Galactic Archaeology

Astronomers are deciphering the violent history of the Milky Way, one star at a time.

The Milky Way Galaxy has settled into a comfortable, sedate life. Our galactic home gives birth to a few stars a year, but not too many. A star occasionally explodes, but not too often. The disk of hot gas around the supermassive black hole at its core sometimes flares up, but not too brightly. All that seems to be missing to complete this picture of contentment is a pair of slippers and a pipe.

That hasn’t always been the case, though. During its early years, the Milky Way was a bit of a hell-raiser. It pulled in several smaller galaxies and ripped them to pieces. The stars and star clusters of the destroyed galaxies were forcibly annexed by the Milky Way, many of them taking up residence in its extended halo. Some of their gas and dust fell into our galaxy’s disk, giving birth to more stars. In short, those smaller galaxies helped mold the Milky Way into the magnificent spiral that spins through the universe today.

“When I was growing up, the idea was that the galaxy formed by the monolithic collapse of a big cloud of gas,” says Simon Schuler, an assistant professor of physics at the University of Tampa. “But evidence now suggests that mergers of smaller galaxies play a very important role in this process.”

Most of those galaxies are long gone (there’s one survivor, but it, too, is doomed). Yet hints of the merged galaxies remain. And like archaeologists, astronomers are digging through the remains to help piece together the history of the Milky Way. They’re learning how the galaxy grew, how its wide disk changed over the eons, and how it’s being influenced by other galaxies even now.

“Galactic archaeology is looking at the fossil record of our galaxy,” says Melissa Ness, an astronomer at Columbia University and the Center for Computational Astrophysics at the Simons Foundation. “We’re winding back the clock to understand what it looked like one billion years ago, two billion years ago, and so on, back to the earliest times.”

Our fossils are rather unusual because we can not hold them in our hands.

“The formation of galactic archaeology has helped astronomers and physicists from disparate fields come together as a community to share data and collaborate, similar to the development of the field heliophysics,” says Samantha Thompson, curator of space history at the National Air and Space Museum. “The data acquired through this effort can therefore help many fields of astronomy. More precise data on many more stars in our galaxy will help astronomers better understand how stars evolve, which impacts their understanding of how the galaxy evolved. The better we understand stars, the better we can understand the planets that orbit them, a key element for understanding stellar systems outside our own.”

Digging for Stars

“Our fossils are rather unusual because we can not hold them in our hands,” says Sven Buder, a research fellow at the Australian National University in Canberra. “That’s because our fossils are stars and the gas and dust between them.”

There are plenty of stellar fossils to examine. The Milky Way is a giant spiral galaxy that contains at least 100 billion stars. Its disk-shape body spans at least 100,000 light-years, with some recent studies putting the diameter at more than twice that. A spherical halo of dark matter—mysterious, unseen matter that accounts for 85 percent of the mass in the universe—and old, faint stars surround the disk, extending hundreds of thousands of light-years in every direction.

Digging through the archaeological record of this enormous island universe requires powerful trowels, picks, and brushes. The toolkit includes telescopes to study the motions, compositions, and ages of millions of stars, and a pioneering space telescope to plot the positions and motions of more than a billion more. Sophisticated computer software hunts for patterns in the mounds of data—stars that move through space together, stars with matching ages, and stars with the same chemistry. The patterns yield not only Big (with a capital B) discoveries—the galactic equivalent of golden idols and hidden burial chambers—but also the trash middens and hearth fires that reveal the details of everyday life.

“The field [of galactic archaeology] had been growing for decades, but the really big change, over the past two decades or so, has been the advent of huge astronomical data sets produced by really large sky surveys,” says Daniel Zucker, an associate professor of astronomy and physics at Macquarie University in Australia. “These enormous data sets are allowing us to disentangle the complex history of the galaxy.”

Teams around the world are conducting several of these surveys, which use a technique called spectroscopy to spread the light from a star into its individual wavelengths or colors. Each chemical element in the star leaves a unique imprint in the resulting spectrum, like a barcode. From those barcodes, astronomers compile a thorough dossier on the star, determining its composition, surface temperature, motion toward or away from us, and whether it has companion stars or planets.

Using stars to trace the history of the galaxy also requires a few other key bits of data: a star’s distance, motion across the sky, and age. Most measurements have come from what may be galactic archaeology’s most-valuable player: the European Space Agency’s Gaia space telescope.

“Using pre-Gaia data, people were making huge inroads in the field,” says Diederik Kruijssen, an astrophysicist at the Heidelberg University in Germany. “Gaia blew them all out of the water. It’s looking at more than a billion stars, giving us their motion in the sky and their exact distance. And for about seven million stars, it’s adding radial velocity [the star’s motion toward or away from us], so we’re getting the 3-D positions and velocities of all these stars. When you combine the Gaia data with the big spectroscopic surveys, which tell you the composition, it’s possible to do archaeology.”

Generation Gaps

Gaia is producing detailed maps of the galaxy. To extend those maps into the fourth dimension—time—scientists also need to know the ages of stars. “In archaeology, one of the most useful things you can learn is how old something is,” says Jennifer Johnson, a professor of astronomy at the Ohio State University and one of the leaders of the Sloan survey. “In galactic archaeology, we’ve learned a lot about how to get ages for stars.”

Until the last decade or so, astronomers settled for estimates based on models of how stars evolve. Today, though, thanks to improved modeling based on a star’s composition and a few other key developments, they can determine the ages of stars—and especially groups of stars—fairly accurately.

One of those other developments came from an unlikely source: the Kepler space telescope. It was designed to hunt for planets orbiting stars outside our own solar system, and after it was launched in 2009, it discovered thousands of confirmed or candidate worlds. To do so, the telescope kept a constant eye on hundreds of thousands of stars for periods of weeks to years. These observations revealed changes on the surfaces of many of the stars, which were produced by sound waves rippling through their interiors. Just as sound waves traveling through Earth enable scientists to probe our planet’s interior, the stellar waves reveal a star’s internal structure. As a star evolves, that structure changes, so mapping its innards provides an accurate measurement of its age.

“Kepler has become a huge boon for galactic archaeology,” says Ness. “It gives us a benchmark standard for measuring the ages of stars,” enabling astronomers to determine the ages of other stars by comparing their characteristics to those in the Kepler sample. In addition, Ness says the Kepler technique provides a “beautiful symbiosis” with techniques that derive ages by measuring stellar chemistry, which is determined by a star’s birthplace and the material it was born from, as well as the subsequent dredge up of material produced in the nuclear reactions that take place in its core.

When a star is born, it consists almost entirely of the two lightest elements—hydrogen and helium—which were created in the Big Bang. Almost every other element in the periodic table is forged in the hearts of stars through nuclear fusion, in which lighter elements combine to make heavier ones (and release the energy that makes a star shine). Over time, some of the newer elements rise to the surface. Measuring the amounts of different elements, combined with a star’s mass and other details, reveals how long the star’s nuclear reactor has been operating.

Each generation of stars produces more heavy elements, while the exploding stars known as supernovas produce even more during their cataclysmic deaths. As they die, they expel much of their material into space, where it can be incorporated into new stars. As a result, each succeeding generation of stars is born with a slightly higher percentage of heavier elements, so precise measurements of a star’s composition help reveal its age.

Smaller galaxies give birth to fewer of the massive stars that die as supernovas, so measuring the ratios of the elements they produce in stars of similar ages can reveal where the stars were born.

Stellar Immigrants

It turns out that quite a few of the stars in our galaxy are immigrants, born in smaller galaxies that were gobbled up by the larger Milky Way.

Astronomers have discovered those immigrants primarily in the galaxy’s halo, either in long streamers of stars or in families of stars known as globular clusters—dense balls of hundreds of thousands of ancient stars. A third or more of the Milky Way’s 157 known globular clusters might have been acquired from other galaxies.

“Globular clusters are super old—almost as old as the universe itself,” says Kruijssen, whose team trained a neural network to relate the properties of globulars to their parent galaxies. “We can measure the number of stars they have, their composition, how they’re moving, and where they’re located in the Milky Way. These properties encode something about where they came from.”

The streamers, on the other hand, are like great freight trains with hundreds or thousands of cars. They were pulled away from their parent galaxies by the Milky Way’s stronger gravity. A streamer’s stars are about the same age and composition, and they orbit the center of the Milky Way together. In some cases, they’re connected to individual globulars.

Combined, these observations have revealed a complex merger history for the first half of the Milky Way’s life.

Our galaxy probably was born about 13.5 billion years ago, only a few hundred million years after the Big Bang. Giant clouds of gas collapsed to form the first stars (including those in globular clusters), which clumped together in relatively small galactic nuggets. In the crowded early universe, many of those nuggets merged to form larger galaxies, including the Milky Way.

Astronomers say it’s impossible to disentangle those earliest mergers. It is possible, however, to trace the merger history back to about 11 billion years ago. There’s evidence of five significant events (plus about 10 smaller ones), all of them involving galaxies that were much smaller than the Milky Way. “The big picture is that the Milky Way formed unusually quickly, and had a very quiescent life after that compared to galaxies of similar size,” says Kruijssen. “I have a hunch that this may be one of the main reasons why we exist. Colliding galaxies are not nice places to live in. You get a lot of new star formation, and a lot of the stars detonate as supernovas. That’s not a human-friendly environment because a close supernova can produce mass-extinction events.”

Cosmic Sausage

The first merger for which astronomers have found evidence was also the most dramatic. Kruijssen and his colleagues predicted the existence of this galaxy in 2019 by comparing their simulations of the formation of the Milky Way and its globular clusters to the observed globular cluster population. They calculated that the total mass of the stars in the merging galaxy, named Kraken, was about only three percent that of the Milky Way (although it had a higher proportion of dark matter, boosting its relative heft). Yet Kraken was the largest merging galaxy relative to the Milky Way at the time of any merger and contributed roughly 13 of the Milky Way’s globular clusters. The debris of Kraken has since been found by Kruijssen’s team as well as by three other, independent groups.

Mergers with two smaller galaxies followed—the Helmi streams and the Sequoia galaxy. And about nine billion years ago, a galaxy about as massive as Kraken crashed into the Milky Way. It’s known as the Gaia Sausage, Gaia-Enceladus, or the Gaia-Enceladus-Sausage (GES). It is thought to have probably contributed about 20 globular clusters to the halo.

“That merger is noteworthy, as it was quite a massive event,” says Sergey Koposov, an astronomer at the University of Edinburgh who has studied the merger.

“We see a substructure of stars that we think, based on their motions, didn’t form in the Milky Way,” says Zucker. “They were absorbed, mixed in. But their motions are sufficiently distinct that they stand out. It’s like the Milky Way ate the sausage, but it’s not completely digested.”

The GES merger might have had a major impact on the Milky Way’s disk as well as the halo. The disk is built sort of like a round ravioli, with a thin disk enfolded in a thicker one. Roughly nine-tenths of the stars reside in the thin disk, where they’re more tightly packed. On average, stars in the thin disk are billions of years younger than those in the thick disk.

Even before GES was discovered, scientists had proposed that the disk gained that layered structure when a smaller galaxy slammed into the Milky Way. As stars plowed through the disk, they stirred up its resident stars and gas clouds, making the disk puffier. In addition, gas from the colliding galaxy fell into the disk, sparking the birth of new stars, especially along the galactic plane. Many of the galaxy’s early stars stayed in the newly created thick disk, while the thin disk gained a new generation of stars.

Although the idea is still “somewhat speculative,” Koposov says, the timing of the GES merger with the Milky Way appears to match the creation of the thick disk, suggesting that the smaller galaxy was responsible for sculpting the disk’s layered structure.

The last big merger began perhaps six billion years ago and is continuing today, although the merging galaxy wasn’t discovered until 1994. That’s because even though the Sagittarius dwarf is the Milky Way’s closest galactic neighbor—about 70,000 light-years from Earth, in the outskirts of the disk—it’s behind the galactic center, so it’s veiled by both bright stars and dark clouds of dust. Since its discovery, astronomers have found “tails” of its stars wrapping all the way around the Milky Way.

Simulations suggest that Sagittarius has looped through the Milky Way three times, stirring up our galaxy with each encounter. It has contributed gas that has given birth to new stars. And like a bowling ball thrown in a pond, Sagittarius has created waves in the Milky Way’s disk that have helped compress its gas clouds and lead to the birth of even more stars.

Beyond the Milky Way

Tracing the history of the Milky Way is only the first step for galactic archaeologists. “At a top level, the goal of the subject is to learn enough about the physical processes that have led, over the history of the universe, to the galaxy which is our Milky Way today—and that we then establish our galaxy as a ‘Rosetta Stone’ to compare with other galaxies across cosmic time,” says Gerry Gilmore, a professor of experimental philosophy at England’s Cambridge University and one of the original proposers of the Gaia mission. “We aspire to learn what is always important in galaxy evolution, what leads to different outcomes, what is structural, what is chance.”

There’s plenty of “future” archaeology for scientists to study as well. Although the modern Milky Way is fairly quiet, its hell-raising days aren’t over. It might someday absorb the Magellanic Clouds in as little as 1.5 billion years.

Simulations suggest that Sagittarius has looped through the Milky Way three times, stirring up our galaxy with each encounter.

And, in about 4.5 billion years, the Milky Way will collide with M31, the Andromeda Galaxy, which is larger and more massive than the Milky Way. For a while, the giant galaxies will look like a pair of dragons locked in mortal combat, with great streamers of stars flung into space and the fires of gigantic stellar nurseries blazing across their bodies. After perhaps several crossings, the galaxies will settle down, forming the most sedate of all galaxies, a giant elliptical. The elliptical will look like a dull, fuzzy lozenge—full of old stars that are slowly fading away. It will once again be time to pull out the slippers and pipe.  

Damond Benningfield is a freelance science writer and radio producer in Austin, Texas.

Trowels, Picks, and Brushes

A guide to the astronomical tools used by galactic archaeologists.

The Galactic Archaeology with HERMES (GALAH) Survey, begun in 2013, seeks to produce in-depth data on one million stars in the Milky Way galaxy. GALAH’s principal instrument is the Anglo-Australian Telescope’s HERMES spectrograph, which enables exquisitely detailed measurements to be made on 400 stars at a time—including each star’s velocity and its composition based on 25 chemical signatures. To date, the project has observed more than 700,000 stars.

The Gaia space observatory, launched by the European Space Agency in 2013, is conducting a census of one billion stars—representing up to one percent of the stars in the Milky Way—by charting their positions, distances, movements, and changes in brightness. Gaia monitors each of its target stars about 70 times over a five-year period. The spacecraft’s payload, consisting of two optical telescopes that work with three science instruments, can measure the distance of nearby stars to an accuracy of 0.001 percent and the distance of stars close the galactic center—about 30,000 light-years away—to an accuracy of 20 percent.

The Gaia-ESO Public Spectroscopic Survey, which is the ground-based extension to the Gaia space mission, is obtaining high-quality spectroscopy of some 100,000 Milky Way stars by means of the Very Large Telescope’s Fibre Large Array Multi Element Spectrograph (FLAMES) in Chile.

The Kepler space telescope, launched by NASA in 2009, spent more than nine-and-a-half years searching for Earth-size exoplanets in the Milky Way. But the telescope also contributed valuable stellar data to the study of galactic archaeology. That’s because Kepler reveals the existence of exoplanets by measuring the reduction in a star’s brightness when a planet passes in front of it (known as the “transit method”). Researchers can further refine the estimated diameter of an exoplanet by calculating the precise size of its star through asteroseismology, which is the measurement of sound waves caused by turbulence in the near-surface layers of stars. Those seismic studies yielded details about the internal structure of tens of thousands of stars, including more precise measurements of their age.

Mark Strauss is the managing editor at Air & Space Quarterly.