Galaxies come in three main types… ellipticals, spirals, and irregulars.
So, from our limited visual perspective, how in the world do we know we are a part of a spiral galaxy?
We have seen it’s spiral arms.
This really is the sort of question easily answered with some simple searching, you know. 
Cute. But that doesn’t at all address the question, since we’ve known the milky Way is a spiral for decades predating this mission.
It looks like a disc with a bulge in the middle, which is what you’d expect if you were at the edge of a spiral galaxy looking in.
I’ve heard there’s a periodicity to the absorption by dust caused by the “bands” of the spiral arms. Not sure if this was found before or after people became convinced the Milky Way was a spiral though.
Not an answer, but to expand the question; it’s been fairly recently discovered that we are in a barred spiral.
What Q.E.D. said.
i did a small bit of searching, (yes, I know about google) but quite frankly, i’ve heard this for decades. And until now, I had never heard it only had 2 arms.
What was the basis for the claim?
One thing I’ve learned over the years is to never assume or expect from space. it’s a guess. it might be an accurate one, but hell, it was a wag.
The original basis for the claim that the Milky Way is a spiral galaxy was based, I believe, on plotting the three-dimensional location of many stars and nebulae and noticing that they lied along spiral-shaped bands.
This could only occur once astronomy had progressed to the point that distance measurements to those objects were consistent and accurate enough, and that happened in the first decades of the 20th century.
Ed
Did the discovery that the Milky Way is a spiral galaxy predate the observation of other galaxies?
If scientists figured out indirectly that our galaxy was some sort of spiral, and then subsequently saw other galaxies that were spirals, too, then that would serve as a great bit of confirmation for a made-from-scratch hypothesis.
If, on the other hand, the determination of our galaxy’s shape was made after seeing the structure of other spirals (like the Andromeda galaxy), then there’s always the chance that scientists were just interpreting the data to see what they expected to see.
The data’s not really that hard to interpret. You just plot the distance and locations of the starts you see, and that spiral shape just jumps right out at you.
Not all stars, but rather the brightest stars, open star clusters, and nebulae. Stars are pretty much uniformly distributed in the galactic disk. There’s no rifts where there are no stars. In fact, I believe the sun is about halfway between two spiral arms.
Spiral arms are density waves in the gas and dust. So nebulae are concentrated in them. New stars are also found in them, since they form in the densest nebulae. When stars form, they almost always form in clusters, so star clusters tend to be found in spiral arms too. The brightest stars have very short lives, on the order of a million years. So they don’t move very far from where they formed before they expire.
Nope. It’s fairly obvious that the Milky Way is some sort of flattish structure and once people began to speculate that some nebulae are similar distant structures, then there was obviously the possibility by the late 19th century that it might also be a spiral structure. But no-one had a way of establishing that spiral nebulae were collections of stars on the necessary scale.
Two events, either side of 1920, change this. First, Harlow Shapley showed that globular clusters are distributed about a centre distant from the Sun. This gives a scale to the Milky Way and strongly suggests that most of it is a flat disk, thin compared to the radius.
Then Hubble shows that the Andromeda Nebula is a similarly scaled structure outside it. That relatively quickly settles the argument about what spiral nebulae are. The Milky Way becomes seen as just one out of many, many large groupings of stars: galaxies.
Galaxies come in different types, so is it a spiral? When Hubble published The Realm of the Nebulae in 1936, he thought this was plausible, but wasn’t sure. I suspect that it was the flatness issue that he found most convincing. He does say that there’s no evidence of a central bulge - even though it was obvious since Shapley’s work in which direction it might lie.
Subsequent work in the Thirties and Forties on stellar orbits sharpen the issue up a bit, showing that all the stars in the disk are rotating in the same plane and the like.
But what settles the basic question was this famous plot, which is Figure 4 from this 1958 paper by Oort, Kerr and Westerhout. Basically they were using radio telescopes to map out where neutral hydrogen is in the plane of the Milky Way. Their introduction has a brief discussion of the question to that date.
If someone actually takes the time to read the article linked, it explains how the spiral structure was originally determined.
Which answers the OP quite nicely.
This is actually a very difficult question for astronomers to answer. It’s tough to measure directly, and - this might shock you - we still aren’t really that sure what our galaxy looks like. The above posters who talk about “mapping” the sky underestimate the difficulty of finding the distance to stars. We don’t really have a great handle on most of that stuff, and if the only method we used were plotting the distance of every star, we wouldn’t have a very clear picture of our galaxy at all. Measurements of even open clusters and O and B stars can based on apparent brightness are uncertain to 10 percent, and parallaxes, even with Hipparcos, can’t go far enough to see much of the galaxy. So we have to focus on studying properties of the density waves and arms of the galaxy rather than directly mapping out the sky.
To really get this, I think you have to understand the spiral arms of galaxies. It is not just the case that the stars are arranged permanently in the spiral arms, since then the spiral form would disintegrate since the interior of a galaxy rotates much faster than the outside. The spiral would disappear quite quickly.
Instead, the spiral arms are believed to be density waves. In other words, there is a difference in the amount of hydrogen and various star-forming gases in the galaxy, and the clumpy part stretches out into a long spiral. This denser band of hydrogen forms a rich environment for young, new stars - and this is why the band appears bright. In addition, the spiral is propelled around the galactic center by explosions of dead stars. Most stars take a long while to wind down, but some, when they are very large, will burn out very quickly and end in a huge explosion. This explosion (since they have a short lifespan) is always juuuust behind the galactic spiral arm, which pushes the whole density field around the galaxy.
So this leaves us with two primary things we can look for to tell whether we live in a spiral:
-Distribution of emission nebulae, since its the ultraviolet radiation of young, hot stars which cause these clouds of gas to glow. If all the emission nebula are distributed in a spiral or in a bar, then the young, hot stars are probably similarly clustered in a spiral. If that’s the case, we have a spiral galaxy.
-Differences in density of hydrogen, which can be measured using radio telescopes, since radio waves pass right through dust, allowing us to study the density of matter in any given direction. The actual method used is called “spectral-line radio astronomy.”
If you are content with what you know so far, stop now. If not, here’s how spectral-line radio astronomy works:
The idea is this: hydrogen atoms have a proton and an electron. Each of these particles has a certain amount of “spin,” as if it were spinning on an axis. Rules of quantum mechanics dictate that the electron can either be spinning in the same direction as the proton, or its opposite - but nothing else. There is a small difference in energy between these two states - same direction is higher energy, opposite is lower. Sometimes, very very rarely, in a hydrogen atom where they are spinning in the same direction, the electron will spontaneously flip to the lower energy state, and spin in the opposite direction. When it does this, the atom emits a photon which carries that extra difference in energy, and it is emitted as radiation with a wavelength of 21 cm.
This is extremely rare. The half-life is about 11 million years for the transition. But there are enough hydrogen atoms in space that we can detect the 21-cm radiation. Happily, the 21-cm radiation is absorbed by virtually everything in the galaxy since it is so low-energy. It can reach us even from the other side of the galaxy, while most light waves only get about 10 percent of the way to the Galactic center, on average, just due to dust.
Also happily, laws of physics and mechanics tell us that gas at the center of a galaxy rotates with a shorter period than gas further from the center - in other words, it is moving more quickly. We can use this information to check for Doppler shifting. If the 21-cm photons emitted at the center of the galaxy, they will have a slightly higher amount of energy, and so they will look like a (for example) 20-cm photon.
So we can map out the density of hydrogen across the entire galaxy using this method - we take the spectrum of these radio photons and then plot their distance from the center of the galaxy. You don’t have to do the work, though. Here’s an atlas of neutral hydrogen across the galaxy, yours for a cool $180!
Today, we’ve learned some interesting properties about the spiral arms of galaxies that allow us to look for more wavelengths than just the 21-cm line, though hydrogen remains important. Some molecules actually can form even in the cold of space by latching onto dust particles, and they also emit spectral lines which are far stronger than hydrogen’s emission. Observations of carbon monoxide, in particular, are useful as a tracer to detect density differences. The details of molecular radio astronomy are beyond me, I’m afraid, but the idea is roughly the same as that of neutral hydrogen radio astronomy. We can then pool our data for molecular and hydrogen clouding to come up with an idea of the density of the galaxy.
Many questions asked here could be searched out on Google. The point, surely, is to avail oneself of the special expertise and knowledge that many members of the board are possessed of and are willing to share. That’s why it’s such a joy to read these threads, one always learns something new and surprising, something it is most unlikely that a Google search would turn up.
Too late for edit window but:
The radiation is absorbed by virtually nothing in the galaxy. :smack: Duh.
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As a teacher, one of the tendencies of today’s society that I consistently fight is the desire for the easy answer. “Just tell me what I want to know,” my students are often saying. Individuals should be encouraged to do their own thinking, sorting, researching, etc. The modern society’s need for immediate answers without much work is to be decried, in my opinion.
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More practically, the compendium of knowledge we have here is more easily and efficiently accessed if it isn’t having to offer up basic knowledge that can be learned in 5 min. of reading the results of a Google search. If, for example, in this case, the original poster reads up on the basics of the idea online, then comes to us with some clarifying questions not as easily answered, then the experts around here have value. So, after reading what he could on line, the Pot might have come here and said something like, “I don’t get it. How does ______ ?” Then the experts would be able to offer some pointed help.
This is correct. And there are 2 main arms. Scutum-Centarus, and Perseus.
There are 2 minor arms, Norma and Sagittarius. Our sun, Sol, is in a minor dead end off the Sagittarius arm called the Orion Spur.
Now, if someone can tell me why a major arm of our galaxy is named Norma, That would be interesting!
Not as exciting as you might think. As with all the other arms, it’s named after an associated constellation. That’s it.
Even if our distance measurements were perfect, this still wouldn’t work. We can only see stars in the plane of our Galaxy if they’re very close by: Too far away, and they get blocked out by the dust in between. The stars that we can see in visible light extend far enough that we can vaguely tell that there’s a sort of flattening to the shape of the Galaxy, but not nearly enough to see a spiral structure. In fact, William Herschel used a method somewhat like this, and produced a map of the Galaxy as he could determine it. As you can see, it doesn’t look much like our modern pictures.
As long as we’re on the subject, how do know we’re looking INTO the exact center of the galaxy?
If we’re in-between arms, wouldn’t it chance that we could be looking at the inside edge of another arm?
Does our night sky ALWAYS give us purchase of the center of the MW?