Monday mornings, we gathered in the student lounge, eagerly discussing whatever mind-blowing filigree of physics had been the subject of that week's episode. I still recall one of the ones that made the biggest impression on me -- the tenth episode, "The Edge of Forever," which included, among many other things, wonderful simulations of the motion of stars within a galaxy, and what happens when two galaxies collide. (You can watch a clip of it here.) The simulations were (at the time) state-of-the-art, and certainly enough to blow the mind of a sophomore physics student like myself; what struck me most was that galaxies aren't rigid, and their constituent stars don't "hang together," but move independently around the massive black hole at the galactic core. This can settle down into a shape that seems pretty stable -- such as the spiral pattern of the Milky Way -- or it can destabilize, flinging stars out into space, exploding the galaxy and scattering its pieces across hundreds of thousands of light years.
Sagan, of course, put it best: "A galaxy is a fluid made of a billion suns, all bound together by gravity."
When galaxies collide, it disrupts both completely; at the same time, collisions between the stars themselves are extremely uncommon. However big the stars are, they're still minuscule with respect to the galaxies that contain them. It's like the atoms writ large, isn't it? The seemingly solid matter around you is made up of tiny charged particles interacting through the force of electromagnetism, but in between those particles is... nothing. Matter is mostly empty space, and only seems solid because you're feeling the mutual repulsion of the electrons in your fingers and the electrons on the surface of whatever you're touching. Likewise, most of interstellar space is very close to nothing, and the galaxies themselves are made up of particles (stars) interacting through a different force (gravity), and separated by vast, empty voids.
Makes you almost think that the pagans might have been on to something with their dictum of "As above, so below."
Map of the Milky Way, as it would look from above the galactic disk [Image licensed under the Creative Commons 鄭興武和馬克 裡德（Mark J. Reid）銀河系棒和旋臂結構遺產性巡天(BeSSeL)項目組/南京大學/哈佛-斯密松天體物理中心., Milky Way large, CC BY-SA 4.0]
The streamer has been named Nyx, after the Greek goddess of the night. 250 stars have been identified as being part of Nyx. "The two possible explanations here are that they are the remnants of a [galactic] merger, or that they are disk stars that got shaken into their new orbits because of a collision with the disk of the Milky Way," said study lead author Lina Necib, in an interview with CNN. The likelihood, though, is the former, something that is expected to be confirmed by chemical analysis of the constituent stars. "Galaxies form by swallowing other galaxies," Necib said. "We've assumed that the Milky Way had a quiet merger history, and for a while it was concerning how quiet it was because our simulations show a lot of mergers. Now, we understand it wasn't as quiet as it seemed. We're at the beginning stages of being able to really understand the formation of the Milky Way."
I think it's stunning that we can figure out this sort of thing at all -- that 250 out of the estimated 250 billion stars in the Milky Way started out somewhere else in the universe. I think that's pretty damn impressive. "This particular structure is very interesting because it would have been very difficult to see without machine learning," Necib said. "I think we reached a point in astronomy where we are not data limited anymore. This project is an example of something that would have not been possible a few years ago, the culmination of developments in data with Gaia, high resolution simulations, and machine learning methods."
How pleased and amazed Carl Sagan would have been. He went a long way toward bringing the wonders of the universe, from the largest scales to the smallest, to laypeople. He certainly blew the minds of me and my friends, and that was back in 1980. Necib's comment, that we're still at the beginning of being able to understand the formation of galaxies, tells us that we have a long way still to go -- and that many, many more eye-opening discoveries are sure to come our way in the next years.
This week's Skeptophilia book of the week is for anyone fascinated with astronomy and the possibility of extraterrestrial life: The Sirens of Mars: Searching for Life on Another World, by Sarah Stewart Johnson.
Johnson is a planetary scientist at Georgetown University, and is also a hell of a writer. In this book, she describes her personal path to becoming a respected scientist, and the broader search for life on Mars -- starting with simulations in the most hostile environments on Earth, such as the dry valleys of central Antarctica and the salt flats of Australia, and eventually leading to analysis of data from the Mars rovers, looking for any trace of living things past or present.
It's a beautifully-told story, and the whole endeavor is tremendously exciting. If, like me, you look up at the night sky with awe, and wonder if there's anyone up there looking back your way, then Johnson's book should be on your reading list.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]