NASA’s Hubble traces the hidden history of the Andromeda Galaxy

NASA Hubble Mission Team

Goddard Space Flight Center

January 16, 2025

In the years following NASA’s launch Hubble Space Telescopeastronomers have identified more than 1,000 billion galaxies in the universe. But only one galaxy stands out as the most important stellar island near our Milky Way: the magnificent Andromeda Galaxy (Messier 31). It can be observed with the naked eye, on a very clear autumn night, as a cigar-shaped object, approximately the apparent angular diameter of our Moon.

A century ago, Edwin Hubble established for the first time that this so-called “spiral nebula” was actually very far from our own galaxy, the Milky Way, at a distance of about 2.5 million light years, or about 25 diameters of the Milky Way. Before that, astronomers had long believed that the Milky Way encompassed the entire universe. Overnight, Hubble’s discovery shook cosmology by revealing an infinitely larger universe.

Now, a century later, the space telescope named in honor of Hubble has made the most comprehensive study of this alluring empire of stars. The Hubble telescope provides new clues about the evolutionary history of Andromeda, and it appears markedly different from the history of the Milky Way.

Without Andromeda as a proxy for the spiral galaxies of the universe as a whole, astronomers would know much less about the structure and evolution of our own Milky Way. This is because we are integrated into the Milky Way. It’s like trying to understand the layout of New York City while standing in the middle of Central Park.

“With Hubble, we can get enormous detail about what is happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,” said lead researcher Ben Williams of the University of Washington. Hubble’s advanced imaging capabilities can detect more than 200 million stars in the Andromeda Galaxy, detecting only stars brighter than our Sun. They look like grains of sand on the beach. But that’s just the tip of the iceberg. Andromeda’s total population is estimated at 1 trillion stars, with many less massive stars falling below Hubble’s sensitivity limit.

Photographing Andromeda was a herculean task, because the galaxy constitutes a much larger target in the sky than the galaxies regularly observed by Hubble, which are often billions of light years away. The complete mosaic was created as part of two Hubble programs. In total, it took more than 1,000 Hubble orbits, spanning more than a decade.

This panorama began with the Hubble Andromeda Panchromatic Treasure (PHAT) program about a decade ago. Images were obtained at near ultraviolet, visible and near infrared wavelengths using the Advanced camera for investigations and the Wide field camera 3 aboard Hubble to photograph the northern half of Andromeda.

This program was followed by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), recently published in The Astrophysics Journal and led by Zhuo Chen of the University of Washington, who added images of about 100 million stars in the southern half of Andromeda. This region is structurally unique and more sensitive to the galaxy’s merger history than the northern disk mapped by the PHAT survey.

The combined programs collectively cover the entire disk of Andromeda, which is viewed almost edge-on, tilted 77 degrees relative to the view from Earth. The galaxy is so large that the mosaic is assembled from about 600 separate fields of view. The mosaic image is made up of at least 2.5 billion pixels.

Hubble’s complementary survey programs provide information on the age, abundance of heavy elements and stellar masses inside Andromeda. This will allow astronomers to distinguish between competing scenarios in which Andromeda would merge with one or more galaxies. Detailed Hubble measurements constrain models of the merger and evolutionary history of Andromeda’s disk.

A galactic “train wreck”

Although the Milky Way and Andromeda likely formed around the same time several billion years ago, observational evidence shows that they have very different evolutionary histories, despite growing up in the same cosmological neighborhood. According to the researchers, Andromeda appears to be more populated with younger stars and unusual features such as coherent streams of stars. This implies that it has a more active recent history of star formation and interactions than the Milky Way.

“Andromeda is a train wreck. It looks like it underwent some sort of event that caused it to form a large number of stars and then stop,” said Daniel Weisz of the University of California, Berkeley. “This was probably due to a collision with another nearby galaxy.”

A possible culprit is the compact satellite galaxy Messier 32which resembles the stripped core of a once-spiral galaxy that may have interacted with Andromeda in the past. Computer simulations suggest that when a close encounter with another galaxy depletes all available interstellar gas, star formation declines.

“Andromeda looks like a transitional type of galaxy, between a star-forming spiral and a kind of elliptical galaxy dominated by aging red stars,” Weisz said. “We can say that there is this large central bulge of older stars and a star-forming disk that is not as active as one would expect given the mass of the galaxy. “

“This detailed examination of resolved stars will help us reconstruct the past history of mergers and interactions of the galaxy,” Williams added.

New Hubble findings will support future NASA observations James Webb Space Telescope and the next Nancy Grace Roman Space Telescope. Essentially a wide-angle version of Hubble (with the mirror the same size), Roman will capture the equivalent of at least 100 high-resolution Hubble images in a single exposure. These observations will complement and extend Hubble’s enormous data set.

The Hubble Space Telescope has been operating for more than three decades and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Astronomical Research, conducts Hubble science operations for NASA.

Explore more

Media contact:

Claire Andreoli (claire.andreoli@nasa.gov)
from NASA Goddard Space Flight Center, Greenbelt, Maryland

Ray Villard
Space Telescope Science Institute, Baltimore, MD