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lostgalaxy
2010-Jul-30, 03:47 PM
Hi all,

Sorry if my question isn't sensible enough. I mean if one star wants to escape a galaxy then what are its main obstacles? Is it dark matter or the central black hole? Or both because actually that black hole is mainly made of dark matter?

It's important for me because it gives me an analogy for a life and buddhist question of liberation. So thank you beforehand for your generous information.

Regards,
LG

korjik
2010-Jul-30, 03:56 PM
Technically, it is all the mass of the galaxy that holds it in. All the stars and all the gas actually is alot more mass than the black hole in the center of the galaxy. Then again, all the stars and all the gas combined are a small mass when compared to the mass of dark matter.

Remember, dark matter is then name for whatever it is that we cant see (cause it is dark) that causes most of the gravitational attraction in the outer areas of the galaxy. It isnt a substance that you can go get. Dark matter is a placeholder for the fact that we dont know what is causing the bulk of the gravity in the galaxy (well, whole universe actually).

antoniseb
2010-Jul-30, 04:16 PM
... if one star wants to escape a galaxy then what are its main obstacles? ...

The things that help a star escape from a galaxy are:
- gravitational interactions with other galaxies
- gravitational interactions with very massive pairs of other bodies

Aside from these things, as korjik says, all the mass of the galaxy (of which the central black hole is usually less than 0.1 percent), keeps the star from wandering too far from its current orbit.

Tim Thompson
2010-Jul-30, 05:25 PM
I mean if one star wants to escape a galaxy then what are its main obstacles? Is it dark matter or the central black hole? Or both because actually that black hole is mainly made of dark matter?
The total mass of the Milky Way, including dark matter, is about 1012 solar masses. The total mass excluding dark matter is about 1011 solar masses, most of that in stars. The mass of the central black hole is about 4x106 solar masses. Even excluding dark matter, the rest of the Galaxy vastly outweighs the central black hole. So in order to escape from the Galaxy altogether, it is the total mass of the Galaxy that is the final arbiter.

But one must keep in kind that stars don't just take off on their own. The observational class of stars moving fast enough to risk escaping from the Galaxy are called hypervelocity stars or runaway stars. In all cases, they are former members of multiple star systems which were flung out of the system by a close encounter in the system (the same "gravitational slingshot (http://en.wikipedia.org/wiki/Gravity_assist)" effect by which we use planets to fling spacecraft around the solar system, but on a much grander scale). So a runaway star must first escape from the gravity of its local environment. If it does so with enough speed it can escape from the Galaxy. The hypervelocity stars HE 0437-5439 appears to be a blue straggler star ejected from the environment of the Galactic central black hole and flung in the direction of the Magellanic Clouds (Brown, et al., 2010 (http://adsabs.harvard.edu/abs/2010ApJ...719L..23B)). Most hypervelocity stars originate from the region around the Galactic central black hole. More general reviews on the topic are found, for instance, in Brown, et al., 2006 (http://adsabs.harvard.edu/abs/2006ApJ...647..303B), Brown, et al., 2007a (http://adsabs.harvard.edu/abs/2007ApJ...660..311B), Brown, et al., 2007b (http://adsabs.harvard.edu/abs/2007ApJ...671.1708B), the summary Brown, 2008 (http://adsabs.harvard.edu/abs/2008arXiv0811.0571B), and citations thereto.

But not all runaways come from the center of the Galaxy. The stars Na'ir al Saif (Iota Orionis (http://en.wikipedia.org/wiki/Iota_Orionis)), Mu Columbae (http://en.wikipedia.org/wiki/Mu_Columbae) and AE Aurigae (http://en.wikipedia.org/wiki/AE_Aurigae) are all runaway stars, apparently ejected from the crowded environment of the Trapezium region (http://en.wikipedia.org/wiki/Trapezium_(astronomy)) of the Orion Nebula Star Cluster a few million years ago.

Cougar
2010-Jul-30, 05:50 PM
The total mass of the Milky Way, including dark matter, is about 1012 solar masses. The total mass excluding dark matter is about 1011 solar masses...

This seems to contradict what I recall reading. For example:



"We find that the galaxy has a much larger mass than the sum of all the stars, dust, and other things we "see." The shortfall is not just a few percentage points, but most of the mass of our galaxy seems to have been left unaccounted." [Rocky Kolb, Blind Watchers of the Sky]

Then David Law at UCLA states in this article: (http://www.npr.org/templates/story/story.php?storyId=122290475)



"While we may not be able to see the dark matter, it's a vitally important component of our galaxy. It makes up the bulk of what the actual stuff is in our galaxy," says David Law, an astrophysicist at the University of California, Los Angeles, who says dark matter accounts for more than 70 percent of the mass in galaxies like the Milky Way.

Of course, I expect that the bulk of the dark matter associated with our galaxy resides well outside the galactic disk, and the disk is typically considered "the galaxy," so... it should probably be clarified that the OP will have to escape the galaxy and its dark matter 'halo.'

forrest noble
2010-Jul-30, 06:33 PM
lostgalaxy,


if one star wants to escape a galaxy then what are its main obstacles?

Your choice of wording seems humorous and colorful since accordingly the star would have no intent of escape or "wanting." There are a few mechanisms of escape that I can think of but they have to do with escape velocities rather than the counter forces of gravity or dark matter. I will mention a few that I can think of. The first would be a rapidly orbiting binary or ternary stellar system that seemingly could provide the velocity needed to member stars, or to a converging star to escape the galaxy via the gravitational influence of their rapid rotation rates, especially in a collapsing system. Next a newly forming stellar black hole, where the surrounding torus could contain a companion star, which might begin to orbit very rapidly as the central star collapses to a relatively tiny very rapidly spinning entity. The great orbital velocity of the torus might catapult much of this material into galactic space. Any companion Stars might gain the momentum needed to overcome the hold of the central star and afterward the galaxy. Some central galactic black holes are believed to also spin at great velocities. In such cases orbiting stars or even smaller black holes might gain the velocities needed via the velocities of materials in the torus, necessary to achieve escape velocity from the galaxy.

lostgalaxy
2010-Jul-31, 03:38 AM
Wow. Lots of things to think about. It pays off even if I appeared "humorous".

It still bugs me about dark matter. Is it possible that dark matter don't pull, but actually push?

While the central black hole and the bulk of stars pull things inwards, the outer dark matter "halo" acts as a guardian ring, it pushes things back into the galaxy.

But if a runaway star reaches the point of no return, its the dark matter itself that propels the poor [or lucky?] thing into empty space.

Oh no what have I just said? I must hyper away, now!

Nereid
2010-Jul-31, 03:55 PM
Wow. Lots of things to think about. It pays off even if I appeared "humorous".

It still bugs me about dark matter. Is it possible that dark matter don't pull, but actually push?
If it did (push, not pull) it wouldn't behave like dark matter! :)


While the central black hole and the bulk of stars pull things inwards, the outer dark matter "halo" acts as a guardian ring, it pushes things back into the galaxy.
In some respects, the use of the word "halo" is unfortunate, because it gives a misleading impression.

The CDM (cold dark matter) "halo" of our galaxy, and (apparently) almost all others, is not something like a hollow ball; instead it is like the Sun - a spherical ball, with a density which increases as you go further towards the centre (of course, the density profile is quite unlike the Sun's, but at a qualitative level it's an OK analogy).



But if a runaway star reaches the point of no return, its the dark matter itself that propels the poor [or lucky?] thing into empty space.

Oh no what have I just said? I must hyper away, now!
Questions are always welcome! :)

neilzero
2010-Jul-31, 08:15 PM
My perception is dark matter is close to irrelevant close to large mass such as the Earth, but becomes key for holding galaxies into the shape that they have. Neil

Shaula
2010-Jul-31, 08:52 PM
My perception is dark matter is close to irrelevant close to large mass such as the Earth, but becomes key for holding galaxies into the shape that they have. Neil
Be truer to say that the effect of Dark matter appears to be too small to be relevant over small scales such as in the Earth's environs, probably due to the fact that it doesn't seem to clump much on these scales. However there is so much of it and it does clump at galatic scales so then it becomes dominant. The Earth is not a large mass, cosmologically!

Tim Thompson
2010-Aug-01, 05:21 PM
The total mass of the Milky Way, including dark matter, is about 1012 solar masses. The total mass excluding dark matter is about 1011 solar masses, ...
This seems to contradict what I recall reading. For example: ...
Dark matter out-masses normal matter by about 10:1 in galaxy clusters (e.g., Zwicky, 1933 (http://adsabs.harvard.edu/abs/1933AcHPh...6..110Z); Zwicky, 1937 (http://adsabs.harvard.edu/abs/1937ApJ....86..217Z) & citations thereto). Xue, et al., 2008 (http://adsabs.harvard.edu/abs/2008ApJ...684.1143X) derive a total Milky Way virial mass of 1.0 (+0.3, -0.2)x1012 solar masses and a virial radius about 250,000 parsecs (815,000 light years), a cold baryonic mass of 6.5x1010 solar masses and a total baryonic mass fraction of 0.17 (that's 83% non baryonic dark matter and 17% baryonic matter). So my assumption that dark matter out-masses baryonic matter by the same 10:1 ratio is off the mark, the correct ration being closer to 5:1. They also estimate 40% of the baryonic matter being in stars, from the cold baryon fraction I think. But it remains the case that dark matter significantly out-masses baryonic matter. The paper already has 99 citations, so there maybe some modification of these numbers in more recent papers, but I doubt the changes will be very large.

Read, et al., 2010 (http://adsabs.harvard.edu/abs/2010AIPC.1240..391R) model the dark matter disk for the Milky Way, but they do it in terms of a non-rotating halo density, which I can't readily convert to as mass fraction. Nevertheless, while the disk might be dominated by baryonic matter, there should still be s significant dark matter disk as well (more likely spatially similar to the thick stellar disk).


Is it possible that dark matter don't pull, but actually push?
No.


... Dark matter appears to be too small to be relevant over small scales such as in the Earth's environs, probably due to the fact that it doesn't seem to clump much on these scales.
The trick is that since non-baryonic dark matter does not interact electromagnetically, it cannot cool by emitting photons, as baryonic matter does. So non-baryonic dark matter can cool only inefficiently, providing a gravitational well to attract baryonic matter, which then cools and condenses into galaxies & clusters, leaving behind a halo of dark matter. Iorio, 2010 (http://adsabs.harvard.edu/abs/2010JCAP...05..018I) argues that the observed variation of the Earth-sun distance (time variability of the astronomical unit) is consistent with theoretical expectations for dark matter accretion in the solar system. There are several studies of dark matter in the solar system, but they are all equally ambiguous. While dark matter makes itself rather obvious over large distance scales, there is no obvious sign of it over solar system distance scales.

forrest noble
2010-Aug-01, 05:34 PM
lostgalaxy,


It still bugs me about dark matter. Is it possible that dark matter don't pull, but actually push?

While the central black hole and the bulk of stars pull things inwards, the outer dark matter "halo" acts as a guardian ring, it pushes things back into the galaxy.



It seems possible to me that dark matter is a pushing of force (if it exists), but its mechanics would then be different than the general hypotheses asserts. The mainstream hypothesis asserts that dark matter is a "simple form of matter" that warps space in the same manner as other matter in accord with the perspectives of General Relativity.