<p>The boundedness or otherwise of clusters remains to be established in most cases. The vast majority of clusters become unbound and disperse at a much younger age than the Pleiades. Or they may be born unbound as you suggest.</p>

<p>The stars in a cluster have a distribution of velocities and there will always be a tail of high speed stars that will be able to escape the gravitational potential in an "evaporation process", even in a cluster which is globally bound (i.e. the sum of kinetic and gravitational potential energies is negative).</p>

<p>It turns out that this isn't usually sufficient to explain the disappearance of clusters. A more effective disruptive mechanism are the Galactic tides. Stars that stray beyond a tidal radius can be stripped from the cluster. The reduction in the cluster's mass then reduces the tidal radius allowing more stars to escape in what eventually would become a runaway process. The tidal radius for a cluster near the Sun is about $5 M^{1/3}$ light years, where $M$ is the cluster mass expressed in units of the solar mass. For the Pleiades, $M \sim 1000 M_{\odot}$, so stars that stray beyond 50 light years can be lost.</p>

<p>In some cases, the evaporation process causes a cluster core collapse. The escaping stars take away kinetic energy and the remaining stars become more tightly bound. This is what has happened in most of the massive and long-lived globular clusters. Less massive open clusters like the Pleiades are much more susceptible to tidal dissipation.</p>

CC BY-SA 3.0
2017-09-15T23:39:33.523Z
2017-09-16T18:32:28.380Z
2017-09-16T18:32:28.380Z
ProfRob
<p>In other words, if a group of stars born within the same nebula, e.g. the Pleiades, are initially gravitationally bound - what causes them to finally get separated and proceed individually? Is it the influence of  &quot;external&quot; mass? Or, are they now and we're they always, even at birth, having the escape velocity? (If the latter is true then I think they should not be considered truly bound from the beginning...)</p>

CC BY-SA 4.0
2021-05-09T09:13:10.990Z
From https://en.wikipedia.org/wiki/Pleiades#Composition The cluster contains over 1,000 statistically confirmed members, a figure that excludes an unresolved likely further number of binary stars.[...] The total mass contained in the cluster is estimated to be about 800 solar masses and is dominated by fainter and redder stars. [...] The cluster contains many brown dwarfs [...] They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.
PM 2Ring
<astrophysics><stellar-dynamics><star-cluster>
Why do stars born in a cluster finally disperse?
Alchimista
# Why do stars born in a cluster finally disperse?

(Asked by: username_0 on 2017-09-15)

In other words, if a group of stars born within the same nebula, e.g. the Pleiades, are initially gravitationally bound - what causes them to finally get separated and proceed individually? Is it the influence of "external" mass? Or, are they now and we're they always, even at birth, having the escape velocity? (If the latter is true then I think they should not be considered truly bound from the beginning...)

**Tags:** astrophysics, stellar-dynamics, star-cluster

> **Comments:**
> **username_1**: From https://en.wikipedia.org/wiki/Pleiades#Composition The cluster contains over 1,000 statistically confirmed members, a figure that excludes an unresolved likely further number of binary stars.[...] The total mass contained in the cluster is estimated to be about 800 solar masses and is dominated by fainter and redder stars. [...] The cluster contains many brown dwarfs [...] They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.  

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## 1 answers

### Answer by username_2 on 2017-09-15. Score: 12 | Accepted answer

The boundedness or otherwise of clusters remains to be established in most cases. The vast majority of clusters become unbound and disperse at a much younger age than the Pleiades. Or they may be born unbound as you suggest.

The stars in a cluster have a distribution of velocities and there will always be a tail of high speed stars that will be able to escape the gravitational potential in an "evaporation process", even in a cluster which is globally bound (i.e. the sum of kinetic and gravitational potential energies is negative).

It turns out that this isn't usually sufficient to explain the disappearance of clusters. A more effective disruptive mechanism are the Galactic tides. Stars that stray beyond a tidal radius can be stripped from the cluster. The reduction in the cluster's mass then reduces the tidal radius allowing more stars to escape in what eventually would become a runaway process. The tidal radius for a cluster near the Sun is about $5 M^{1/3}$ light years, where $M$ is the cluster mass expressed in units of the solar mass. For the Pleiades, $M \sim 1000 M\_{\odot}$, so stars that stray beyond 50 light years can be lost.

In some cases, the evaporation process causes a cluster core collapse. The escaping stars take away kinetic energy and the remaining stars become more tightly bound. This is what has happened in most of the massive and long-lived globular clusters. Less massive open clusters like the Pleiades are much more susceptible to tidal dissipation.


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