First of all scientists have developed maps of the galaxy based on observations from Earth and extrapolation, this is one from ESA from 2013
Artist's impression of the Milky Way
You can only measure distances to the nearest stars directly, through parallax, and to go further you use the Cosmic distance ladder
This video shows the galaxy in 3D, from different angles:
See 3D Map of Milky Way Galaxy Reveals Peanut-Shaped Core (Video)
To make that map, they used a special class of Red Giant stars with known physical properties, 22 million of them which lets you calculate the distance to them precisely from their brightness
We can measure the distances to clusters and star forming regions and denser parts of the nearby galaxy, and several spiral arms have been mapped out that way, though of course we see them best in the region closest to the Earth. So that helps us to fill in the details, like this:
from Astronomers Construct 3D Image of the Milky Way Galaxy
Or this version from Wikipedia
(for some reason they have it the other way up, with the sun at the top. It shows more information, for the individual arms, see e.g. Norma Arm)
The ESA Gaia mission will make this more precise. It aims to find out the precise distance to a billion stars. It can find the distance to stars at the centre of the galaxy, 30,000 light years away to an accuracy of 20%, and for stars close to the solar system to an accuracy of 0.001%. Gaia overview
This is an artist's impression of the satellite
Artist's impression of Gaia
See also the Wikipedia article about it: Gaia (spacecraft)
It will be able to create a much more detailed 3D map of the galaxy.
In future maybe with some "super Gaia" we can map the exact positions of all the stars we can see, both visible, also infra red, x-ray sources etc, right out to the edge of the galaxy. After all Gaia can already go 30,000 light years, and the galaxy is only 100,000 light years across, doesn't seem too impossible that some day we can do an exact map of the whole thing.
Artist's impression of the Milky Way
You can only measure distances to the nearest stars directly, through parallax, and to go further you use the Cosmic distance ladder
This video shows the galaxy in 3D, from different angles:
See 3D Map of Milky Way Galaxy Reveals Peanut-Shaped Core (Video)
To make that map, they used a special class of Red Giant stars with known physical properties, 22 million of them which lets you calculate the distance to them precisely from their brightness
We can measure the distances to clusters and star forming regions and denser parts of the nearby galaxy, and several spiral arms have been mapped out that way, though of course we see them best in the region closest to the Earth. So that helps us to fill in the details, like this:
Or this version from Wikipedia
The ESA Gaia mission will make this more precise. It aims to find out the precise distance to a billion stars. It can find the distance to stars at the centre of the galaxy, 30,000 light years away to an accuracy of 20%, and for stars close to the solar system to an accuracy of 0.001%. Gaia overview
This is an artist's impression of the satellite
See also the Wikipedia article about it: Gaia (spacecraft)
It will be able to create a much more detailed 3D map of the galaxy.
In future maybe with some "super Gaia" we can map the exact positions of all the stars we can see, both visible, also infra red, x-ray sources etc, right out to the edge of the galaxy. After all Gaia can already go 30,000 light years, and the galaxy is only 100,000 light years across, doesn't seem too impossible that some day we can do an exact map of the whole thing.
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