotter in a predictable pattern as they age.
What this text seems to tell us is that the behaviour of all T tauri stars is identical, and that as a result we can calculate their age. One important issue is: how do you know that the size of individual stars varies during their life time. Next is written:
If you know the surface temperature and luminosity, you can tell how old it is.
Exactly. In theory.
I realized that the collective set of ages of all these stars in a cluster would reveal the star formation history of the group - when and at what rate the member stars formed over time.
Palla and I found that for all groups that still posses copious cloud gas, the total star-formation rate has been increasing with time.
IMO this means that there are more young stars than old stars
In 2007 Eric Huff and I constructed a theoretical model of the parent cloud of the Orion nebula cluster.
For more information read this: Wikipedia Orion Nebula
Next is written:
Our model included the forces of contraction and expansion postulated by my theorie.
In computer simulations based on the model the simulated cloud contracted just as we predicted it should.
More information is required. One important question to answer is: is this whole simulation solely based on Newton's Law.
One important issue are the initial conditions.
The overall message is that the fact that the cloud contracts does says to much.
A much more important issue is: what is the reason that the molecular cloud is there anyway. What happened before.
The book "Universe" by Kaufmann, chapter The Birth of stars" writes:
In fact, an infant star going through its T Tauri stage can lose as much as .4 solar masses before it settles down on the main sequence.
T Tauri stars have relative low masses of rouhly 0.2 to 2 Solar Masses. Young stars that are hotter and more massive than T Tauris propel their mass loss by vigorous stellar winds.
What this last sentence implies is that those young stars keep the cloud in life.
Next is written:
We then applied an emperical description known as the Schmidt_Kennicutt law, derived from Schmidt's Observations and many subsequent ones, to show how the increase in density in a parcel of the cloud over time would affect the local star-formation rate.
IMO you should only use Newton's Law in order to describe the behaviour of star clusters including molecular clouds.
The problem is that a molecular cloud in principle is unstable. To make the cloud stable you have to add additional stars which rotate around a common center. Changes in the positions of these stars will trickel through the cloud and will induce density changes which inturn will cause new stars to be born.
Next is writen:
This additional finding further corroborated the force-balance theory's assumption that parent clouds contract in the early stages of cluster evolution.
The orion constellation is part of our galaxy. The description of the evolution of the Orion nebula cluster as such is part of the evolution of our galaxy as a whole and can not be described locally.

Cluster expansion

So far the early stages of open-cluster evolution remain inaccessible even to direct observation.
The observation of the differnt stages in the evolution of all types of stars clusters i.e how they evolved in time from one into the other over a long period, is impossible.
It is possible, however, to model the evolution of an open cluster whose parent cloud has already vanished using so-called N body simulations.
It would be better to write: to model the behaviour of an open cluster.
What the author should have done is to use this same aproach also for the orion nebula cluster.
Next is written:
This aproach has elucidated what happens in open clusters after the initial star-forming contraction proposed by my theorie.
The problem with the proposed theorie is that it has to be supported by observations. The fact that you can simulate contraction is no proof that star formation evolves in the same matter.
Although open clusters are remarkably stable, they are not static.
Clusters are neither stable nor static.
Our strategy was to guess an arbitrary initial configuration for the cluster and then let it evolve for 125 million years.
It is more interesting to start from the present and to calculate this initial condition.
What we saw surprised us. It seems that while remaining gravitational bound the Pleiades cluster has expanded more or less uniformely since its cloud dispersed.
This finding suggests that the clasical analysis overlooked some critical factor in the balance of forces shaping cluster evolution. What drives the uniform expansion of open clusters?
The fact that this is observed in a simulation does not mean that the same happens in the reality (at the same rate). What the author did is, he started from an initial condition such that the size of the cluster was smaller than the present size of the Pleiades cluster.

When you perform a simulation of a cluster of identical stars, in general, first the cluster contracts and then expands. This contracting strongly depends on the initial conditions.
See also The virial Theorem for a critical discussion and to do your own simulations.
Next is written:

This results conflicts with prior analysis which had predicted that the stars in open clusters would slowly segregate into an inner clump of heavier ones and an outer envelope of relative light ones.
If you start with a large cluster (the name open cluster is very misleading) and you allow the stars to merge than such a scenario could easily happen.
Next is written:
Globular clusters are known to behave this way. Yet even when we let out n-body simulation run for 900 million years expansion continued uniformly, showing what an inflated but still intact Pleiades will look like at the age of a billion years.
The simulations mentioned above show the same result. The whole issue is the size of the cluster and the mixture of stars involved.
Simulations performed by Douglas Heggie showed in the mid 1970's that when a third star approaches such a pair the three engage in a complicated dance after which the lightest of the three is usually ejected at high speed.
This last fact only happens very rarely. Specific this means that the speed v1 of a star at a distance d from a binary pair is much smaller than the speed v2 of that same star away from this pair at the same distance d.
In fact there are four different scenarios assuming a binary pair and a third star:
For example, astronomers should look for ways to observe the uniform expansion of open clusters predicted by my studies.
First you need a good definition of what is an open cluster and what is not. IMO the only workable definition is a group of stars with almost all identical stars.
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Created: 13 March 2013

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