There is an often-ignored giant stretching across our sky beyond the Milky-Way galaxy. If dark matter were visible, the halo around this giant would have covered a full quarter of our sky.
Imaging the central portion of the visible core of this system, the Andromeda galaxy, tens of times smaller than its dark matter halo, required a mosaic of 600 images from the Hubble Space Telescope, which was just made public by NASA over the past week. This panoramic image includes 2.5 billion pixels, a thousand times more than a typical cell phone image.
Andromeda resembles the Milky-Way in its general disk-like structure and its total mass, a few trillion solar masses. It is fair to consider Andromeda as our sister galaxy, in which alien astronomers might have similar thoughts to ours as they use their own space telescope to construct a panoramic image of the Milky-Way. It would take our communication signals 2.5 million years to reach them, comparable in duration to the entire history of the human species on Earth.
Andromeda is heading towards the Milky-Way. The merger of these two sister galaxies will give birth to a giant elliptical galaxy in the future. I dubbed the merger product Milkomeda in a paper that I wrote with my postdoc T.J. Cox in 2007. As a prelude for this merger, the dark matter halo of Andromeda is already starting to overlap with the Milky-Way halo. The ultimate merger will roughly double the stellar and dark-matter reservoirs of the Milky Way. But the rest of the Universe will accelerate away, leaving empty space around Milkomeda in the distant future. If all scientific archives will be lost by some cataclysmic event, will archeology of Milkomeda by future astronomers be able to reveal that it originated from the Big Bang, compared to an alternative hypothesis of an island galaxy that was embedded in vacuum for a long time? A paper that I published showed that hypervelocity stars ejected by Milkomeda can be used to trace the cosmic expansion and inform future astronomers of the global cosmic context.
This is the only paper in my publication record that has a chance of being cited trillions of years from now.
Evidence of the gravitational attraction between the Milky-Way and Andromeda comes from their relative motions. The galaxies are falling toward each other at nearly 270,000 mph, given that spectral lines of Andromeda’s light appear to be blue-shifted — displaced toward the blue end of the spectrum — by the Doppler effect. In contrast, all galaxies beyond the local group are flying away from the Milky-Way.
Numerical simulations are indispensable for understanding galactic mergers. One of the distinguishing characteristics of galaxy interactions is the appearance of long streams of stars and gas that stretch from one or both of the participant galaxies. These features are commonly labeled tidal tails. As the tails form, stars and gas are ripped from the host galaxy into intergalactic space.
As the Local Group of galaxies evolves, the Milky-Way and Andromeda will begin to have a dynamic impact upon each other owing to their mutual gravity. As a result, it is possible that the Sun — along with the Earth — will be dragged into a tidal tail. During this period, a solar system astronomer would have a unique vantage point. Torn shreds of the Milky Way will fill a large fraction of the night sky as our galaxy experiences its gravitational dance with Andromeda. Since only a small fraction of a galaxy’s mass ends up in tidal tails, it is more likely the Sun will go for a much less dramatic ride. Most of the stars in merging galaxies remain relatively close to their host galaxies.
The Sun’s trajectory will be far more chaotic than it is now owing to the rapid fluctuations in gravity induced by the merger. What would this mean for Earth and its future residents? Our research suggested the Milky Way and Andromeda will begin to interact strongly 2 billion years from now, and then complete the merger in about 5 billion years. These milestones overlap with the Sun’s remaining life span of 7.6 billion years.
Although the Milky-Way and Andromeda will merge, stars within the two galaxies, such as our Sun, will not physically collide. This is because of the extremely large distances between individual stars in galaxies. For example, if the Sun were the size of a ping-pong ball, the nearest star, Proxima Centauri, would be another ping-pong ball 715 miles away.
Our simulation indicated that the Sun’s ultimate orbit inside Milkomeda will look very different. The Milky-Way and Andromeda host most stars in a planar disk on nearly circular orbits around the galactic center. In contrast, Milkomeda will be nearly spherical in shape and much smoother in appearance than any spiral galaxy. Stars within Milkomeda will follow complex orbits, with many of them spending brief periods of time near the dense galactic center, but orbit much farther away most of the time.
The spheroidal shape of Milkomeda is not unusual, as it characterizes a major class of galaxies called “elliptical galaxies.” Presumably, many of these galaxies in the present-day Universe formed out of mergers between galactic disks at earlier cosmic times.
A recent paper adopted the latest and most accurate observations by the Gaia and Hubble space telescopes, along with recent mass estimates to derive possible future scenarios, taking account of other massive members of the Local Group of galaxies, such as M33 and the Large Magellanic Cloud. Whereas including M33 increases the merger probability, the orbit of the Large Magellanic Cloud runs perpendicular to the Milky-Way & Andromeda orbit and makes their merger less likely. These calculations indicate that uncertainties in the present positions, motions, and masses of all the local galaxies allow different possible outcomes, with nearly half the future scenarios including no Milky-Way & Andromeda merger during the next 10 billion years. Time will tell.
Throughout the past 13.8 billion years after the Big Bang, mergers like the local one were very common. They occurred trillions of times within the observable cosmic volume and perhaps an infinite number of times beyond it. However, for humans this merger will have dramatic consequences only once.
The 1984 novel titled “The Unbearable Lightness of Being,” by Milan Kundera starts with Friedrich Nietzsche’s notion that everything in life happens an infinite number of times, causing the “heaviest of burdens.” Therefore, a personal life in which everything happens only once constitutes “the unbearable lightness of being.”
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
