So how many arms does our galaxy have? I see artist's impressions have 2, then scientists in 2013 suggesting 4. How is it so unclear? We can see galaxies hundreds of times further away and see their structure, why can't we determine the shape of our own backyard? Making a 3D map doesn't sound hard, all you need is the distance and direction of a percentage of every star in the galaxy, wouldn't our satellites have this information readily available? Thanks for your response in advanced! :)
Interesting that you bring this up because Phil Plait @badastronomy recently published over at @slateinbrief a piece in response to and rebuttal of the Tweetstorm of ignorance released into the social mediasphere by Ken Ham (yes, that Ham).
Amidst this collection of face palm-inducing “gotcha” Tweets, Ken doll set Phil up to address the complexity of galactic evolution by sharing the below episode (#37) of Crash Course Astronomy where the Milky Way’s spiral arms are discussed in great detail.
“When you look at the spiral of cream in your coffee cup after you stir it, the inner part does spin faster than the outer parts. But galaxies aren’t like that. The arms aren’t coherent structures that spin like cream in your mug o’ java; they’re more like traveling traffic jams. By the way, scientists have known for nearly a century that spiral arms don’t and shouldn’t wind themselves up.”
The way that Phil addresses people who disseminate bad science while having a voice and influence over others - especially in Ham’s case with his taking to the stage to “debate” Bill Nye on evolution, among other things - is quite humbling and inspiring to see during an age of information so widely accessible, searchable, and shareable that we choose not to thoroughly educate ourselves at our own peril and the human beings whom we reach.
You can read the full editorial by @badastronomy HERE. He also published a brilliant piece called ‘Answers for Creationists’ simply to use a platform for science education and critical thinking.
“I’ve said this before, but it bears repeating: I think that people have the right to believe what they want. But when that belief clearly contradicts what we know to be true due to our observations of the Universe, and someone is vocal about it, well, we’re going to have a problem.”– Phil Plait
In regards to your question about why we can’t see the shape of the Milky Way and why our details are so fuzzy on this seemingly simple question (considering the Milky Way is “our” home), reference the below multiwavelength image from NASA GSFC:
We view the Milky Way essentially edge-on from our perspective near the plane of the disk and 28,000 light years from the Galactic center. (x)
Each picture is a panoramic photo—if you want an idea of what we really see, imagine taking each picture and wrapping it in a big circle around you. The photographer has simply chosen to “cut” and “unfold” this circle such that the Galactic Center (the brightest part of the Milky Way) is in the picture’s center. This might give the illusion that we are looking at an edge-on picture of the Milky Way from the outside, but in reality we are not—the left and right edges of this picture simply represent material in the Milky Way that is located in the opposite direction of the Galactic Center from our point of view, and since we ourselves are pretty close to the edge of the Milky Way, there isn’t much to see in this direction.
You can also see from these pictures how much “stuff” there is in the Milky Way which prevents us from seeing through to the opposite side. For example, the optical picture has a bunch of dark dust clouds that almost completely block our view of anything within the galaxy. Luckily, these dust clouds are much more transparent to other types of light (for example, infrared and radio) so we can use these wavelengths to look at objects on the other side of the galaxy from us. (x)
Coconut Science Lab shared a brief animation regarding the data-backed hypothesis that the Milky Way is actually a Barred Spiral Galaxy:
Credit: ESO, NASA JPL Caltech, M. Kornmesser, R. Hurt, University of Central Lancashire, V. Debattista (simulation), L. Noel (animation).
Though we tend to think of the Milky Way as a pure spiral galaxy, new research indicates it is actually a barred-spiral, with a peanut-like, or x-shaped, central core. This inner region of our galaxy has the shape of a peanut in its shell when seen from the side, and of a highly elongated bar shape when seen from above.Barred spirals are quire common in the Universe, though this is the first time that this shape has been clearly seen in our own Milky Way. Simulations have shown that this shape is characteristic of a barred galaxy that started out as a pure disc of stars.
Learn more HERE via NASA/Hubble Space Telescope.
Astronomers now think that the Milky Way started out as a pure disc of stars which formed a flat bar billions of years ago. The inner part of the bar then buckled to form the three-dimensional peanut-shape seen in the new observations.
The results are based on data from two groups of astronomers, who used ESO telescopes to make a three-dimensional map of the central region of the Milky Way. The peanut-shape was mapped using public data from ESO’s VISTA survey telescope, along with measurements of the motions of hundreds of stars in the galactic central bulge. (x)
Also read ‘SO, WHY IS THE MILKY WAY A BARRED SPIRAL?’ via Robert Massey @caltech
Related: ‘Carl Sagan visits his childhood classroom, and discusses images returned from the Voyager spacecraft, answers questions, and communicates our place in space’ from Cosmos: A Personal Voyage
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