The stars of our galaxy keep records of its past. By reading these stories, astronomers will learn more about how the Milky Way came to be and the galaxy we live in today.
New observations are forcing astronomers to take a fresh look at the creation of our galaxy and reconsider the layout of the modern Milky Way.
Late on the evening of October 5, 1923, Edwin Hubble sat at the eyepiece of the Hooker Telescope at Mount Wilson Observatory, located on a mountaintop overlooking the Los Angeles Valley. He observed an object in the northern sky. To the naked eye it was visible as a faint spot. But in a telescope it turned into a bright ellipse called the Andromeda Nebula. To resolve the dispute about the size of the Milky Way, which was then considered the entire Universe, Hubble needed to determine the distance to Andromeda from us.
In the telescope’s field of view, Andromeda was a giant. Hubble patiently made several exposures on many glass photographic plates, and in the early morning of October 6, he took a 45-minute exposure on a small glass plate and scratched an “N” where he saw three new stars, or novas. But when he compared his image with photographs taken by other astronomers, he realized that one of the new stars was actually a Cepheid variable star. This type of star can be used to measure astronomical distances.
He crossed out one letter “N” and wrote “VAR!”
Using this pulsating star, Hubble calculated that Andromeda is 1 million light-years from Earth, i.e. at a distance significantly greater than the diameter of the Milky Way (he was slightly wrong: Andromeda is about 2.5 million light years away). And he realized that Andromeda is not just a nebula, but an entire “island universe” – a galaxy different from ours.
In 1923, astronomer Edwin Hubble changed our understanding of the cosmos by measuring the distance to neighboring Andromeda and discovering that it was a galaxy in its own right.
After the division of space into our native Galaxy and the larger Universe, we could seriously begin to study our limited-sized home – and how it exists within the framework of this Universe. And now, a century later, astronomers continue to make unexpected discoveries about the only island in space on which we will live. They may be able to explain some of the Milky Way’s features by re-imagining how it formed and grew in the early Universe, taking a close look at its jagged shape and its ability to form planets. The latest results, obtained over the past four years, provide a picture of our home as a unique place at a unique time.
It seems we’re lucky enough to live next to a particularly quiet star on the tranquil outskirts of a middle-aged galaxy with an odd tilt and a loose spiral that has remained alone for most of its existence.
Our Island Universe
From the surface of the Earth – if you are in a very dark place – you can only see a bright strip of the galactic disk of the Milky Way, and from the edge. But the Galaxy in which we live is much more complex.
A supermassive black hole rotates at its center, surrounded by a “bulge” – a ball of stars that contains the oldest stellar inhabitants of the galaxy. Next comes the “thin disk” – the structure we see in which most of the Milky Way’s stars, including the Sun, are divided into giant spiral arms. The thin disk is surrounded by a wider “thick disk” that contains older stars that are more evenly spaced. Finally, surrounding these structures is a spherical halo composed primarily of dark matter, but also containing stars and diffuse hot gas.
To map these structures, astronomers study individual stars. The composition of each star records its birthplace, age and initial ingredients, so the study of starlight allows for galactic cartography as well as genealogy. By determining the position of stars in time and place, astronomers can trace history and infer how the Milky Way gradually came into being over billions of years.
The first serious efforts to study the formation of the primordial Milky Way came in the 1960s, when Olin Eggen, Donald Linden-Bell, and Alan Sandage, a former graduate student of Edwin Hubble, argued that the Galaxy was a collapsed, rotating cloud of gas. For a long time after this, astronomers believed that the first structure that arose in our Galaxy was a halo, and then a bright dense disk of stars. With the advent of more powerful telescopes, astronomers built increasingly accurate maps and began to refine their understanding of how the Galaxy formed.
Everything changed in 2016, when the first data from the European Space Agency’s Gaia satellite arrived on Earth. Gaia accurately measures the trajectories of millions of stars across the galaxy, allowing astronomers to know where these stars are, how they move through space, and at what speed. With Gaia’s help, astronomers were able to piece together a clearer picture of the Milky Way, which revealed many surprises.
The bulge is not spherical, but more like a peanut, and is part of a larger stripe running through the middle of our galaxy. The galaxy itself is twisted like the brim of an old cowboy hat. The thick disk is also uneven, becoming thicker towards the edges – perhaps it formed before the halo. However, astronomers are not even sure how many spiral arms our Galaxy actually has.
The map of our island Universe is not as neat and calm as it once seemed.
“If you look at the traditional picture of the Milky Way, you see a spherical halo and a regular disk, and everything is sort of settled and motionless. But now we know that the Galaxy is in a state of disequilibrium,” says Charlie Conroy, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “Over the last few years, this picture of simplicity and orderliness has been completely abandoned.”
New map of the Milky Way
Three years after Edwin Hubble realized that Andromeda was a separate galaxy, he and other astronomers began surveying and classifying hundreds of island universes. These galaxies seemed to have a few predominant shapes and sizes, so Hubble developed a basic classification scheme known as a fork diagram: It divides galaxies into two categories—elliptical and spiral.
Astronomers still use this scheme to classify galaxies, including ours. At the moment, the Milky Way is considered a spiral galaxy with arms that are the main breeding ground for stars (and therefore planets). For half a century, astronomers believed that there were four main arms – the Sagittarius, Orion, Perseus and Cygnus arms (we live in a smaller branch, unimaginatively called the Local Arm). However, new measurements of supergiant stars and other objects paint a different picture, and astronomers can no longer agree on the number of arms, their size, or even whether our Galaxy can be considered unusual compared to other cosmic islands.
“Strikingly, almost no outer galaxy has four spirals extending from its center to its outer regions,” Xu Ye, an astronomer at China’s Purple Mountain Observatory, said in an email.
To trace the spiral arms of the Milky Way, Ye and his colleagues used Gaia and ground-based radio telescopes to search for young stars. They found that, like other spiral galaxies, the Milky Way has only two main arms – Perseus and Norma. Several long, irregularly shaped arms also swirl around its core: the Centauri, Sagittarius, Carina, Outer and Local arms. It appears that, at least in shape, the Milky Way may be more similar to distant space islands than astronomers thought.
“Studying the spiraling Milky Way may reveal whether it is unique among the billions of galaxies in the observable universe,” Yeh writes.
Hubble’s study of Andromeda and its variable star was prompted by his bitter rivalry with another famous Mount Wilson astronomer, Harlow Shapley. Harvard astronomer Henrietta Swan Lewitt pioneered the use of Cepheid variable stars to measure distances, and using her method, Shapley calculated that the Milky Way was 300,000 light-years across—a surprising assertion in 1919, when most astronomers believed the Sun to be at the center of the galaxy, and the entire galaxy extends over 3,000 light years. Thus, Shapley insisted that the other “spiral nebulae” must be gas clouds rather than individual galaxies, since their size would mean they were impossibly distant.
Henrietta Swan Lewitt developed a key method for measuring astronomical distances based on the pulsations of Cepheid variable stars.*
Hubble, in turn, recorded the results of measurements of variable stars and convinced everyone that Andromeda was indeed a separate galaxy. “Here is the letter that destroyed my universe,” Shapley said after reviewing the Hubble data.
However, from the point of view of astronomical distances, Shapley may not have been so wrong. Over the past century, astronomers have estimated that the Milky Way’s bulge is about 12,000 light-years across, its disk is 120,000 light-years across, and its halo of dark matter and ancient star clusters extends hundreds of thousands of light-years in all directions.
Recent observation showed that some stars in the halo are scattered over distances of up to 1 million light years – halfway to Andromeda – suggesting that the halo, and therefore the galaxy, is not an independent island of the Universe.
Astronomers led by Jesse Han, a graduate student at the Harvard-Smithsonian Center for Astrophysics, recently determined that the stellar halo is not spherical, as previously thought, but rather shaped like a football. IN workpublished Sept. 14, Hahn and his team also showed that the dark matter halo may be tilted by about 25 degrees, causing the entire galaxy to appear twisted.
And although this may seem rather strange, the tilt itself may indicate the Milky Way’s turbulent past.
Disturbance in the Galaxy
Many centuries before Hubble sat down to the eyepiece, millennia before the birth of the Sun, long before the appearance of the Milky Way, the Big Bang tore apart all matter and scattered it randomly throughout the newborn cosmos. Eventually, the first galaxies formed from pieces of random debris, beginning the 13 billion-year history that led to us. Astronomers debate the subtleties of these events, but it is known that the galaxy in which we now live was formed as a result of a complex process involving mergers and acquisitions.
Throughout the Universe, galaxies collide and combine into unimaginably huge structures. The telescope, named after Edwin Hubble, constantly records these cosmic clutter. And while the Milky Way is relatively quiet today, it is no exception: By sifting through archaeological records left behind by stars, gas flows, globular clusters made up of thousands or millions of stars, and even the shadows of consumed dwarf galaxies, scientists are learning more about how the Milky Way evolved Path.
The first hints of a violent past came when, in 1992, astronomers looking through Palomar Observatory’s famed 200-inch telescope (first used by Hubble) discovered evidence that the Milky Way was tearing apart some of the globular clusters in its halo. The Sloan Digital Sky Survey confirmed this observation, and radio telescopes later discovered that the galaxy was also sucking up streams of nearby gas.
The Milky Way stream sweeps across the dark sky, accompanied by the Large and Small Magellanic Clouds. Eventually, our galaxy will devour its smaller neighbors.
By mid-2018, astronomers concluded that the Milky Way had merged with several small galaxies during its existence, but most of them were quite small. The largest merger, which occurred 10 billion years ago, was thought to involve the Sagittarius dwarf elliptical galaxy, which sent streams of gas and clusters of stars into the Milky Way’s stellar halo. But astronomers weren’t able to study these objects more closely until the Gaia satellite released a second set of data in 2018.
As astronomers studied detailed data on the movements and positions of a billion stars, there were signs of serious disruption in the galaxy. Scientists saw galactic debris in the halo. There, some stars rotate at extreme angles and have different compositions than others, suggesting they originated somewhere else.
Astronomers have taken these strange stars as evidence of a titanic collision between the Milky Way and another galaxy. The merger, which probably occurred between 8 and 11 billion years ago, would have catastrophically destroyed the young Milky Way, tearing the other galaxy apart and causing a storm of new star formation.
The remnants of the colliding galaxy are now called Gaia-Sausage-Enceladus, the result of the independent discovery of merger remnants by two teams of researchers. One team named it after the Greek deity Gaia, the primordial mother of Earth and all life, and her son Enceladus. Another commented that the leftovers looked like sausage. (Some astronomers dispute version that only a galaxy arriving from outside participated in the merger, and suggest that many smaller collisions over a longer period could lead to the formation of the structures that we see now).
The merger changed everything: the halo, the inner bulge and the oblate disk of the Milky Way.
Astronomers are now using a variety of instruments to understand when the Gaia-Sausage-Enceladus merger occurred and how the Milky Way was born and grew as a result.
In March 2022, Maosheng Xiang and Hans-Walter Rix of the Max Planck Institute for Astronomy began with a description of the Milky Way 1.0, a protogalaxy that existed before the mergers. To do this, they used ancient subgiant stars, which were smaller in size than the Sun, have used up their hydrogen fuel and are now increasingly inflated. The brightness of a subgiant star corresponds to its age, and its light serves as an imprint of the material being born. When Xiang and Ricks used these clues to infer the migration history of a quarter of a million subgiant stars, they found that the thick disk formed earlier than theories of galaxy formation had predicted—13 billion years ago, literally just after the Big Bang.
According to popular cosmological theories, such large, well-defined structures should have taken longer to form after the Big Bang. Yet they continue to appear in observations of distant galaxies using the James Webb Space Telescope, says Rosemary Wise, an astrophysicist at Johns Hopkins University.
“It’s possible to piece together how we think our galaxy formed with what Webb sees. Can we get a holistic picture of galaxy formation? Is our galaxy typical?” – she said.
The thick disk may have existed before the main merger, and the appearance of the thin disk coincided with the appearance of Gaia-Sausage-Enceladus, Xiang and Ricks discovered. This double-sided assembly process, resulting in the formation of individual stellar disks, may be common, and it may be crucial for the onset of star formation. Since then, the number of star births has decreased, but the Milky Way still produces about 10-20 new stars per year.
The neighboring Andromeda galaxy is a spiral similar to our Milky Way. But she has a more troubled past. Eventually, Andromeda and the Milky Way will merge into one.
Yuxie (Lucy) Lu, who had just transferred from Columbia University to the American Museum of Natural History, wanted to understand the history of the galactic disk and how it had changed over time. To do this, she studied how chemical changes during the life of stars can help determine where they were born. She focused her attention on such swollen subgiant stars, and in new, unpublished work, she found that metal-rich subgiants – stars with large amounts of elements heavier than helium – began to grow actively around the time of the Gaia-Sausage-Enceladus merger, between 11 and 8 billions of years ago.
Evidence for the existence of Gaia-Sausage-Enceladus continues to accumulate. But astronomers still cannot understand why everything has been calm since then. The Milky Way’s chemical history and structural history appear atypical, Lu says.
Andromeda, for example, has a much more tumultuous history than the Milky Way. It would be strange for our galaxy to remain alone for so long, given the history of other galaxies and the prevailing cosmological model in which galaxies grow by colliding with each other, Wise said. “The story of its merger is unusual. I would say that the question of whether we are truly an unusual phenomenon in the universe remains open,” she said.
Birth of a new island
While some astronomers are uncovering bits and pieces of the galaxy’s past, others are studying how neighboring galaxies may differ from each other, raising questions about the distribution of planets (and perhaps life) throughout the galaxy.
Here, around one particular star in the Local Arm – the Sun – eight planets have formed, four rocky and four gas. But in other sleeves everything may be different. These environments can give rise to different populations of stars and planets, just as specialized flora and fauna develop on continents with different biospheres.
“Perhaps life can only arise in a very quiet galaxy. Perhaps life can only arise around a very quiet star,” says Jesse Christiansen, an astronomer at the California Institute of Technology who studies galactic conditions and their influence on planet formation. “It’s so difficult to assess with just one statistical sample; everything [в нашей галактике] It may be important, or it may be nothing.”
A century after Edwin Hubble scrawled “VAR!” on a glass plate, the panorama of galaxies resolved in JWST’s field of view is changing our understanding of the cosmos and our place in it. Just as we can use the Milky Way as an astrophysical observatory to understand the wider Universe, we can also use the wider Universe and its billions of galaxies to understand our home and how we came to be.
Astronomers continue to study Andromeda, a faint ellipse in the northern sky, following Hubble’s lead. As with Gaia, the Dark Energy Spectroscopic Instrument at Kitt Peak National Observatory will measure individual stars in Andromeda and closely study their motion, age and chemical abundance. Wise also plans to study individual stars in a nearby galaxy using the Subaru Telescope on Mauna Kea.
This will provide new insight into Andromeda’s past and compare it with our galaxy. In addition, this will allow us to look into the very distant future. Our galaxy will eventually destroy two small nearby galaxies – the Large and Small Magellanic Clouds, which are screaming across space towards us. Our galaxy is already beginning to digest them.
“If we were to observe this in a billion years, it would look much more disorderly,” Conroy said. “We appeared at a time when everything was relatively calm.”
Next, Andromeda will join us. The galaxy captured on Edwin Hubble’s glass plates will cease to be an island universe. Andromeda and the Milky Way will spiral towards each other, their star halos intertwined. On timescales unfathomable to us, the disks will also coalesce, heating the cold gas, causing it to condense and ignite new stars. New suns will appear at the edges of the next structure, and with them new planets. But for now, here, in the Local Arm of the only galaxy we will ever know, everything is calm.