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Galactic archaeologists trace the evolution of the Milky Way

8 November 2022

Improved methods of estimating the ages of stars, combined with new precision position and velocity data, are yielding insights into the major events that shaped our galaxy billions of years ago.

Gaia's view of the galaxy.
This map of the galaxy encompasses more than 1.8 billion stars that have been tracked by the European Space Agency’s Gaia satellite. Credit: ESA/Gaia/DPAC, A. Moitinho; CC BY-SA 3.0 IGO

Archaeologists reconstruct the history of previous civilizations. Researchers in the new field of galactic archaeology have even greater ambitions: They intend to reconstruct the history of the whole Milky Way.

To accomplish that goal, galactic archaeologists need to do the equivalent of excavating stratified layers of earth. Instead of using trowels, they are working with increasingly powerful telescopes and analytical tools that can unearth the ages, chemical compositions, and motions of some of the 400 billion stars that make up our galaxy. With increasing amounts of data and new methods for estimating the ages of stars, the last five years have been a particularly fruitful time in the field, bringing us closer than ever to excavating the galaxy’s past.

Origins of the Milky Way

The formation of galaxies starts with gravity pulling together small clouds of stars and dust to form larger spinning conglomerates. In the case of the Milky Way, radiometric dating of isotopes in select stars suggests the galaxy was born soon after the Big Bang.

Observations since the 1920s have confirmed that our galaxy is a vast rotating spiral disk about 100 000 light-years across. The disk is organized into four spiral arms that contain a relatively high density of stars and interstellar gas and dust. Our solar system sits on a smaller, offshoot arm that is halfway from the galactic center to the visible edge. A central, dense “galactic bulge” of stars, dust, and gas that was identified in the 1940s obscures some of the distant reaches of the galaxy from our view.

Structure of the Milky Way.
This illustrated side view of the Milky Way depicts our galaxy’s basic structure. Features such as the bulge and the disk (which itself consists of a thin, concentrated disk surrounded by a more diffuse one) provide hints about the galaxy’s history. Credit: Adapted from European Space Agency

By the 1980s astronomers had identified more galactic structures. The Milky Way’s spiral disk of stars can be divided into a thin inner disk, about 1000 light-years above and below the galactic plane, and a thicker, enveloping disk that extends 3500 light-years on either side of the plane. A halo of globular star clusters surrounds the thick disk. The thin disk contributes about 85% of the stars in the galactic plane and 95% of the total disk stars. Many of the stars are relatively young, suggesting they were created from gases captured by the galaxy in its later stages, after the formation of the thick disk.

The Milky Way’s major structures didn’t simply arise gradually over time. The galaxy we see today was formed through a turbulent sequence of collisions with other galaxies. To understand the Milky Way’s history, galactic archaeologists need to unpack a jumble of stellar ages and motions and from those reconstruct a chronology of collision events and their role in forming the galaxy we see today.

Determining stars’ ages

Directly estimating the ages of stars is “impossible,” says Benoît Mosser of the Paris Observatory. Rather, ages must be indirectly inferred from observed parameters and models.

One of those parameters is chemical composition, which is obtained from spectroscopic data. In the early days of the universe, elements heavier than hydrogen and helium were made only from the fusion reactions occurring within the first stars. Those initial-generation stars died quickly via type II supernova explosions, which supplied sufficient energy to fuse alpha particles and produce oxygen and other elements such as carbon, magnesium, and silicon. Over time, lower-mass stars formed and then died in type Ia supernovae, which produced even heavier elements, including iron. All those elements became part of the interstellar medium, with small traces incorporated into new stars. The balance of the elements in those stars, determined via measurement of the ratio of oxygen to iron, is age dependent.

In limited circumstances, astronomers can also date stars based on their position on the Hertzsprung–Russell plot of temperature versus luminosity. The method works only for stars that have used up their hydrogen and are becoming giants, a point known as the main sequence turn-off. Modeling shows that at this stage in their evolution, stars of different ages vary in their masses in a predictable way. Later, as they evolve into red giants, it again becomes difficult to differentiate the stars by age.

Hertzsprung–Russell diagram for stars at various ages.
The Hertzsprung–Russell plot of stellar evolution provides few hints about the ages of stars that are still on the main sequence or that have become red giants. But at the point at which stars exhaust the hydrogen in their cores, known as the main sequence turn-off, the relationship between their surface temperatures and luminosities varies by age in a measurable way. Credit: Luca Casagrande

However, there are problems with both the spectroscopy and the Hertzsprung–Russell approaches. The measured temperatures and chemical compositions are based on observations of the stars’ surfaces, which don’t necessarily reflect the properties of the entire star. In addition, stars can undergo changes that mask their true age. For example, one of the stars in a binary system can accrete material from its partner, potentially making it appear younger.

A new approach

To improve their star-dating data, galactic archaeologists have latched on to a technique that was developed to study the Sun: asteroseismology. In the 1960s astronomers started measuring periodic oscillations in our star’s brightness and using the information to model the Sun’s internal structure. In the past decade it became evident that asteroseismology could be done for stars farther afield (see the article by Conny Aerts, Physics Today, May 2015, page 36).

Bill Chaplin of the University of Birmingham in the UK led one of the earliest groups to analyze the natural oscillations of the Sun and other stars. The oscillations occur because of convection on the stars’ surfaces, which generates pressure changes, in the form of sound and buoyancy waves, deep within the star. The frequency of the internal standing waves depends on the star’s size and composition. For example, as the star ages and converts more hydrogen into helium, the speed at which the waves can move decreases, as does the resonant frequency of the oscillations. Chaplin compares the process to that in a resonating musical instrument, where the oscillation of a string at the surface sets up standing waves within the instrument.

The waves cause the star to expand and contract, which leads to observable periodic changes in brightness “on the order of maybe 1 part in 1000,” Chaplin says. Using theoretical models, the brightness patterns can be related to the star’s mass and then its age. Asteroseismology ages have an uncertainty of about 10%, Chaplin says, compared with 25–30% for other methods. “In the end, that gives us a way that is much more precise because it’s . . . inferred by knowing [the star’s] internal structure,” says Andrea Miglio, an astrophysicist at the University of Bologna in Italy.

Miglio has pioneered the use of asteroseismology in galactic archaeology, starting by identifying oscillations in a handful of stars from ground observations. The breakthrough came in 2009 when NASA launched the Kepler space telescope. The probe’s photometer monitored the brightness of approximately 500 000 stars within a fixed field of view. Although Kepler was designed to search for exoplanets orbiting those stars, it also turned out to be ideal for asteroseismology because its measurements of the same stars over a four-year period provided precise data on oscillations (see Physics Today, March 2018, page 16).

Another important step came with the realization that asteroseismology could be used to estimate the ages of red giants, stars that have burned all the hydrogen in their cores. It is difficult to do but valuable to galactic archaeologists because the stars are so luminous—their light is visible from greater distances, and thus further back in time. “So if we study red giants, then we have access to the temporal evolution of the galaxy,” says Luca Casagrande of the Australian National University in Canberra. Between Kepler and its successor, the Transiting Exoplanet Survey Satellite, launched in 2018, galactic archaeologists can now calculate the ages of nearly 100 000 stars, most of them red giants, Chaplin says.

Fresh insights

The ages of stars alone cannot tell the story of the galaxy. Astronomers also need detailed information on the stars’ positions and orbits. That’s where the European Space Agency satellite Gaia comes in. Launched in 2013, it has two telescopes and a billion-pixel camera that precisely track the paths of hundreds of millions of stars. The latest data release came in June. “Gaia has been a revolution in the way we know the Milky Way,” says Miglio. “Before Gaia, we only knew the distances to stars in our backyard.” Now he and his colleagues have access to a detailed galactic map, complete with velocity data to reconstruct stellar orbits.

Combining information on age and orbits, galactic archaeologists now can compare stars in different parts of the galaxy, looking for clues to how various regions formed and for local anomalies in ages or motion. The work has enabled the development of a preliminary chronology of events.

One of the major issues for galactic archaeology now is to understand the Milky Way’s two disks. Stars in the thin disk are at most 8 billion years old, whereas the thick disk contains much older stars. Says Casagrande, “One of the questions that have been around for several decades is: How did this thick disk form?”

An answer could come from the 2018 discovery of the remains of the Gaia–Enceladus galaxy, a former dwarf galaxy whose stars are now incorporated into the Milky Way. The velocity distributions of those stars form a sausage-like shape, so the stars are also known collectively as the Gaia Sausage. The dwarf galaxy’s stars “don’t quite follow the same orbits as the [other] stars,” Miglio says. And because they were born in a stellar medium different from that of the Milky Way’s stars, their chemical compositions as a function of age also differ. On the basis of modeling estimates, at least eight globular clusters and 50 billion solar masses’ worth of stars were added to the Milky Way in the collision, which occurred 8–11 billion years ago.

The motion of a star from the "Sausage" galaxy.
This 3D visualization of the orbits of three Milky Way stars was constructed using data from the Gaia satellite. The star depicted in red likely formed in the Gaia–Enceladus galaxy, which is thought to have merged with the Milky Way at least 8 billion years ago. The star’s highly radial orbit is characteristic of such stars, which inspired the term Gaia Sausage. Credit: A. Deason (Durham, UK), SDSS, and Gaia/ESA

Dating this event more precisely would help us better understand the chronology of the galaxy. Last year a team including Miglio, Mosser, and Chaplin published a survey of 100 red giants in the thick disk that were identified as part of the Gaia–Enceladus galaxy. They found the captured stars were of a similar age as, or younger than, those that started their lives in the Milky Way. The results indicate that most of the stars that are now found in the thick disk had already formed when the merger occurred.

Another study, led by Chaplin, pinpointed a single bright giant star known as ν Indi, whose original motion in the Milky Way had clearly been thrown off course by the Gaia–Enceladus collision. By using asteroseismology to determine an age of 11 ± 0.8 billion years, the researchers set an earliest date for the merger of about 13.2 billion years ago, and probably no earlier than 11.6 billion years ago.

Casagrande says evidence now points to the merger being a crucial event in the evolution of the two disks we see today. One interpretation is that the collision “puffed up” the galactic disk of the time to create what we now call the thick disk of older, pre-collision stars. The collision may have also brought an infusion of new gas, providing material for the birth of many of the younger stars now found in the thin disk.

Piecing together more clues

The information made available in the past five years has sparked the field, but there are parts of the galaxy that remain difficult to study. Astronomers still haven’t unlocked the chronology of the earliest stages of the Milky Way’s evolution. The inner bulge region near the galactic center is thought to contain the oldest population of stars, but that’s not a sure thing. Spectroscopic evidence obtained in 2017, for example, suggests that about 15% of the stars in that region are less than 5 billion years old, leading to questions of their origins. The galactic center “is probably the part that is still the least explored or understood,” says Miglio. “It is in a region where there’s still a lot of gas and dust. So even with Gaia, getting precise and accurate distances, let alone ages, of [stars] is still limited.”

Cristina Chiappini of the Leibniz Institute for Astrophysics Potsdam, in Germany, is trying to improve on the age estimates of those shrouded stars using a computer modeling tool called StarHorse. The software combines data from Gaia and IR surveys to improve estimates of stellar surface temperatures. She is also hoping the greater sensitivity will help assess the ages of another elusive set of galactic stars. Crossing through the galactic bulge is a large, dense assembly of stars and gas with cigar-shaped orbits, forming a bar that rotates rigidly around the center of the galaxy. “That’s where I believe there are the pristine stars of our galaxy,” says Chiappini. The bar’s existence was ascertained in the 1990s from modeling of gas motions in the bulge region, but little is known about how the bar relates to the formation of the bulge.

Understanding the galaxies that surround us will also give clues to some of the general principles that guided the formation of our galaxy. Mosser is using asteroseismology to study red giants in the Large Magellanic Cloud, one of the galaxies closest to the Milky Way, around 160 000 light-years from Earth. Look any further and “even the brightest red giants will be too dim,” he says.

Galactic center in IR.
This IR image of the bustling center of the Milky Way, combining observations from the Hubble, Spitzer, and Chandra space telescopes, illustrates the challenges facing astronomers who want to determine the characteristics of individual stars. Credit: NASA, ESA, SSC, CXC, STScI

The first data released from NASA’s James Webb Space Telescope in July are also being met with excitement by the galactic archaeology community. The large aperture and IR sensitivity of the multibillion-dollar telescope enables observations of some of the first galaxies in the universe. In early images those galaxies look like puffy blobs, as opposed to our evolved spiral galaxy, though there are claims of the presence of central bars. Chiappini hopes to observe features that resemble the Milky Way’s disks and bulge.

Chiappini is also helping to write proposals for a mission, to be launched around 2050, that would obtain asteroseismology ages for red giants in dense stellar fields and deep inside the galactic bulge. “I will not be here to work on those data,” she says. But like others before her, she will leave behind the knowledge and tools for the next generation to take us farther back into our galaxy’s past.

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