This article about a “break” in the Sagittarius arm gets into direct measurements of distances a bit. Basically we can’t measure everything, but given what we can see and the measurements we can make (especially with the Gaia mission), we can work out the spiral arm structure nearby and infer the larger structure based on what we see in other galaxies.
In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure.
“Distances are among the most difficult things to measure in astronomy,” said co-author Alberto Krone-Martins, an astrophysicist and lecturer in informatics at the University of California, Irvine and a member of the Gaia Data Processing and Analysis Consortium (DPAC). “It is only the recent, direct distance measurements from Gaia that make the geometry of this new structure so apparent.”
filladelp t1_j4lf5ad wrote
Reply to How do we know that we are in a certain place inside our galaxy? and how do we know how big it is just by looking at the cross section we are in? by friday_panda
This article about a “break” in the Sagittarius arm gets into direct measurements of distances a bit. Basically we can’t measure everything, but given what we can see and the measurements we can make (especially with the Gaia mission), we can work out the spiral arm structure nearby and infer the larger structure based on what we see in other galaxies.
https://www.nasa.gov/feature/jpl/astronomers-find-a-break-in-one-of-the-milky-way-s-spiral-arms
In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure.
“Distances are among the most difficult things to measure in astronomy,” said co-author Alberto Krone-Martins, an astrophysicist and lecturer in informatics at the University of California, Irvine and a member of the Gaia Data Processing and Analysis Consortium (DPAC). “It is only the recent, direct distance measurements from Gaia that make the geometry of this new structure so apparent.”
Also take a look at https://en.m.wikipedia.org/wiki/Stellar_kinematics