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tbartman
2007-Feb-07, 02:43 AM
I know a tad about astronomy, and listen to AstronomyCast dutifully, but my 9-year-old has a tendency to ask me brilliant questions. This one stumped me: are there any stars that aren't part of a galaxy?

I would think that as matter condensed after the Big Bang, that there could have been cases where a star was formed but wasn't close enough to the gravitational field of a forming galaxy. Another thing I could imagine is if two galaxies pass close enough to each other, one could "rip" some stars at the outer edge of the other off of their trajectory around their galaxy and send them flying into open space.

Can anyone confirm or refute my fantastic ideas?

Thanks,

Tom Bartman

Nowhere Man
2007-Feb-07, 03:09 AM
Golliding galaxies is a good example of stars being ripped from their galaxy. Look for images of "The Antennae" (NGC 4038 & 4039) and "The Mice" (NGC 4676), two pairs of interacting galaxies.

Another possibility is two or more stars interacting in such a way that one of them is gravitationally thrown out of its galaxy. I believe some such have been detected in the Milky Way.

Fred

triclon
2007-Feb-07, 03:10 AM
Globular clusters (dense clusters of very old stars) exist within the halos of galaxies, which is outside of the galaxy proper.

You are also right about galaxies colliding. When they collide, stars can be thrown out of the galaxies into intergalactic space. http://csep10.phys.utk.edu/astr162/lect/galaxies/colliding.html shows several examples of this.

peter eldergill
2007-Feb-07, 03:51 AM
But wouldn't they still be bound to the galaxy? As in a VERY long orbit?

Pete

Tim Thompson
2007-Feb-07, 05:26 AM
I would think that as matter condensed after the Big Bang, that there could have been cases where a star was formed but wasn't close enough to the gravitational field of a forming galaxy.
I think this is highly unlikely, though I can't say it is impossible. Galaxies are really big things, even the dwarf galaxies, and their gravitational fields are hard to avoid.


Another thing I could imagine is if two galaxies pass close enough to each other, one could "rip" some stars at the outer edge of the other off of their trajectory around their galaxy and send them flying into open space.
But this one is a winner. Colliding & interacting galaxies definitely create large populations of intergalactic stars (i.e., Sun, et al., 2005 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005ApJ...630L.133S&db_key=AST&d ata_type=HTML&format=&high=4366fa465117079), Zibetti, et al., 2005 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.358..949Z&db_key=AST&d ata_type=HTML&format=&high=4366fa465117079)). There may be cases of stars ejected from a galaxy by close encounters with other stars. There are a number of these "runaway stars (http://en.wikipedia.org/wiki/Runaway_star)" in the Milky Way, but it is not clear if they actually make escape velocity, or just wind up in the halo of the galaxy.

And for globular clusters ...

But wouldn't they still be bound to the galaxy? As in a VERY long orbit?
Maybe, maybe not. Globular cluster NGC2419 (http://en.wikipedia.org/wiki/NGC_2419) is about 300,000 light years from the center of the Milky Way. Although it has been dubbed "The Intergalactic Wanderer", it most likely resides in the "soft" gravitational field of the Milky Way, where it could be stripped away by local group tidal forces.

ATKINS
2007-Feb-07, 10:47 PM
Globular clusters (dense clusters of very old stars) exist within the halos of galaxies, which is outside of the galaxy proper.

But are globular clusters subject to the same gravitational forces which are supposed to keep the "galaxy proper" together? If so, how do these forces work in the case of the globular cluster?

Nereid
2007-Feb-08, 02:09 AM
But are globular clusters subject to the same gravitational forces which are supposed to keep the "galaxy proper" together?Yes.
If so, how do these forces work in the case of the globular cluster?The globular cluster (GC) acts, for the most part, as a single, (gravitationally) bound object. You can imagine replacing the tens of thousands to millions of stars in a GC with a single, point mass, at the GC's centre of mass. This point mass then moves about in the (parent) galaxy's gravitational well.

Of course, GC's are not point masses; as they orbit their parent, stars can be tidally stripped off, creating tidal streams, such as that discovered by SDSS associated with Pal 5 (http://www.sdss.org/news/releases/20020603.pal5.html).

ATKINS
2007-Feb-08, 10:54 AM
Originally Posted by ATKINS
But are globular clusters subject to the same gravitational forces which are supposed to keep the "galaxy proper" together?

If so, how do these forces work in the case of the globular cluster?
Yes.The globular cluster (GC) acts, for the most part, as a single, (gravitationally) bound object. You can imagine replacing the tens of thousands to millions of stars in a GC with a single, point mass, at the GC's centre of mass. This point mass then moves about in the (parent) galaxy's gravitational well.


Sorry, my question wasn't clear enough. I know the theory of how the stars in the globular cluster act as a single, (gravitationally) bound object. My question was specifically about how the clusters move with respect to the galaxy as a whole. Your remark that "This point mass then moves about in the (parent) galaxy's gravitational well" is perhaps a little too succinct.

My question pehaps should have been: what is the centre of gravity for each globular cluster's orbit? As I understand it, it has to be the galactic centre. But in that case, at each half revolution (lasting hundreds of millions of years), a given globular cluster necessarily passes through the galactic disc such that it will then be observed below the galactic plane if it was originally above it and vice versa.

Is that correct?

Nereid
2007-Feb-08, 01:42 PM
Sorry, my question wasn't clear enough. I know the theory of how the stars in the globular cluster act as a single, (gravitationally) bound object. My question was specifically about how the clusters move with respect to the galaxy as a whole. Your remark that "This point mass then moves about in the (parent) galaxy's gravitational well" is perhaps a little too succinct.

My question pehaps should have been: what is the centre of gravity for each globular cluster's orbit? As I understand it, it has to be the galactic centre. But in that case, at each half revolution (lasting hundreds of millions of years), a given globular cluster necessarily passes through the galactic disc such that it will then be observed below the galactic plane if it was originally above it and vice versa.

Is that correct?More or less.

Some MW GCs' proper motions have been estimated (via observation) with sufficient accuracy to allow their (recent) orbits to be calculated; the same applies to some of the nearer MW satellite galaxies.

These motions, and the estimated orbits, are very useful for getting a handle on the Milky Way's gravitational well, which in turn helps constrain the distribution of dark matter.

To a first approximation, GCs and satellite galaxies which are gravitationally bound orbit the centre of mass of the MW.

GCs do, periodically, pass through the disk of the Milky Way; in fact, IIRC, at least one can be seen doing just that now.

trinitree88
2007-Feb-09, 12:45 PM
I know a tad about astronomy, and listen to AstronomyCast dutifully, but my 9-year-old has a tendency to ask me brilliant questions. This one stumped me: are there any stars that aren't part of a galaxy?

I would think that as matter condensed after the Big Bang, that there could have been cases where a star was formed but wasn't close enough to the gravitational field of a forming galaxy. Another thing I could imagine is if two galaxies pass close enough to each other, one could "rip" some stars at the outer edge of the other off of their trajectory around their galaxy and send them flying into open space.

Can anyone confirm or refute my fantastic ideas?

Thanks,

Tom Bartman

Tom. One of the types of stars that galaxies contain is neutron stars formed in supernovae. When birthed they can achieve velocities well over 1000 km/sec, though most of them tend to be in the vicinity of ~250 km/sec or less.
A neutron star can escape from a galaxy and become free roaming in space if it's velocity exceeds escape veocity. That's calculated by Vesc = (2GM/r)1/2....where G is the universal gravitational constant...~6.67 X 10-11 N.m2/ (kg)2

and M is the Mass of the object you are escaping (galaxy), and r is your distance from the center of mass of that object. For the Milky Way, it runs ~ 500 km/sec. So if Sirius at a distance of ~ 8 light years goes supernova, we could watch it escape from the Milky Way over the next 200,000,000 years.

Most neutron stars circle like a cloud of bees into the galactic halo, then down through the disk to the other side of the halo, until their final demise.

So, with regards your nine year old's question: Yes there are stars not bound to galaxies.
P.S. People who think that these high gravitational redshift objects cannot be ejected from galaxies close to us (low redshift galaxies) are not doing the numbers from the data.....and there are a lot of them.;) Pete.

see:http://arxiv.org/abs/astro-ph/0611671

Nereid
2007-Feb-09, 01:02 PM
This 2MASS page (http://www.ipac.caltech.edu/2mass/gallery/images_globs.html) has images of some of the GCs seen in the MW plane, including some only recently discovered ...

Kaptain K
2007-Feb-09, 04:38 PM
P.S. People who think that these high gravitational redshift objects cannot be ejected from galaxies close to us (low redshift galaxies) are not doing the numbers from the data...
OK, where are the high blue shift objects? Otherwise, you are postulating that every single one of these objects is ejected away from us!

neilzero
2007-Feb-11, 04:43 AM
Not bound to the galaxy seems to have different meanings for different experts. A star moving at greater than the escape velocity of the galaxy could be considered not bound, even though it will be millions of years before it reaches the halo of that galaxy. A star well outside any galaxy could be considered bound to a galaxy it will enter a billion = 10E9 years from now. I suspect there are trillions (perhaps as much as 0.0001%) of not bound stars, but they will become compact stars before they travel even one galaxy radius, unless they are class M stars which stay on main sequence up to a trillion = 10E12 years. Neil

trinitree88
2007-Feb-12, 03:07 PM
OK, where are the high blue shift objects? Otherwise, you are postulating that every single one of these objects is ejected away from us!

Kaptain K. Cougar posted Kip Thorne's point that a neutron star with ~ twice the Chandrasekhar circumference can have an ~ 40 % redshift of it's light frequencies. For the sake of argument I'll make it monochromatic.
Since no pulsar has been observed yet with a higher transverse velocity than in the high 2000's of km/sec (3000 km/sec = ~ 1% the speed of light), then even if such a pulsar were emitting and simultaneously traveling towards us from the Large Cloud (our nearest neighboring galaxy)...the Doppler shift of said pulsars radiation would be ~ 1% max due to kinematic effects....woefully short of out-compensating an ~40% gravitational redshift. All those babies will be net redshifted, unless you are viewing them from the surface of an identical neutron star (which is impossible). So I'd say your argument that there ought to be some blue-shifted ones is specious . Pete

Nereid
2007-Feb-12, 03:16 PM
Kaptain K. Cougar posted Kip Thorne's point that a neutron star with ~ twice the Chandrasekhar circumference can have an ~ 40 % redshift of it's light frequencies. For the sake of argument I'll make it monochromatic.
Since no pulsar has been observed yet with a higher transverse velocity than in the high 2000's of km/sec (3000 km/sec = ~ 1% the speed of light), then even if such a pulsar were emitting and simultaneously traveling towards us from the Large Cloud (our nearest neighboring galaxy)...the Doppler shift of said pulsars radiation would be ~ 1% max due to kinematic effects....woefully short of out-compensating an ~40% gravitational redshift. All those babies will be net redshifted, unless you are viewing them from the surface of an identical neutron star (which is impossible). So I'd say your argument that there ought to be some blue-shifted ones is specious . PeteThis of course assumes that the lines we use for measuring redshift originate on the surface of the neutron star.

Do they?

trinitree88
2007-Feb-12, 06:06 PM
This of course assumes that the lines we use for measuring redshift originate on the surface of the neutron star.

Do they?

That's a good question Nereid. Some atoms might be emitting fairly far removed from the surface of a neutron star. But, the argument in the sixties against the gravitational redshift of light was that a compact massive object would soon deplete it's reservoir of gas (it was mostly hydrogen spectral lines being argued about)...and would soon disappear. That wasn't seen to happen, so the concensus was that therefore it wasn't a compact object with a gas cloud in close proximity that was emitting. Hubble ruled.
Now we know that compact massive objects can be found traveling with substantial transverse velocities. We also know that gas clouds can be light years in length. So a neutron star booking it along through an extended predominantly hydrogen gas cloud can spend decades emitting redshifted spectra. Then it can run out of cloud and virtually disappear as it's luminosity wanes.
As far as I know there is nothing to prevent the gas from accreting on the surface of a pulsar, as this is the accepted mechanism by which white dwarfs in a binary system with a red giant successfully accumulate their hydrogen outer layers which they eventually detonate in a type 1a supernova...No?
So proposing that the gas emits highly redshifted light in close proximity to the surface of a compact massive object...a la Kip Thorne (referred to by Cougar) is a small extension of a pretty mainstream concept. Pete

closetgeek
2007-Feb-16, 03:06 PM
You have to love kids for their questions. I don't know if you tried to look the answers up yourself, sometimes finding the correct phrase can be difficult. However, a great way to give them a nudge in the right direction, plus, it makes for great quality time is to answer, "you know what? I don't know, let's trying and find out." I suggest using a child safe search engine like AOL's KOL-research and learn because sometimes the hits can be horrifying. My three have a running "stump mommy" contest going. It makes them think more because it is flattering when they can ask an adult a question that the supposed "all-knowing" figure can't answer. Sometimes their "what if's" can be mindnumbing, but sometimes I say I don't know, even when I do, just to see how they search. In a lot of the cases, they ask questions that would never even dawn on me and I have expanded my knowledge as a result.


I know a tad about astronomy, and listen to AstronomyCast dutifully, but my 9-year-old has a tendency to ask me brilliant questions. This one stumped me: are there any stars that aren't part of a galaxy?

I would think that as matter condensed after the Big Bang, that there could have been cases where a star was formed but wasn't close enough to the gravitational field of a forming galaxy. Another thing I could imagine is if two galaxies pass close enough to each other, one could "rip" some stars at the outer edge of the other off of their trajectory around their galaxy and send them flying into open space.

Can anyone confirm or refute my fantastic ideas?

Thanks,

Tom Bartman