<blockquote>
<p>The exact form of the Milky Way was figured out the in early fifties of the 20-th century.</p>
</blockquote>
<p>The <strong>exact</strong> form of the Milky Way was not figured out in the early 1950s. It remains a matter of debate. Astronomers agree that the Milky Way is a barred spiral galaxy. How many arms? That's still debated.</p>
<p>Stars in the Andromeda Galaxy were not visible to humans until recently, about 100 years ago. The <a href="https://apod.nasa.gov/debate/debate20.html" rel="noreferrer">Shapley–Curtis Debate</a> occurred 101 years ago, in April 1920. Up until then, the predominant theory was that the Milky Way was the universe. This debate marked the start of modern astronomy. It wasn't until the late 1920s when Hubble's work on Cepheid variable stars convinced most (but certainly not all) astronomers that the Andromeda Galaxy was indeed well removed from the Milky Way.</p>
<p>It wasn't until the 1950s that radio astronomy became well-used. Infrared astronomy was even later. Radio astronomy and infrared astronomy were what enabled astronomers to see beyond the dust clouds that obscure much of the Milky Way in the visible portion of the spectrum.</p>
<p>It wasn't until the 1970s that distance estimates were good enough to yield a partial picture of the Milky Way. Distance remains a challenge, and without distance, there is no way to develop a picture of the galaxy. Quoting from <a href="https://iopscience.iop.org/article/10.1088/0004-637X/700/1/137" rel="noreferrer">Trigonometric parallaxes of massive star-forming regions. VI. Galactic structure, fundamental parameters, and noncircular motions</a>, which was published in 2009,</p>
<blockquote>
<p>The Milky Way is known to possess spiral structure. However, revealing the nature of this structure has proven to be elusive for decades. The Georgelin &amp; Georgelin (1976) study of H ii regions produced what has been generally considered the &quot;standard model&quot; for the spiral structure of the Galaxy. However, after decades of study there is little agreement on this structure. Indeed, we do not really know the number of spiral arms (Simonson 1976; Cohen et al. 1980; Bash 1981; Vallée 1995; Drimmel 2000; Russeil 2003) or how tightly wound is their pattern. The primary reason for the difficulty is the lack of accurate distance measurements throughout the Galaxy. Photometric distances are prone to calibration problems, which become especially severe when looking through copious dust to distant objects in the plane of the Galaxy. Thus, most attempts to map the Galaxy rely on radio frequency observations and kinematic distances, which involve matching source Doppler shifts with those expected from a model of Galactic rotation. However, because of distance ambiguities in the first and fourth quadrants (where most of the spiral arms are found) and the existence of sizeable noncircular motions, kinematic distances can be highly uncertain (Burton &amp; Bania 1974; Liszt &amp; Burton 1981; Gómez 2006).</p>
</blockquote>
<p>Astronomers did not know the exact form of the Milky Way in the early 1950s. They did not know the exact form of the Milky Way in 2009. They still do not know the exact form of the Milky Way to this day as there is a 20° gap on the far side of the galaxy that is obscured by the center of the galaxy.</p>

CC BY-SA 4.0
2021-06-24T15:45:35.307Z
+1 great answer for not accepting the false premise of the question and giving historical evidence!
Daddy Kropotkin
CC BY-SA 4.0
2021-06-24T16:11:10.973Z
Great to read that it isnt even known today (the number of arms). Its ineed difficult to know if you are fixed and your vision is obscured on top of that!
Deschele Schilder
CC BY-SA 4.0
2021-06-24T14:54:15.883Z
2021-06-24T14:54:15.883Z
David Hammen
<p>The question sort of makes sense, although with inaccurate historical information. It is based on a premise that since we are located inside the Milky Way, it is hard to figure out the exact form of it. Many years ago during my visit to an observatory in northern Virginia, one of the staff working there told us something related to this question. The original statement I have already forgotten, but it is something like the following: since we are located inside it, we are unable to see how it looks like.</p>
<p>If you look at the image of UGC 12158 galaxy taken by Hubble, you can see the full distant view of it, showing a barred spiral galaxy. The fact that Hubble can take the image of UGC 12158 is because Hubble is located outside of and far from that galaxy. The UGC 12158 is called the Milky Way's (almost) identical twin (<a href="https://www.discovermagazine.com/the-sciences/the-milky-ways-almost-identical-twin#more-25551" rel="nofollow noreferrer">https://www.discovermagazine.com/the-sciences/the-milky-ways-almost-identical-twin#more-25551</a>), which is inspirational for scientists trying to figure out the exact form of Milky Way. Imagine aliens living on a &quot;sister earth&quot; inside the UGC 12158 galaxy take the image of our Milky Way and use it as an inspiration trying to figure out the exact form of their own galaxy.</p>
<p>As we are located inside the gigantic Milky Way, we cannot launch a telescope to fly out of the Milky Way to take its picture and send the data back, and therefore have to rely on many observations and models. One observation can be our view of the hazy band of white light in the night sky. Other various advanced measurements including optical, radio, infrared, etc. are the further extensions of our eyes. Then scientists try to piece the information together and apply theoretical models to figure out the possible form of Milky Way galaxy. You see the logic here, as we are inside it we cannot see its entire form, and therefore have to rely on some observations and theoretical models to figure it out.</p>
<p>Over the centuries our knowledge of the Milky Way has changed considerably. The image of flat spiral in school textbooks is an illustration based on scientists' current knowledge. It is absolutely not the exact form of Milky Way. Scientists are still making new discoveries on the form of Milky Way. For instance, in a recent Science article (<a href="https://www.science.org/doi/10.1126/science.aau3181" rel="nofollow noreferrer">https://www.science.org/doi/10.1126/science.aau3181</a>), scientists built a 3D model to demonstrate that the Milky Way is not even flat but warped disc. See the penultimate video for the 3D structure of the Milky Way at <a href="https://phys.org/news/2019-08-d-milky-galaxy-cepheids.html" rel="nofollow noreferrer">https://phys.org/news/2019-08-d-milky-galaxy-cepheids.html</a>.</p>
<p>In conclusion, it is hard to figure out the form of the Milky Way, partly because of our location inside it, and partly because of the bottleneck of data and models (see <a href="https://sedigism.mpifr-bonn.mpg.de/workshop_talks/A_Pettitt_SEDIGISM_Workshop.pdf" rel="nofollow noreferrer">https://sedigism.mpifr-bonn.mpg.de/workshop_talks/A_Pettitt_SEDIGISM_Workshop.pdf</a>). The word &quot;exact&quot; makes it even more challenging.</p>

CC BY-SA 4.0
2022-01-02T04:36:32.907Z
2022-01-04T05:34:08.227Z
2022-01-04T05:34:08.227Z
Josiah_H
<p>The exact form of the Milky Way was figured out the in early fifties of the 20-th century. Why did it take so long to see?</p>
<p>Is it because we are in the &quot;middle&quot; of it? In general, it's easier to find out the shape of something if you place yourself at a distance from it. Galactic forms were discovered long before the shape of our own was discovered (it can't be observed directly, obviously). Have these shapes helped to find its shape (how big it is seems no problem to see, or is it?)? The fact of being inside seems an obstacle (while it facilitates other stuff). How was this obstacle removed?</p>

CC BY-SA 4.0
2021-06-23T17:09:30.230Z
These questions might be better suited for History of S&M?
Daddy Kropotkin
CC BY-SA 4.0
2021-06-23T17:14:37.623Z
@DaddyKropotkin I think you might be right. I asked a similar question once on the physicd site (why it took so long to build an atomic bomb). It indeed ended up on the hos site. Not that I wished it could be built faster but it could have changed history, like many things
Deschele Schilder
CC BY-SA 4.0
2021-06-23T21:48:36.330Z
It took a while for it to sink in that there was more than one galaxy. People thought that the spiral nebulae were inside the Milky Way. When I was a kid, it was still quite common for books to use that terminology, eg they'd talk about the Andromeda nebula, rather than the Andromeda galaxy.
PM 2Ring
CC BY-SA 4.0
2021-06-23T21:58:41.933Z
@PM2Ring Did people think it was inside the Milky Way? I never thought about that... Even though stars were visible inside it?
Deschele Schilder
CC BY-SA 4.0
2021-06-23T22:39:15.553Z
That's what I said. ;) The very idea that the Milky Way wasn't the whole universe was novel. But once it was realised how far away the "spiral nebulae" actually were, and how large they must be, people started calling the Milky Way "our island universe".
PM 2Ring
CC BY-SA 4.0
2021-06-23T22:40:22.993Z
It's not easy to see that there are stars inside a "spiral nebula". You need a very good telescope. But that's not really an issue. There are plenty of star clusters inside the Milky Way: globular clusters, and open clusters (eg, the Pleiades).
PM 2Ring
CC BY-SA 4.0
2021-06-23T22:49:43.873Z
@PM2Ring I dont  know why but I pictured you much younger... Strange how a universe can change with time. Though I dont think the future has any possible change left.
Deschele Schilder
CC BY-SA 4.0
2021-06-24T03:19:26.777Z
Stars in the Andromeda Galaxy were not visible to humans until recently, about 100 years ago. The [Shapley–Curtis Debate](https://apod.nasa.gov/debate/debate20.html) occurred 101 years ago. Up until then, the predominant theory was that the Milky Way was the universe. This debate marked the start of modern astronomy. It wasn't until the late 1920s when Hubble's work on  Cepheid variable stars convinced most (but certainly not all) astronomers that the Andromeda Galaxy was indeed well removed from the Milky Way.
David Hammen
CC BY-SA 4.0
2021-06-25T17:25:31.943Z
@Deschele Schilder See my answer at:  https://hsm.stackexchange.com/questions/13303/why-did-it-take-so-long-to-discover-the-shape-of-the-milky-way/13310#13310
M. A. Golding
CC BY-SA 4.0
2021-06-25T17:26:21.210Z
There is an answer to a similar question at: https://hsm.stackexchange.com/questions/13303/why-did-it-take-so-long-to-discover-the-shape-of-the-milky-way/13310#13310
M. A. Golding
CC BY-SA 4.0
2021-06-25T17:34:03.960Z
@M.A.Golding It is the same question. Ii asked on that site too. :)
Deschele Schilder
CC BY-SA 4.0
2022-01-02T14:17:28.073Z
The question should be rephrased "Why *is* it so difficult to figure out the form of the Milky Way?" We're not 100% certain about it.
Zac67
<galaxy><milky-way>
Why was it so difficult to figure out the form of the Milky Way?
Deschele Schilder
# Why was it so difficult to figure out the form of the Milky Way?

(Asked by: username_0 on 2021-06-23)

The exact form of the Milky Way was figured out the in early fifties of the 20-th century. Why did it take so long to see?

Is it because we are in the "middle" of it? In general, it's easier to find out the shape of something if you place yourself at a distance from it. Galactic forms were discovered long before the shape of our own was discovered (it can't be observed directly, obviously). Have these shapes helped to find its shape (how big it is seems no problem to see, or is it?)? The fact of being inside seems an obstacle (while it facilitates other stuff). How was this obstacle removed?

**Tags:** galaxy, milky-way

> **Comments:**
> **username_1**: These questions might be better suited for History of S&M?  
> **username_0**: @username_1 I think you might be right. I asked a similar question once on the physicd site (why it took so long to build an atomic bomb). It indeed ended up on the hos site. Not that I wished it could be built faster but it could have changed history, like many things  
> **username_2**: It took a while for it to sink in that there was more than one galaxy. People thought that the spiral nebulae were inside the Milky Way. When I was a kid, it was still quite common for books to use that terminology, eg they'd talk about the Andromeda nebula, rather than the Andromeda galaxy.  
> **username_0**: @username_2 Did people think it was inside the Milky Way? I never thought about that... Even though stars were visible inside it?  
> **username_2**: That's what I said. ;) The very idea that the Milky Way wasn't the whole universe was novel. But once it was realised how far away the "spiral nebulae" actually were, and how large they must be, people started calling the Milky Way "our island universe".  
> **username_2**: It's not easy to see that there are stars inside a "spiral nebula". You need a very good telescope. But that's not really an issue. There are plenty of star clusters inside the Milky Way: globular clusters, and open clusters (eg, the Pleiades).  
> **username_0**: @username_2 I dont  know why but I pictured you much younger... Strange how a universe can change with time. Though I dont think the future has any possible change left.  
> **username_3**: Stars in the Andromeda Galaxy were not visible to humans until recently, about 100 years ago. The [Shapley–Curtis Debate](https://apod.nasa.gov/debate/debate20.html) occurred 101 years ago. Up until then, the predominant theory was that the Milky Way was the universe. This debate marked the start of modern astronomy. It wasn't until the late 1920s when Hubble's work on  Cepheid variable stars convinced most (but certainly not all) astronomers that the Andromeda Galaxy was indeed well removed from the Milky Way.  
> **username_4**: @username_0 See my answer at:  https://hsm.stackexchange.com/questions/13303/why-did-it-take-so-long-to-discover-the-shape-of-the-milky-way/13310#13310  
> **username_4**: There is an answer to a similar question at: https://hsm.stackexchange.com/questions/13303/why-did-it-take-so-long-to-discover-the-shape-of-the-milky-way/13310#13310  
> **username_0**: @username_4 It is the same question. Ii asked on that site too. :)  
> **username_5**: The question should be rephrased "Why *is* it so difficult to figure out the form of the Milky Way?" We're not 100% certain about it.  

---

## 2 answers

### Answer by username_3 on 2021-06-24. Score: 10

> The exact form of the Milky Way was figured out the in early fifties of the 20-th century.

The **exact** form of the Milky Way was not figured out in the early 1950s. It remains a matter of debate. Astronomers agree that the Milky Way is a barred spiral galaxy. How many arms? That's still debated.

Stars in the Andromeda Galaxy were not visible to humans until recently, about 100 years ago. The [Shapley–Curtis Debate](https://apod.nasa.gov/debate/debate20.html) occurred 101 years ago, in April 1920. Up until then, the predominant theory was that the Milky Way was the universe. This debate marked the start of modern astronomy. It wasn't until the late 1920s when Hubble's work on Cepheid variable stars convinced most (but certainly not all) astronomers that the Andromeda Galaxy was indeed well removed from the Milky Way.

It wasn't until the 1950s that radio astronomy became well-used. Infrared astronomy was even later. Radio astronomy and infrared astronomy were what enabled astronomers to see beyond the dust clouds that obscure much of the Milky Way in the visible portion of the spectrum.

It wasn't until the 1970s that distance estimates were good enough to yield a partial picture of the Milky Way. Distance remains a challenge, and without distance, there is no way to develop a picture of the galaxy. Quoting from [Trigonometric parallaxes of massive star-forming regions. VI. Galactic structure, fundamental parameters, and noncircular motions](https://iopscience.iop.org/article/10.1088/0004-637X/700/1/137), which was published in 2009,

> The Milky Way is known to possess spiral structure. However, revealing the nature of this structure has proven to be elusive for decades. The Georgelin & Georgelin (1976) study of H ii regions produced what has been generally considered the "standard model" for the spiral structure of the Galaxy. However, after decades of study there is little agreement on this structure. Indeed, we do not really know the number of spiral arms (Simonson 1976; Cohen et al. 1980; Bash 1981; Vallée 1995; Drimmel 2000; Russeil 2003) or how tightly wound is their pattern. The primary reason for the difficulty is the lack of accurate distance measurements throughout the Galaxy. Photometric distances are prone to calibration problems, which become especially severe when looking through copious dust to distant objects in the plane of the Galaxy. Thus, most attempts to map the Galaxy rely on radio frequency observations and kinematic distances, which involve matching source Doppler shifts with those expected from a model of Galactic rotation. However, because of distance ambiguities in the first and fourth quadrants (where most of the spiral arms are found) and the existence of sizeable noncircular motions, kinematic distances can be highly uncertain (Burton & Bania 1974; Liszt & Burton 1981; Gómez 2006).

Astronomers did not know the exact form of the Milky Way in the early 1950s. They did not know the exact form of the Milky Way in 2009. They still do not know the exact form of the Milky Way to this day as there is a 20° gap on the far side of the galaxy that is obscured by the center of the galaxy.

> **Comments:**
> **username_1**: +1 great answer for not accepting the false premise of the question and giving historical evidence!  
> **username_0**: Great to read that it isnt even known today (the number of arms). Its ineed difficult to know if you are fixed and your vision is obscured on top of that!  

---

### Answer by username_6 on 2022-01-02. Score: 3

The question sort of makes sense, although with inaccurate historical information. It is based on a premise that since we are located inside the Milky Way, it is hard to figure out the exact form of it. Many years ago during my visit to an observatory in northern Virginia, one of the staff working there told us something related to this question. The original statement I have already forgotten, but it is something like the following: since we are located inside it, we are unable to see how it looks like.

If you look at the image of UGC 12158 galaxy taken by Hubble, you can see the full distant view of it, showing a barred spiral galaxy. The fact that Hubble can take the image of UGC 12158 is because Hubble is located outside of and far from that galaxy. The UGC 12158 is called the Milky Way's (almost) identical twin (<https://www.discovermagazine.com/the-sciences/the-milky-ways-almost-identical-twin#more-25551>), which is inspirational for scientists trying to figure out the exact form of Milky Way. Imagine aliens living on a "sister earth" inside the UGC 12158 galaxy take the image of our Milky Way and use it as an inspiration trying to figure out the exact form of their own galaxy.

As we are located inside the gigantic Milky Way, we cannot launch a telescope to fly out of the Milky Way to take its picture and send the data back, and therefore have to rely on many observations and models. One observation can be our view of the hazy band of white light in the night sky. Other various advanced measurements including optical, radio, infrared, etc. are the further extensions of our eyes. Then scientists try to piece the information together and apply theoretical models to figure out the possible form of Milky Way galaxy. You see the logic here, as we are inside it we cannot see its entire form, and therefore have to rely on some observations and theoretical models to figure it out.

Over the centuries our knowledge of the Milky Way has changed considerably. The image of flat spiral in school textbooks is an illustration based on scientists' current knowledge. It is absolutely not the exact form of Milky Way. Scientists are still making new discoveries on the form of Milky Way. For instance, in a recent Science article (<https://www.science.org/doi/10.1126/science.aau3181>), scientists built a 3D model to demonstrate that the Milky Way is not even flat but warped disc. See the penultimate video for the 3D structure of the Milky Way at <https://phys.org/news/2019-08-d-milky-galaxy-cepheids.html>.

In conclusion, it is hard to figure out the form of the Milky Way, partly because of our location inside it, and partly because of the bottleneck of data and models (see <https://sedigism.mpifr-bonn.mpg.de/workshop_talks/A_Pettitt_SEDIGISM_Workshop.pdf>). The word "exact" makes it even more challenging.


---

