# Jeans mass

by Ranku
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 P: 115 What happens when the mass of a system is below Jeans mass? Will the system dissipate mass? Are all gravitationally bound systems required to be precisely the Jeans mass, to avoid gravitational collapse or dissipation of mass?
 Sci Advisor P: 1,686 In the early universe, when a system is below the Jeans mass, it undergoes oscillations. When it is greater than the Jeans mass, it becomes gravitationally unstable.
P: 115
 Quote by bapowell In the early universe, when a system is below the Jeans mass, it undergoes oscillations. When it is greater than the Jeans mass, it becomes gravitationally unstable.
Ok, and what about in the present universe? And what does oscillations mean?

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Jeans mass

 Quote by Ranku What happens when the mass of a system is below Jeans mass? Will the system dissipate mass? Are all gravitationally bound systems required to be precisely the Jeans mass, to avoid gravitational collapse or dissipation of mass?
It doesn't have to be precisely at the Jeans mass for part of the mass in a volume to gravitationally collapse into a denser state. Down to about half the Jeans mass a system can collapse, but more matter is thrown off the lower the initial mass. That's due to the "oscillations" the other poster mentioned, which is a series of radial pulsations (if the system is spherical) that lose mass as the system collapses. Remember that the Jeans mass is achieved when gravitational self-attraction of a system causes it to collapse faster than pressure changes can smooth out over-dense regions i.e. sound/pressure waves can't smooth it out quick enough.
P: 115
 Quote by qraal It doesn't have to be precisely at the Jeans mass for part of the mass in a volume to gravitationally collapse into a denser state. Down to about half the Jeans mass a system can collapse, but more matter is thrown off the lower the initial mass. That's due to the "oscillations" the other poster mentioned, which is a series of radial pulsations (if the system is spherical) that lose mass as the system collapses. Remember that the Jeans mass is achieved when gravitational self-attraction of a system causes it to collapse faster than pressure changes can smooth out over-dense regions i.e. sound/pressure waves can't smooth it out quick enough.
Is this the same oscillation seen in stellar evolution, where a star collapses alternately contracting and expanding, radiating away half of the released gravitational energy in accordance to the virial theorem?
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 Quote by Ranku Is this the same oscillation seen in stellar evolution, where a star collapses alternately contracting and expanding, radiating away half of the released gravitational energy in accordance to the virial theorem?
Which bit of stellar evolution do you mean?
P: 115
 Quote by qraal Which bit of stellar evolution do you mean?
I guess when a star is gravitationally collapsing.
P: 775
 Quote by Ranku I guess when a star is gravitationally collapsing.
IN that case AFAIK the collapse process doesn't involve pulsation so much as jet formation and the intense early stellar wind, both of which throw off excess angular momentum that stars need to be rid of to collapse. But there's all sorts of complicated opacity effects too which slow down the escape of heat from the star. It's complicated.

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