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All the stars we see are in our galaxy. Or not...?


binarymizar
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Hi informed Astronuts,

I had a bit of a debate the other day with a mate about stars, galaxies etc. I said to him that all the stars we can see in the night sky are in the Milky Way, as there are no such things as stars that just sit in space on their own outside a galaxy.

He disagrees. Who is right and who is wrong?

And another question, just from me!

We know that we are in the milky way, and that informed thinking says that the milky way probably looks similar to, say, the Andromeda nebula. In other words a bright nucleus with spirals. That being the case, why can we not see this bright nucleus if we look towards the centre of the Milky Way? Even as a faint white smudge?

To most of you these are probably inane questions :D Sorry!

Ali

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There are stars floating in between galaxies, most having been chucked out of a galaxy for insulting a black hole at an office party. They are too far away for us to see with our eyes or amateur equipment. I am not sure if any individual stars are visible in the Clouds of Magellan, the two small galaxies visible in the southern hemisphere. I don't think there are, but if you can, they are outside our galaxy.

There is too much stuff between us and the core of the Milky Way for it to be visible. The galactic centre is near Sagittarius, but it looks like just more Milky Way.

The Milky Way galaxy is actually a 'barred spiral' according to the latest measurements. It doesn't look exactly like the Andromeda galaxy, but has a bar of stars at the centre, with spiral arms coming off it.

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While expressed in his unique vernacular, WH is essentially correct. (Who's to say the black hole didn't start it?) But I'd just like to add/clarify a bit.

Individual stars are indeed visible in the Magellanic Clouds The "Jewel Box" cluster, among dozens of others, are easily resolved from southern climes. While technically "outside" our galaxy, both clouds are in the process of being devoured by the Milky Way.

The "technical" term for WH's "stuff" between us and the core of the MW is "dust". The stars at the core can be seen only at non-visual wavelengths. But while you can't see the core itself, you can see the central bulge. Compare the width of the MW in the Sagittarius region with the MW near Orion. You'll easily see it's much wider in Sagittarius. That's the central bulge, even if the core is obscured.

For your friend, you are right. At a public star party once, I told someone the same thing-that all we've looked at so far was inside our own galaxy. He seemed crestfallen! It was quite a blow to him, so I started pointing out galaxies external to ours, just for him. I started close, at ~11 million LY and moved outward to ~60 million LY. He got a big kick out of that, even though in the C8 they looked like little smudges. The idea I gave was that the light from that galaxy has travelled virtually unimpeded through space for 60 million years to land on his eyeball tonight. Works every time! 8)

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Riiiggghhhht...... Mmmm, pretty clear so far. I must admit magellanic clouds has confused the issue a bit for me. I always thought that m/c`s were a part of an existing galaxy, not a separate thing altogether. Shows how much I know! OK, here`s another one from a thickie:

A nebula is....what? An exploded star? I thought stars generally went through a stage near the end of their life of becoming a red giant, and then a white dwarf before dissappearing when everything has been burnt up. So why would it explode?

And, bringing it back to my original question, nebulas we can see are presumably all in the Milky Way?

Lor, I`d forgotten just what a completely fascinating subject The Universe is! And how little I understood how it worked...

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Hope I didn't confuse you too much. Sounds like you've got the MC relationship, though. It's similar to the M32 and M110 galaxies around Andromeda-they're separate galaxies, but gravitationally bound to the larger galaxy.

There are several types of nebulae-Reflection, Emission, Absorption and Planetary. Reflection nebulae are caused by light scattered off dust in the area of usually a bright, young star. The area around Merope in M45 for instance, or The Witch Head nebula around Rigel are reflection neb's. Emission nebs are molecular clouds of gas and dust, and the birthplace of new stars. The Eagle and Orion neb's are examples. They glow from light emitted from new stars within. Absorption nebs are regions of dark, dense dust that block light from other nebulae behind them. The Horsehead and Snake nebulae for instance. Planetary nebs are as you described-very old stars that have exhausted thier fuel and leave the white dwarf withing exposed to light up the gas in the stars atmosphere. The Ring and Dumbell nebulae show these well.

The Tarantula nebula is an emission nebula in the LMC, and some other stuff in the SMC, but just about all the nebulae you can see, at least with the naked eye, are within the MW. There are vast regions of similar types of emission nebs in other galaxies you can see with a telescope. NGC 204 in Andromeda and other such regions in M101, M33 and so on.

IHTH

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I said to him that all the stars we can see in the night sky are in the Milky Way, as there are no such things as stars that just sit in space on their own outside a galaxy.

He disagrees. Who is right and who is wrong?

Short answer: Your mate is right.

I asked this exact question on the Sky At Night site a while back, here is the reply I got back:

When galaxies interact (often described as 'colliding galaxies', although

the number of actual stellar collisions will be very low), the enormous

gravitational forces involved will cause severe tidal disruption. This will

often lead to stars in both galaxies being dragged out of their host galaxy

and ejected into Intergalactic space.

These 'loose' stars could be dragged back to their original host (if their

velocity is less than the escape velocity of the body), they might transfer

over into the other galaxy, or they might be thrown out into inter-galactic

space. So there should indeed be 'rogue stars', roaming the lonely spaces

between galaxies, as a result of violent tidal interactions.

The first detections of such 'rogue stars' (spotted ~300,000 light years

from the nearest visible galaxy) were made in HST observations of the Virgo

galaxy cluster in 1997. It was estimated that up to 10% of the cluster mass

could be in the form of such stars, stripped free of their original galaxies

by interactions and now moving under the gravitational influence of the

entire cluster of ~2,500 galaxies.

It may also be possible for stars to form from lone gas clouds drifting

between galaxies, if something (e.g. close approach to a galaxy or another

gas cloud) could provide the force needed to initiate the collapse of the

cloud and start star formation.

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TBF: That's a fairly loose translation. I don't personally have HST at my disposal to detect or observe rogue stars 300,000 LY from an interacting galaxy, and while there are indeed such stars, the question is, can I observe such stars from my back yard with my scope? The answer is, NO. The Milky Way is not interacting with any galaxies of sufficient mass or relative velocity to strip off stars and create rogues. It is absorbing the Magellanic Clouds, but the closing speed is relatively small. If you include the qualifier, "with the naked eye", the answer becomes even more obvious.

Kain: Globulars do orbit the core of the MW, but in a more or less spherical cloud, rather than aligned with the disk, like open clusters are. They are still withing the gravitational influence of the MW, so are part of it.

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So is M13 and M92 exampled of the the swarm around the core of the MW?

Thats quite cool though, to think when your looking at them your looking at something orbiting the core of the MW

I suppose in a way were orbiting the core too..

Kain

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TBF: That's a fairly loose translation. I don't personally have HST at my disposal to detect or observe rogue stars 300,000 LY from an interacting galaxy, and while there are indeed such stars, the question is, can I observe such stars from my back yard with my scope? The answer is, NO.

I was just repsonding to the bit about whether any stars exist outside of galaxies.

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I was just repsonding to the bit about whether any stars exist outside of galaxies.

I said to him that all the stars we can see in the night sky are in the Milky Way, as there are no such things as stars that just sit in space on their own outside a galaxy.

Ok, so you're both wrong, and you're both right. Depends on how you frame the question.

There are rogue stars between galaxies, but it takes extraordinary circumstances to put them there, and most people will not see them first hand.

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