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IAmAPhysicsGuy t1_j4jydnx wrote

If you were a passenger on a merry-go-round, would you be able to work out your position based on the movement or location of the other riders? Or when you look around and see other people on a different ride or a ferris wheel?

Your brain can calculate distance because of the parallax that you get from having two eyes. Or you can see that people look smaller when they are further away (assuming you are familiar with the average size of a person). You can also hear sounds change pitch as you move towards or away from them.

Imagine these same principles applying to different positions in orbit around the Sun throughout the year, type 1A supernovae, and red / blue shift of light, and you can begin to map images of stars and galaxies pretty well.

We can see that we are in a spiral shaped galaxy, and that we are not in a globular galaxy. Our spiral galaxy has arms with varying density, and we can also determine that we are in an area that is relatively less dense, meaning that we are between arms. We also can look towards the center of our galaxy and measure our distance from the middle

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chechomsky t1_j4lhozv wrote

Our galactic “year” (time it takes for the sun to revolve around the center of the galaxy) is 230 million earth years. Is there enough rotation (I assume about 50 years of data) for us to use parallax to get a sense of our location?

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TwentyninthDigitOfPi t1_j4ljh2a wrote

The parallax isn't from the solar system's rotation around the galaxy, it's from Earth's rotation around the sun.

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[deleted] t1_j4mfxnt wrote

Yeah, you'd have to wait like 200 million years for any valuable parallax data from the Sun's orbit around the galaxy.

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Colon t1_j4myfry wrote

sounds like ancient alien knowledge becomes more and more valuable as time goes on

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cantonic t1_j4mb9mn wrote

They will look at a star when the earth is on one side of the sun, then look at the same star when the earth is on the other side of the sun, 6 months later. The change in position is about 180 million miles. How the position of the star has changed in that 6 months gives them enough information to calculate how far away the star is.

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Ordoshsen t1_j4kck6e wrote

> Your brain can calculate distance because of the parallax that you get from having two eyes.

This only works for stars that are very close to us, not for most of the stars we see.

> Or you can see that people look smaller when they are further away (assuming you are familiar with the average size of a person).

If you use average size you're going to have a large error. You need to know how large the star actually is to determine how far it is. You can't just assume all stars are the same on average because we know that's not the case. Also I assume you're actually talking about apparent and absolute magnitude here.

I'll just add that this can be part of the puzzle when using dynamic paralax, but it doesn't work as you have worded it.

> Imagine these same principles applying to different positions in orbit around the Sun throughout the year

Yes, parallax works like that. But you will see no change in brightness of a star on our orbit.

> type 1A supernovae, and red / blue shift of light

Both of these are used for measuring distances to other galaxies. You can tell how far a Ia supernova (standard candle) has occured. But if you happen to see one from our own galaxy, you just know there was one binary star there but generally we just look at a bunch of galaxies and hope to see one there. But you probably didn't even know there was a star there before so you can't use that to measure stars you actually knew about.

red blue shift can tell you distance only of galaxies outside our neighborhood. You can't use it for measuring distance of stars that are next to us.

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mojowind t1_j4kmzg3 wrote

The Gaia satellite is able to measure parallax with an unprecedented precision. Parallax is possible for the thousands that can be seen with the naked eye, and potentially any star in the Milky Way which we have a line of sight view.

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craigiest t1_j4lccoh wrote

Are you just nitpicking details or are you arguing that the answerer is wrong in principle and that we don’t now the size of the Galaxy or our place in it?

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Ordoshsen t1_j4pebpg wrote

We do know the size. And I was wrong with how well we can measure the parallax. But other than the parallax the explanation was still mostly incorrect as far as I can tell.

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e_j_white t1_j4kcf6n wrote

Great answers here, just want to add that we don't "know" for sure. We're still trying to measure the size of Milky Way every year.

We thought it was 100K ly across, but some researchers in 2015 claimed it's 150K ly.

Just recently, a new publication is now saying it could be 200K ly across!

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jeweliegb t1_j4kqde3 wrote

It better not be! It'll render Monty Python's Galaxy Song completely wrong and that's the only way I remember any of those space-related numbers.

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VoilaVoilaWashington t1_j4law8c wrote

> We thought it was 100K ly across, but some researchers in 2015 claimed it's 150K ly.

Just for clarity, this doesn't mean that the measured outer edge is actually that much farther away, but rather that we're finding things farther away.

It's a bit like saying a city is 10km across, but then realizing that there are actually buildings outside that radius. You didn't measure the original wrong, you simply expanded the definition.

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Crowasaur t1_j4kuf6z wrote

Not to mention the expanding Fermi-bubbles out of the core and the remnants of past absorbed galaxies orbiting in, around and through us in long strings

The Milky way looks more like "Deep Space Nine" than a bulging disk.

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kcalb33 t1_j4mjsz6 wrote

I read an article today saying there are milky way stars half way towards andromeda

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Oknight t1_j4mon6h wrote

But remember the size isn't really a determined thing... the galaxy doesn't have "start and end" points it's a vague cloud mostly of surrounding dark matter in a gigantic halo. There's a "center" to the spiral structure of the areas that are most actively forming stars and where gas and dust are densest but that isn't a strongly defined point the way the Sun is for the Solar System.

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holynuggetsandcrack t1_j4kjv5q wrote

So this is a nice question, considering we can figure out a lot of things about other galaxies that seem so unfathomably far away it'd make sense we could figure things out about our own — right?!

Astronomy/astrophysics is an observational science. We do not have the capacity to carry out experiments by nature, of, well, being small humans, meaning we only learn about things by looking at them. What can we learn about our Galaxy looking at it? Turns out, not much.

We know its shape, that's very evident from just the night sky alone. We can observe the black hole at its center, but we don't really know a whole lot about black holes, so all we can say about this one for sure is the one thing we know for all the others: they're old! And with a certain degree of accuracy we can determine its mass, by looking at how other things behave and move around it.

Do we know the galaxy's age? We in fact do! We look at the oldest globular clusters we can find in the milky way, figure out how many heavier elements they have (particularly Be), and from then we can figure out how many stars existed before these and thus — the age.

What about the size and shape, and our position in it? This is difficult... We can look at Cepheids (special types of stars that change brightness allowing us to discern their distance and the distance of objects around them) on the other end of the milky way and determine how far those are, but this really is not accurate. It's like measuring a basketball court by taking the distance between two furthest players. Not very accurate is it? We can't know the way it looks, zero way to do this without stepping outside, and we can somewhat discern where we are...in comparison to other objects in the galaxy? This is also a very rough estimate.

In short, the only real things we can know about the galaxy is the type and age. We can figure out a whole lot about objects in it though. :)

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[deleted] t1_j4jqryb wrote

[removed]

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filladelp t1_j4lf5ad wrote

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

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seriousnotshirley t1_j4kxbsk wrote

We look for radiation from hydrogen that occurs when the electron’s spin flips. This releases a photon with a very precise frequency that is seen with a radio telescope. It’s very rare for any single hydrogen atom to exhibit this spin flip but there’s a lot of hydrogen in space so it happens regularly.

While the frequency of the photon that’s emitted is precise the frequency we observe is not. The frequency we observe can be blue shifted or red shifted by the hydrogen moving towards or away from us as we and the hydrogen move around the galaxy.

With an estimate of the gravity of the center of the milky way we can estimate how from out the hydrogen we observe must be for it to be moving at the velocity that it does. That provides a basic model of the structure of the Galaxy.

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BrooklynVariety t1_j4lvkib wrote

> With an estimate of the gravity of the center of the milky way we can estimate how from out the hydrogen we observe must be for it to be moving at the velocity that it does.

This would be a terrible way of doing this since you have know the geometry and mass distribution of the galaxy to have a model of how stars should be rotating. On top of that, even if you had a good model, you only measure line-of-sight velocities, making this pretty useless.

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[deleted] t1_j4kkfuq wrote

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BrooklynVariety t1_j4lvzg4 wrote

> Stuff farther away moves faster away than stuff closer to us. So we know relative distances to us.

I blame this on poor science communication, but I see people talking about redshift being used to measure distances in all the wrong contexts.

Redshift ONLY works when measuring the distances to GALAXIES outside our local group. So relative velocities are meaningless even when talking about andromeda, much less stars in the milkyway.

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Puppy-Zwolle t1_j4mxv09 wrote

I was not talking red shift. This is just one of the methods. Not for our neighborhood but does paint a picture we can apply to our neck of the woods.

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BrooklynVariety t1_j4n0tdd wrote

Not really, the whole “things moving faster further away from us” only applies to the things furthest away from us, not at all our neck of the woods. The physics that governs that phenomenon is irrelevant within the local group and certainly within our galaxy, so it doesn’t really tell us anything about how we map our own galaxy.

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Oknight t1_j4mpblo wrote

Problem is, what do you mean by the "size" of the galaxy? Where is the edge of a cloud?

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Puppy-Zwolle t1_j4myljc wrote

You mention one of the issues. This one is among the rather fluid definitions like; ''How big is a planet ''and ''How small is a moon''.

Size is a relative as big and small. The moon is smaller than the sun and bigger than a tomato. In galactic perspective that's already pretty close.

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