This is the spiral galaxy NGC 2835, imaged by the impeccable, seemingly timeless Hubble Space Telescope.
And this is NGC 1132, an elliptical galaxy captured through the tandem efforts of Hubble and NASA's Chandra X-ray Observatory.
We know what both of these breathtakingly beautiful galaxies look like because we can see them from afar. How then do we know what our own Milky Way galaxy looks like, seeing as how we are inside it?
While we've never been able to zoom out and take a true galactic selfie, there are numerous observations that clue us in to the structure of our home galaxy. The greatest hint comes from looking at other galaxies. While there are perhaps two trillion in our observable universe, surprisingly, they only seem to come in three discernable varieties: spiral, elliptical, and irregular. Spiral galaxies have a mostly flat disk with a bright central bulge and arms that swirl out from the middle. Elliptical galaxies tend to be round or oval with a uniform distribution of stars. Irregular galaxies are like stellar splotches in space, with little structure at all.
Gazing skyward from our vantage point on and around Earth, there are clear signs that the Milky Way is a spiral galaxy. You can see one sign with the naked eye! The Milky Way appears in our sky as a relatively flat disk.
Using more sophisticated methods, astrophysicists and astronomers have provided two more clues to the structure of the Milky Way.
"When we measure velocities of stars and gas in our galaxy, we see an overall rotational motion that differs from random motions," Sarah Slater, a graduate student in cosmology at Harvard University, wrote. "This is another characteristic of a spiral galaxy."
Moreover the gas proportions, colors, and dust content are similar to other spiral galaxies, she added.
Aside from these lines of evidence, astronomers are also using their tools in ingenious ways to map the structure of the Milky Way. Just this year, scientists used two radio astronomy projects from different parts of the globe to measure the parallaxes – differences in the apparent positions of objects viewed along two different lines of sight – from masers shooting off electromagnetic radiation in numerous massive star forming regions in our galaxy.
"These parallaxes allow us to directly measure the forms of spiral arms across roughly one-third of the Milky Way, and we have extended the spiral arm traces into the portion of the Milky Way seen from the Southern Hemisphere using tangencies along some arms based on carbon monoxide emission," the researchers explained. They coupled these observations with other gathered data points to construct a new image of the Milky Way. This is our home galaxy, in all its resplendant glory: