<p>Update for clarity:</p>

<p>For the visible part of a standalone galaxy, the stars can all be measured to orbit that galaxy's core. So if you wanted to measure the furthest extent at this simplified level, it would be very easy. The problem is that there is a lot of mass which is not stars, and which is dark matter. Some of it is so far out, it may be impossible to calculate what it is orbiting.</p>

<p>If you read the paper, you will see that they chose a limit beyond which the effect of the negligible mass further out was not relevant to this calculation. </p>

<p>A galaxy can be thought of as a flattened sphere, as regards mass. Stars, at least for spiral galaxies, tend to lie in a plane, but there is mass orbiting the common centre in all planes. So there is no start point, there is just a need to decide how far out we want to say the galaxy extends. </p>

<p>For the Milky Way and Andromeda, a decision has been made that is the same for both (the exact decision doesn't appear to have been published for this paper, but as long as it is consistent, then the relative masses will be correct)</p>

<p>For galaxies that are colliding or near to each other it is much more difficult - do you calculate which galaxy a star belongs to by the direction of its motion? Stars may swap from one to the other. From <a href="https://en.wikipedia.org/wiki/Milky_Way">Wikipedia's Milky Way page:</a></p>

<blockquote>
  <p>Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 ly (55 kpc).[51] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way.</p>
</blockquote>

<p>tl;dr - it's a pretty arbitrary decision :-)</p>

CC BY-SA 3.0
2014-07-30T09:26:17.787Z
Thanks for the answer.When we are saying Andromeda is neighbour to our galaxy then isnt this mean that there is something that separated these 2 galaxies.Otherwise why we call these as 2 different galaxies ,we can mix it to single one
SpringLearner
CC BY-SA 3.0
2014-07-30T10:12:07.310Z
For the bulk of the visible galaxy, each star orbits the centre, but for dark matter, or other non-star mass, it can be very tricky to assess.
Rory Alsop
CC BY-SA 3.0
2014-07-30T10:16:20.657Z
so we can know the width between Andromeda and our galaxy
SpringLearner
CC BY-SA 3.0
2014-07-30T10:44:51.753Z
We know the distance between the cores - that's easy - but the diameter is a bit arbitrary. A quick google will show you reputable sources quoting 90 - 120 thousand light years for the Milky Way
Rory Alsop
CC BY-SA 3.0
2014-07-30T09:21:15.207Z
2014-07-30T10:56:29.997Z
Rory Alsop
<p>As already answered, the definition of the size of a galaxy must always to some extend be arbitrary. In astronomy, several definitions are used, according to the context in which it's used, e.g.:</p>

<ul>
<li>$R_{\mathrm{vir}}$ (the virial radius): Used when considering the galaxy's dynamics; defined by $GM_{\mathrm{vir}}/R_{\mathrm{vir}} \sim V^2$, where $G$ is the gravitational constant, $M_{\mathrm{vir}}$ is the mass inside the virial radius, and $V$ is the circular velocity (for disk galaxies) or velocity dispersion (for elliptical or irregular galaxies). For instance, if an initial density perturbation is smaller than its virial radius, it will collapse to form a galaxy. This radius is also the radius within particles will be gravitationally bound if their velocity does not exceed $V$.</li>
<li>$R_{1/2}$ (the half-light radius): The radius within which half of the total observed light is emitted. This definition is useful when comparing the luminosity of galaxies.</li>
<li>$R_{200}$: The radius within which the average density is equal to 200 times the average density of the Universe. Similarly, sometimes $R_{500}$ or $R_{1000}$ are used. These definitions make sense when discussing the mass of a galaxy, since beyond they sort of blend in to the intergalactic medium.</li>
</ul>

<p>Although these definition give different sizes, they are all of the same order of magnitude.</p>

CC BY-SA 3.0
2015-01-21T10:55:40.933Z
2015-02-18T10:06:00.320Z
2015-02-18T10:06:00.320Z
pela
<p>Here's a rough sketch of the <a href="//en.wikipedia.org/wiki/Milky_Way" rel="noreferrer">Milky Way</a> and the <a href="//en.wikipedia.org/wiki/Andromeda_Galaxy" rel="noreferrer">Andromeda galaxy</a>,<sup>*</sup> showing their approximate sizes and distance from each other to scale:</p>

<p>&nbsp; <img src="https://i.sstatic.net/mZkT8.png" alt="Sketch of the Milky Way and Andromeda galaxies, to scale"></p>

<p>What the picture (hopefully) illustrates is the incredibly vast gap of empty space &mdash; around 2.5 million light years, to be exact &mdash; between the galaxies, each of which has a diameter of only(!) around 100 thousand light years or so.</p>

<p>While both galaxies are shaped pretty much like "fuzzy whirlpools" of dust and gas (and, apparently, dark matter), with no precisely defined sharp outer edge, they are still pretty unambiguously separated.  While we may not be able to point to a specific line in space and say that "the Milky Way ends right here", it's still clear that the Milky Way is over <em>here</em>, while the Andromeda galaxy is over <em>there</em>, and there's a huge gap of pretty much absolutely nothing at all in between.</p>

<p>(Of course, there is <em>some</em> stuff even in <a href="//en.wikipedia.org/wiki/Intergalactic_space" rel="noreferrer">intergalactic space</a>, including some <em>very</em> diffuse gas, a little bit of dust and even the occasional <a href="//en.wikipedia.org/wiki/Intergalactic_star" rel="noreferrer">stray star</a>.  Still, compared to the galaxies themselves &mdash; which, from a human viewpoint, are <em>already</em> pretty full of empty space &mdash; the intergalactic medium can be pretty well described as empty.)</p>

<p><sub>*) The little specks in the picture around each major galaxy are meant to represent their smaller satellite galaxies, such as the lesser and greater <a href="//en.wikipedia.org/wiki/Magellanic_Clouds" rel="noreferrer">Magellanic clouds</a>. Their relative positions and distances, alas, are probably not very accurate.</sub></p>

CC BY-SA 3.0
2014-07-30T15:11:33.850Z
2014-07-30T15:11:33.850Z
Ilmari Karonen
<p>I was reading this <a href="http://zeenews.india.com/news/space/milky-way-is-way-smaller-than-thought_951214.html">article</a>
 and it says the following:</p>

<blockquote>
  <p>Researchers measured the mass of the Milky Way and found that our galaxy is approximately half the weight of a <strong>neighbouring galaxy</strong> known as Andromeda which has a similar structure to our own. </p>
</blockquote>

<p>So I was thinking, if we consider a galaxy neighbor to us, then our galaxy should be having an endpoint and the neighboring galaxy should be having a start point. So how do we know which is the starting and ending point of a galaxy, and how do we calculate it?</p>

CC BY-SA 3.0
2016-11-13T19:10:13.063Z
The furthest objects from the center are perhaps some heavy planets with high inertia that have been flung away from the galaxy by initial kinetic excitation from supernova followed by a sling shot from one or two large stars straight away from the furthest star formations in the galaxy.
bandybabboon
CC BY-SA 3.0
2014-07-30T10:29:14.483Z
Related question: [Why is it so hard to figure out if Voyager 1 has left the solar system?](http://space.stackexchange.com/questions/2696)
James Jenkins
<galaxy><milky-way><intergalactic-space>
Where does the Milky Way end?
SpringLearner
# Where does the Milky Way end?

(Asked by: username_0 on 2014-07-30)

I was reading this [article](http://zeenews.india.com/news/space/milky-way-is-way-smaller-than-thought_951214.html)
and it says the following:

> Researchers measured the mass of the Milky Way and found that our galaxy is approximately half the weight of a **neighbouring galaxy** known as Andromeda which has a similar structure to our own.

So I was thinking, if we consider a galaxy neighbor to us, then our galaxy should be having an endpoint and the neighboring galaxy should be having a start point. So how do we know which is the starting and ending point of a galaxy, and how do we calculate it?

**Tags:** galaxy, milky-way, intergalactic-space

> **Comments:**
> **username_2**: Related question: [Why is it so hard to figure out if Voyager 1 has left the solar system?](http://space.stackexchange.com/questions/2696)  
> **username_1**: The furthest objects from the center are perhaps some heavy planets with high inertia that have been flung away from the galaxy by initial kinetic excitation from supernova followed by a sling shot from one or two large stars straight away from the furthest star formations in the galaxy.  

---

## 3 answers

### Answer by username_5 on 2014-07-30. Score: 27 | Accepted answer

Here's a rough sketch of the [Milky Way](//en.wikipedia.org/wiki/Milky_Way) and the [Andromeda galaxy](//en.wikipedia.org/wiki/Andromeda_Galaxy),\* showing their approximate sizes and distance from each other to scale:

![Sketch of the Milky Way and Andromeda galaxies, to scale](https://i.sstatic.net/mZkT8.png)

What the picture (hopefully) illustrates is the incredibly vast gap of empty space — around 2.5 million light years, to be exact — between the galaxies, each of which has a diameter of only(!) around 100 thousand light years or so.

While both galaxies are shaped pretty much like "fuzzy whirlpools" of dust and gas (and, apparently, dark matter), with no precisely defined sharp outer edge, they are still pretty unambiguously separated. While we may not be able to point to a specific line in space and say that "the Milky Way ends right here", it's still clear that the Milky Way is over *here*, while the Andromeda galaxy is over *there*, and there's a huge gap of pretty much absolutely nothing at all in between.

(Of course, there is *some* stuff even in [intergalactic space](//en.wikipedia.org/wiki/Intergalactic_space), including some *very* diffuse gas, a little bit of dust and even the occasional [stray star](//en.wikipedia.org/wiki/Intergalactic_star). Still, compared to the galaxies themselves — which, from a human viewpoint, are *already* pretty full of empty space — the intergalactic medium can be pretty well described as empty.)

\*) The little specks in the picture around each major galaxy are meant to represent their smaller satellite galaxies, such as the lesser and greater [Magellanic clouds](//en.wikipedia.org/wiki/Magellanic_Clouds). Their relative positions and distances, alas, are probably not very accurate.


---

### Answer by username_3 on 2014-07-30. Score: 5

Update for clarity:

For the visible part of a standalone galaxy, the stars can all be measured to orbit that galaxy's core. So if you wanted to measure the furthest extent at this simplified level, it would be very easy. The problem is that there is a lot of mass which is not stars, and which is dark matter. Some of it is so far out, it may be impossible to calculate what it is orbiting.

If you read the paper, you will see that they chose a limit beyond which the effect of the negligible mass further out was not relevant to this calculation.

A galaxy can be thought of as a flattened sphere, as regards mass. Stars, at least for spiral galaxies, tend to lie in a plane, but there is mass orbiting the common centre in all planes. So there is no start point, there is just a need to decide how far out we want to say the galaxy extends.

For the Milky Way and Andromeda, a decision has been made that is the same for both (the exact decision doesn't appear to have been published for this paper, but as long as it is consistent, then the relative masses will be correct)

For galaxies that are colliding or near to each other it is much more difficult - do you calculate which galaxy a star belongs to by the direction of its motion? Stars may swap from one to the other. From [Wikipedia's Milky Way page:](https://en.wikipedia.org/wiki/Milky_Way)

> Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 ly (55 kpc).[51] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way.

tl;dr - it's a pretty arbitrary decision :-)

> **Comments:**
> **username_0**: Thanks for the answer.When we are saying Andromeda is neighbour to our galaxy then isnt this mean that there is something that separated these 2 galaxies.Otherwise why we call these as 2 different galaxies ,we can mix it to single one  
> **username_3**: For the bulk of the visible galaxy, each star orbits the centre, but for dark matter, or other non-star mass, it can be very tricky to assess.  
> **username_0**: so we can know the width between Andromeda and our galaxy  
> **username_3**: We know the distance between the cores - that's easy - but the diameter is a bit arbitrary. A quick google will show you reputable sources quoting 90 - 120 thousand light years for the Milky Way  

---

### Answer by username_4 on 2015-01-21. Score: 3

As already answered, the definition of the size of a galaxy must always to some extend be arbitrary. In astronomy, several definitions are used, according to the context in which it's used, e.g.:

* $R\_{\mathrm{vir}}$ (the virial radius): Used when considering the galaxy's dynamics; defined by $GM\_{\mathrm{vir}}/R\_{\mathrm{vir}} \sim V^2$, where $G$ is the gravitational constant, $M\_{\mathrm{vir}}$ is the mass inside the virial radius, and $V$ is the circular velocity (for disk galaxies) or velocity dispersion (for elliptical or irregular galaxies). For instance, if an initial density perturbation is smaller than its virial radius, it will collapse to form a galaxy. This radius is also the radius within particles will be gravitationally bound if their velocity does not exceed $V$.
* $R\_{1/2}$ (the half-light radius): The radius within which half of the total observed light is emitted. This definition is useful when comparing the luminosity of galaxies.
* $R\_{200}$: The radius within which the average density is equal to 200 times the average density of the Universe. Similarly, sometimes $R\_{500}$ or $R\_{1000}$ are used. These definitions make sense when discussing the mass of a galaxy, since beyond they sort of blend in to the intergalactic medium.

Although these definition give different sizes, they are all of the same order of magnitude.


---

