Look at What Happens When Two Galaxies Collide

Look at What Happens When Two Galaxies Collide

Gravity can do some pretty astonishing things out there in the universe. When it’s not ensuring the downward trajectory of your spilled coffee directly onto your shirt here on Earth, the invisible force is playing arts and crafts with cosmic matter: crushing gas and dust into radiant new stars, smoothing clumpy rock into spherical planets, and, my personal favorite, smushing entire galaxies together. Galaxies are pushed toward each other by gravity, often two or more, until they collide, with their contents quieling and merging into one giant galactic ball.

Astronomers have observed such events, known as mergers, in nearly every stage of the process. When the galaxies become clustered together , it’s as though they are attending a space conference. In the thick of it, when gravity has begun to stretch them out of their original shapes. The end is when all that remains is a chaotic sphere. a faint glow of star material surrounding the orb is the last sign that there has been a merger.

The latest image at the top, taken by the Gemini North telescope from Hawaii, shows the start of a merger with sparkling details. The two galaxies involved–NGC 4567 at top, and NGC 4568 at bottom–will swing around each other, jostling existing stars and sparking new ones, until everything coalesces in about 500 million years. They look almost like little paper hearts for now.

Galaxy merging are among the most imaginative events in all of universe. Sure, supernovas are cool, and so are collisions of black holes. Galaxy mergers offer all this and more. (Zoom into this shot from Gemini North and you’ll spot the afterglow of an exploding star.) Galactic collisions also provide great material for daydreams about extraterrestrial life far beyond Earth. Consider that NGC 4568 and NGC 4567 are full of stars, and most stars, as we’ve learned from observations of our own galaxy, have planets. That means–if we could fantasize for just a bit–the NGC galaxies could be, to someone else, home. What might it be like to exist in the midst of a galactic merger?

The nighttime view “would be quite spectacular,” Vicente Rodriguez-Gomez, an astronomer at the National Autonomous University of Mexico, told me. It would appear that the sky was filled with new stars and there would be warped streams and stars of gas and dust running across it. This view is especially impressive if one lives near the edges of the galaxy. There, the stars would not be as densely packed in the sky than the center. The galaxy that merges with yours would shine in darkness and be brighter than any other star. You would see the big, old galaxy in the darkness as a simple fact of your existence. It’s like our cratered moon.

Even more exciting, you could take in that view mostly unperturbed, because, despite the galactic jumble, the possibility of your sun smashing into another would be extremely unlikely, Moiya McTier, an astrophysicist and the author of The Milky Way: An Autobiography of Our Galaxy, told me. “Have you ever watched a really good marching band, a performance where two groups walk through each other?” she asked me. That’s how stars move in galaxy mergers, passing one another as seamlessly as uniformed musicians gliding across the grass. McTier stated that there will be many more stars, but space remains vast and open. “Most of these stars do not have the potential to collide with another star.” (If your planet was too far out in the boondocks, however, you might be in trouble: The hustle can untether stars from the very edges of a galaxy and fling them out into the depths of intergalactic space. )

Even though the stars are arranged in a straight line, there can be some chaos between them. Jeyhan Kartaltepe (a Rochester Institute of Technology astrophysicist who studies galaxy formation) said that galaxies contain huge clouds of dust and gas. This would result in hazy areas of dust and gas that will eventually fall under their own weight, and then ignite to make new stars. Astronomers on Earth can detect these starburst regions in their snapshots of galaxy mergers, such as this one, captured by the Hubble Space Telescope. These are the Antennae Galaxies; they began to collide about 600 million years ago, and their spiral structures have already been smudged away, yielding to clusters of new stars, seen here as light-blue shimmers.

As the stellar stuff of colliding galaxies migrates in, so will the gigantic black holes that scientists believe sit at the centers of most large galaxies. These dense and invisible objects, which are invisible to the naked eye, will be able to plow through galaxies like meteors. They may also drag stars with them, or swallow some. According to Chiara Mingarelli (an astrophysicist from the University of Connecticut, who studies supermassive dark holes in galaxy mergers), “when two galaxies merge, their central supramassive supermassive Black holes sink to center of newly formed galaxy” and merge into one larger black hole. Three colliding galaxies are most likely to form a binary system. Mingarelli explained that if two supermassive black holes find one another, it is more likely that they will interact with each other. “In the end you could have a random supermassive dark hole just roaming the universe.”

All of this action–the flash of new stars, roving black holes–would unfold over millions, even billions, of years. A hypothetical inhabitant of the NGC galaxies wouldn’t notice any changes in her already brilliant night sky in her lifetime, but she could very well be aware that she’s living within a merger. Hypothetical astronomers in her world could sort through archival observations from earlier generations and collect data for future scientists. They would be able to understand their galaxy’s past, just as astronomers here on Earth have figured out that the Milky Way has previously experienced tiny mergers and absorbed smaller galaxies. These astronomers would be able to outline their future galaxy just like astronomers here on Earth discovered that the Milky Way is heading toward a major merger that will change everything.

Our Milky Way is on a collision course with another spiral galaxy called Andromeda. Today Andromeda is visible as a speck of light in the night sky, but about 5 billion years from now, it will be tangled up with us. Our galaxy’s spiral arms will disappear, and so will our supermassive black hole. Andromeda’s central black hole has the mass of 100 million suns, and it will quickly swallow up our own, which has a comparatively tiny mass of 4 million suns. “It’s going to be like, bloop, done,” Mingarelli said.

Although mergers won’t result in perceptible changes in one person’s lifetime, they could still offer intriguing opportunities for any astronomers living within them. A disadvantage to being in an unentangled, lonely galaxy is that it can make you feel isolated. McTier stated that Earth’s location in the Milky Way is not the most ideal for viewing our galaxy’s home. There’s too much dust and gas in the way. “We have to study other spiral galaxies to learn about the behavior and evolution of spiral galaxies like the one we live in,” she said. She said that “if another spiral galaxy was closer than yours, and it was angled so that you could see the most of it,” then it would be easier to study it.

In other ways, being an astronomer in the middle of a galaxy merger would be frustrating; the night sky would simply be too crowded to observe very distant targets. Rodriguez-Gomez stated that it is difficult to see lines that aren’t contaminated by the merging system. Maybe someone has seen the Milky Way while perched in the midst of a cosmic, radiant collision. Perhaps they have sighed and wondered, as if daydreaming, about what the future might look like.

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