<p>There are lots of reasons. First of all because we can see that a typical galaxy is a spiral galaxy and we seem to live somewhere pretty typical.</p>

<p>Secondly because we can see the arms of the spiral through close examination of the Milky Way (ie the band in the sky) - through stellar distances and measurements of hydrogen cloud red shifts and so on.</p>

<p>But you your question is a perfectly good one because, ultimately, we are guessing a bit as we cannot see the galaxy from anywhere other than one vantage point.</p>

CC BY-SA 3.0
2014-09-27T21:13:48.523Z
I hate to knock on your answer, but I disagree with your first point. How do we know spiral galaxies are "typical", and that the Milky Way is "typical"? And dust in the spiral arms blocks a good deal of light.
HDE 226868
CC BY-SA 3.0
2014-09-28T08:56:42.023Z
We obviously cannot be sure. But applying the Copernican/cosmological principle that we are perfectly ordinary is a good place to start.
adrianmcmenamin
CC BY-SA 3.0
2014-09-27T12:01:49.640Z
2014-09-27T12:01:49.640Z
adrianmcmenamin
<p>Measure the distance to a star and its direction, and plot it based on these numbers in a 3D plot. Repeat for a gazillion other stars.</p>

<p>From its appearance in the sky we can already infer that the Milky Way is rather flat; most stars (and gas clouds) can be found in a rather narrow band in the sky. That the Milky Way is a spiral galaxy is something you can't see with the naked eye (nor with a telescope), but it's something you can derive from the 3D model.<br>
The 3D model also tells us where the sun is in the Milky Way: there are far more stars in one direction than the other, and the former ones are also farther way. This way we know we're about 2/3 from the center of the galaxy.</p>

CC BY-SA 3.0
2014-09-29T17:04:57.913Z
2014-09-29T17:04:57.913Z
stevenvh
<p>Milky Way is a young galaxy, filled with neutral hydrogen along its arms.
This hydrogen can undergo state transition, a rare event, which is compensated by the huge amount of hydrogen itself, i.e., even if this transition is rare it is still strong because a large amount of atoms can exhibit it.
The basics of this transition is explained by quantum physics through state-change in the energy levels of the hydrogen atoms, and can be summarized in the following picture:</p>

<p><img src="https://i.sstatic.net/Fumwe.jpg" alt="enter image description here"></p>

<p>The electron in the atom transits from a level with parallel spin (with respect to the nuclear proton), to the lower state, where the electron spin is antiparallel.
When this happens, the atom emits a photon, known as the <a href="http://en.wikipedia.org/wiki/Hydrogen_line" rel="nofollow noreferrer">21 cm radiation</a>, or <em>hydrogen line</em>.
This line falls into the radio domain, which can be observed by radio telescopes.</p>

<p>Since this radiation comes from rotating arms, <a href="http://csep10.phys.utk.edu/astr162/lect/light/doppler.html" rel="nofollow noreferrer">doppler shift</a> of the line can trace the distance of the emitter region.</p>

<p>And voilà, you have your map.</p>

<p>EDIT: ops, I missed the previous links where the answer was already given.</p>

CC BY-SA 3.0
2014-10-01T01:26:47.377Z
Mmm, please downvoter come out, tell me all your truth.
Py-ser
CC BY-SA 3.0
2014-09-23T08:46:48.257Z
2014-09-23T08:46:48.257Z
Py-ser
<p>How can we know how does the Milky Way look like if we are in it?
Sorry if the answer is evident, I am not an expert.</p>

CC BY-SA 3.0
2014-09-13T21:27:07.940Z
Hi, Eduardo. Can you clarify a bit? Do you mean its structure (in which case http://astronomy.stackexchange.com/questions/1225/how-can-we-tell-that-the-milky-way-is-a-spiral-galaxy and http://astronomy.stackexchange.com/questions/1269/how-do-we-know-milky-way-is-a-barred-spiral-galaxy could help).
HDE 226868
CC BY-SA 3.0
2014-09-25T17:17:08.423Z
Voting to close as duplicate of the second question I listed, but I will retract that if Eduardo clarifies the question a bit.
HDE 226868
<galaxy><milky-way>
Milky way: How do we know its appearance?
Eduardo Casseres
# Milky way: How do we know its appearance?

(Asked by: username_0 on 2014-09-13)

How can we know how does the Milky Way look like if we are in it?
Sorry if the answer is evident, I am not an expert.

**Tags:** galaxy, milky-way
> **Comments:**
> **username_1**: Hi, Eduardo. Can you clarify a bit? Do you mean its structure (in which case http://astronomy.stackexchange.com/questions/1225/how-can-we-tell-that-the-milky-way-is-a-spiral-galaxy and http://astronomy.stackexchange.com/questions/1269/how-do-we-know-milky-way-is-a-barred-spiral-galaxy could help).  
> **username_1**: Voting to close as duplicate of the second question I listed, but I will retract that if Eduardo clarifies the question a bit.  

---

## 3 answers

### Answer by username_2 on 2014-09-27. Score: 1

There are lots of reasons. First of all because we can see that a typical galaxy is a spiral galaxy and we seem to live somewhere pretty typical.

Secondly because we can see the arms of the spiral through close examination of the Milky Way (ie the band in the sky) - through stellar distances and measurements of hydrogen cloud red shifts and so on.

But you your question is a perfectly good one because, ultimately, we are guessing a bit as we cannot see the galaxy from anywhere other than one vantage point.

> **Comments:**
> **username_1**: I hate to knock on your answer, but I disagree with your first point. How do we know spiral galaxies are "typical", and that the Milky Way is "typical"? And dust in the spiral arms blocks a good deal of light.  
> **username_2**: We obviously cannot be sure. But applying the Copernican/cosmological principle that we are perfectly ordinary is a good place to start.  

---

### Answer by username_3 on 2014-09-29. Score: 0

Measure the distance to a star and its direction, and plot it based on these numbers in a 3D plot. Repeat for a gazillion other stars.

From its appearance in the sky we can already infer that the Milky Way is rather flat; most stars (and gas clouds) can be found in a rather narrow band in the sky. That the Milky Way is a spiral galaxy is something you can't see with the naked eye (nor with a telescope), but it's something you can derive from the 3D model.  
The 3D model also tells us where the sun is in the Milky Way: there are far more stars in one direction than the other, and the former ones are also farther way. This way we know we're about 2/3 from the center of the galaxy.


---

### Answer by username_4 on 2014-09-23. Score: -1

Milky Way is a young galaxy, filled with neutral hydrogen along its arms.
This hydrogen can undergo state transition, a rare event, which is compensated by the huge amount of hydrogen itself, i.e., even if this transition is rare it is still strong because a large amount of atoms can exhibit it.
The basics of this transition is explained by quantum physics through state-change in the energy levels of the hydrogen atoms, and can be summarized in the following picture:

![enter image description here](https://i.sstatic.net/Fumwe.jpg)

The electron in the atom transits from a level with parallel spin (with respect to the nuclear proton), to the lower state, where the electron spin is antiparallel.
When this happens, the atom emits a photon, known as the [21 cm radiation](http://en.wikipedia.org/wiki/Hydrogen_line), or *hydrogen line*.
This line falls into the radio domain, which can be observed by radio telescopes.

Since this radiation comes from rotating arms, [doppler shift](http://csep10.phys.utk.edu/astr162/lect/light/doppler.html) of the line can trace the distance of the emitter region.

And voilà, you have your map.

EDIT: ops, I missed the previous links where the answer was already given.

> **Comments:**
> **username_4**: Mmm, please downvoter come out, tell me all your truth.  

---

