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Galaxy Formation and Evolution

Galaxies are home to most of the stars in the universe, and they form the beads of the cosmic jewelry that defines structure on the largest scales. But galaxies haven’t always been around, and they have changed over the universe’s 13.8 billion-year history. Astronomers study the ways galaxies form and evolve by comparing the different shapes across the history of the cosmos, and tracing how they came to look the way they do.

Our Work

Center for Astrophysics | Harvard & Smithsonian scientists study galaxy formation and evolution in a variety ways:

  • Looking for hidden structures and unusual stars that reveal the Milky Way’s history. Since our galaxy grew by merging with and eating other galaxies, traces of that violent past are visible in the form of streams of stars that were pulled from other places. In particular, the Milky Way is still stripping stars from its satellite galaxies, producing the “tidal streams” astronomers have observed.
    Farthest Stars in Milky Way Might Be Ripped from Another Galaxy

  • Combining theory and observational data to understand how spiral galaxies grow and maintain their spiral arms. These structures are defined by bright young stars, and sometimes — but not always — occur in galaxies that are gravitationally interacting with nearby galaxies. Astronomers look for differences and common features that might reveal the origins of spiral arms.
    New Insights on How Spiral Galaxies Get Their Arms

  • Searching for the earliest galaxies in the universe, which were the progenitors of modern galaxies. These objects are too far to be seen directly, but closer-by galaxies and clusters magnify their light through gravitational lensing.
    Discovering Distant Radio Galaxies via Gravitational Lensing

  • Studying the interactions between the Milky Way’s supermassive black hole Sagittarius A* and the rest of the galaxy. The central part of our galaxy is difficult to observe, since a lot of stars and other matter lie between Earth and Sagittarius A*, with the very nucleus of the Milky Way being particularly jam-packed. For that reason, astronomers are still finding new objects near the black hole, and signs from when Sagittarius A* was a more active black hole in the past.
    Milky Way Had a Blowout Bash 6 Million Years Ago

  • Observing the complex effects supermassive black holes have on their host galaxies. Particularly, black holes can either boost or suppress star formation, depending on the particular circumstances. Using NASA’s Chandra X-ray Observatory, astronomers have identified black-hole powered feedback loops in some galaxies that stifle the creation of new stars.
    NASA's Chandra Observatory Finds Cosmic Showers Halt Galaxy Growth

  • Creating theoretical models to describe the interactions between black holes and their galaxies, in part to understand how those black holes got so huge. Since the matter feeding a supermassive black hole comes from its surroundings, a merger of galaxies potentially provides a lot of new food. Researchers use observational data to understand the way this happens, and extrapolate to testable theories about black hole growth.
    Dark Matter Guides Growth of Supermassive Black Holes

Our Galactic Home and Its Place in the Cosmos

Even though we can’t see the Milky Way from the outside, astronomers have been able to deduce its shape and many of its details from our inside perspective. Researchers also can compare the unique information we have from living inside the galaxy to what we observe in other galaxies.

 

Building a Galaxy

Galaxies are a diverse bunch, though astronomers group them into a few categories according to features they have in common. Spiral galaxies, a category including the Milky Way, are the most common large galaxies in the universe. Researchers observe huge numbers of these galaxies to understand how their spiral arms form and how long those arms last.

Studies show that big galaxies are made from smaller galaxies, based on how populations of stars are distributed. Astronomers use that knowledge look for the oldest and farthest galaxies, which are the building blocks for the modern versions. While most of those are too small and far away to see directly, a few are visible when magnified by gravitational lensing. In other cases, astronomers can piece together indirect evidence about them without seeing the galaxies themselves.

The largest galaxies in the cosmos, the giant ellipticals, are probably made from mergers between many smaller galaxies. Researchers perform galactic archeology to find signs of those older galaxies within the giant ellipticals.

Finally, researchers also look for galaxies in the process of merging or eating each other. These events can last a very long time, so any particular one we witness is more or less frozen at a moment in time during the merger. Looking at many of these, along with traces of past mergers, helps fill in the rest of the details, showing how the history of each galaxy gives it the unique appearance it has.

 

The Black Hole-Galaxy Connection

Nearly every large galaxy, including the Milky Way, is host to at least one supermassive black hole. Astronomers discovered that these black holes help shape their home galaxies by generating flows of material both toward and away from the galactic center.

Today, the Milky Way’s black hole is very quiet, but there’s evidence that in the past it was “active”, jetting out matter and churning the central parts of the galaxy. That’s in keeping with observations of other galaxies, which in earlier times were often host to the very active black holes known as quasars, blazars, and other phenomena. The Milky Way’s black hole, known as Sagittarius A* — pronounced “A star” — was likely one of these billions of years ago.

Understanding how black holes shape their host galaxies is part of the study of galactic structure and evolution. Astronomers have seen signs showing how black holes can throttle star formation or give it a boost, depending on circumstances. Additionally, when galaxies merge or eat each other, that changes the environment of the black holes. At least in some cases, both supermassive black holes end up at the center, where they might merge into a single even larger object.

The galaxy M101 as seen by NASA's Hubble Space Telescope reveals its beautiful and complex spiral arms. Though spiral galaxies are the most common type of large galaxy, many details of how they form and evolve are still mysterious.

Credit: NASA, ESA, CXC, SSC, and STScI