We may never leave our Milky Way galaxy and see it from the outside, but the Hubble Space Telescope at least helps us imagine that view.
A newly shared Hubble image that NASA posted on Friday takes a look at NGC 3432. This not-too-distant celestial body (it’s only 42 million light-years away!) is a spiral galaxy much like Earth’s own Milky Way. But because of where we are and how it’s situated, Hubble’s photo amounts to a side view.
You might be wondering how we even know this is what a spiral galaxy looks like from the side, since we’re only able to look at it (and pretty much every other distant thing in outer space) from one angle. NASA, as it turns out, has some experience in the arena of looking at space photos.
“Dark bands of cosmic dust, patches of varying brightness and pink regions of star formation help with making out the true shape of NGC 3432 — but it’s still somewhat of a challenge,” NASA’s Friday post reads. “Because observatories such as the NASA/ESA Hubble Space Telescope have seen spiral galaxies at every kind of orientation, astronomers can tell when we happen to have caught one from the side.”
If you’re anything like me, this photo also got you wondering how we could possibly know that the Milky Way is a spiral galaxy. I’m sure it’s been addressed elsewhere, but I found an explanation on Cornell’s website from former student Sara Slater, who studied physics at Harvard and now works as a researcher at M.I.T.’s Kavli Institute.
Although we don’t have a way to actually snap a photo of the Milky Way, Slater points to three clues that out our home galaxy’s general shape. First, researchers are able to make out the galaxy’s rough shape in silhouette by looking to the galaxy’s center. Our view of it is similar to Hubble’s edge-on look at NGC 3432, with the Milky Way appearing as a long, thin strip with a bulge at the center.
“This is a bit of a tipoff,” Slater wrote. She also noted that the measured velocity of stars and gas in our galaxy suggests “an overall rotational motion that differs from random motions,” and that’s “another characteristic of a spiral galaxy.” And finally, the observed gas and dust content of the Milky Way also falls in line with that of other spiral galaxies.
“So, overall, it’s a pretty convincing argument,” Slater concluded. “Of course, we have to assume our galaxy is not completely unlike the other galaxies we see—once a civilization has accepted that it does not occupy any special place in the Universe, arguments about similarity seem sensible.”