It seems rather strange to me that we know do accurately how it is shaped. Would it not be impossible to see the other side of it? I am pretty sure we never saw it from the outside either.
I honestly have troubles of understanding it besides “we simulated with lots of maths” which I’m not even sure is how we did it.
In: Physics
Well, the first hint that it’s a disc, and that we are sat in that disc, is that we see a dense strip in the night sky. This suggests we are looking at a disc from inside it, as it’s almost flat.
There are also various ways of measuring distances to all the stars we can see, which all suggest they live in this spiral galaxy.
Observations and simulations (usually with gravity) allow us to conclude the Milky Way is a spiral galaxy, without ever having seen it from the outside.
Astronomers can get pretty accurate readings for how far away a star is from earth by taking measurements 6 months apart and comparing. This means you can plot that stars position in relation to our star and most of the other stars in our galaxy and from that we can work out the shape of the galaxy. Like plotting thousands of points on a 3 axis graph will build a picture up this is the same. But even then it’s a lot of guess work.
Edit: [article on it](https://www.space.com/amp/milky-way-3d-map-warped-shape.html) [And image of what we have mapped](https://cdn.mos.cms.futurecdn.net/Ggg8bCsJyLdrXQtpsE9GpQ-1200-80.jpg)
>Would it not be impossible to see the other side of it?
That’s not as hard as you would think. There’s a lot of empty space between those stars, and everything’s moving. It would be practically impossible for anything to be hidden behind something else 100% of the time.
> I honestly have troubles of understanding it besides “we simulated with lots of maths” which I’m not even sure is how we did it.
Take a look at the room you’re in. You can’t see it from the top, but just looking at what direction everything is from where you’re sitting and how far it is, you should have a decent idea of what it looks like from above.
We do the same with the stars. We have a decent idea of how far they are, and it’s easy to tell what direction they’re in based on what time of year we can see them. Putting those pieces of information together we can make a map of what our galaxy looks like from above.
Actually, it was radio astronomy that first gave us the concept that we were living in a spiral-armed Galaxy. We couldn’t get there just from visual observation of the Milky Way.
As a teenager, I had the incredible privilege of recreating a piece of the early radio telescope observations with one of the guys who discovered the shape of the galaxy. Now I’m just a 60-year-old retired former English professor and lawyer with a faulty memory of the physics stuff I knew when I was a smart teenager, but I can try to explain.
When you point your radio telescope at the night sky, you pick up emissions all along the radio spectrum. Hydrogen clouds emit radiation at a known spectrum, but these emissions get Doppler-shifted up and down in frequency depending on the movement of the clouded with respect to the Earth.
(The Doppler effect is what makes an ambulance siren change in pitch as it goes by – high-pitched as the siren approaches, squeezing the sound waves, lower pitched as it drives away, stretching them out.)
So back to the radio telescope. When you map out your emissions along the line in which you were pointing your telescope, you get several big bumps at certain frequencies. These are clumps of hydrogen clouds at certain distances. Actually, they represent a cross section of the spiral arms of our galaxy. But to build the whole picture, you have to repeat the process pointing your radio telescope at every point of the Milky Way.
When they put this all together back in the day, they got a picture of vague lumpy and broken bands of matter forming sort of elliptical ripples. It takes a lot of imagination (or genius) to see those early maps and say, aha, the galaxy is in the form of a spiral!
It was very exciting, as telescopes and astronomical photography improved, to find evidence of other spiral galaxies in the universe. Today you can see a picture of dozens of spiral galaxies in a single frame.
Radio astronomy has also improved. You have to know a lot of chemistry and mathematics to interpret the radio emissions it picks up, but it is how scientists detect and assess the chemical compositions of remote objects and clouds of gas. A lot of deep space images you see are visual reconstructions of data gleaned in this way.
There – I’ve tapped out my knowledge in this area and probably committed some howlers in describing what I never completely understood even back in the day (I always sucked at chemistry) but hopefully enough for you to research more if you’re interested.
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