[Eternal Student (OP)]

Hi.

    I'm a bit lazy, so instead of using Google, I thought I'd ask for some pointers here.

How did Astronomers measure the size and general shape of the Milky Way Galaxy?

1.  Parallax will help with some close stars.  Earth orbits around the sun in 1 year and you get some parallax... I know that bit.

2.   I believe they were also using Cepheid variables to estimate some these distances... I know that bit.

However the following issues seem to confound the exercise:
1.    What if the galaxy had an outer edge that is just extremely deficient in Cepheids?   Do you just not measure the distance to that bit or just not include any of those stars in the galaxy?  Is it just that Cepheids are sufficiently common that was unlikely to happen?
2.    Where and for what reason do you decide the galaxy has ended?    If you get a better set of telescopes and start finding more Cepheids, why or how do you decide they aren't in our galaxy?    Is it just that there is a clear border, some big void where stars aren't found and it's just convenient to say the galaxy ends there?
3.    Dark matter is the new confounding problem.   There seems to be some in a halo around our galaxy but, by definition, you can't see it.  Annoyingly it seems to extend a bit further out beyond the visible stars in our galaxy.  None the less it's presumably important for contributing to the how and why this galaxy formed originally.  It's also presumably gravitationally bound by the mass in this galaxy and stays with it like most of the other things that we decide are in this galaxy system.   We probably should include some of that dark matter halo as being something that is in this galaxy rather than some other galaxy.  How do we now extend the definition or size estimates for the galaxy to include some of that dark matter?

    That's it....  so another way of stating the question would be something like this:   How is the border or edge of a galaxy decided upon?  How do you know a galaxy has ended?  For what reason, both historically and with the benefit of our modern understanding, would you say a galaxy ends here -->      rather then     here --------->     ?

I'd be grateful for any answers or references.
Best Wishes.

[Halc]

In the spirit of your OP, I am answering without any consultation of google or other resources. This is just my thoughts, so no references.

1.  Parallax will help with some close stars.  Earth orbits around the sun in 1 year and you get some parallax... I know that bit.

2.   I believe they were also using Cepheid variables to estimate some these distances... I know that bit.

These are good for distances to individual bits, which I think is sort beside the point that you're asking.

Is it just that Cepheids are sufficiently common that was unlikely to happen?

If you really want to use this method, I suppose you could look at something like Andromeda, figure we're sort of similar in distribution, see how far out its Cepheids go in relation to its designated 'size' and figure a similar rule for us.

2.    Where and for what reason do you decide the galaxy has ended?

That's a better question. They just sort of fade away, and hardly end abruptly. Similar question can be asked of the size of the solar system.

I would think a better measure would be where obvious orbits end. They've measured velocity of a lot of stars and from this you get an idea of the hill curve of the galaxy, so they might just draw a line where the orbital speed becomes indistinct (might not result in an actual orbit, or isn't reflective of the average motion of all the material at that distance). At some point, things are not gravitationally bound to the galaxy. How about the small orbiting galaxies? They're bound to us (for now), but are they then part of the galaxy or are they their own things? This seems an arbitrary human designation, much like most nouns.

3.    Dark matter is the new confounding problem.   There seems to be some in a halo around our galaxy but, by definition, you can't see it.

But they map where it is by that rotation curve, and this differs from one galaxy to the next. Fair chance that we lose a bunch of our collective dark matter when we collide with Andromeda. Friction will slow much of the colliding gas clouds but the dark matter won't slow at all. Some galaxies that do direct hits manage to lost almost all their dark matter that way.

Annoyingly it seems to extend a bit further out beyond the visible stars in our galaxy.

It extends forever. It's not like there's no dark matter between the galaxies. But the stuff that isn't bound to any particular gravity well doesn't really count, does it?

That's it....  so another way of stating the question would be something like this:   How is the border or edge of a galaxy decided upon?  How do you know a galaxy has ended?

That's like asking how much of you is the mass of you, and how much is just the stuff you ingested. What's your unladen weight? When does food become part of you? There's no obvious answer. It's just a matter of human language designation, sort of a vote I guess.

At exactly what point will Andromeda and the Milky Way cease being separate galaxies?

[evan_au]

Parallax will help with some close stars

Parallax got a whole lot better in the past decade - the Gaia spacecraft (launched in 2014) took it to a whole new level, even measuring the distance to some individual stars in the dwarf galaxies orbiting the Milky Way galaxy.
They are aiming to measure the distance and velocity of over a billion stars.
https://en.wikipedia.org/wiki/Gaia_(spacecraft)#Significant_results

Where and for what reason do you decide the galaxy has ended?

The stars just get thinner and thinner.
One of the problems for our galaxy is that we are looking out through the plane of our galaxy which is filled with stars and dust, and trying to spot rare distant stars in a field that is full of near stars.
Even for distant galaxies seen side-on, this is a difficult problem, because the light from sparse stars (the "surface brightness") is less intense than the glow from Earth's atmosphere. It is still a problem with space telescopes, because thermal noise in the detectors provides a lower limit to the intensity of light that can be detected.

For those galaxies that have an active central black hole, the relativistic jets often project out farther than the visible size of the galaxy.
https://en.wikipedia.org/wiki/Astrophysical_jet#Relativistic_jets

the small orbiting galaxies...they're bound to us (for now), but are they then part of the galaxy or are they their own things?

Gaia has also identified streams of stars that are definitely part of our galaxy (they are within the plane of the galaxy), but their common orbit around the center of the galaxy suggests they are remnants of previous dwarf galaxies that have physically joined the Milky Way, but are still distinguishable by their velocity.

[Eternal Student (OP)]

Hi again.
   Well, instead of being completely lazy, I've started to try and look for answers.   Just thought I should mention that before anyone spends too long here.

   @Halc ,  you're too fast, you do your job too well.   However I'm very grateful for your time and effort.  I've gone through what you've said and it does seem reasonable, thanks.

If anyone else was ever considering the same issue then this reference seemed pretty good to me:
http://www.astronomycast.com/2008/07/ep-99-the-milky-way/#transcript.
The podcast itself will take a half-hour but the narrators are pleasant, you can read the transcript in less time, of course.   It was made in 2008 but the age hardly matters since it still serves to show the historical development.

ES asked:   Is it just that Cepheids are sufficiently common that was unlikely to happen?

Halc replied with a good idea for seeing how Cepheids are actually distributed.
   It seems that Cepheids aren't ubiquitous within our own galaxy, in particular many of the globular clusters of stars above and below us (Earth) in the Milky way have few, if any, Cepheids.   Trying to find the distance to those globular clusters was just hard work and the application of lots of different methods (like parallax and then making best guesses).
    According to the information from the link presented above, determining the shape and size of the Milky Way was a long journey and many features are still uncertain.  It wasn't elegant, there was no easy method, it involved just getting some rough ideas and then taking the time to look at absolutely everything you could see in the sky with a telescope (especially infrared and radio wavelengths) and continually revising those best guesses.
    There were plenty of mistakes made along the way:  The Milky way was once thought to have 4 spiral arms but now we are fairly sure there are only 2 arms, for example.

(About where galaxies are said to end)   They just sort of fade away, and hardly end abruptly. Similar question can be asked of the size of the solar system.

    That's what I would have guessed and seems to be supported from what info. I've seen.

How about the small orbiting galaxies? They're bound to us (for now), but are they then part of the galaxy or are they their own things? This seems an arbitrary human designation, much like most nouns.

    Yes, all seems absolutely correct.   Some dwarf galaxies are currently being "eaten" by the Milky Way and these tidal streams of matter falling into the Milky way make the estimate of the size of the Milky Way very imprecise.   You can include the tidal streams if you want to.

So where does the Milky Way galaxy (or any other galaxy) end?

It's just a matter of human language designation, sort of a vote I guess.

    Yes, there are some vague possibilities based on stuff you (Halc) mentioned like identifying who or what is gravitationally bound to who or what.
    Here's how Wikipedia define a galaxy:
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.

  If you take that as the absolute, literal, defining feature that makes something a galaxy and watch the path of everything for long enough (astronomical time) then quite possibly there is just one super-big galaxy in our region which will be the merger of everything in the local group.
    I suspect the separation of space into distinct units we call galaxies is just for convenience and based on human bias.   On short time scales, there seems to be some orbiting and shapes of star distribution over there that is vaguely like the thing over here, with large voids between the two places, so we just declare the two things to be different galaxies.
    I've used the term "galaxy" many times but never really stopped to think how it is properly defined or how the early Astronomers came to decide where to chop the universe up into different galaxies.

Best Wishes.


LATE EDITING:   Evan_au has also written some good stuff before I finished this and posted it.    Thanks Evan.  @Halc has been mentioned several times already.   I'm just going to give @evan_au  the "best answer" award because I haven't commented enough on what they've written and said thanks.

[Halc]

There were plenty of mistakes made along the way:  The Milky way was once thought to have 4 spiral arms but now we are fairly sure there are only 2 arms, for example.

Well there's an awful lot of names for just two arms.
The main ones seem to be Scutum-Crux to our inside and Perseus to our outside, but between those are Carina-Sagittarius and Norma/Outer. Then there's the minor spur arms like the Orion-Cygnus arm that we're currently in, and probably more that are really hard to see from our vantage.

Here's how Wikipedia define a galaxy:
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.
  If you take that as the absolute, literal, defining feature that makes something a galaxy and watch the path of everything for long enough (astronomical time) then quite possibly there is just one super-big galaxy in our region which will be the merger of everything in the local group.

That's right. The local group is just one galaxy by that definition, and we're not part of say Virgo only because of dark energy.

I find the definition not particularly useful. A galaxy is a sort of self contained spirally shaped thingy (assuming it has reasonable angular momentum), and if there's two bound objects each with its own spirally shape, we call them two galaxies, not one. Where they end is another story, and not an important one. It's not like the dynamics changes beyond some definable boundary. Most galaxies have a size that is specified to barely one significant digit. It just isn't important beyond that. You can say that one is a bit bigger than another without having to take an exact size measurement. Andromeda has a diameter of around 220 KLY (more than 1 sig digit!), over twice our size. It has a central mass that's an order of magnitude more massive than Sgr-A, and yet we're not sure if the galaxy masses more than ours or not. It was long thought so, but evidence to the contrary is turning up.

I suspect the separation of space into distinct units we call galaxies is just for convenience and based on human bias.

As I suggested above, the same could be argued for a great percentage of nouns.

I'm just going to give evan_au the "best answer" award because ...

Hah! I'm a moderator and can move that designation to my post. Wait, so is Evan. I can see a tug-of-war brewing....